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SDLRC - Scientific Articles all years by Author - Li+
The Sheahan Diamond Literature Reference Compilation
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcementscalled the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Resource Center
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
The SDLRC provides 3 types of references identified in the reference code. DS for scientific article, DM for a media article, and DC for a corporate announcement. Consider DS0512-0001. The DS stands for "diamond scientific". 05 stands for 2005, the year the reference was posted. 12 represents the month the reference was posted. For all years prior to 2015 the default month is 12. -0001 is the reference's identifier and it does not mean anything. The number below the refence code, ie 2015, is the year the article was published. Note that the posted year may sometimes be later than the published year.
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Geochronology and geochemistry of felsic xenoliths in lamprophyre dikes from the southeastern margin of the North Chin a Craton: implications for the interleaving of the Dabie Sulu orogenic crust.
International Geology Review, Vol. 57, 9-10, pp. 1305-1325.
Earth and Planetary Science Letters, Vol. 494, 1, pp. 92-98.
Mantle
water
Abstract: In this study, we present new experimental constraints on the phase stability and thermal equation of state of an important hydrous phase, ?-AlOOH, using synchrotron X-ray diffraction up to 142 GPa and 2500 K. Our experimental results have shown that ?-AlOOH remains stable at the whole mantle pressure-temperature conditions above the D? layer yet will decompose at the core-mantle boundary because of a dramatic increase in temperature from the silicate mantle to the metallic outer core. At the bottom transition zone and top lower mantle, the formation of ?-AlOOH by the decomposition of phase Egg is associated with a ?2.1-2.5% increase in density (?) and a ?19.7-20.4% increase in bulk sound velocity (V?). The increase in ? across the phase Egg to ?-AlOOH phase transition can facilitate the subduction of ?-AlOOH to the lower mantle. Compared to major lower-mantle phases, ?-AlOOH has the lowest ? but greatest V?, leading to an anomalous low ? /V? ratio which can help to identify the potential presence of ?-AlOOH in the region. More importantly, water released from the breakdown of ?-AlOOH at the core-mantle boundary could lower the solidus of the pyrolitic mantle to cause partial melting and/or react with Fe in the region to form the low-velocity FeO2Hx phase. The presence of partial melting and/or the accumulation of FeO2Hx phase at the CMB could be the cause for the ultra-low velocity zone. ?-AlOOH is thus an important phase to transport water to the lowermost mantle and helps to understand the origin of the ultra-low velocity zone.
Earth and Planetary Science Letters, Vol. 490, 1, pp. 161-169.
Mantle
geothermometry
Abstract: In this study, we have determined the phase boundary between Mg0.735Fe0.21Al0.07Si0.965O3-Bm and PPv and the thermal equations of state of both phases up to 202 GPa and 2600 K using synchrotron X-ray diffraction in laser heated diamond anvil cells. Our experimental results have shown that the combined effect of Fe and Al produces a wide two-phase coexistence region with a thickness of 26 GPa (410 km) at 2200 K, and addition of Fe lowers the onset transition pressure to 98 GPa at 2000 K, consistent with previous experimental results. Furthermore, addition of Fe was noted to reduce the density (?) and bulk sound velocity () contrasts across the Bm-PPv phase transition, which is in contrast to the effect of Al. Using the obtained phase diagram and thermal equations of state of Bm and PPv, we have also examined the effect of composition variations on the ? and
profiles of the lowermost mantle. Our modeling results have shown that the pyrolitic lowermost mantle should be highly heterogeneous in composition and temperature laterally to match the observed variations in the depth and seismic signatures of the D? discontinuity. Normal mantle in a pyrolitic composition with ?10% Fe and Al in Bm and PPv will lack clear seismic signature of the D? discontinuity because the broad phase boundary could smooth the velocity contrast between Bm and PPv. On the other hand, Fe-enriched regions close to the cold slabs may show a seismic signature with a change in the velocity slope of the D? discontinuity, consistent with recent seismic observations beneath the eastern Alaska. Only regions depleted in Fe and Al near the cold slabs would show a sharp change in velocity. Fe in such regions could be removed to the outer core by strong core-mantle interactions or partitions together with Al to the high-pressure phases in the subduction mid ocean ridge basalts. Our results thus have profound implication for the composition of the lowermost mantle.
Abstract: The redox state of Earth’s upper mantle in several tectonic settings, such as cratonic mantle, oceanic mantle, and mantle wedges beneath magmatic arcs, has been well documented. In contrast, oxygen fugacity (graphic) data of upper mantle under orogens worldwide are rare, and the mechanism responsible for the mantle graphic condition under orogens is not well constrained. In this study, we investigated the graphic of mantle xenoliths derived from the southern Tibetan lithospheric mantle beneath the Himalayan orogen, and that of postcollisional ultrapotassic volcanic rocks hosting the xenoliths. The graphic of mantle xenoliths ranges from ?FMQ = +0.5 to +1.2 (where ?FMQ is the deviation of log graphic from the fayalite-magnetite-quartz buffer), indicating that the southern Tibetan lithospheric mantle is more oxidized than cratonic and oceanic mantle, and it falls within the typical range of mantle wedge graphic values. Mineralogical evidence suggests that water-rich fluids and sediment melts liberated from both the subducting Neo-Tethyan oceanic slab and perhaps the Indian continental plate could have oxidized the southern Tibetan lithospheric mantle. The graphic conditions of ultrapotassic magmas show a shift toward more oxidized conditions during ascent (from ?FMQ = +0.8 to +3.0). Crustal evolution processes (e.g., fractionation) could influence magmatic graphic, and thus the redox state of mantle-derived magma may not simply represent its mantle source.
Abstract: Cratons are old and strong continental cores where the lithosphere is thick and remains largely undeformed for 2-3 b.y. Unlike typical cratons, the Wyoming craton underwent pervasive deformation ca. 80-55 Ma during the Laramide orogeny in the west-central United States, and has been subsequently encroached upon by the Yellowstone hotspot since 2.0 Ma. However, the mechanism for the deformation and the craton-hotspot interaction are not well understood. We present here a three-dimensional shear wave velocity model beneath the Wyoming craton constrained from Rayleigh wave data, which reveal new details about the cratonic lithosphere. The average lithosphere thickness beneath the craton is ?150 km, significantly thinner than a normal cratonic root (>200 km). Continuous low velocities are observed beneath the Yellowstone hotspot and the Cheyenne belt. A low-velocity column is also present in the central-eastern craton at depths of 115-250 km. These low velocities can be explained by hot temperature and partial melting, implying mantle upwelling. A high-velocity anomaly with a dripping shape in central Wyoming extends to 200-250 km depth, indicating mantle downwelling and lithosphere erosion. Our model provides the first seismic evidence for complex small-scale mantle convection beneath the Wyoming craton. The convection probably developed during the subduction of the Farallon plate and has been reinforced by the Yellowstone hotspot. We propose that the combination of flat-slab subduction, small-scale convection, and hotspot activity can lead to massive destruction of a cratonic lithosphere.
Earth and Planetary Science Letters, Vol. 412, pp. 42-51.
Mantle
Coesite
Abstract: Compressional and shear wave velocities of coesite have been measured using ultrasonic interferometry in a multi-anvil apparatus up to 12.6 GPa at room temperature for the first time. While the P wave velocity increases continuously with pressure, the S wave exhibits an anomalous softening and the velocity decreases continuously with pressure. Finite strain analysis of the data yielded KS0=103.6(4) GPaKS0=103.6(4) GPa, G0=61.6(2) GPaG0=61.6(2) GPa and View the MathML sourceK0?=2.9(1), View the MathML sourceG0?=0.3(1) for the bulk and shear moduli and their pressure derivatives, respectively. The anomalous elastic behavior of coesite results in large velocity and impedance contrasts across the coesite–stishovite transition, reaching ?39% and ?48% for P and S wave velocity contrasts, and ?70% and 78% for P and S wave impedance contrasts, respectively, at pressure ?8 GPa, with P and S wave velocity perturbations showing no apparent dependence on depths (i.e., View the MathML sourcedln?V(PorS)/dh?0) within 8–12 GPa. These unusually large contrasts and depth independent characteristics render the transition between the two silica polymorphs one of the most plausible candidates for the cause of the seismically observed X-discontinuity. The current P and S wave velocity perturbation dependences on the SiO2 content, d(ln?VP)/d(SiO2)?0.43 (wt%)?1d(ln?VP)/d(SiO2)?0.43 (wt%)?1 and d(ln?VS)/d(SiO2)?0.60 (wt%)?1d(ln?VS)/d(SiO2)?0.60 (wt%)?1, can serve as a geophysical probe to track ancient subducted eclogite materials to gain insights on the geodynamics of the mantle.
Earth-Science Reviews, Vol. 210, doi.org/10.1016 /j.earscirev.2020. 103356 27p. Pdf
Mantle
UHP, geodynamics
Abstract: Eclogites are witnesses of geodynamic processes that are commonly related to subduction of oceanic crust. Information on the part of these processes that refers to the burial of this rock type is rarely published but stored in the eclogitic garnet core and inclusions therein. To better understand general aspects of the burial process, a literature search on the chemical characteristics of garnet in worldwide occurrences of eclogite was undertaken. In most cases extended garnet cores show either a prograde growth zoning with increasing Mg, starting at a few percent of pyrope component, and decreasing Mn contents (type I eclogite) or a (nearly) constant chemical composition frequently with pyrope contents significantly above 10 percent (eclogites of types II and III). Only in minor cases, it is difficult to assign the reported garnet core to an eclogite type. The growth zoning of garnet was thermodynamically modelled for the chemical composition of a basalt following different burial paths. These paths are characterized either by a trajectory along a low geothermal gradient (type I eclogite), as expected for the subducting upper portion of oceanic crust, or a one characterized by nearly isothermal burial at temperatures above 500 °C reaching peak pressures up to 2.1 GPa (type III eclogite), as possibly due to crustal thickening during continent-continent collision, or more (type II eclogite) when basic rocks are tectonically eroded from the overriding continental plate before deep subduction. In addition, diffusion modelling was undertaken on mm-sized garnet demonstrating that the characteristics of the core zoning are not fully obliterated even during residence at temperatures of 800-850 °C within 10 million years. The scrutiny of more than 200 eclogites reported in the literature led to the following result: about half of them are type II eclogites; a third and a sixth can be related to type I and type III, respectively. Among type III are almost all of the few Proterozoic eclogites considered. To demonstrate the benefit of our study, we link the core zoning of eclogitic garnet from various (ultra)high-pressure terranes in Phanerozoic orogenic belts to the geodynamics shaping corresponding orogens. The eclogites in these belts are dominated by type II. Thus, we propose that some of the material of the lower portion of the overriding continental crust was tectonically eroded by a subducted oceanic plate and brought to great depth. Afterwards, this material was exhumed first in a deep subduction channel and then in an exhumation channel during continent-continent collision where a contact with the upper continental plate was re-established. Furthermore, we suggest that type II eclogite can also occur in extrusion wedges as far as oblique subduction took place.
Journal of Materials Research and Technology, Vol. 12, pp. 1473-1485.
China
nanodiamonds
Abstract: In this work, Ni/diamond composite coatings have been synthesized by electrodeposition in direct current mode. The effects of mechanical and ultrasonic agitations on the microstructural, surface characteristics and electrochemical properties have been comparatively investigated by various methods. Results show that diamond nanoparticles have been evenly dispersed in Ni metallic matrix, which could reinforce their performances. The coatings prepared under ultrasonic and mechanical agitation both exhibit compact, dense and hill-valley like morphology with pyramid-like nickel crystallite grains. The relative texture coefficient (RTC) values show that the preferred orientation of the Ni/diamond coating was (200) texture. From 3 to 5 A dm?2, the crystallite sizes of ultrasonic conditions were 59.2-81.7 nm, which were smaller than 76.3-83.2 nm of magnetic agitations. The average roughness (Ra = 78.9-133 nm) of ultrasonic-assisted coatings were lower than 103-139 nm of magnetic conditions. The mechanism of the co-electrodeposition process was proposed. Electrochemical impedance spectroscopy (EIS) results illustrate that the ultrasonic-assisted electrodeposited Ni/diamond coating has better corrosion resistance than that prepared under mechanical stirring conditions. The Ni/diamond composite coatings could be applied as protective materials in harsh mediums.
Geochemistry of late Mesozoic adakites from the Sulu belt, China: magma genesis and implications for crustal recycling beneath continental collisional orogens.
Wang, Q., Wyman, D.A., Xu, J., Jian, P., Zhao, Z., Li, C., Xu, W., Ma, J., He, B.
Early Cretaceous adakitic granites in the northern Dabie Complex, central China: implications for partial melting and delamination of thickened lower crust.
Geochimica et Cosmochimica Acta, Vol. 71, 10, May 15, pp. 2609-2636.
Lower crustal melting via magma underplating: elemental Sr Nd Pb isotopic constraints from late Mesozoic intermediate felsic volcanic rocks in NE Chin a block.
Abstract: A fundamental goal of mineralogy and petrology is the deep understanding of mineral phase relationships and the consequent spatial and temporal patterns of mineral coexistence in rocks, ore bodies, sediments, meteorites, and other natural polycrystalline materials. The multi-dimensional chemical complexity of such mineral assemblages has traditionally led to experimental and theoretical consideration of 2-, 3-, or n-component systems that represent simplified approximations of natural systems. Network analysis provides a dynamic, quantitative, and predictive visualization framework for employing “big data” to explore complex and otherwise hidden higher-dimensional patterns of diversity and distribution in such mineral systems. We introduce and explore applications of mineral network analysis, in which mineral species are represented by nodes, while coexistence of minerals is indicated by lines between nodes. This approach provides a dynamic visualization platform for higher-dimensional analysis of phase relationships, because topologies of equilibrium phase assemblages and pathways of mineral reaction series are embedded within the networks. Mineral networks also facilitate quantitative comparison of lithologies from different planets and moons, the analysis of coexistence patterns simultaneously among hundreds of mineral species and their localities, the exploration of varied paragenetic modes of mineral groups, and investigation of changing patterns of mineral occurrence through deep time. Mineral network analysis, furthermore, represents an effective visual approach to teaching and learning in mineralogy and petrology.
Abstract: An important issue in Earth’s earliest history is the timing and mixing history of the late accreted material that supplied highly siderophile elements to Earth’s mantle after core segregation. Previously, constraints on ancient mantle processes could only be obtained indirectly from mantle-derived magmas such as basalts or komatiites. Relics of Eoarchean (older than 3.8 Ga) mantle were proposed to occur within the Eoarchean terrains of western Greenland. Here we provide geochemical evidence, including combined platinum group element (PGE) and Re-Os isotope data, showing that modern mantle-like peridotites occur at two localities in southwest Greenland. Rhenium-depletion model ages of these peridotites are mostly of Eoarchean age, in accord with U-Pb zircon ages of crosscutting granitoid intrusives. PGE abundances and patterns are similar to those of modern depleted mantle peridotites. For the first time, such patterns provide conclusive evidence for preservation of Eoarchean depleted mantle rocks that are clearly distinguishable from magmatic cumulates or komatiites. Abundances of Os, Ir, and Ru combined with Os isotope compositions in the Greenland peridotites reveal that primitive late accreted material appears to have been efficiently mixed into the sampled mantle domains by Eoarchean time.
Functional Diamond, Vol. 1, 1, pp. 63-82. doi.org/10.1080/ 26941112.2021.1877021
Global
nitrogen
Abstract: The burgeoning multi-field applications of diamond concurrently bring up a foremost consideration associated with nitrogen. Ubiquitous nitrogen in both natural and artificial diamond in most cases as disruptive impurity is undesirable for diamond material properties, eg deterioration in electrical performance. However, the feat of this most common element-nitrogen, can change diamond growth evolution, endow diamond fancy colors and even give quantum technology a solid boost. This perspective reviews the understanding and progress of nitrogen in diamond including natural occurring gemstones and their synthetic counterparts formed by high temperature high pressure (HPHT) and chemical vapor deposition (CVD) methods. The review paper covers a variety of topics ranging from the basis of physical state of nitrogen and its related defects as well as the resulting effects in diamond (including nitrogen termination on diamond surface), to precise control of nitrogen incorporation associated with selective post-treatments and finally to the practical utilization. Among the multitudinous potential nitrogen related centers, the nitrogen-vacancy (NV) defects in diamond have attracted particular interest and are still ceaselessly drawing extensive attentions for quantum frontiers advance.
Abstract: Magnetic monopoles play a central role in various areas of fundamental physics, ranging from electromagnetism to topological states of matter. While their observation is elusive in high-energy physics, monopole sources of artificial gauge fields have been recently identified in synthetic matter. String theory, a potentially unifying framework that encompasses quantum mechanics, promotes the conventional \emph{vector} gauge fields of electrodynamics to \emph{tensor} gauge fields, and predicts the existence of more exotic \emph{tensor monopoles} in 4D space. Here we report on the characterization of a tensor monopole synthesized in a 4D parameter space by the spin degrees of freedom of a single solid-state defect in diamond. Using two complementary methods, we characterize the tensor monopole by measuring its quantized topological charge and its emanating Kalb-Ramond field. By introducing a fictitious external field that breaks chiral symmetry, we further observe an intriguing transition in the spectrum, characterized by spectral rings protected by mirror symmetries. Our work represents the first detection of tensor monopoles in a solid-state system and opens up the possibility of emulating exotic topological structures inspired by string theory.
Petrogenesis of Cretaceous adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province: implications for geodynamics and Cu-Au mineralization.
Geochimica et Cosmochimica Acta, Vol. 295, pp. 207-223.
China
nephelinites, basanites
Abstract: Widespread Cenozoic intraplate basalts from eastern China offer the opportunity to investigate the consequences of interaction between the stagnant Pacific slab and overlying asthenosphere and chemical heterogeneity within this “big mantle wedge”. We present and compile a comprehensive study of highly siderophile elements and Mg-Zn isotopes of this magmatic suite (60 samples including nephelinites, basanites, alkali basalts and tholeiites). The large-scale Mg-Zn isotopic anomalies documented in these basalts have been ascribed to mantle hybridization by recycled Mg-carbonates from the stagnant western Pacific plate. Our results reveal that the nephelinites and basanites are characterized by unfractionated platinum-group element (PGE) patterns normalized to primitive upper mantle (PUM) (e.g., PdN/IrN normalized to PUM?=?1.1?±?0.8, 1?), relatively high total PGE contents (e.g., Ir?=?0.25?±?0.14?ppb) and modern mantle-like 187Os/188Os (0.142?±?0.020). These characteristics are coupled with lighter Mg isotope (?26Mg?=??0.48?±?0.07‰) and heavier Zn isotope (?66Zn = +0.46?±?0.06‰) compositions compared to the mantle values (?26Mg: ?0.25?±?0.07‰; ?66Zn: +0.18?±?0.05‰). Together, these data are interpreted to reflect the oxidative breakdown of low proportions of mantle sulfides in the sources of these small-degree melts, likely caused by recycled carbonates, which then release chalcophile-siderophile elements into carbonatitic melts. By contrast, the contemporaneous alkali basalts and tholeiites are characterized by highly fractionated PGE patterns (e.g., PdN/IrN?=?4.4?±?3.3; Ir?=?0.037?±?0.027?ppb) and radiogenic 187Os/188Os (0.279?±?0.115) coupled with less fractionated Mg-Zn isotope compositions (?26Mg: ?0.39?±?0.05‰; ?66Zn: +0.35?±?0.03‰). In combination with other isotopic (e.g., Sr-Nd) and chemical (SiO2, Ce/Pb, Ba/Th, Fe/Mn) constraints, the alkali basalts and tholeiites were derived from higher degree melting of ancient pyroxenite-bearing mantle in addition to mixing with the aforementioned nephelinitic and basanitic melts. Collectively, we suggest that deep recycled carbonates promoted melting within the "big mantle wedge" leading to the generation of Cenozoic intraplate basalts across eastern China and the "redox freezing of carbonates" may cause the oxidation of Fe0 and S2-. This process may provide an important mechanism to oxidize mantle sulfides and transfer precious metals from deep mantle to crust.
Contribution to Mineralogy and Petrology, Vol. 177, 19 , 18p. Pdf
Global
garnet
Abstract: Garnet is a common U-bearing mineral in various magmatic and metamorphic rocks with a high U-Pb closure temperature (>?850 °C), rendering it a potentially valuable U-Pb geochronometer. However, a high U (>?10 ppm) garnet reference material that suits both quadrupole and/or multi-collector inductively coupled plasma mass spectrometry (ICP-MS) is yet to be established. This study evaluates a potential reference material for in situ garnet U-Pb analysis with anomalously high U content from the Prairie Lake alkaline complex, Canada. The PL57 garnet, occurring in a calcite ijolite, has high TiO2 (6.5-15.0 wt%, average 12.7 wt%) and Fe2O3 (17.1-21.3 wt%) contents and is a member of the andradite (26-66 mol.%)-morimotoite (18-41 mol.%)-schorlomite (16-35 mol.%) solid solution series. Four samples were dated by U-Pb ID-TIMS to assess reproducibility. Twelve TIMS analyses produced concordant, equivalent results. Garnet PL57 yielded a concordant age of 1156.2?±?1.2 Ma (2?, n?=?10, MSWD?=?1.0), based on ten analyses with two results discarded due to possible mineral inclusions (if included, the concordia age is 1156.6?±?1.8 Ma; n?=?12, MSWD?=?2.0). PL57 had 27-76 ppm (average 41 ppm) U with Th/U of 0.51-0.68 (average 0.63). The total common Pb content ranged from 0.4 to 3.9 pg (average 1.1 pg). Laser ablation coupled with ICP-MS and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging provide direct evidence that U is incorporated and homogeneously distributed within the garnet lattice rather than as defects or pore spaces. Published garnet samples and standards were then tested by calibrating the Willsboro, Mali, Qicun, and Tonglvshan garnet against PL57, which gave accurate ages within the recommended values. Case studies of garnet from the Archean Musselwhite orogenic gold deposit in Canada and the Cenozoic Changanchong and Habo skarn deposits in China yield reliable ages. This suggests that PL57 is a robust U-Pb isotope reference material. The limited variations of U and Pb isotopic ratios, together with the high U concentration and extremely low initial common Pb, make PL57 an ideal calibration and monitor reference material for in situ measurements.
Environmental Forensics, Vol. 22, 3-4, pp. 340-350. abstract only
China
synthetics
Abstract: In this study, soil and sediment samples along with groundwater samples were collected and analyzed from an abandoned synthetic diamond production plant in Anhui Province, South China. Chemical analysis, pollution characteristics analysis, and correlation analysis were conducted to assess and to determine the source(s) of the toxic metal and organic pollutions in the study sites. The Co and Ni concentrations of soil samples collected from the production area exceed the risk screening value for contaminated development land in Soil Environment Quality Standards for soil pollution risk control on construction land (Trial) of China, while the concentrations of other toxic elements such as Cr, Cu, and Zn are lower than the screening value. The PCA and HCA results are consistent with the correlation coefficient analysis and indicate that industrial activities are the main sources of Co and Ni. The chemical composition and source analysis results of soil and groundwater show that toxic metals originating from catalyst and low pH value from acid waste water should be the main point of concern in the synthetic diamond production plant.
Abstract: Porphyritic olivine kimberlitic breccia, discovered in the Dörbed Banner of Inner Mongolia, Western China, is referred to as Longtou Shan Kimberlite in our study. This kimberlite occurs as a pipe in the Halahuogete Formation of Bayan Obo Group. Zircon U-Pb ages of Longtou Shan Kimberlite reveals a Mesoproterozoic age of ~1,552 Ma, constraining the deposition age of Halahuogete Formation to the Mesoproterozoic. Compared with Mesoproterozoic kimberlite of the ancient landmass, it can be inferred that the North China Craton is a member of the Ur ancient continent of the Columbia supercontinent. Furthermore, according to the tectonic background of the Bayan Obo Group, we raise this possibility that “Bayan Obo Aulacogen” should be renamed the “Bayan Obo Continental Rift.”
Abstract: In the Rodinia supercontinent, Laurentia is placed at the center because it was flanked by late Neoproterozoic rifted margins; however, the conjugate margin for western Laurentia is still enigmatic. In this study, new paleomagnetic results have been obtained from 15 ca. 775 Ma mafic dikes in eastern Hebei Province, North China craton (NCC). Stepwise thermal demagnetization revealed a high-temperature component, directed northeast or southwest with shallow inclinations, with unblocking temperatures of as high as 580 °C. Rock magnetism suggests the component is carried by single-domain and pseudo-single-domain magnetite grains. Its primary origin is supported by a positive reversal test and regional remanence direction correlation test, and the paleomagnetic pole (29.0°S, 64.7°E, A95 = 5.4°) is not similar to any published younger poles of the NCC. Matching the late Mesoproterozoic to early Neoproterozoic (ca. 1110-775 Ma) apparent polar wander paths of the NCC and Laurentia suggests that the NCC could have been the conjugate margin for northwestern Laurentia in Rodinia, rather than sitting off the northeast coast of the main Rodinian landmass. Geological data indicate that breakup of the NCC and Laurentia occurred between ca. 775 and 720 Ma.
Abstract: Syn-exhumation mafic magmatism during continental collision provides insights into the crust-mantle reaction during deep subduction and the nature of orogenic lithospheric mantle in collisional orogens. In this study, we present a comprehensive data set of zircon U-Pb ages and whole-rock major-trace elements as well as Sr-Nd-Pb isotopes of alkaline mafic rocks from the southern Liaodong Peninsula, eastern China. Zircon U-Pb analyses yield Late Triassic age of 213 ± 3 to 217 ± 3 Ma, younger than the Middle Triassic ultrahigh-pressure metamorphic rocks of the Dabie-Sulu orogen. Thus, the alkaline mafic rocks are products of syn-exhumation magmatism during continental collision of the South and North China blocks. The rocks show shoshonitic affinities with high K2O (3.78-5.23 wt%) and K2O/Na2O (0.71-1.22). They are characterized by arc-like trace-element patterns with enriched LILE, Pb, and LREE, and depleted HFSE. They exhibit enriched Sr-Nd isotopic compositions with high initial 87Sr/86Sr isotopic ratios of 0.7058-0.7061 and negative ?Nd(t) values of ?13.0 to ?15.1. These results suggest involvement of recycled continental crust in their mantle source. The mantle source likely formed by the metasomatic reaction of subducted continental crust-derived melts with the overlying subcontinental lithospheric mantle during the Triassic continental collision. Decompressional melting of this metasomatized mantle formed syn-exhumation mafic magmas during the transition from convergent to extensional tectonics in the Late Triassic. Accordingly, mafic rocks from the southern Liaodong Peninsula provide a geochemical record of the subduction and recycling of continental crust into the mantle and melt-mantle reaction induced metasomatism within the orogen.
Abstract: Earth’s inner core (IC) is less dense than pure iron, indicating the existence of light elements within it1. Silicon, sulfur, carbon, oxygen and hydrogen have been suggested to be the candidates2,3, and the properties of iron-light-element alloys have been studied to constrain the IC composition4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19. Light elements have a substantial influence on the seismic velocities4,5,6,7,8,9,10,11,12,13, the melting temperatures14,15,16,17 and the thermal conductivities18,19 of iron alloys. However, the state of the light elements in the IC is rarely considered. Here, using ab initio molecular dynamics simulations, we find that hydrogen, oxygen and carbon in hexagonal close-packed iron transform to a superionic state under the IC conditions, showing high diffusion coefficients like a liquid. This suggests that the IC can be in a superionic state rather than a normal solid state. The liquid-like light elements lead to a substantial reduction in the seismic velocities, which approach the seismological observations of the IC20,21. The substantial decrease in shear-wave velocity provides an explanation for the soft IC21. In addition, the light-element convection has a potential influence on the IC seismological structure and magnetic field.
Abstract: Porphyritic olivine kimberlitic breccia, discovered in the Dörbed Banner of Inner Mongolia, Western China, is referred to as Longtou Shan Kimberlite in our study. This kimberlite occurs as a pipe in the Halahuogete Formation of Bayan Obo Group. Zircon U–Pb ages of Longtou Shan Kimberlite reveals a Mesoproterozoic age of ~1,552 Ma, constraining the deposition age of Halahuogete Formation to the Mesoproterozoic. Compared with Mesoproterozoic kimberlite of the ancient landmass, it can be inferred that the North China Craton is a member of the Ur ancient continent of the Columbia supercontinent. Furthermore, according to the tectonic background of the Bayan Obo Group, we raise this possibility that “Bayan Obo Aulacogen” should be renamed the “Bayan Obo Continental Rift.”
Abstract: Mineral water contents, together with the major and trace element compositions of minerals and whole-rock, were determined for garnet pyroxenites enclosed by ultrahigh-pressure (UHP) metamorphic gneiss at Hujialin in the Sulu orogen. The garnet pyroxenites have low SiO2 contents of 40.25 to 46.68 wt% and MgO contents of 10.99 to 14.79 wt%. They are characterized by enrichment in LREE and LILE (Ba, Sr, Pb) but depletion in HFSE (Nb, Zr) and HREE. They were generated in the Triassic by metasomatic reaction of the mantle wedge peridotite with hydrous felsic melts derived from partial melting of the deeply subducted continental crust. Measured water contents vary from 523 to 1213 ppm for clinopyroxene, and 55 to 1476 ppm for garnet. These mineral water contents are not only correlated with mineral major and trace element abundances but also relatively homogenous within single mineral grains. Such features preclude significant disturbance of the mineral water contents during pyroxenite exhumation from the mantle depth to the surface and thus indicate preservation of the primary water contents for the UHP metasomatites. The garnet pyroxenites are estimated to have bulk water contents of 424-660 ppm, which are higher than those for the MORB source, similar to or higher than those for the OIB sources and close to the lower limit for the arc magma source. The relationships between contents of mineral water and some elements suggest that the high water contents of garnet pyroxenites are primarily determined by the abundance of water-rich clinopyroxene. Garnet also has the high water contents, suggesting its importance in hosting water at mantle depths. Calculated whole-rock H2O/Ce ratios are 63-145, higher than those for Hawaiian garnet pyroxenites and SWIR abyssal pyroxenites. These observations suggest that metasomatic pyroxene-rich lithologies have the capacity to contribute high H2O concentrations and variable H2O/Ce ratios to the mantle. This lends support to the interpretation that the source of some intraplate basalts may be a heterogeneous mixture of peridotite and pyroxenite. On the other hand, the high water contents of garnet pyroxenites suggest that the presence of ultramafic metasomatites in the mantle wedge would enhance its water storage and thus reduce the water transport into deeper mantle by subduction.
Polat, A., Herxberg, C., Munker, C., Rodgers, R., Kusky, T., Li, J., Fryer, B.
Geochemical and petrological evidence for a supra subduction zone origin of Neoarchean (ca 2.5 Ga) peridotites, central orogenic belt, North Chin a craton.
Geological Society of America Bulletin, Vol. 118, 7, July pp. 771-784.
Science China Earth Sciences, Vol. 59, 3, pp. 573-582.
China
Craton, North China
Abstract: Present-day hot spots and Phanerozoic large igneous provinces (LIPs) and kimberlites mainly occur at the edges of the projections of Large Low Shear Wave Velocity Provinces (LLSVPs) on the earth’s surface. If a plate contains accurately dated LIPs or kimberlites, it is possible to obtain the absolute paleoposition of the plate from the LIP/kimberlite and paleomagnetic data. The presence of Middle Ordovician kimberlites in the North China Block provides an opportunity to determine the absolute paleoposition of the block during the Middle Ordovician. In addition to paleobiogeographical information and the results of previous work on global plate reconstruction for the Ordovician Period, we selected published paleomagnetic data for the North China Block during the Middle Ordovician and determined the most reasonable absolute paleoposition of the North China Block during the Middle Ordovician: paleolatitude of approximately 16.6°S to 19.1°S and paleolongitude of approximately 10°W. The block was located between the Siberian Plate and Gondwana, close to the Siberian Plate. During the Cambrian and Ordovician periods, the North China Block may have moved toward the Siberian Plate and away from the Australian Plate.
Abstract: Carbonates are common rock-forming minerals in the Earth’s crust and act as sinks of atmospheric carbon dioxide. Subduction of hydrothermally altered oceanic lithosphere returns carbon to the interior, where more than three quarters of Earth’s carbon is stored. The contribution of subducted carbonates to the Earth's long-term deep carbon cycle is uncertain and has recently emerged as a topic of intense debate [1]. Moreover, mantle-slab interaction has been proposed as a mechanism to produce super-deep diamonds, thus questioning the use of certain mineral inclusions to infer lower-mantle origin [2]. Here we report new data on the chemical stability and reaction kinetics of carbonates in the mantle from multianvil and diamond-anvil-cell experiments. Our results suggest that carbon can be sequestered into deep Earth through reaction freezing and that the index minerals for super-deep diamonds are not reliable indicators for their formation depths.
Geophysical Research Letters, Vol. 46, 4, pp. 1984-1992.
Mantle
diamond genesis
Abstract: Superdeep diamonds originate from great depths inside Earth, carrying samples from inaccessible mantle to the surface. The reaction between carbonate and iron may be an important mechanism to form diamond through interactions between subducting slabs and surrounding mantle. Interestingly, most superdeep diamonds formed in two narrow zones, at 250-450 and 600-800 km depths within the ~2,700?km?deep mantle. No satisfactory hypothesis explains these preferred depths of diamond formation. We measured the rate of a diamond forming reaction between magnesite and iron. Our data show that high temperature promotes the reaction, while high pressure does the opposite. Particularly, the reaction slows down drastically at about 475(±55) km depth, which may explain the rarity of diamond formation below 450 km depth. The only exception is the second zone at 600-800 km, where carbonate accumulates and warms up due to the stagnation of subducting slabs at the top of lower mantle, providing more reactants and higher temperature for diamond formation. Our study demonstrates that the depth distribution of superdeep diamonds may be controlled by reaction rates.
Geochemical Perspectives Letters, Vol. 11, pp. 18-22.
Mantle
nitrogen
Abstract: Nitrogen and carbon are essential elements for life, and their relative abundances in planetary bodies are important for understanding planetary evolution and habitability. The high C/N ratio in the bulk silicate Earth (BSE) relative to chondrites has been difficult to explain through partitioning during core formation and outgassing from molten silicate. Here we propose a new model that may have released nitrogen from the metallic cores of accreting bodies during impacts with the early Earth. Experimental observations of melting in the Fe-N-C system via synchrotron X-ray radiography of samples in a Paris-Edinburgh press reveal that above the liquidus, iron-rich melt and nitrogen-rich liquid coexist at pressures up to at least 6 GPa. The combined effects of N-rich supercritical fluid lost to Earth’s atmosphere and/or space as well as N-depleted alloy equilibrating with the magma ocean on its way to the core would increase the BSE C/N ratio to match current estimates.
Abstract: Reactions involving carbon in the deep Earth have limited manifestations on Earth's surface, yet they have played a critical role in the evolution of our planet. The metal-silicate partitioning reaction promoted carbon capture during Earth's accretion and may have sequestered substantial carbon in Earth's core. The freezing reaction involving iron-carbon liquid could have contributed to the growth of Earth's inner core and the geodynamo. The redox melting/freezing reaction largely controls the movement of carbon in the modern mantle, and reactions between carbonates and silicates in the deep mantle also promote carbon mobility. The 10-year activity of the Deep Carbon Observatory has made important contributions to our knowledge of how these reactions are involved in the cycling of carbon throughout our planet, both past and present, and has helped to identify gaps in our understanding that motivate and give direction to future studies.
Geochimica et Cosmochimica Acta, Vol. 281, pp. 67-90. pdf
Russia, Siberia
deposit - Udachnaya
Abstract: Cratonic lithospheric mantle is believed to have been formed in the Archean, but kimberlite-hosted coarse peridotites from Udachnaya in the central Siberian craton typically yield Paleoproterozoic Re-depletion Os isotope ages (TRD). By comparison, olivine megacrysts from Udachnaya, sometimes called “megacrystalline peridotites”, often yield Archean TRD ages, but the nature of these rare materials remains enigmatic. We provide whole-rock (WR) Re-Os isotope and PGE analyses for 24 olivine-rich xenoliths from Udachnaya as well as modal and petrographic data, WR and mineral major and trace element compositions. The samples were selected based on (a) high olivine abundances in hand specimens and (b) sufficient freshness and size to yield representative WR powders. They comprise medium- to coarse-grained (olivine??1?cm) dunite, olivine megacrysts and low-orthopyroxene (11-21% opx) harzburgites equilibrated at 783-1154?°C and 3.9-6.5 GPa; coarse dunites have not been previously reported from Udachnaya; two xenoliths contain ilmenite. The harzburgites and dunites have similar WR variation ranges of Ca, Al, Fe, Cr and Mg# (0.917-0.934) typical of refractory cratonic peridotites, but the dunites tend to have higher MgO, NiO and Mg/Si. Mineral abundances and those of Ca and Al are not correlated with Mg#WR; they are not due to differences in melting degrees but are linked to metasomatism. Several samples with high 187Re/188Os show a positive linear correlation with 187Os/188Os with an apparent age of 0.37?Ga, same as eruption age of host kimberlite. Robust TRD ages were obtained for 16 xenoliths with low 187Re/188Os (0.02-0.13). TRD ages for low-opx harzburgites (1.9-2.1?Ga; average 2.0?±?0.1?Ga, 1 ?) are manifestly lower than for dunites and megacrysts (2.4-3.1?Ga); the latter define two subsets with average TRD of 2.6?±?0.1?Ga and 3.0?±?0.1?Ga, and TMA of 3.0?±?0.2?Ga and 3.3?±?0.1?Ga, respectively. Differences in olivine grain size (coarse vs. megacrystalline) are not related to age. The age relations suggest that the dunites and megacrysts could not be produced by re-melting of harzburgites, e.g. in arc settings, nor be melt channel materials in harzburgites. Instead, they are relict fragments of lithospheric mantle formed in the Archean (likely in two events at or after 2.6?Ga and 3.0?Ga) that were incorporated into cratonic lithosphere during the final assembly of the Siberian craton in the Paleoproterozoic. A multi-stage formation of the Siberian lithospheric mantle is consistent with crustal basement ages from U-Pb dating of zircons from crustal xenoliths at Udachnaya and detrital zircons from the northern Siberian craton (1.8-2.0, 2.4-2.8 and 3.0-3.4?Ga). The new data from the Siberian and other cratons suggest that the formation of strongly melt-depleted cratonic lithosphere (e.g. Mg# ?0.92) did not stop at the Archean-Proterozoic boundary as is commonly thought, but continued in the Paleoproterozoic. The same may be valid for the transition from the ‘Archean’ (4-2.5?Ga) to modern tectonic regimes.
Journal of Metamorphic Geology, Vol. 38, pp. 593-627.
Australia
geochronology
Abstract: The final assembly of the Mesoproterozoic supercontinent Nuna was marked by the collision of Laurentia and Australia at 1.60 Ga, which is recorded in the Georgetown Inlier of NE Australia. Here, we decipher the metamorphic evolution of this final Nuna collisional event using petrostructural analysis, major and trace element compositions of key minerals, thermodynamic modelling, and multi?method geochronology. The Georgetown Inlier is characterised by deformed and metamorphosed 1.70-1.62 Ga sedimentary and mafic rocks, which were intruded by c. 1.56 Ga old S?type granites. Garnet Lu-Hf and monazite U-Pb isotopic analyses distinguish two major metamorphic events (M1 at c. 1.60 Ga and M2 at c. 1.55 Ga), which allows at least two composite fabrics to be identified at the regional scale—c. 1.60 Ga S1 (consisting in fabrics S1a and S1b) and c. 1.55 Ga S2 (including fabrics S2a and S2b). Also, three tectono?metamorphic domains are distinguished: (a) the western domain, with S1 defined by low?P (LP) greenschist facies assemblages; (b) the central domain, where S1 fabric is preserved as medium?P (MP) amphibolite facies relicts, and locally as inclusion trails in garnet wrapped by the regionally dominant low?P amphibolite facies S2 fabric; and (c) the eastern domain dominated by upper amphibolite to granulite facies S2 foliation. In the central domain, 1.60 Ga MP-medium?T (MT) metamorphism (M1) developed within the staurolite-garnet stability field, with conditions ranging from 530-550°C at 6-7 kbar (garnet cores) to 620-650°C at 8-9 kbar (garnet rims), and it is associated with S1 fabric. The onset of 1.55 Ga LP-high?T (HT) metamorphism (M2) is marked by replacement of staurolite by andalusite (M2a/D2a), which was subsequently pseudomorphed by sillimanite (M2b/D2b) where granite and migmatite are abundant. P-T conditions ranged from 600 to 680°C and 4-6 kbar for the M2b sillimanite stage. 1.60 Ga garnet relicts within the S2 foliation highlight the progressive obliteration of the S1 fabric by regional S2 in the central zone during peak M2 metamorphism. In the eastern migmatitic complex, partial melting of paragneiss and amphibolite occurred syn? to post? S2, at 730-770°C and 6-8 kbar, and at 750-790°C and 6 kbar, respectively. The pressure-temperature-deformation-time paths reconstructed for the Georgetown Inlier suggest a c. 1.60 Ga M1/D1 event recorded under greenschist facies conditions in the western domain and under medium?P and medium?T conditions in the central domain. This event was followed by the regional 1.56-1.54 Ga low?P and high?T phase (M2/D2), extensively recorded in the central and eastern domains. Decompression between these two metamorphic events is ascribed to an episode of exhumation. The two?stage evolution supports the previous hypothesis that the Georgetown Inlier preserves continental collisional and subsequent thermal perturbation associated with granite emplacement.
Geochimica et Cosmochimica Acta, Vol. 294, pp. 215-231. pdf
Mantle
bridgmanite
Abstract: The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth’s lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (?-factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its ?-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller ?-factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰.
Abstract: To investigate lateral and depth variations of seismic anisotropy beneath the central-western United States, we determined a detailed 3-D model of P-wave anisotropic tomography by inverting a large number of arrival-time data of local and teleseismic events. Our results reveal significant azimuthal anisotropies in the crust and lithosphere, which are associated with ancient orogenic collisional and magmatic activities. As depth increases, the fast-velocity direction (FVD) pattern becomes gradually trended and small features fade away. There is a boundary in the FVD distribution, which separates the tectonically active region in the west from the stable cratonic region in the east. Frozen-in anisotropy with a NW-SE FVD is preserved in the thick Wyoming cratonic lithosphere that exhibits as a high-velocity (high-V) anomaly to a depth of ~250 km. In the asthenosphere beneath the western thin lithosphere, FVDs are generally parallel with the absolute motion direction of the North American plate due to shearing between the plate and the asthenosphere. In the deeper areas, the subducted and fragmented slab exhibiting as high-V anomalies leads to slab-related mantle flows. These results indicate that seismic anisotropies exist in both the lithosphere and asthenosphere with different geodynamic mechanisms and it is feasible to link the P-wave azimuthal anisotropy to lithospheric deformations, fossil anisotropy in the lithosphere, and flows in the asthenosphere.
U-Pb zircon geochronology of coesite bearing eclogites from the southern Dulan areas of the North Qaidam UHP terrane, northwestern China: spatially and temporally
Journal of Metamorphic Geology, Vol. 28, 9, pp. 955-978.
Ore Geology Reviews, doi.org/10.1016/j.oregeorev.2021.104284 11p. Pdf
China
REE
Abstract: The carbonatites in the southern margin of the North China Craton are distinguishable by containing abundant quartz and are closely spatially associated with Mo-(REE) deposits. Unveiling the nature of ore-forming fluids is key to understand the genesis of these Mo-(REE) deposits and to explore their potential genetic relationships with the quartz-rich carbonatites, but such issues were currently not convincingly addressed. Here, we provide detailed petrographic, microthermometric and LA-ICP-MS analyses of the fluid inclusions hosted in the primary quartz from the carbonatites in the Huangshuian Mo-(REE) deposit which is the largest Mo-(REE) one in the region, containing 0.4 million tons of Mo metal with REEs as the major by-product. Our results show that the fluid inclusions in the quartz of the carbonatites are two- and three-phase CO2-bearing types with high homogenization temperatures (average at 396 °C) and low salinities (average at 3.88 wt% NaCl equiv). The LA-ICP-MS analyses of these inclusions reveal that the primary fluids contain high concentrations of La, Ce, Pr, Nd, Sr, and Ba, similar to typical carbonatite-related fluids. In addition, they are characterized by high Y, Cu, Pb, and Zn. Such a metal association is broadly consistent with the mineral assemblages of the Huangshuian Mo-(REE) deposit, such as the widespread barite, bastnäsite, xenotime, chalcopyrite, galena, and sphalerite, strongly supporting the close genetic relation of the deposit with the quartz-rich carbonatites. Although the concentrations of Mo are extremely low in these inclusions (below the detect limit), it was constrained to be gradually enriched in evolved fluids. Considering that the recorded fluids in quartz represent earliest generation of fluids exsolved from carbonatite magmas, our new results highlight that quantifying metal budgets of fluid inclusions could be a robust way to evaluate fertility of carbonatites that are widespread in the southern margin of the North China Craton.
Contributions to Mineralogy and Petrology, in press available 19p.
Asia, Tibet
Melting
Abstract: Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F-Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800-850 °C, >15 kbar). Zircons show four main age groupings: 1.0-0.5 Ga, 50-45, 35-20, and 16-13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50-45 Ma zircons match the crystallization age and juvenile character (?Hfi +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35-20 Ma Miocene ages are derived from zircons with similar Hf-O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon ?18O (+8.73-8.97 ‰) values, and extremely low ?Hfi (?12.7 to ?9.4) values. Such evolved Hf-O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low ?18O (+6 to 7.5 ‰), intermediate (?18O +8.5 to 9.0 ‰), and high ?18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with ?18O 10-12 ‰. Both the low and intermediate ?18O groups show significant variation in Fe content, whereas the two high ?18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate ?18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high ?18O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35-20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust-mantle hybridization in the deep Tibetan crust.
Geophysical Research Letters, Vol. 46, 2, pp. 678-688.
Global
craton
Abstract: Low??18O magma has received great attention and it has profound implications on geological and climate evolution. Neoproterozoic era is a unique period to breed low??18O magmas and snowball Earth. This manuscript first report Neoproterozoic moderately 18O?depleted zircons from the central part of the Cathaysia Block in South China, and it builds a four end?member Hf?O isotopic mixing model to explain the global low??18O magmas at Neoproterozoic era. Our compilation of low??18O zircon data and our new data confirms that globally Neoproterozoic 18O?depleted magmatic activities generally began after 800 Ma and reached a peak at 780-760 Ma. This provides new information on the rifting of Rodinia supercontinent and suggests close connections between northwest India, Madagascar, and South China in the Rodinia supercontinent. This manuscript deals with the hot?debated topics on oxygen isotopes and supercontinent cycle. We believe that this manuscript will attract international readers from a wide scope of geosciences.
Earth and Planetary Science Letters, Vol. 516, pp. 190-201.
Mantle
carbon
Abstract: A long-standing unresolved problem in understanding Earth's deep carbon cycle is whether crustal carbon is recycled beyond arc depths. While isotopic signatures of eclogitic diamonds and their inclusions suggest deep recycling of crustal material, the crustal carbon source remains controversial; seafloor sediment - the widely favored crustal carbon source - cannot explain the combined carbon and nitrogen isotopic characteristics of eclogitic diamonds. Here we examined the carbon and oxygen isotopic signatures of bulk-rock carbonate for 80 geographically diverse samples from altered mafic-ultramafic oceanic crust (AOC), which comprises 95 vol% of the crustal material in subducting slabs. The results show: (i) AOC contains carbonate with C values as low as ?24‰, indicating the presence of biogenic carbonate; (ii) carbonate in AOC was mainly formed during low-temperature (<100 °C) alteration processes. Modeling accounting for this newly recognized carbon source in the oceanic crust with formation temperatures <100 °C yields a global carbon influx of 1.5±0.3 × 1012 mol C/yr carried by subducting AOC into the trench, which is 50-90% of previous estimates, but still of the same order of the carbon influx carried by subducting sediments into the trench. The AOC can retain carbon better than sediment during subduction into the asthenosphere, transition zone and lower mantle. Mixing of asthenospheric and AOC fluids provides the first consistent explanation of the diverse record of carbon and nitrogen isotopes in diamonds, suggesting that AOC, instead of sediment, is the key carrier of crustal carbon into the deep mantle.
www.minsocam.org/ MSA/Centennial/ MSA_Centennial _Symposium.html The next 100 years of mineral science, June 20-21, p. 22. Abstract
Mantle
diamond inclusions
Abstract: Much of the temporal record of Earth’s evolution, including its trace of plate tectonics, is blurred due to the dynamic nature of the crust-mantle system. While zircon provides the highest fidelity crustal record, diamond takes over in the mantle as the go-to mineral, capable of retaining critical information for a variety of geochemical proxies, over billion year timescales. Here we use diamond and its inclusions to tell the story of the recycling of C, N, O, H and B from the crust to various depths in Earth’s mantle. In this story, altered oceanic crust (AOC) and lithospheric mantle will play a prominent role. The carbon isotope record of diamond has long been thought to reflect the mixing of primitive mantle carbon with carbon recycled from isotopically light organic material originating from the crust. A major difficulty has been reconciling this view with the highly varied nitrogen and carbon isotope signatures in diamonds of eclogitic paragenesis, which cannot be interpreted by the same mechanism. Recent work on AOC of igneous origin (Li et al., EPSL in press) shows how isotopically varied carbon and nitrogen can be subducted to great depth and retained in spatial juxtaposition with the mafic silicate component of AOC to form the complex C-N isotope systematics observed in diamonds and the varied O isotope compositions of their inclusions. In this model a large portion of the 13C depleted carbon originated from biogenic carbonate within the AOC rather than from overlying sediments. Metamorphosed and partially devolatilized AOC will have very variable C/N ratios and highly variable nitrogen isotopes, explaining why simple two component mixing between organic matter and convecting upper mantle cannot explain the complexity of C-N isotope systematics in diamonds. Igneous AOC and its underlying altered mantle are considerably more efficient than subducted sediment at retaining their volatile inventory when recycled to transition zone and even lower mantle depths. Hence, this combination of mixing between AOC-derived volatiles and those from the convecting mantle produces the isotopic fingerprints of superdeep diamonds and their inclusions. These amazing diamonds, some worth millions of dollars, can contain pristine ultra-high pressure mineral phases never before seen in terrestrial samples. The first hydrous ringwoodite found in Earth provides evidence in support of a locally water-saturated transition zone that may result from altered oceanic lithospheric mantle foundering at that depth in the mantle. The O isotope composition of deep asthenosphere and transition zone phases document clearly crustal precursors that have interacted with the hydrosphere before residing hundreds of km deep within the Earth. Finally, spectacular blue diamonds contain boron, an element of strong crustal affinities, transported into the deep Earth along with crustal carbon, by the plate tectonic conveyor system. Diamond - such a simple mineral - and its inclusions, will continue to provide a unique, brightly illuminating light into the darkest recesses of Earth’s mantle for many years to come.
Diamond & Related Materials, Vol. 81, pp. 161-167.
Technology
microdiamonds
Abstract: Polylactic acid (PLA)-based composites filled with 20 or 50 ?m-diameter microdiamond are synthesized by hot pressing. Through improving the interface compatibility between the filler and the matrix enabled by octadecylamine (ODA) coating on the microdiamond particles, the maximum thermal conductivity of the composites is 2.22 Wm? 1 K? 1, which is a ~ 10-fold increase in comparison with that of pure PLA. According to the analysis on the glass transmission of the composites and the surface chemistry of the fillers using DSC, FI-IR, and Raman microscopy, it is found out that ODA is connected with the -OH group on the microdiamond surface through hydrogen bonding and an interfacial structure of PLA/ODA/microdiamond is formed. Thus, the interfacial thermal transport between PLA and microdiamond is significantly improved, leading to the enhancement of the thermal conductivity of the composites. Our work presents a simple method to modify the surface chemistry of microdiamond and to improve the interface compatibility between microdiamond and PLA. The microdiamond/PLA composites with large thermal conductivity are promising thermal management materials used for modern electronic products.
Bulk organic geochemistry and U-Pb zircon geochronology of the Wombat sedimentary fill.
2018 Yellowknife Geoscience Forum , p. 98-99. abstract
Canada, Northwest Territories
deposit - Wombat
Abstract: The Wombat locality (64.73°N, 110.59°W) is a diamondiferous kimberlite in the Lac de Gras kimberlite field of Northwest Territories. Two drill cores, CH 93-29 and DDH 0-005, intersect the Wombat crater facies and include 195 m of well preserved, undisturbed lake sediment fill. Bulk sediment elemental analysis, C isotope composition, and Rock-Eval pyrolysis, together with inferences from microfossils, are used to characterize conditions of sedimentation and paleoenvironment in the maar lake. Bulk sediment C/N, hydrogen index (HI), and ?13C indicate material derived from C3 land plants dominates the sedimentary organic matter, with a minor algal contribution. The ?13C values range from -25.3 ‰ to -30.2 ‰ (average -26.6 ‰) and are typical for C3 land plants, with fluctuations in ?13C likely related to shifts in the proportions of land-derived material and algal organic matter. An overall trend of higher ?13C towards the top of the core suggests increasing autochthonous organic matter production. 18 samples analyzed by Rock-Eval pyrolysis all plot in the Type III kerogen field for HI vs. Tmax,with average Tmax values ~425 °C indicative of the low thermal maturity of organic matter. Total organic carbon (TOC) averages 3.6 wt.% and average total carbonate content is 14.1 wt.%, indicating bottom water anoxia and substantial carbonate input from weathering of overlying carbonate cover rocks, respectively. Together with well-preserved freshwater microfossils (e.g. diatoms, chrysophytes, synurophytes), the results indicate deposition in a non-marine setting. The age of the Wombat maar lake sediments is determined using MC-LA-ICP-MS U-Pb zircon geochronology from two distal rhyolitic tephra beds found in the core DDH 0-005, yielding a date of 82.97±0.60 Ma (MSWD = 1.7, n=18 of 33 grains analyzed). This minimum age suggests that Wombat kimberlite pipe emplacement occurred during the Late Cretaceous, with sedimentation in the maar beginning shortly thereafter. Though our geochronology is preliminary at this point, our findings from the Wombat pipe post-eruptive lake sediment fill provide direct evidence for a non-marine environment in the Lac De Gras area during the Late Cretaceous. Furthermore, microfossils in the Wombat pipe sediment fill likely include the oldest-known occurrence of freshwater diatoms.
Earth and Planetary Science Letters, Vol. 516, pp. 190-201.
Mantle
carbon
Abstract: A long-standing unresolved problem in understanding Earth's deep carbon cycle is whether crustal carbon is recycled beyond arc depths. While isotopic signatures of eclogitic diamonds and their inclusions suggest deep recycling of crustal material, the crustal carbon source remains controversial; seafloor sediment - the widely favored crustal carbon source - cannot explain the combined carbon and nitrogen isotopic characteristics of eclogitic diamonds. Here we examined the carbon and oxygen isotopic signatures of bulk-rock carbonate for 80 geographically diverse samples from altered mafic-ultramafic oceanic crust (AOC), which comprises 95 vol% of the crustal material in subducting slabs. The results show: (i) AOC contains carbonate with C values as low as ?24‰, indicating the presence of biogenic carbonate; (ii) carbonate in AOC was mainly formed during low-temperature (<100 °C) alteration processes. Modeling accounting for this newly recognized carbon source in the oceanic crust with formation temperatures <100 °C yields a global carbon influx of 1.5±0.3 × 1012 mol C/yr carried by subducting AOC into the trench, which is 50-90% of previous estimates, but still of the same order of the carbon influx carried by subducting sediments into the trench. The AOC can retain carbon better than sediment during subduction into the asthenosphere, transition zone and lower mantle. Mixing of asthenospheric and AOC fluids provides the first consistent explanation of the diverse record of carbon and nitrogen isotopes in diamonds, suggesting that AOC, instead of sediment, is the key carrier of crustal carbon into the deep mantle.
www.minsocam.org/ MSA/Centennial/ MSA_Centennial _Symposium.html The next 100 years of mineral science, June 20-21, p. 22. Abstract
Mantle
diamond inclusions
Abstract: Much of the temporal record of Earth’s evolution, including its trace of plate tectonics, is blurred due to the dynamic nature of the crust-mantle system. While zircon provides the highest fidelity crustal record, diamond takes over in the mantle as the go-to mineral, capable of retaining critical information for a variety of geochemical proxies, over billion year timescales. Here we use diamond and its inclusions to tell the story of the recycling of C, N, O, H and B from the crust to various depths in Earth’s mantle. In this story, altered oceanic crust (AOC) and lithospheric mantle will play a prominent role. The carbon isotope record of diamond has long been thought to reflect the mixing of primitive mantle carbon with carbon recycled from isotopically light organic material originating from the crust. A major difficulty has been reconciling this view with the highly varied nitrogen and carbon isotope signatures in diamonds of eclogitic paragenesis, which cannot be interpreted by the same mechanism. Recent work on AOC of igneous origin (Li et al., EPSL in press) shows how isotopically varied carbon and nitrogen can be subducted to great depth and retained in spatial juxtaposition with the mafic silicate component of AOC to form the complex C-N isotope systematics observed in diamonds and the varied O isotope compositions of their inclusions. In this model a large portion of the 13C depleted carbon originated from biogenic carbonate within the AOC rather than from overlying sediments. Metamorphosed and partially devolatilized AOC will have very variable C/N ratios and highly variable nitrogen isotopes, explaining why simple two component mixing between organic matter and convecting upper mantle cannot explain the complexity of C-N isotope systematics in diamonds. Igneous AOC and its underlying altered mantle are considerably more efficient than subducted sediment at retaining their volatile inventory when recycled to transition zone and even lower mantle depths. Hence, this combination of mixing between AOC-derived volatiles and those from the convecting mantle produces the isotopic fingerprints of superdeep diamonds and their inclusions. These amazing diamonds, some worth millions of dollars, can contain pristine ultra-high pressure mineral phases never before seen in terrestrial samples. The first hydrous ringwoodite found in Earth provides evidence in support of a locally water-saturated transition zone that may result from altered oceanic lithospheric mantle foundering at that depth in the mantle. The O isotope composition of deep asthenosphere and transition zone phases document clearly crustal precursors that have interacted with the hydrosphere before residing hundreds of km deep within the Earth. Finally, spectacular blue diamonds contain boron, an element of strong crustal affinities, transported into the deep Earth along with crustal carbon, by the plate tectonic conveyor system. Diamond - such a simple mineral - and its inclusions, will continue to provide a unique, brightly illuminating light into the darkest recesses of Earth’s mantle for many years to come.
Journal of Geophysical Research , Vol. 124, 5, pp. 4617-4638.
Mantle
melting
Abstract: The oxidation state of the Earth`s mantle, often expressed as oxygen fugacity (fO2), could control the behavior of multivalent elements and thus exert a significant influence on the formation of magmatic ore deposits and the secular evolution of Earth`s atmosphere. Whether arc mantle is more oxidized than oceanic mantle remains a controversial topic. As a multivalent element, partitioning behavior of vanadium is fO2 sensitive and is capable of tracking mantle redox state. However, except fO2, other factors (temperature, pressure, and phase composition) that may affect vanadium partitioning behavior have not been clearly evaluated. Here we conducted high temperature and pressure experiments to determine partition coefficients of vanadium during mantle melting under various fO2 conditions. Combining our and published data, we evaluated the effects of fO2, T, P, and compositions of mineral and melt on the vanadium partitioning using multiple linear regressions. The results indicate that, in addition to fO2, temperature exerts a significant control on the vanadium partitioning. Additionally, we estimated fO2 of the arc mantle via numerical modelling using appropriate partition coefficients for vanadium. Our results clarify and reconcile the discrepancies between previous studies and reveal that arc mantle is generally ~10 times more oxidized than oceanic mantle.
Li, L-M., Sun, M., Wang, Y., Xing, G., Zhao, G., Cai, K., Zhang, Y.
Geochronological and geochemical study of Paleproterozoic gneissic granites and clinopyroxenite xenolths from NW Fujian: implications for crustal evol.
Journal of Asian Earth Sciences, Vol. 41, 2, pp. 204-212.
Geophysical Research Letters, Vol. 43, 8, pp. 3693-3697.
Mantle
Melting
Abstract: Seismic tomography models reveal two large low shear velocity provinces (LLSVPs) that identify large-scale variations in temperature and composition in the deep mantle. Other characteristics include elevated density, elevated bulk sound speed, and sharp boundaries. We show that properties of LLSVPs can be explained by the presence of small quantities (0.3-3%) of suspended, dense Fe-Ni-S liquid. Trapping of metallic liquid is demonstrated to be likely during the crystallization of a dense basal magma ocean, and retention of such melts is consistent with currently available experimental constraints. Calculated seismic velocities and densities of lower mantle material containing low-abundance metallic liquids match the observed LLSVP properties. Small quantities of metallic liquids trapped at depth provide a natural explanation for primitive noble gas signatures in plume-related magmas. Our model hence provides a mechanism for generating large-scale chemical heterogeneities in Earth's early history and makes clear predictions for future tests of our hypothesis.
Geophysical Research Letters, Vol. 43, 8, pp. 3693-3699.
Mantle
Melting
Abstract: Seismic tomography models reveal two large low shear velocity provinces (LLSVPs) that identify large-scale variations in temperature and composition in the deep mantle. Other characteristics include elevated density, elevated bulk sound speed, and sharp boundaries. We show that properties of LLSVPs can be explained by the presence of small quantities (0.3 -3%) of suspended, dense Fe-Ni-S liquid. Trapping of metallic liquid is demonstrated to be likely during the crystallization of a dense basal magma ocean, and retention of such melts is consistent with currently available experimental constraints. Calculated seismic velocities and densities of lower mantle material containing low-abundance metallic liquids match the observed LLSVP properties. Small quantities of metallic liquids trapped at depth provide a natural explanation for primitive noble gas signatures in plume-related magmas. Our model hence provides a mechanism for generating large-scale chemical heterogeneities in Earth's early history and makes clear predictions for future tests of our hypothesis.
Earth and Planetary Science Letters, Vol. 470, pp. 54-63.
Mantle
geophysics - seismic
Abstract: Seismic tomography resolves anomalies interpreted as oceanic lithosphere subducted deep into Earth's lower mantle. However, the fate of the compositionally distinct oceanic crust that is part of the lithosphere is poorly constrained but provides important constraints on mixing processes and the recycling process in the deep Earth. We present high-resolution seismic array analyses of anomalous P-waves sampling the deep mantle, and deterministically locate heterogeneities in the lowermost 300 km of the mantle. Spectral analysis indicates that the dominant scale length of the heterogeneity is 4 to 7 km. The heterogeneity distribution varies laterally and radially and heterogeneities are more abundant near the margins of the lowermost mantle Large Low Velocity Provinces (LLVPs), consistent with mantle convection simulations that show elevated accumulations of deeply advected crustal material near the boundaries of thermo-chemical piles. The size and distribution of the observed heterogeneities is consistent with that expected for subducted oceanic crust. These results thus suggest the deep mantle contains an imprint of continued subduction of oceanic crust, stirred by mantle convection and modulated by long lasting thermo-chemical structures. The preferred location of the heterogeneity in the lowermost mantle is consistent with a thermo-chemical origin of the LLVPs. Our observations relate to the mixing behaviour of small length-scale heterogeneity in the deep Earth and indicate that compositional heterogeneities from the subduction process can survive for extended times in the lowermost mantle.
Earth and Planetary Science Letters, Vol. 478, pp. 47-58.
Mantle
plumes
Abstract: Mantle plumes are thought to originate from thermal boundary layers such as Earth's core-mantle boundary (CMB), and may cause intraplate volcanism such as large igneous provinces (LIPs) on the Earth's surface. Previous studies showed that the original eruption sites of deep-sourced LIPs for the last 200 Myrs occur mostly above the margins of the seismically-observed large low shear velocity provinces (LLSVPs) in the lowermost mantle. However, the mechanism that leads to the distribution of the LIPs is not clear. The location of the LIPs is largely determined by the source location of mantle plumes, but the question is under what conditions mantle plumes form outside, at the edges, or above the middle of LLSVPs. Here, we perform 3D geodynamic calculations and theoretical analyses to study the plume source location in the lowermost mantle. We find that a factor of five decrease of thermal expansivity and a factor of two increase of thermal diffusivity from the surface to the CMB, which are consistent with mineral physics studies, significantly reduce the number of mantle plumes forming far outside of thermochemical piles (i.e., LLSVPs). An increase of mantle viscosity in the lowermost mantle also reduces number of plumes far outside of piles. In addition, we find that strong plumes preferentially form at/near the edges of piles and are generally hotter than that forming on top of piles, which may explain the observations that most LIPs occur above LLSVP margins. However, some plumes originated at pile edges can later appear above the middle of piles due to lateral movement of the plumes and piles and morphologic changes of the piles. ?65-70% strong plumes are found within 10 degrees from pile edges in our models. Although plate motion exerts significant controls over the large-scale mantle convection in the lower mantle, mantle plume formation at the CMB remains largely controlled by thermal boundary layer instability which makes it difficult to predict geographic locations of most mantle plumes. However, all our models show consistently strong plumes originating from the lowermost mantle beneath Iceland, supporting a deep mantle plume origin of the Iceland volcanism.
Physics of the Earth and Planetary Interiors, Vol. 277, 1, pp. 10-29.
Mantle
Geothermometry
Abstract: The dynamics of Earth’s lowermost mantle exert significant control on the formation of mantle plumes and the core-mantle boundary (CMB) heat flux. However, it is not clear if and how the variation of CMB heat flux and mantle plume activity are related. Here, we perform geodynamic model experiments that show how temporal variations in CMB heat flux and pulses of mantle plumes are related to morphologic changes of the thermochemical piles of large-scale compositional heterogeneities in Earth’s lowermost mantle, represented by the large low shear velocity provinces (LLSVPs). We find good correlation between the morphologic changes of the thermochemical piles and the time variation of CMB heat flux. The morphology of the thermochemical piles is significantly altered during the initiation and ascent of strong mantle plumes, and the changes in pile morphology cause variations in the local and the total CMB heat flux. Our modeling results indicate that plume-induced episodic variations of CMB heat flux link geomagnetic superchrons to pulses of surface volcanism, although the relative timing of these two phenomena remains problematic. We also find that the density distribution in thermochemical piles is heterogeneous, and that the piles are denser on average than the surrounding mantle when both thermal and chemical effects are included.
Earth and Planetary Science Letters, Vol. 500, pp. 86-96.
Mantle
geothermometry
Abstract: The seismically-observed large low shear velocity provinces in the Earth's lowermost mantle have been hypothesized to be caused by thermochemical piles of compositionally distinct, more-primitive material which may be remnants of Earth's early differentiation. However, one critical question is how the Earth's thermal evolution is affected by the long-term presence of the large-scale compositional heterogeneity in the lowermost mantle. Here, we perform geodynamical calculations to investigate the time evolution of the morphology of large-scale compositional heterogeneity and its influence on the Earth's long-term thermal evolution. Our results show that a global layer of intrinsically dense material in the lowermost mantle significantly suppresses the CMB heat flux, which leads to faster cooling of the background mantle relative to an isochemical mantle. As the background mantle cools, the intrinsically dense material is gradually pushed into isolated thermochemical piles by cold downwellings. The size of the piles also decreases with time due to entraining of pile material into the background mantle. The morphologic change of the accumulations of intrinsic dense material eventually causes a gradual increase of CMB heat flux, which significantly reduces the cooling rate of Earth's mantle.
Abstract: The time and processes of hydrothermal mineralization are long-standing problems in geology. This work addresses these questions with reference to the Maoniuping giant rare earth elements (REE) deposit (southwest China), which has rare earth oxides (REO) reserves of 3.17 million tons with an average grade of 2.95 wt%. Bastnäsite is the dominant economic mineral, occurring as four distinct paragenetic types in the Maoniuping syenite-carbonatite complex: (1) primary euhedral bastnäsite (type-A) in syenite, with isolated melt inclusions; (2) macro-crystalline tabular euhedral bastnäsite (type-B) in pegmatitic dikes, with a diverse variety of fluid inclusions; (3) fine-grained, anhedral veinlet-disseminated bastnäsite (type-C) in syenite; and (4) coarse-grained anhedral bastnäsite (type-D) in carbonatite dikes, occurring as veinlets or interstitial to calcite, fluorite, and barite. From the paragenetic and compositional variations, it is inferred that type-A bastnäsite is of primary magmatic origin, whereas the other three types have characteristics of hydrothermal origins. In situ LA-ICP-MS U-Pb geochronology of the four types of bastnäsite results in lower intercept ages of 28.2 ± 0.5 Ma (n = 95, MSWD = 5.10), 27.8 ± 0.4 Ma (n = 43, MSWD = 0.73), 26.8 ± 0.7 Ma (n = 50, MSWD = 0.83), and 25.8 ± 0.7 Ma (n = 55, MSWD = 1.70), respectively, which are consistent with the weighted average 206Pb/238U and 208Pb/232Th ages by 207Pb-correction method. Compositional variations of clinopyroxene and apatite from the associated syenite, pegmatitic and carbonatitic dikes indicate a genetic relationship of the Maoniuping alkaline complex. The compositions of clinopyroxene range from Ae44-67Di14-18Hd17-41 in pegmatitic dikes, Ae43-66Di6-20Hd21-38 in carbonatitic dikes to Ae68-90Di0-3Hd10-30 in syenite. Apatites in the pegmatitic and carbonatitic dikes have similar compositions with higher F, total REE, and Sr, and lower CaO contents than those in the syenite, which suggests a cogenetic origin for the associated pegmatite and carbonatite. Clinopyroxene and apatite compositions suggest that the pegmatitic melt might differentiate directly from the initial carbonatitic melt rather than the syenitic magma. The bastnäsite U-Pb geochronology and minerals data indicate continuous magmatic-hydrothermal evolution for the REE mineralization in the Maoniuping alkaline complex.
Fundamental Research , 10.1016/j.fmre.2022.04.004 34p. Pdf
Mantle
REE
Abstract: Exploitable or potentially exploitable deposits of critical metals, such as rare-earth (REE) and high-field-strength elements (HFSE), are commonly associated with alkaline or peralkaline igneous rocks. However, the origin, transport and concentration of these metals in peralkaline systems remains poorly understood. This study presents the results of a mineralogical and geochemical investigation of the Na-metasomatism of alkali amphiboles from a barren peralkaline granite pluton in NE China, to assess the remobilization and redistribution of REE and HFSE during magmatic-hydrothermal evolution. Alkali amphiboles from the peralkaline granites show evolutionary trends from calcic through sodic-calcic to sodic compositions, with increasing REE and HFSE concentrations as a function of increasing Na-index (Na#, defined as molar Na/(Na+Ca) ratios). The Na-amphiboles (i.e., arfvedsonite) can be subsequently altered, or breakdown, to form Na-clinopyroxene (i.e., aegirine) during late- or post-magmatic alteration. Representative compositions analyzed by in-situ LA-ICPMS show that the alkali amphiboles have high and variable REE (1295-2218 ppm) and HFSE (4194-16,862 ppm) contents, suggesting that these critical metals can be scavenged by alkali amphiboles. Compared to amphiboles, the early replacement aegirine (Aeg-I, Na#?=?0.91-0.94) has notably lower REE (577-797) and HFSE (4351-5621) contents. In contrast, the late hydrothermal aegirine (Aeg-II, Na#?=?0.92-0.96) has significantly lower REE (127-205 ppm) and HFSE (6.43-72.2 ppm) contents. Given that the increasing Na# from alkali amphibole to aegirine likely resulted from Na-metasomatism, a scavenging-release model can explain the remobilization of REE and HFSE in peralkaline granitic systems. The scavenging and release of REE and HFSE by alkali amphiboles during Na-metasomatism provides key insights into the genesis of globally significant REE and HFSE deposits. The Na-index of alkali amphibole-aegirine might be useful as a geochemical indicator in the exploration for these critical-metals.
Abstract: Carbonatitic magmatism in subduction zones provides extremely valuable information on the cycling, behavior and storage of deep carbon within the Earth. It may also shed light on insights into crust-mantle interaction and mantle metasomatism within subduction zones. Origin of carbonatite has long been debated: all hypotheses need to reflect the different mineral assemblages and geochemical compositions of carbonatites and their diverse tectonic settings. Here we present a petrological, geochronological, geochemical and isotopic study of carbonatite bodies associated with orogenic peridotites, which occur as stocks or dykes with widths of tens to hundreds of meters in the Luliangshan region, North Qaidam, northern Tibet, China. On the basis of modal olivine (Ol) content, the studied samples were subdivided into two groups: Ol-poor carbonatite and Ol-rich carbonatite. Zircon grains from the Ol-poor carbonatite show detrital features, and yield a wide age spectrum between 400?Ma and 1000?Ma with a pronounced peak at ca. 410-430?Ma. By contrast, oscillatory zoned zircons and inherited cores show two relatively small Neoproterozoic age peaks at ca. 920 and 830?Ma. Zircon grains from the Ol-rich carbonatite sample are also distributed in a wide spectrum between 400 and 1000?Ma, with a pronounced peak at ca. 440?Ma and a slightly inferior peak at ca. 410?Ma. The oscillatory zoned zircons and inherited cores exhibit a smaller Neoproterozoic age peak at ca. 740?Ma. The pronounced peaks ranging from 430 to 410?Ma are consistent with the deep subduction and mantle metasomatic events recorded in associated ultramafic rocks. Both groups of carbonatites are characterized by enrichment of light rare earth elements (LREEs) with high (La/Yb)N values and pronounced negative Eu anomalies. They show high 87Sr/86Sr values (0.708156-0.709004), low 143Nd/144Nd values (0.511932-0.512013) and high ?18OV-SMOW values (+17.9 to +21.3‰). This geochemical and isotopic evidence suggests that these carbonatites were derived from remobilized sedimentary carbonate rocks. We propose that the primary carbonatite magma was formed by partial melting of sedimentary carbonates with mantle contributions. Sedimentary carbonates were subducted into the shallow upper mantle where they melted and formed diapirs that moved upwards through the hot mantle wedge. The case presented provides a rare example of carbonatite originating from sedimentary carbonates with mantle contributions and relevant information on the mantle metasomatism within a subduction zone.
Chen, Y.X., Zheng, Y-F., Chen, R-X., Zhang, S-B., Li, Q., Dai, M., Chen, L.
Metamorphic growth and recrystallization of zircons in extremely 18 O depleted rocks during eclogite facies metamorphism: evidence from U-Pb ages, trace elements and O-Hf isotopes.
Geochimica et Cosmochimica Acta, Vol. 75, 17, pp. 4877-4898.
Precisely dating Paleozoic kimberlites in the North Chin a craton and Hf isotopic constraints on the evolution of the subcontinental lithospheric mantle.
Abstract: Kimberlite field is an example of widespread Mesoproterozoic intracontinental magmatism. Recent studies have identified deep subcontinental lithospheric mantle as a source region of the kimberlite magmatism while timing, origin, and processes responsible for the generation of coeval lamprophyres remain poorly constrained. Here, we present and discuss new petrological and geochemical data for two lamprophyre dykes from the Wajrakarur kimberlite field and assess their petrogenetic relation to the kimberlite occurrences. Based on mineral compositional and whole?rock geochemical characters, it is suggested that lamprophyres are formed through low degrees of partial melting of “enriched” lithospheric mantle that was modified and metasomatized by melts derived from recycled crust. This differs from geochemical imprints found in coeval kimberlites, where a crustal source component appears to be absent and is more consistent with rock derivation from “depleted” lithosphere which has experienced interaction with asthenosphere?derived melts. An apparent lack of garnet in the mantle sources of lamprophyres is suggestive of melting at comparatively shallow depth (~100 km) relative to the kimberlites. Hence, these geochemically contrasting rocks, although have formed at the same time, are derived from vertically heterogeneous lithospheric mantle sources and can be explained through and linked with a thermal anomaly in the underlying convective asthenosphere. We suggest that the deeper mantle source region of the kimberlites was more pristine and devoid of subduction?related signatures, whereas the shallower mantle source region of the lamprophyres seems to have preserved imprints of plate convergence and subduction associated with the evolution of the Dharwar Craton.
Precise age determin ation of the Paleozoic kimberlites in North Chin a craton and Hf isotopic constraint on the evolution of its subcontinental lithospheric mantle.
Destructive of the North Chin a craton: delamin ation or thermal/chemical erosion? Mineral chemistry and oxygen isotope insights from websterite xenoliths.
Mesoproterozoic U-Pb ages, trace element and Sr-Nd isotopic composition of perovskite from kimberlites of the Eastern Dharwar craton, southern India: distinct mantle sources and a Wide spread 1.1 Ga Tectonomagmatic event.
Abstract: Alkaline rock and carbonatite complexes, including the Prairie Lake complex (NW Ontario), are widely distributed in the Canadian region of the Midcontinent Rift in North America. It has been suggested that these complexes were emplaced during the main stage of rifting magmatism and are related to a mantle plume. The Prairie Lake complex is composed of carbonatite, ijolite and potassic nepheline syenite. Two samples of baddeleyite from the carbonatite yield U-Pb ages of 1157.2±2.3 and 1158.2±3.8 Ma, identical to the age of 1163.6±3.6 Ma obtained for baddeleyite from the ijolite. Apatite from the carbonatite yields the same U-Pb age of ~1160 Ma using TIMS, SIMS and laser ablation techniques. These ages indicate that the various rocks within the complex were synchronously emplaced at about 1160 Ma. The carbonatite, ijolite and syenite have identical Sr, Nd and Hf isotopic compositions with a 87Sr/86Sr ratio of ~0.70254, and positive ?Nd(t)1160 and ?Hf(t)1160 values of ~+3.5 and ~+4.6, respectively, indicating that the silicate and carbonatitic rocks are co-genetic and related by simple fractional crystallization from a magma derived from a weakly depleted mantle. These age determinations extend the period of magmatism in the Midcontinent Rift in the Lake Superior area to 1160 Ma, but do not indicate whether the magmatism is associated with passive continental rifting or the initial stages of plume-induced rifting.
Abstract: Baddeleyite has been recognized as a key mineral to determine the crystallization age of silica-undersaturated igneous rocks. Here we report a new occurrence of baddeleyite identified from REE-Nb-Th-rich carbonatite in the world's largest REE deposit, Bayan Obo, in the North China Craton (China). U-Th-Pb dating of three baddeleyite samples yields crystallization ages of 310–270 Ma with the best estimated crystallization age of ca. 280 Ma. These ages are significantly younger than the ca. 1300 Ma Bayan Obo carbonatites, but broadly coeval to nearby Permian granitoids intruding into the carbonatites. Hence, the Bayan Obo baddeleyite did not crystallize from the carbonatitic magma that led to the formation of the Bayan Obo carbonatites and related REE-Nb-Th deposit. Instead, it crystallized from hydrothermal fluids and/or a reaction involving zircon and dolomite during contact metamorphism related to the Permian granitoid emplacement. This is in agreement with the results of electron microprobe analysis that show humite inclusions in baddeleyite, since humite is a typical magnesian skarn mineral and occurs in close proximity to the intrusive contacts between carbonatites and granitoids. Our results show that baddeleyite can be used for dating hydrothermal and contact metamorphic processes.
Journal of Asian Earth Sciences, Vol.213, 104750, 12p.pdf
India
deposit -P12
Abstract: Petrology, bulk-rock geochemistry, and perovskite U-Pb age for the P-12 kimberlite pipe from the Wajrakarur kimberlite field, Eastern Dharwar craton (EDC) of southern India is reported. Perovskites yielded a high-precision U-Pb age of 1122 ± 7.7 Ma, taken to be an emplacement age of the host P-12 kimberlite pipe. The groundmass of coherent facies P-12 kimberlite contains monticellite, clinopyroxene, andradite, atoll spinel with titanomagnetite trend, and perovskite with an elevated REE contents. Phlogopite shows restricted Al2O3 and TiO2 contents. Furthermore, olivines with a wider and higher range of core compositions (i.e. Mg# = 84-94) and multi-granular nodules are the hallmark features of the P-12 pipe. This assorted primary mineral content and its composition indicates the transitional nature of the P-12 towards the Kaapvaal lamproites. However, concentrations of bulk-rock major and trace elements in the P-12 and other Wajrakarur kimberlites are similar to the global hypabyssal magmatic kimberlites. Large ion lithophile and high field strength elements (e.g. Ba and Nb) and their ratios (e.g. La/Nb and Th/Nb) suggest the presence of a heterogeneous and lithosphere influenced mantle source region which have been severely overprinted by metasomatizing fluids/melts emanating from the deep sourced upwelling mantle. The presence of such mixed and metasomatized mantle source regions likely to be an important factor for the transitional nature of the P-12 and other Mesoproterozoic kimberlites. Based on the availability of the newest emplacement ages, we propose a geodynamic model for the origin of kimberlites in the Indian subcontinent. The U-Pb age of 1122 ± 7.7 Ma for the P-12 pipe shows its close temporal association to the emplacement of the recently proposed 1110 Ma Large Igneous Province (LIP), with plume center beneath the NW part of the Kalahari craton. Emplacement of the P-12 and other contemporaneous Indian kimberlites, therefore, marks the impingement of mantle plume which contributed heat and triggered partial melting of metasomatized lithospheric mantle without melt input. The eruption phase of ~ 100 million years (i.e. 1050-1153 Ma), for the kimberlites and related rocks in the Indian shield, does not appear to be continuous and can be separated into several short-durational magmatic events. For this reason, small-volume, volatile-rich magmatism during the Mesoproterozoic time in India is linked to the presence of a number of LIPs and associated mantle plumes during Columbia to Rodinia supercontinent transition and assembly of cratonic blocks of the latter.
Nature Communications, 10.10.1038/s4167-021-266668-z 14p. Pdf
Mantle
olivine
Abstract: Oxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities. Here we provide an alternative view for the origin of light oxygen-isotope signatures in mantle-derived magmas using kimberlites, carbonate-rich magmas that assimilate mantle debris during ascent. Olivine grains in kimberlites are commonly zoned between a mantle-derived core and a magmatic rim, thus constraining the compositions of both mantle wall-rocks and melt phase. Secondary ion mass spectrometry (SIMS) analyses of olivine in worldwide kimberlites show a remarkable correlation between mean oxygen-isotope compositions of cores and rims from mantle-like 18O/16O to lower ‘crustal’ values. This observation indicates that kimberlites entraining low-18O/16O olivine xenocrysts are modified by assimilation of low-18O/16O sub-continental lithospheric mantle material. Interaction with geochemically-enriched domains of the sub-continental lithospheric mantle can therefore be an important source of apparently ‘crustal’ signatures in mantle-derived magmas.
Contributions to Mineralogy and Petrology, in press available 19p.
Asia, Tibet
Melting
Abstract: Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F-Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800-850 °C, >15 kbar). Zircons show four main age groupings: 1.0-0.5 Ga, 50-45, 35-20, and 16-13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50-45 Ma zircons match the crystallization age and juvenile character (?Hfi +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35-20 Ma Miocene ages are derived from zircons with similar Hf-O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon ?18O (+8.73-8.97 ‰) values, and extremely low ?Hfi (?12.7 to ?9.4) values. Such evolved Hf-O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low ?18O (+6 to 7.5 ‰), intermediate (?18O +8.5 to 9.0 ‰), and high ?18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with ?18O 10-12 ‰. Both the low and intermediate ?18O groups show significant variation in Fe content, whereas the two high ?18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate ?18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high ?18O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35-20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust-mantle hybridization in the deep Tibetan crust.
Abstract: The dynamical evolution and exhumation mechanisms of oceanic?derived eclogites are controversial conundrums of oceanic subduction zones. The previous studies indicated that density is the primary factor controlling the exhumation of oceanic rocks. To explore their density evolution, we systematically investigate the phase relations and densities of different rock types in oceanic crust, including mid ocean ridge basalt (MORB), serpentinite, and global subducting sediments (GLOSS). According to the density of eclogites, these currently exposed natural eclogites can be classified into two categories: the self?exhumation of eclogites (?MORB < ?Mantle) and the carried exhumation of eclogites (?MORB > ?Mantle). The depth limit for an exhumation of oceanic?derived eclogites solely driven by their own buoyancies is 100-110 km, and it increases with the lithospheric thickness of the overriding plate. The parameters of carried?exhumation, that is, KGLOSS and KSerp, are defined in order to quantitatively evaluate the assistance ability of GLOSS and serpentinites for carrying the denser eclogites. KGLOSS is mainly controlled by pressure, whereas KSerp is dominantly affected by temperature. Using 2?D thermomechanical models, we demonstrate that the presences of low?density, low?viscosity GLOSS and seafloor serpentinites are the prerequisites for the exhumation of oceanic?derived eclogites. Our results show that oceanic?derived eclogites should be stalled and exhumed slowly at the Moho and Conrad discontinuities (named Moho/Conrad stagnation). We propose that oceanic?derived eclogites should undergo a two?stage exhumation generally, that is, early fast exhumation driven by buoyancy at mantle levels, and final exposure to surface actuated by tectonic exhumation facilitated by divergence between upper plate and accretionary wedge or by rollback of lower plate.
Abstract: The finite element method is used to simulate the steady-state temperature field in diamond synthesis cell. The 2D and 3D models of the China-type cubic press with large deformation of the synthesis cell was established successfully, which has been verified by situ measurements of synthesis cell. The assembly design, component design and process design for the HPHT synthesis of diamond based on the finite element simulation were presented one by one. The temperature field in a high-pressure synthetic cavity for diamond production is optimized by adjusting the cavity assembly. A series of analysis about the influence of the pressure media parameters on the temperature field are examined through adjusting the model parameters. Furthermore, the formation mechanism of wasteland was studied in detail. It indicates that the wasteland is inevitably exists in the synthesis sample, the distribution of growth region of the diamond with hex-octahedral is move to the center of the synthesis sample from near the heater as the power increasing, and the growth conditions of high quality diamond is locating at the center of the synthesis sample. These works can offer suggestion and advice to the development and optimization of a diamond production process.
Contributions to Mineralogy and Petrology, Vol. 175, 22p. Pdf.
Russia
deposit - Obnazhennaya
Abstract: The petrology, mineral major and trace-element concentrations, and garnet oxygen isotopic composition of low-MgO (11-16 wt%) eclogites from the Obnazhennaya kimberlite, Siberian craton, are used to infer their petrogenesis. These eclogites contain two types of compositionally distinct garnet: granular coarse garnet, and garnet exsolution (lamellae and fine-grained garnet) in clinopyroxene. The former record higher temperatures at lower pressures than the latter, which record the last stage of equilibrium at moderate pressure-temperature conditions 2.3-3.7 GPa and 855-1095 °C in the upper mantle at the time of entrainment. Although derived from the garnet stability field, these rocks have low-pressure cumulate protoliths containing plagioclase, olivine, and clinopyroxene as reflected by pronounced positive Eu and Sr anomalies in all eclogites, and low heavy rare earth element (HREE) contents in both minerals and reconstructed bulk rocks for a number of samples. Major elements, transition metals, and the HREE compositions of the reconstructed whole rocks are analogous to modern oceanic gabbro cumulates. Despite geochemical signatures supporting an oceanic crust origin, mantle-like ?18O of the garnets (5.07-5.62‰) for most samples indicates that the protoliths either did not interact with seawater or have coincidently approximately normal igneous values. Some of the eclogite xenoliths have lower SiO2 contents and depleted light REE ((Nd/Yb)N?
Minerals MDPI, Vol. 10, 1117, doi:10.3390/ min10121117 20p. Pdf
China
deposit - Bayan Obo
Abstract: The Bayan Obo rare earth element (REE) deposit in Inner Mongolia, northern China, is the largest REE deposit in the world, whose mineralization process remains controversial. There are dozens of carbonatite dykes that are tightly related to the deposit. Here we report the petrological and mineralogical characteristics of a typical dolomite carbonatite dyke near the deposit. The dolomite within the dyke experienced intense post-emplacement fluids metasomatism as evidenced by the widespread hydrothermal REE-bearing minerals occurring along the carbonate mineral grains. REE contents of bulk rocks and constituent dolomite minerals (>90 vol.%) are 1407-4184 ppm and 63-152 ppm, respectively, indicating that dolomite is not the dominant mineral controlling the REE budgets of the dyke. There are three types of apatite in the dyke: Type 1 apatite is the primary apatite and contains REE2O3 at 2.35-4.20 wt.% and SrO at 1.75-2.19 wt.%; Type 2 and Type 3 apatites are the products of replacement of primary apatite. The REE2O3 (6.10-8.21 wt.%) and SrO (2.83-3.63 wt.%) contents of Type 2 apatite are significantly elevated for overprinting of REE and Sr-rich fluids derived from the carbonatite. Conversely, Type 3 apatite has decreased REE2O3 (1.17-2.35 wt.%) and SrO (1.51-1.99 wt.%) contents, resulting from infiltration of fluids with low REE and Na concentrations. Our results on the dyke suggest that post-magmatic fluids expelled from the carbonatitic melts dominated the REE mineralization of the Bayan Obo deposit, and a significant fluid disturbance occurred but probably provided no extra REEs to the deposit.
Abstract: The petrology, mineral major and trace element concentrations, and garnet oxygen isotopic composition of low-MgO (11-16 wt.%) eclogites from the Obnazhennaya kimberlite, Siberian craton, are used to infer their petrogenesis. These eclogites equilibrated at moderate pressure-temperature conditions 2.3-3.7 GPa and 855- 1095?C at the time of entrainment. Although derived from the garnet stability field, these rocks have low-pressure cumulate protoliths containing plagioclase, olivine, and clinopyroxene as reflected by pronounced positive Eu and Sr anomalies in all eclogites, and low heavy rare earth element (HREE) contents in both minerals and reconstructed bulk rocks for a number of samples. Major elements, transition metals, and the HREE compositions of the reconstructed whole rocks are analogous to modern oceanic gabbro cumulates. Despite geochemical signatures supporting an oceanic crust origin, mantle-like ?18O of the garnets (5.07-5.62 ‰ ) for most samples indicates that the protoliths either did not interact with seawater or have coincidently approximately normal igneous values. Some of the eclogite xenoliths have lower SiO2 contents and depleted light REE ((Nd/Yb)N ? 1) compared to modern oceanic gabbros, suggesting that they experienced partial melting. Positively inclined middle to heavy-REE patterns ((Dy/Yb)N ?1) of the reconstructed bulk rocks mostly result from repeated partial melting in the eclogite stability field, based on melting model calculations. We therefore suggest that the Obnazhennaya low-MgO eclogites may represent the gabbroic section of subducted or foundered basaltic crust that underwent continued partial melting processes at high pressures where garnet was the main residual phase.
Carboniferous and Triassic eclogites in the Western Dabie Mountains east central Chin a: evidence for protracted convergence of the North and South Chin a Blocks.
Journal of Metamorphic Geology, Vol. 20, 9, pp. 873-886.
Evolution from oceanic subduction to continental collision: a case study from the northern Tibetan Plateau based on geochemical and geochronological data.
Two stage collision related extrusion of the western Dabie HP-UHP metamorphic terranes, centra China: evidence from quartz c-axis fabrics and structures.
Abstract: The dynamics of formation and exhumation of high-pressure (HP) and ultra-high pressure (UHP) metamorphic orogens in double subduction-collision zones remain enigmatic. Here we employ two-dimensional thermo-mechanical numerical models to gain insights on the exhumation of HP-UHP metamorphic rocks, as well as their deformation during the collision of a micro-continent with pro- and retro-continental margins along two subduction zones. A three-stage collisional process with different convergence velocities is tested. In the initial collisional stage, a fold-and-thrust belt and locally rootless superimposed folds are developed in the micro-continent and subduction channel, respectively. In the second (exhumation) stage of HP-UHP rocks, a faster convergence model results in upwelling of the asthenosphere, which further leads to a detachment between the crust and lithospheric mantle of the micro-continent. A slower convergence model results in rapid exhumation of HP-UHP rocks along the north subduction channel and a typical piggy-back thrusting structure in the micro-continent. A non-convergence model produces a slab tear-off, leading to the rebound of residual lithosphere of the micro-continent. In the third and final stage, a series of back and ramp thrusts are formed in the micro-continent with the pro-continent re-subducted. Based on an analogy of our numerical results with the Western Dabie Orogen (WDO), we suggest that: (1) slab tear-off results in a rebound of residual lithosphere, which controls the two-stage syn-collisional exhumation process of HP-UHP rocks in the WDO; and (2) in contrast to the single subduction-collision system, the exhumation range of the partially molten rocks with lower viscosity and density is restricted to a specific region of the micro-continent by the Mianlue and Shangdan subduction zones, which generated the complex deformation features in the WDO.
Geochimica et Cosmochimica Acta, Vol. 295, pp. 207-223.
China
nephelinites, basanites
Abstract: Widespread Cenozoic intraplate basalts from eastern China offer the opportunity to investigate the consequences of interaction between the stagnant Pacific slab and overlying asthenosphere and chemical heterogeneity within this “big mantle wedge”. We present and compile a comprehensive study of highly siderophile elements and Mg-Zn isotopes of this magmatic suite (60 samples including nephelinites, basanites, alkali basalts and tholeiites). The large-scale Mg-Zn isotopic anomalies documented in these basalts have been ascribed to mantle hybridization by recycled Mg-carbonates from the stagnant western Pacific plate. Our results reveal that the nephelinites and basanites are characterized by unfractionated platinum-group element (PGE) patterns normalized to primitive upper mantle (PUM) (e.g., PdN/IrN normalized to PUM?=?1.1?±?0.8, 1?), relatively high total PGE contents (e.g., Ir?=?0.25?±?0.14?ppb) and modern mantle-like 187Os/188Os (0.142?±?0.020). These characteristics are coupled with lighter Mg isotope (?26Mg?=??0.48?±?0.07‰) and heavier Zn isotope (?66Zn = +0.46?±?0.06‰) compositions compared to the mantle values (?26Mg: ?0.25?±?0.07‰; ?66Zn: +0.18?±?0.05‰). Together, these data are interpreted to reflect the oxidative breakdown of low proportions of mantle sulfides in the sources of these small-degree melts, likely caused by recycled carbonates, which then release chalcophile-siderophile elements into carbonatitic melts. By contrast, the contemporaneous alkali basalts and tholeiites are characterized by highly fractionated PGE patterns (e.g., PdN/IrN?=?4.4?±?3.3; Ir?=?0.037?±?0.027?ppb) and radiogenic 187Os/188Os (0.279?±?0.115) coupled with less fractionated Mg-Zn isotope compositions (?26Mg: ?0.39?±?0.05‰; ?66Zn: +0.35?±?0.03‰). In combination with other isotopic (e.g., Sr-Nd) and chemical (SiO2, Ce/Pb, Ba/Th, Fe/Mn) constraints, the alkali basalts and tholeiites were derived from higher degree melting of ancient pyroxenite-bearing mantle in addition to mixing with the aforementioned nephelinitic and basanitic melts. Collectively, we suggest that deep recycled carbonates promoted melting within the "big mantle wedge" leading to the generation of Cenozoic intraplate basalts across eastern China and the "redox freezing of carbonates" may cause the oxidation of Fe0 and S2-. This process may provide an important mechanism to oxidize mantle sulfides and transfer precious metals from deep mantle to crust.
Abstract: Syn-exhumation mafic magmatism during continental collision provides insights into the crust-mantle reaction during deep subduction and the nature of orogenic lithospheric mantle in collisional orogens. In this study, we present a comprehensive data set of zircon U-Pb ages and whole-rock major-trace elements as well as Sr-Nd-Pb isotopes of alkaline mafic rocks from the southern Liaodong Peninsula, eastern China. Zircon U-Pb analyses yield Late Triassic age of 213 ± 3 to 217 ± 3 Ma, younger than the Middle Triassic ultrahigh-pressure metamorphic rocks of the Dabie-Sulu orogen. Thus, the alkaline mafic rocks are products of syn-exhumation magmatism during continental collision of the South and North China blocks. The rocks show shoshonitic affinities with high K2O (3.78-5.23 wt%) and K2O/Na2O (0.71-1.22). They are characterized by arc-like trace-element patterns with enriched LILE, Pb, and LREE, and depleted HFSE. They exhibit enriched Sr-Nd isotopic compositions with high initial 87Sr/86Sr isotopic ratios of 0.7058-0.7061 and negative ?Nd(t) values of ?13.0 to ?15.1. These results suggest involvement of recycled continental crust in their mantle source. The mantle source likely formed by the metasomatic reaction of subducted continental crust-derived melts with the overlying subcontinental lithospheric mantle during the Triassic continental collision. Decompressional melting of this metasomatized mantle formed syn-exhumation mafic magmas during the transition from convergent to extensional tectonics in the Late Triassic. Accordingly, mafic rocks from the southern Liaodong Peninsula provide a geochemical record of the subduction and recycling of continental crust into the mantle and melt-mantle reaction induced metasomatism within the orogen.
Abstract: This paper presents the first major and trace element compositions of mantle-derived garnet xenocrysts from the diamondiferous No. 30 kimberlite pipe in the Wafangdian region, and these are used to constrain the nature and evolution of mantle metasomatism beneath the North China Craton (NCC). The major element data were acquired using an electron probe micro-analyzer and the trace element data were obtained using laser ablation inductively coupled plasma-mass spectrometry. Based on Ni-in-garnet thermometry, equilibrium temperatures of 1107-1365 °C were estimated for peridotitic garnets xenocrysts from the No. 30 kimberlite, with an average temperature of 1258 °C, and pressures calculated to be between 5.0 and 7.4 GPa. In a CaO versus Cr2O3 diagram, 52% of the garnets fall in the lherzolite field and 28% in the harzburgite field; a few of the garnets are eclogitic. Based on rare earth element patterns, the lherzolitic garnets are further divided into three groups. The compositional variations in garnet xenocrysts reflect two stages of metasomatism: early carbonatite melt/fluid metasomatism and late kimberlite metasomatism. The carbonatite melt/fluids are effective at introducing Sr and the light rare earth elements, but ineffective at transporting much Zr, Ti, Y, or heavy rare earth elements. The kimberlite metasomatic agent is highly effective at element transport, introducing, e.g., Ti, Zr, Y, and the rare earth elements. Combined with compositional data for garnet inclusions in diamonds and megacrysts from the Mengyin and Wafangdian kimberlites, we suggest that these signatures reflect a two-stage evolution of the sub-continental lithospheric mantle (SCLM) beneath the NCC: (1) early-stage carbonatite melt/fluid metasomatism resulting in metasomatic modification of the SCLM and likely associated with diamond crystallization; (2) late-stage kimberlite metasomatism related to the eruption of the 465 Ma kimberlite.
Common Pb isotope mapping of UHP metamorphic zones in Dabie orogen, central China: implication for Pb isotopic structure of subducted continental crust.
Geochimica et Cosmochimica Acta, Vol. 143, pp. 115-131.
Abstract: The Mg isotopic compositions of garnet and clinopyroxene mineral separates and whole rocks from 21 xenolithic eclogites (11 low-MgO eclogites and 10 high-MgO eclogites) from the Koidu kimberlite complex, erupted within the Archean Man Shield, Sierra Leone, West Africa, provide new evidence bearing on the origin of cratonic eclogites. Garnet and clinopyroxene in both low-MgO and high-MgO eclogites generally record equilibrium inter-mineral Mg isotope partitioning, with ?26Mg varying from -2.15‰ to -0.46‰ in garnets and from -0.49‰ to +0.35‰ in clinopyroxenes. Bulk ?26Mg values (-1.38‰ to +0.05‰), constructed from garnet and clinopyroxene data, are similar to results from rock powders (-1.60‰ to +0.17‰), suggesting that kimberlite infiltration has had negligible influence on the Mg isotopic compositions of the xenoliths. The ?26Mg values of low-MgO eclogites (-0.80‰ to +0.05‰) exceed the range of mantle peridotite xenoliths (-0.25‰ ± 0.04‰), consistent with the eclogite’s derivation from recycled altered oceanic crust. Similarly variable ?26Mg values in high-MgO eclogites (-0.95‰ to -0.13‰), together with their high MgO and low FeO contents, suggest that high-MgO eclogites were produced by Mg-Fe exchange between partially molten low-MgO eclogites and surrounding peridotites. Our study shows that cratonic xenolithic eclogites preserve a record of Mg isotopic compositions produced by low-pressure, surficial isotope fractionations. The recycling of oceanic crust therefore increases the Mg isotope heterogeneity of the mantle.
Abstract: China covers approximately 10?million?km2 and its crust has a complicated evolution of amalgamation, igneous activity, and sedimentation. Many studies address various aspects of China's crust, but few provide a simple geological and geophysical overview that is accessible to students and non-specialists; Filling this void is the objective of this review. China is characterized by thick (40-75?km) crust in the west due to Cenozoic collision with India and thin (30-40?km thick) crust in the east due to E-W Mesozoic-Cenozoic backarc extension. In contrast, overall crustal fabric trends E-W, defined by ophiolite belts and ultra-high pressure metamorphic rocks. This crustal fabric indicates that China has grown like a sandwich, with crust progressively added through Phanerozoic time by closing various E-W oriented Tethys oceans and seaways. In map view, China consists of five E-W trending tiers. Tier 1 is defined by the Central Asian Orogenic Belt (CAOB) along the northern margin of China, which consists of the Xing'an-Mongolia orogenic belt in the NE and the Tianshan Orogen in the NW. The CAOB formed during ~1000?Ma to ~250?Ma and is an accretionary orogen of mostly Paleozoic age that formed through closure of the Paleo-Asian Ocean and collision between the Siberian Craton and Archean-Paleoproterozoic crust to the south, which constitutes Tier 2. The CAOB has a strong aeromagnetic signature. Sediments from the Amur River show detrital U-Pb zircon age peaks at 2.8-2.3?Ga, 1.8?Ga, 450-250?Ma, and 200-100?Ma, which is expected for erosion of the Xing'an-Mongolia belt. Tier 1 igneous rocks are mainly Paleozoic except in the NE (Xing'an-Mongolia orogenic belt) and reflect subduction of the Paleo-Asian Ocean and associated accretion events, whereas Paleozoic CAOB crust in the east is overprinted by Jurassic and Cretaceous igneous rocks related to subduction of ancient Pacific basin oceanic lithosphere. Tier 2 includes the North China Craton (NCC) to the east and Tarim Craton to the west. The NCC contains the oldest rocks in China and is dominated by Archean and Paleoproterozoic ages. The extent of Archean rocks in the NCC may have been overestimated, as suggested by detrital zircons from the Yellow River, which flows across the craton, showing age peaks at 2.5-2.2?Ga, ~1.9?Ga, 500-400?Ma, and 300-200?Ma. The Tarim Craton is dominated by Palaeoproterozoic- Mesoproterozoic metamorphic strata along with a significant proportion of Neoproterozoic (~0.8?Ga) rocks. U-Pb ages for detrital zircons from Tarim River sediments reflect this basement geology, with strong peaks of Early and Late Paleozoic age, less abundant Neoproterozoic ages, and scattered ages back to the Archean. The NCC also was affected by abundant Mesozoic igneous activity with voluminous Early Cretaceous rocks that are associated with lithospheric thinning and decratonization. Tier 3 - also known as the Central China Orogen - is composed of the Sulu-Dabie-Qinling-Kunlun Orogen and records closing of an arm of Prototethys during the Ordovician to Silurian and Paleotethys during the Triassic. Tier 3 contains one of Earth's three giant ultra-high pressure (UHP) terranes with well-documented peak metamorphism of 650-850?°C and 4?GPa, indicating that some of these rocks were deeply subducted and then exhumed from depths of over 120?km in Triassic time. Tier 3 magmatism occurred in two episodes, early-middle Paleozoic and Triassic. Tier 4 contains blocks rifted from Gondwana, which include the Songpan-Ganzi, Qiangtang, and Lhasa terranes of Tibet in the west and the South China Block in the east. These terranes are marked by broad magnetic anomalies with a NE-SW trend along the Pacific margin, and a broad N-S trending anomaly between Tibet and South China. The South China Block is made up of Proterozoic and minor Archean crust of the Yangtze and Cathaysia blocks, which collided at 1.0-0.8?Ga to form the Jiangnan Orogen and the South China Block. Age spectra for detrital zircons from the Yangtze and Pearl Rivers shows major peaks at ~1.8?Ga, 900-800?Ma, ~400?Ma, and 300-150?Ma, which is consistent with the age of South China Block crust. Early-Middle Paleozoic igneous rocks are also found in South China. Mesozoic igneous rocks are widespread in both South China and Tibet and are related to subduction of the Paleo-Pacific and Tethyan oceanic plates, respectively. The accretion of Tibetan terranes to southern Eurasia occurred in the Mesozoic before collision with India at ~55?Ma. Tier 5 is represented by the island of Taiwan on the SE margin of China and marks where China crust continues to grow. Taiwan lies on a complex convergent boundary between the South China Block to the NW, the Philippine Sea Plate to the SE, and the Sunda Plate to the SW.
Abstract: Petrological, geochemical, and zircon U-Pb geochronological features of Archean ultramafic-mafic complexes formed in subduction?related settings provide significant insights into mantle source and geodynamic processes associated with subduction-accretion?collision events in the early Earth. Here, we investigate a suite of serpentinized dunite, dunite, pyroxenite, and clinopyroxenite from an ultramafic complex along the collisional suture between the Western Dharwar Craton (WDC) and the Central Dharwar Craton (CDC) in southern India. We present petrology, mineral chemistry, zircon U-Pb geochronology, rare earth element (REE), Lu-Hf isotopes, and whole?rock geochemistry including major, trace element, and platinum?group element (PGE) data with a view to investigate the magmatic and metasomatic processes in the subduction zone. Mineral chemistry data from chromite associated with the serpentinised ultramafic rocks show distinct characteristics of arc?related melt. Zircon U-Pb data from the ultramafic suite define different age populations, with the oldest ages at 2.9 Ga, and the dominant age population showing a range of 2.8-2.6 Ga. The early Paleoproterozoic (ca. 2.4 Ga) metamorphic age is considered to mark the timing of collision of the two WDC and CDC. Zircon REE patterns suggest the involvement continental crust components in the magma source. Zircon Lu-Hf analysis yields both positive and negative ?Hf(t) values from ?3.9 to 1.5 with Hf?depleted model ages (TDM) of 3,041-3,366 Ma for serpentinised dunite and ?0.2-2.0 and 2,833-2,995 Ma for pyroxenite, suggesting that the magma was sourced from depleted mantle and was contaminated with the ancient continental crust. Geochemical data show low MgO/SiO2 values and elevated Al2O3/TiO2 ratios, implying subduction?related setting. The serpentinized dunites and dunites show mild LREE enrichment over HREE, with relatively higher abundance of LILE (Ba, Sr) and depletion in HFSE (Nb, Zr), suggesting fluid-rock interaction, melt impregnation, and refertilization processes. The PGE data suggest olivine, chromite, and sulphide fractionations associated with subduction processes. Our study on the Mesoarchean to Neoarchean ultramafic complex provides important insights to reconstruct the history of the crust-mantle interaction in an Archean suprasubduction zone mantle wedge.
Earth Science Reviews, in press available 41p. Pdf
Mantle
plate tectonics
Abstract: The Earth as a planetary system has experienced significant change since its formation c. 4.54 Gyr ago. Some of these changes have been gradual, such as secular cooling of the mantle, and some have been abrupt, such as the rapid increase in free oxygen in the atmosphere at the Archean-Proterozoic transition. Many of these changes have directly affected tectonic processes on Earth and are manifest by temporal trends within the sedimentary, igneous, and metamorphic rock record. Indeed, the timing of global onset of mobile-lid (subduction-driven) plate tectonics on our planet remains one of the fundamental points of debate within the geosciences today, and constraining the age and cause of this transition has profound implications for understanding our own planet's long-term evolution, and that for other rocky bodies in our solar system. Interpretations based on various sources of evidence have led different authors to propose a very wide range of ages for the onset of subduction-driven tectonics, which span almost all of Earth history from the Hadean to the Neoproterozoic, with this uncertainty stemming from the varying reliability of different proxies. Here, we review evidence for paleo-subduction preserved within the geological record, with a focus on metamorphic rocks and the geodynamic information that can be derived from them. First, we describe the different types of tectonic/geodynamic regimes that may occur on Earth or any other silicate body, and then review different models for the thermal evolution of the Earth and the geodynamic conditions necessary for plate tectonics to stabilize on a rocky planet. The community's current understanding of the petrology and structure of Archean and Proterozoic oceanic and continental crust is then discussed in comparison with modern-day equivalents, including how and why they differ. We then summarize evidence for the operation of subduction through time, including petrological (metamorphic), tectonic, and geochemical/isotopic data, and the results of petrological and geodynamical modeling. The styles of metamorphism in the Archean are then examined and we discuss how the secular distribution of metamorphic rock types can inform the type of geodynamic regime that operated at any point in time. In conclusion, we argue that most independent observations from the geological record and results of lithospheric-scale geodynamic modeling support a global-scale initiation of plate tectonics no later than c. 3 Ga, just preceding the Archean-Proterozoic transition. Evidence for subduction in Early Archean terranes is likely accounted for by localized occurrences of plume-induced subduction initiation, although these did not develop into a stable, globally connected network of plate boundaries until later in Earth history. Finally, we provide a discussion of major unresolved questions related to this review's theme and provide suggested directions for future research.
SHRIMP U Pb zircon dating of the Rongcheng eclogite and associated peridotite: new constraints for UHP metamorphism of mantle derived mafic ultramafic bodies
Geological Society of America Special Paper, No. 403, pp. 115-126.
Chemical composition and ultrahigh P metamorphism of garnet peridotites from the Sulu UHP terrane, China: investigation of major, trace elements and Hf isotopes
Chemical composition and ultrahigh P metamorphism of garnet peridotites from the Sulu UHP terrane, China: investigation of major trace elements and Hf isotopes.
Chemical Geology, Vol. 255, 1-2, Sept. 30, pp. 250-264.
Science China Earth Sciences, Vol. 59, 3, pp. 573-582.
China
Craton, North China
Abstract: Present-day hot spots and Phanerozoic large igneous provinces (LIPs) and kimberlites mainly occur at the edges of the projections of Large Low Shear Wave Velocity Provinces (LLSVPs) on the earth’s surface. If a plate contains accurately dated LIPs or kimberlites, it is possible to obtain the absolute paleoposition of the plate from the LIP/kimberlite and paleomagnetic data. The presence of Middle Ordovician kimberlites in the North China Block provides an opportunity to determine the absolute paleoposition of the block during the Middle Ordovician. In addition to paleobiogeographical information and the results of previous work on global plate reconstruction for the Ordovician Period, we selected published paleomagnetic data for the North China Block during the Middle Ordovician and determined the most reasonable absolute paleoposition of the North China Block during the Middle Ordovician: paleolatitude of approximately 16.6°S to 19.1°S and paleolongitude of approximately 10°W. The block was located between the Siberian Plate and Gondwana, close to the Siberian Plate. During the Cambrian and Ordovician periods, the North China Block may have moved toward the Siberian Plate and away from the Australian Plate.
ISPRS Journal of Photogrammetry and Remote Sensing, Vol. 146, pp. 91-107.
Mantle
remote sensing
Abstract: Since Google Earth was first released in 2005, it has attracted hundreds of millions of users worldwide and made a profound impact on both academia and industry. It can be said that Google Earth epitomized the first-generation of Digital Earth prototypes. The functionalities and merits that have sustained Google Earth’s lasting influence are worth a retrospective review. In this paper, we take the liberty to conduct a bibliometric study of the applications of Google Earth during 2006-2016. We aim first to quantify the multifaceted impacts, and then to develop a structured understanding of the influence and contribution associated with Google Earth. To accomplish these objectives, we analyzed a total of 2115 Scopus publication records using scientometric methods and then proceed to discussion with a selected set of applications. The findings and conclusions can be summarized as follows: (1) the impact of Google Earth has been profound and persistent over the past decade. Google Earth was mentioned in an average of 229 publications per year since 2009. (2) Broadly, the impact of Google Earth has touched upon most scientific disciplines. Specifically, during 2006-2016, Google Earth has been mentioned in 2115 publications covering all of Scopus’s 26 subject areas; (3) the influence of Google Earth has largely concentrated in GIScience, remote sensing and geosciences. The extended influence of Google Earth has reached a wider range of audiences with a concentration in fields such as human geography, geoscience education and archaeology.
Journal of Geophysical Research: Solid Earth, Vol. 124, 7, pp. 6490-6503.
Mantle
peridotite
Abstract: Subducted sediments play an important role in the transport of incompatible elements back into the Earth's mantle. In recent years, studies of volcanic rocks from Samoan (Jackson et al., 2007, https://doi.org/10.1038/nature06048), NE China (Wang, Chen, et al., 2017, https://doi.org/10.1016/j.epsl.2017.02.028), and Gaussberg, Antarctica (Murphy et al., 2002, https://doi.org/10.1093/petrology/43.6.981), have shown geochemical records of a sediment?influenced mantle source from the deep Earth. However, experimental studies on the partial melting behavior of mixed sediment?peridotite mantle beyond subarc depths are very rare. In this study, we conducted experiments to investigate the partial melting behavior of mixed sediment?peridotite mantle at 4-15 GPa and 1200-1800 °C. The experimental solidi of mixed sediment?peridotite and K?feldspar?peridotite systems (Mixes A and B) cross the hot mantle geotherm at depths of around the X discontinuity (seismic discontinuity, ~300?km depth). The trace element compositions of the corresponding partial melts in Mix A showed similar characteristics to those of the Samoan basaltic lavas, potassic basalts from NE China, and Gaussberg lamproites. Therefore, the experimental results provide a possible explanation for the origin of some unusual mantle?derived volcanic rocks that contain recycled sediment signatures and have very deep origins. At depths of ~300 km (X discontinuity), a mixed sediment?peridotite source was heated by a hot?upwelling mantle and produced enriched melt. The enriched melt may interact with the surrounding mantle before incorporated into the upwelling mantle plume and becoming involved in the origin of some volcanic rocks. The experiments also provide a possible link between the enriched?mantle source in the deep mantle and the X discontinuity.
Environmental Forensics, Vol. 22, 3-4, pp. 340-350. abstract only
China
synthetics
Abstract: In this study, soil and sediment samples along with groundwater samples were collected and analyzed from an abandoned synthetic diamond production plant in Anhui Province, South China. Chemical analysis, pollution characteristics analysis, and correlation analysis were conducted to assess and to determine the source(s) of the toxic metal and organic pollutions in the study sites. The Co and Ni concentrations of soil samples collected from the production area exceed the risk screening value for contaminated development land in Soil Environment Quality Standards for soil pollution risk control on construction land (Trial) of China, while the concentrations of other toxic elements such as Cr, Cu, and Zn are lower than the screening value. The PCA and HCA results are consistent with the correlation coefficient analysis and indicate that industrial activities are the main sources of Co and Ni. The chemical composition and source analysis results of soil and groundwater show that toxic metals originating from catalyst and low pH value from acid waste water should be the main point of concern in the synthetic diamond production plant.
Contributions to Mineralogy and Petrology, Vol. 171, 7, 14p.
China
Geothermometry
Abstract: The large equilibrium Mg isotope fractionation between clinopyroxene and garnet observed in eclogites makes it a potential high-precision geothermometer, but calibration of this thermometer by natural samples is still limited. Here, we report Mg isotopic compositions of eclogite whole rocks as well as Mg and O isotopic compositions of clinopyroxene and garnet separates from 16 eclogites that formed at different temperatures from the Dabie orogen, China. The whole-rock ?26Mg values vary from ?1.20 to +0.10 ‰. Among them, 11 samples display limited ?26Mg variations from ?0.36 to ?0.17 ‰, similar to those of their protoliths. The mineral separates exhibit very different ?26Mg values, from ?0.39 to +0.39 ‰ for clinopyroxenes and from ?1.94 to ?0.81 ‰ for garnets. The clinopyroxene -garnet Mg isotope fractionation (?26Mgclinopyroxene -garnet = ?26Mgclinopyroxene -?26Mggarnet) varies from 1.05 to 2.15 ‰. The clinopyroxene -garnet O isotope fractionation (?18Oclinopyroxene -garnet = ?18Oclinopyroxene -?18Ogarnet) varies from ?1.01 to +0.98 ‰. Equilibrium Mg isotope fractionation between clinopyroxene and garnet in the investigated samples is selected based on both the ?26Mgclinopyroxene versus ?26Mggarnet plot and the state of O isotope equilibrium between clinopyroxene and garnet. The equilibrium ?26Mgclinopyroxene -garnet and corresponding temperature data obtained in this study, together with those available so far in literatures for natural eclogites, are used to calibrate the clinopyroxene -garnet Mg isotope thermometer. This yields a function of ?26Mgclinopyroxene -garnet = (0.99 ± 0.06) × 106/T 2, where T is temperature in Kelvin. The refined function not only provides the best empirically calibrated clinopyroxene -garnet Mg isotope thermometer for precise constraints of temperatures of clinopyroxene- and garnet-bearing rocks, but also has potential applications in high-temperature Mg isotope geochemistry.
Barium isotopic composition of the mantle constrained by carbonatites.
Goldschmidt Conference, 1p. Abstract
Africa, Tanzania, east Africa, Canada, Europe, Germany, Greenland
carbonatite
Abstract: Deep mantle origin and ultra-reducing conditions in podiform chromitite: diamonds, moissanite, and other unusual minerals in podiform chromitites from the Pozanti-Karsanti ophiolite, southern Turkey
Geochimica et Cosmochimica Acta, in press available doi.org/10.1016 / j.gca.2019.06.041 36p.
Africa, Tanzania, Canada, East Africa, Europe, Germany, Greenland
deposit - Oldoinyo Lengai
Abstract: To investigate the behaviour of Ba isotopes during carbonatite petrogenesis and to explore the possibility of using carbonatites to constrain the Ba isotopic composition of the mantle, we report high-precision Ba isotopic analyses of: (1) carbonatites and associated silicate rocks from the only active carbonatite volcano, Oldoinyo Lengai, Tanzania, and (2) Archean to Cenozoic carbonatites from Canada, East Africa, Germany and Greenland. Carbonatites and associated phonolites and nephelinites from Oldoinyo Lengai have similar ?137/134Ba values that range from +0.01 to +0.03‰, indicating that Ba isotope fractionation during carbonatite petrogenesis is negligible. The limited variation in ?137/134Ba values from ?0.03 to +0.09‰ for most carbonatite samples suggests that their mantle sources have a relatively homogeneous Ba isotopic composition. Based on the carbonatites investigated in this work, the average ?137/134Ba value of their mantle sources is estimated to be +0.04?±?0.06‰ (2SD, n?=?16), which is similar to the average value of +0.05?±?0.06‰ for mid-ocean ridge basalts. The lower ?137/134Ba value of ?0.08‰ in a Canadian sample and higher ?137/134Ba values of +0.14‰ and?+?0.23‰ in two Greenland samples suggest local mantle isotopic heterogeneity that may reflect the incorporation of recycled crustal materials in their sources.
Geochimica et Cosmochimica Acta, Vol. 278, pp. 235-243.
Mantle
carbonatite
Abstract: To investigate the behaviour of Ba isotopes during carbonatite petrogenesis and to explore the possibility of using carbonatites to constrain the Ba isotopic composition of the mantle, we report high-precision Ba isotopic analyses of: (1) carbonatites and associated silicate rocks from the only active carbonatite volcano, Oldoinyo Lengai, Tanzania, and (2) Archean to Cenozoic carbonatites from Canada, East Africa, Germany and Greenland. Carbonatites and associated phonolites and nephelinites from Oldoinyo Lengai have similar ?137/134Ba values that range from +0.01 to +0.03‰, indicating that Ba isotope fractionation during carbonatite petrogenesis is negligible. The limited variation in ?137/134Ba values from ?0.03 to +0.09‰ for most carbonatite samples suggests that their mantle sources have a relatively homogeneous Ba isotopic composition. Based on the carbonatites investigated in this work, the average ?137/134Ba value of their mantle sources is estimated to be +0.04?±?0.06‰ (2SD, n?=?16), which is similar to the average value of +0.05?±?0.06‰ for mid-ocean ridge basalts. The lower ?137/134Ba value of ?0.08‰ in a Canadian sample and higher ?137/134Ba values of +0.14‰ and?+?0.23‰ in two Greenland samples suggest local mantle isotopic heterogeneity that may reflect the incorporation of recycled crustal materials in their sources.
Abstract: To investigate the behaviour of Ba isotopes during carbonatite petrogenesis and to explore the possibility of using carbonatites to constrain the Ba isotopic composition of the mantle, we report high-precision Ba isotopic analyses of: (1) carbonatites and associated silicate rocks from the only active carbonatite volcano, Oldoinyo Lengai, Tanzania, and (2) Archean to Cenozoic carbonatites from Canada, East Africa, Germany and Greenland. Carbonatites and associated phonolites and nephelinites from Oldoinyo Lengai have similar ?137/134Ba values that range from +0.01 to +0.03‰, indicating that Ba isotope fractionation during carbonatite petrogenesis is negligible. The limited variation in ?137/134Ba values from ?0.03 to +0.09‰ for most carbonatite samples suggests that their mantle sources have a relatively homogeneous Ba isotopic composition. Based on the carbonatites investigated in this work, the average ?137/134Ba value of their mantle sources is estimated to be +0.04?±?0.06‰ (2SD, n?=?16), which is similar to the average value of +0.05?±?0.06‰ for mid-ocean ridge basalts. The lower ?137/134Ba value of ?0.08‰ in a Canadian sample and higher ?137/134Ba values of +0.14‰ and?+?0.23‰ in two Greenland samples suggest local mantle isotopic heterogeneity that may reflect the incorporation of recycled crustal materials in their sources.
Sun, N., Wei, W., Han, S., Song, J., Li, X., Duan, Y., Prakapenka, V.B., Mao, Z.
Phase transition and thermal equations of state of (Fe, Al) -bridgmanite and post perovskite: implication for the chemical heterogeneity at the lowermost mantle.
Earth Planetary Science Letters, Vol. 490, pp. 161-169.
Geostandards and Geoanalytical Research, Vol. 43, 2, pp. 291-300.
Global
carbonatites
Abstract: This study presents a high?precision method to measure barium (Ba) isotope compositions of international carbonate reference materials and natural carbonates. Barium was purified using chromatographic columns filled with cation exchange resin (AG50W?X12, 200-400 mesh). Barium isotopes were measured by MC?ICP?MS, using a 135Ba-136Ba double?spike to correct mass?dependent fractionation during purification and instrumental measurement. The precision and accuracy were monitored by measuring Ba isotope compositions of the reference material JCp?1 (coral) and a synthetic solution obtained by mixing NIST SRM 3104a with other matrix elements. The mean ?137/134Ba values of JCp?1 and the synthetic solution relative to NIST SRM 3104a were 0.21 ± 0.03‰ (2s, n = 16) and 0.02 ± 0.03‰ (2s, n = 6), respectively. Replicate measurements of NIST SRM 915b, COQ?1, natural coral and stalagmite samples gave average ?137/134Ba values of 0.10 ± 0.04‰ (2s, n = 18), 0.08 ± 0.04‰ (2s, n = 20), 0.27 ± 0.04‰ (2s, n = 16) and 0.04 ± 0.03‰ (2s, n = 20), respectively. Barium mass fractions and Ba isotopes of subsamples drilled from one stalagmite profile were also measured. Although Ba mass fractions varied significantly along the profile, Ba isotope signatures were homogeneous, indicating that Ba isotope compositions of stalagmites could be a potential tool (in addition to Ba mass fractions) to constrain the source of Ba in carbonate rocks and minerals.
Abstract: Carbonatites and related alkaline rocks host most REE resources. Phosphate minerals, e.g., apatite and monazite, commonly occur as the main REE-host in carbonatites and have been used for tracing magmatic and mineralization processes. Many carbonatite intrusions undergo metamorphic and/or metasomatic modification after emplacement; however, the effects of such secondary events are controversial. In this study, the Miaoya and Shaxiongdong carbonatite-alkaline complexes, in the South Qinling Belt of Central China, are selected to unravel their magmatic and hydrothermal remobilization histories. Both the complexes are accompanied by Nb-REE mineralization and contain apatite and monazite-(Ce) as the major REE carriers. Apatite grains from the two complexes commonly show typical replacement textures related to fluid metasomatism, due to coupled dissolution-reprecipitation. The altered apatite domains, which contain abundant monazite-(Ce) inclusions or are locally surrounded by fine-grained monazite-(Ce), have average REE concentrations lower than primary apatite. These monazite-(Ce) inclusions and fine-grained monazite-(Ce) grains are proposed to have formed by the leaching REE from primary apatite grains during fluid metasomatism. A second type of monazite-(Ce), not spatially associated with apatite, shows porous textures and zoning under BSE imaging. Spot analyses of these monazite-(Ce) grains have variable U-Th-Pb ages of 210-410 Ma and show a peak age of 230 Ma, which is significantly younger than the emplacement age (440-430 Ma) but is roughly synchronous with a regionally metamorphic event related to the collision between the North China Craton and Yangtze Block along the Mianlue suture. However, in situ LA-MC-ICP-MS analyses of those grains show that they have initial Nd values same as those of magmatic apatite and whole rock. We suggest these monazite-(Ce) grains crystallized from the early Silurian carbonatites and have been partially or fully modified during a Triassic metamorphic event, partially resetting U-Pb ages over a wide range. Mass-balance calculations, based on mass proportions and the REE contents of monazite-(Ce) and apatite, demonstrate that the quantity of metasomatized early Silurian monazite-(Ce) is far higher than the proportion of monazite-(Ce) resulting from the metasomatic alteration of the apatite. Therefore, Triassic metamorphic events largely reset the U-Th-Pb isotopic system of the primary monazite-(Ce) and apatite but only had limited or local effects on REE remobilization in the carbonatite-alkaline complexes in the South Qinling Belt. Such scenarios may be widely applicable for other carbonatite and hydrothermal systems.
Precisely dating Paleozoic kimberlites in the North Chin a craton and Hf isotopic constraints on the evolution of the subcontinental lithospheric mantle.
Precise age determin ation of the Paleozoic kimberlites in North Chin a craton and Hf isotopic constraint on the evolution of its subcontinental lithospheric mantle.
In situ U Pb dating and Sr Nd isotopic analysis of perovskite: constraints on the age and petrogenesis of the Kuruman kimberlite province, Kaapvaal Craton, South Africa.
Geochemical and Sr-Nd-Hf-O-C isotopic constraints on the origin of the Neoproterozoic Qieganbulake ultramafic carbonatite complex from the Tarim block, northwest China.
Nature Geoscience, doi.org/10.1038/s41561-019-0410-y 10p pdf
Mantle
Plumes, hotspots
Abstract: The thermal and chemical state of the early Archaean deep mantle is poorly resolved due to the rare occurrences of early Archaean magnesium-rich volcanic rocks. In particular, it is not clear whether compositional heterogeneity existed in the early Archaean deep mantle and, if it did, how deep mantle heterogeneity formed. Here we present a geochronological and geochemical study on a Palaeoarchaean ultramafic-mafic suite (3.45-Gyr-old) with mantle plume signatures in Longwan, Eastern Hebei, the North China Craton. This suite consists of metamorphosed cumulates and basalts. The meta-basalts are iron rich and show the geochemical characteristics of present-day oceanic island basalt and unusually high mantle potential temperatures (1,675?°C), which suggests a deep mantle source enriched in iron and incompatible elements. The Longwan ultramafic-mafic suite is best interpreted as the remnants of a 3.45-Gyr-old enriched mantle plume. The first emergence of mantle-plume-related rocks on the Earth 3.5-3.45?billion years ago indicates that a global mantle plume event occurred with the onset of large-scale deep mantle convection in the Palaeoarchaean. Various deep mantle sources of these Palaeoarchaean mantle-plume-related rocks imply that significant compositional heterogeneity was present in the Palaeoarchaean deep mantle, most probably introduced by recycled crustal material.
Abstract: Very few zircon-bearing, kimberlite-hosted mantle eclogite xenoliths have been identified to date; however, the zircon they contain is crucial for our understanding of subcratonic lithospheric mantle evolution and eclogite genesis. In this study, we constrain the characteristics of zircon from mantle eclogite xenoliths based on existing mineralogical and geochemical data from zircons from different geological settings, and on the inferred origin of mantle eclogites. Given the likely origin and subsequent evolution of mantle eclogites, we infer that the xenoliths can contain zircons with magmatic, metamorphic and xenogenic (i.e. kimberlitic zircon) origins. Magmatic zircon can be inherited from low-pressure mafic oceanic crust precursors, or might form during direct crystallization of eclogites from primary mantle-derived melts at mantle pressures. Metamorphic zircon within mantle eclogites has a number of possible origins, ranging from low-pressure hydrothermal alteration of oceanic crustal protoliths to metasomatism related to kimberlite magmatism. This study outlines a possible approach for the identification of inherited magmatic zircon within subduction-related mantle eclogites as well as xenogenic kimberlitic zircon within all types of mantle eclogites. We demonstrate this approach using zircon grains from kimberlite-hosted eclogite xenoliths from the Kasai Craton, which reveals that most, if not all, of these zircons were most likely incorporated as a result of laboratory-based contamination.
Mo, X., Zhao, Z., Deng, J., Flower, M., Yu, X., Luo, Z., Li, Y., Zhou, S., Deng, G., Zhu, D.
Petrology and geochemistry of post collisional volcanic rocks from the Tibetan plateau: implications for lithosphere heterogeneity and collision induced mantle
Geological Society of America, Special Paper, No. 409, pp. 507-530.
Abstract: Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100-200 µm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm?1 and 1333 cm?1, mostly at 1331.51 cm?1 or 1326.96 cm?1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones (SSZ), where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.
Abstract: The abundances of volatile elements in the Earth’s mantle have been attributed to the delivery of volatile-rich material after the main phase of accretion1, 2, 3. However, no known meteorites could deliver the volatile elements, such as carbon, nitrogen, hydrogen and sulfur, at the relative abundances observed for the silicate Earth4. Alternatively, Earth could have acquired its volatile inventory during accretion and differentiation, but the fate of volatile elements during core formation is known only for a limited set of conditions4, 5, 6, 7, 8. Here we present constraints from laboratory experiments on the partitioning of carbon and sulfur between metallic cores and silicate mantles under conditions relevant for rocky planetary bodies. We find that carbon remains more siderophile than sulfur over a range of oxygen fugacities; however, our experiments suggest that in reduced or sulfur-rich bodies, carbon is expelled from the segregating core. Combined with previous constraints9, we propose that the ratio of carbon to sulfur in the silicate Earth could have been established by differentiation of a planetary embryo that was then accreted to the proto-Earth. We suggest that the accretion of a Mercury-like (reduced) or a sulfur-rich (oxidized) differentiated body—in which carbon has been preferentially partitioned into the mantle—may explain the Earth’s carbon and sulfur budgets.
Abstract: Carbonatitic magmatism in subduction zones provides extremely valuable information on the cycling, behavior and storage of deep carbon within the Earth. It may also shed light on insights into crust-mantle interaction and mantle metasomatism within subduction zones. Origin of carbonatite has long been debated: all hypotheses need to reflect the different mineral assemblages and geochemical compositions of carbonatites and their diverse tectonic settings. Here we present a petrological, geochronological, geochemical and isotopic study of carbonatite bodies associated with orogenic peridotites, which occur as stocks or dykes with widths of tens to hundreds of meters in the Luliangshan region, North Qaidam, northern Tibet, China. On the basis of modal olivine (Ol) content, the studied samples were subdivided into two groups: Ol-poor carbonatite and Ol-rich carbonatite. Zircon grains from the Ol-poor carbonatite show detrital features, and yield a wide age spectrum between 400?Ma and 1000?Ma with a pronounced peak at ca. 410-430?Ma. By contrast, oscillatory zoned zircons and inherited cores show two relatively small Neoproterozoic age peaks at ca. 920 and 830?Ma. Zircon grains from the Ol-rich carbonatite sample are also distributed in a wide spectrum between 400 and 1000?Ma, with a pronounced peak at ca. 440?Ma and a slightly inferior peak at ca. 410?Ma. The oscillatory zoned zircons and inherited cores exhibit a smaller Neoproterozoic age peak at ca. 740?Ma. The pronounced peaks ranging from 430 to 410?Ma are consistent with the deep subduction and mantle metasomatic events recorded in associated ultramafic rocks. Both groups of carbonatites are characterized by enrichment of light rare earth elements (LREEs) with high (La/Yb)N values and pronounced negative Eu anomalies. They show high 87Sr/86Sr values (0.708156-0.709004), low 143Nd/144Nd values (0.511932-0.512013) and high ?18OV-SMOW values (+17.9 to +21.3‰). This geochemical and isotopic evidence suggests that these carbonatites were derived from remobilized sedimentary carbonate rocks. We propose that the primary carbonatite magma was formed by partial melting of sedimentary carbonates with mantle contributions. Sedimentary carbonates were subducted into the shallow upper mantle where they melted and formed diapirs that moved upwards through the hot mantle wedge. The case presented provides a rare example of carbonatite originating from sedimentary carbonates with mantle contributions and relevant information on the mantle metasomatism within a subduction zone.
Geochimica et Cosmochimica Acta, Vol. 251. pp. 87-115.
Mantle
nitrogen
Abstract: Nitrogen, the most dominant constituent of Earth’s atmosphere, is critical for the habitability and existence of life on our planet. However, its distribution between Earth’s major reservoirs, which must be largely influenced by the accretion and differentiation processes during its formative years, is poorly known. Sequestration into the metallic core, along with volatility related loss pre- and post-accretion, could be a critical process that can explain the depletion of nitrogen in the Bulk Silicate Earth (BSE) relative to the primitive chondrites. However, the relative effect of different thermodynamic parameters on the alloy-silicate partitioning behavior of nitrogen is not well understood. Here we present equilibrium partitioning data of N between alloy and silicate melt () from 67 new high pressure (P?=?1-6?GPa)-temperature (T?=?1500-2200?°C) experiments under graphite saturated conditions at a wide range of oxygen fugacity (logfO2????IW ?4.2 to ?0.8), mafic to ultramafic silicate melt compositions (NBO/T?=?0.4 to 2.2), and varying chemical composition of the alloy melts (S and Si contents of 0-32.1?wt.% and 0-3.1?wt.%, respectively). Under relatively oxidizing conditions (??IW ?2.2 to ?0.8) nitrogen acts as a siderophile element ( between 1.1 and 52), where decreases with decrease in fO2 and increase in T, and increases with increase in P and NBO/T. Under these conditions remains largely unaffected between S-free conditions and up to ?17?wt.% S content in the alloy melt, and then drops off at >?20?wt.% S content in the alloy melt. Under increasingly reduced conditions (
Geochimica et Cosmochimica Acta, Vol. 251, pp. 87-115.
Mantle
nitrogen
Abstract: Nitrogen, the most dominant constituent of Earth’s atmosphere, is critical for the habitability and existence of life on our planet. However, its distribution between Earth’s major reservoirs, which must be largely influenced by the accretion and differentiation processes during its formative years, is poorly known. Sequestration into the metallic core, along with volatility related loss pre- and post-accretion, could be a critical process that can explain the depletion of nitrogen in the Bulk Silicate Earth (BSE) relative to the primitive chondrites. However, the relative effect of different thermodynamic parameters on the alloy-silicate partitioning behavior of nitrogen is not well understood. Here we present equilibrium partitioning data of N between alloy and silicate melt () from 67 new high pressure (P?=?1-6?GPa)-temperature (T?=?1500-2200?°C) experiments under graphite saturated conditions at a wide range of oxygen fugacity (logfO2????IW ?4.2 to ?0.8), mafic to ultramafic silicate melt compositions (NBO/T?=?0.4 to 2.2), and varying chemical composition of the alloy melts (S and Si contents of 0-32.1?wt.% and 0-3.1?wt.%, respectively). Under relatively oxidizing conditions (??IW ?2.2 to ?0.8) nitrogen acts as a siderophile element ( between 1.1 and 52), where decreases with decrease in fO2 and increase in T, and increases with increase in P and NBO/T. Under these conditions remains largely unaffected between S-free conditions and up to ?17?wt.% S content in the alloy melt, and then drops off at >?20?wt.% S content in the alloy melt. Under increasingly reduced conditions (
Earth and Planetary Science Letters, Vol. 482, pp. 556-566.
Mantle
nitrogen
Abstract: Understanding the evolution of nitrogen (N) across Earth's history requires a comprehensive understanding of N's behaviour in the Earth's mantle - a massive reservoir of this volatile element. Investigation of terrestrial N systematics also requires assessment of its evolution in the Earth's atmosphere, especially to constrain the N content of the Archaean atmosphere, which potentially impacted water retention on the post-accretion Earth, potentially causing enough warming of surface temperatures for liquid water to exist. We estimated the proportion of recycled N in the Earth's mantle today, the isotopic composition of the primitive mantle, and the N content of the Archaean atmosphere based on the recycling rates of N in modern-day subduction zones. We have constrained recycling rates in modern-day subduction zones by focusing on the mechanism and efficiency of N transfer from the subducting slab to the sub-arc mantle by both aqueous fluids and slab partial melts. We also address the transfer of N by aqueous fluids as per the model of Li and Keppler (2014). For slab partial melts, we constrained the transfer of N in two ways - firstly, by an experimental study of the solubility limit of N in melt (which provides an upper estimate of N uptake by slab partial melts) and, secondly, by the partitioning of N between the slab and its partial melt. Globally, 45-74% of N introduced into the mantle by subduction enters the deep mantle past the arc magmatism filter, after taking into account the loss of N from the mantle by degassing at mid-ocean ridges, ocean islands and back-arcs. Although the majority of the N in the present-day mantle remains of primordial origin, our results point to a significant, albeit minor proportion of mantle N that is of recycled origin (% or % of N in the present-day mantle has undergone recycling assuming that modern-style subduction was initiated 4 or 3 billion years ago, respectively). This proportion of recycled N is enough to cause a departure of N isotopic composition of the primitive mantle from today's N of ?5‰ to ‰ or ‰. Future studies of Earth's parent bodies based on the bulk Earth N isotopic signature should take into account these revised values for the N composition of the primitive mantle. Also, the Archaean atmosphere had a N partial pressure of 1.4-1.6 times higher than today, which may have warmed the Earth's surface above freezing despite a faint young Sun.
Abstract: Natural emeralds from 11 mining areas were studied using an infrared spectrometer. The results showed different spectroscopic characteristics for emerald from different mine regions. Infrared absorption is mainly attributed to the vibration of Si-O lattice, channel water, alkaline cations, and molecules such as CO2, [Fe2(OH)4]2+, etc. Both near-infrared and mid-infrared spectra showed that the differences in band positions, intensities, and shapes are related to the mixed ratio of the two types of channel water. Accordingly, emerald and its mining regions can be divided into 3 types: H2O I, H2O II, and transition I-II. Furthermore, the study indicates that the relative amounts of the two different orientations of channel water molecules are mainly affected by the presence of (Mg + Fe)2+ in the host rock or in the mineralizing fluid. Therefore, the mineralization environment type (alkali-poor, alkali-rich, and transitional types) of emerald can be preliminarily identified from IR spectroscopy. This can be useful for determining the origin of emeralds.
Journal of Geophysical Research: Solid Earth, Vol. 124, 7, pp. 6504-6522.
Mantle
subduction
Abstract: Mantle wedge hybridization by crust?derived melt is a crucial mechanism responsible for arc lavas. However, how the melt?rock reactions proceed in the mantle wedge and affect melt compositions is poorly understood. Garnet peridotites from Jiangzhuang in the Sulu orogen (eastern China) host garnetite and pyroxenite veins formed by slab?mantle interactions at different melt/rock ratios. The Jiangzhuang peridotites consist mainly of garnet lherzolites and minor harzburgites and represent a fragment of the mantle wedge influenced by ultrahigh?pressure metamorphism (5.2-6.1 GPa) in the subduction channel. Petrography, major and trace element geochemistry, and in situ clinopyroxene Sr isotope values of the garnetite and pyroxenite veins reveal their derivation from interactions between mantle wedge peridotites and deeply subducted crust?derived melts. The two veins share a common metamorphic and metasomatic history and have similar mineral assemblages and compositions, enriched isotope signatures, and formation P?T conditions, indicating the same source for their reacting melts. The different mineral proportions and microtextures between the garnetite and pyroxenite veins are ascribed to different melt/rock ratios. The garnetite vein formed at relatively high melt/rock ratios (>1:1), which would likely produce hybrid slab melts with Mg?rich, high?silica adakitic signatures. In contrast, the pyroxenite vein formed at low melt/rock ratios (<1:1), and the expected hybrid slab melts would evolve into high?Mg andesites. Moreover, recycled heterogeneous garnetite and pyroxenite could contribute to the mantle sources of intraplate magmas. Therefore, slab?mantle interactions at different melt/rock ratios could be an important crustal input to lithological and geochemical heterogeneities in the mantle.
Minerals MDPI, Vol. 10, 1117, doi:10.3390/ min10121117 20p. Pdf
China
deposit - Bayan Obo
Abstract: The Bayan Obo rare earth element (REE) deposit in Inner Mongolia, northern China, is the largest REE deposit in the world, whose mineralization process remains controversial. There are dozens of carbonatite dykes that are tightly related to the deposit. Here we report the petrological and mineralogical characteristics of a typical dolomite carbonatite dyke near the deposit. The dolomite within the dyke experienced intense post-emplacement fluids metasomatism as evidenced by the widespread hydrothermal REE-bearing minerals occurring along the carbonate mineral grains. REE contents of bulk rocks and constituent dolomite minerals (>90 vol.%) are 1407-4184 ppm and 63-152 ppm, respectively, indicating that dolomite is not the dominant mineral controlling the REE budgets of the dyke. There are three types of apatite in the dyke: Type 1 apatite is the primary apatite and contains REE2O3 at 2.35-4.20 wt.% and SrO at 1.75-2.19 wt.%; Type 2 and Type 3 apatites are the products of replacement of primary apatite. The REE2O3 (6.10-8.21 wt.%) and SrO (2.83-3.63 wt.%) contents of Type 2 apatite are significantly elevated for overprinting of REE and Sr-rich fluids derived from the carbonatite. Conversely, Type 3 apatite has decreased REE2O3 (1.17-2.35 wt.%) and SrO (1.51-1.99 wt.%) contents, resulting from infiltration of fluids with low REE and Na concentrations. Our results on the dyke suggest that post-magmatic fluids expelled from the carbonatitic melts dominated the REE mineralization of the Bayan Obo deposit, and a significant fluid disturbance occurred but probably provided no extra REEs to the deposit.
Abstract: Magnetic data are sensitive to both the induced magnetization in rock units caused by the present earth's magnetic field and the remanent magnetization acquired by rock units in past geologic time. Susceptibility is a direct indicator of the magnetic mineral content, whereas remanent magnetization carries information about the formation process and subsequent structural movement of geologic units. The ability to recover and use total magnetization, defined as the vectorial sum of the induced and remanent magnetization, therefore enables us to take full advantage of magnetic data. The exploration geophysics community has achieved significant advances in inverting magnetic data affected by remanent magnetization. It is now feasible to invert any magnetic data set for total magnetization. We provide an overview of the state of the art in magnetization inversion and demonstrate the informational value of inverted magnetization through a set of case studies from mineral exploration problems. We focus on the methods that recover either the magnitude of the total magnetization or the total magnetization vector itself.
Geochemistry, Geophysics, Geosystems, 10.1029/2021GC009710 20p. Pdf
United States, Canada
geophysics- seismic
Abstract: We probe the properties of upper mantle rocks beneath northeastern North America using the observations of seismic waves from distant earthquakes. We examine signals of converted P-S waves that originate from locations of rapid vertical or directional changes in seismic velocities. These abrupt velocity boundaries are thought to originate from rock deformation, variations in composition, temperature, or melt content. The sharp transitions detectable by this method are compositionally more plausible within the cold tectonic plate than within the hot convecting asthenosphere. Previous studies in this region that analyzed the same type of seismic data report boundaries with sharp downward reduction in seismic velocities between the depths of 60 and 100 km. Their widespread distribution and local consistency with seismic velocity models was used as evidence of them marking the transition between the cold tectonic plate and the hot convecting asthenosphere. Here we expand our search to other types of boundaries and find numerous examples at much greater depths (down to ?185 km). These deeper boundaries primarily reflect changes in directional variation of seismic velocities (anisotropy). The distribution of our deep boundaries broadly agrees with lithospheric thickness estimates in global upper mantle models that consider seismic, gravity, and heat flow data.
Physics and Chemistry of Minerals, Vol. 48, 10, 6p. Pdf s00269-021-Q1163-5
Mantle
bridgmanite
Abstract: Phase D is proposed to be the most important hydrous phase at the upper part of the lower mantle, and it has been shown to coexist with bridgmanite (Brg), the most abundant mineral and main host for Al2O3 in the lower mantle. The concentration of Al in Phase D could significantly increase the thermal stability field of Phase D, therefore, partitioning of Al between Brg and Phase D is of particular importance to constrain water distribution in the deep mantle. Here, we performed high P-T experiments in MgO-Al2O3-SiO2-H2O system to investigate the partitioning of Al between Brg and Phase D up to 32 GPa and 1350 °C. Our results indicated that Al distributes strongly into Phase D relative to Brg and the partition coefficient slightly decreases with increasing temperature. Al-bearing Phase D exhibits a very high thermal stability region, but it completely decomposed around 28 GPa and 1350 °C, at which point Brg coexisted with a large amount of melt. The depth?~?850 km (28 GPa) is thus proposed to be the second choke point for hydrous minerals. This may shed new lights on several important geophysical observations in subduction zones.
Journal of Asian Earth Sciences, Vol. 172, pp. 56-65.
China, Mongolia
deposit - Bayan Obo
Abstract: In the Bayan Obo REE deposit in Inner Mongolia, Northern China, three major orebodies are hosted in dolomite marble of the Bayan Obo Group. There are carbonatite dikes in the ore district. Apatite is a common accessary mineral in the ore-hosting dolomite marble (DM apatite) and in carbonatite dikes (IC apatite). These two types of apatite are both fluorapatite, and have low SiO2, uniform P2O5, and variable CaO contents. Total REY (REEs?+?Y) contents are correlated with Na2O contents, indicating that REY of both types of apatite enter lattice via the substitution reaction: Na+ + (REY)3+ = 2Ca2+. These features, combined with high REY (6230-18,906?ppm) and Sr (9653-17,200?ppm) contents of DM apatite, indicate that DM apatite likely had a carbonatite origin. Some DM apatite grains are partially replaced by albite and quartz. Fluid inclusions crosscutting both apatite and albite or quartz indicate that they formed later than quartz and albite replacement. The back-scattered electron images show that DM apatite grains contain many micro-pores (fluid inclusions), and monazite inclusions formed from the fluid inclusions. However, no monazite inclusions are observed within quartz and albite, excluding the possibility that the monazite inclusions were precipitated directly from the fluids. The monazite inclusions were therefore formed during fluid-induced dissolution-reprecipitation processes, where DM apatite served as the source of LREEs. This also explains the depletion of some LREEs in DM apatite. The formation of monazite inclusions in apatite requires fluids with relatively low Na and Si concentrations, different from the fluids responsible for quartz and albite replacement. DM apatite was affected by two stages of fluid activities: the first stage of metasomatism by alkaline fluids that were likely derived from carbonatite magmas when the deposit first formed (represented by quartz and albite replacement), followed by a second stage of modification that caused LREEs depletion and the formation of new REE minerals. Thus, the Bayan Obo REE ore deposit was modified by a significant thermal event after the formation, which provided negligible or only small amounts of REEs.
Polyphase formation and exhumation of high to ultrahigh pressure rocks in continental subduction zones: numerical modeling and application to the Sulu ultrahigh pressure terrane in eastern China.
Journal of Geophysical Research, Vol. 114. B9, B09406
Abstract: The role of fluid(s) in the formation of different lithological facies of kimberlites is still poorly understood. The uncertainty in the composition of kimberlite melts hampers understanding the composition of volatiles, the depth of exsolution, and the effect on magma ascent and fragmentation. Recent estimates of H2O and CO2 solubility in kimberlite-like magmas suggest very shallow exsolution of fluid, while many features of kimberlites indicate the presence of significant fluid fraction at depth. Deep magmatic fluid produces negative trigonal etch pits on natural diamonds, the characteristics of which depend on the temperature and composition of the fluid. Positively oriented trigonal etch pits are very rare on natural diamonds and are likely a feature of resorption events unique to only some kimberlite magmas. Here we present the first systematic study of positively oriented trigonal etch pits on natural diamonds from Snap Lake kimberlite dike, Northwest Territories, Canada. The study used 91 micro-diamonds selected from a population of 251 diamonds representative of all six kimberlite litho-facies identified in the Snap Lake dike. We established that unlike the majority of diamonds from kimberlite pipes in the Northwest Territories, every studied Snap Lake diamond shows positively oriented trigons. These trigons cover the whole diamond surface starting from the {111} faces and continuing over the resorbed edges. They overprint negatively oriented trigons and modify them into hexagons. Atomic force microscopy obtained detailed geometry of 154 positive trigons on 14 diamonds. Three distinct trigon morphologies dependent on the type of the crystal lattice defect were recognized. The point-bottomed shape and positive correlation between the depth and diameter of the individual pits suggest a high CO2 content in the fluid. Comparison with the existing experimental data on positive trigons implies resorption at low-pressure conditions in the 800-1000 °C temperature range by trapped magmatic fluid after the dike emplacement. The intensity of this late resorption event (and the size of the positive trigons) increases from the dike contact with the country rock into the interior of the dike. Such a late resorption event is absent in the majority of kimberlites, which form pipes, and might be a specific feature of hypabyssal kimberlite bodies (dikes). The absence of positive trigons on diamonds from the majority of kimberlites suggests very quick magma cooling below ?800 °C after the pipe emplacement, precluding the development of any late resorption features. Our study shows that for kimberlitic magmas, for which mineral chemistry is unable to provide a robust record of magmatic fluid, morphological details of dissolution features on the surface of diamond and other mantle-derived minerals can serve as a fluid proxy. Better constraints of the pressure, temperature, and oxygen fugacity of the reversal in the trigon orientation on diamond may help to reconstruct the emplacement path of geologically diverse kimberlite bodies.
Abstract: Synthetic diamonds have inspired much interest for their unique photophysical properties and versatile potential applications, but their phosphorescent phenomenon and mechanism have been paid much less attention. Here, phosphorescent diamonds with a lifetime of 5.4?s were synthesized by high-pressure and high-temperature method, and the diamonds exhibit an emission band at around 468?nm under the excitation wavelength of 230?nm. The quantum yield of the phosphorescent diamonds is about 4.7% at ambient temperature and atmosphere, which is the first report on the quantum yield of diamonds. The unique phosphorescence emission can be attributed to the radiative recombination from iron related donors and boron related acceptors.
Abstract: Metasomatic processes modify the composition of the subcratonic lithospheric mantle and can either form or destroy diamonds. The composition of these metasomatic agents is uncertain and has been mostly deduced from chemical zonation and overprints recorded by associated mantle silicates. Diamonds experience partial dissolution (resorption) during their residence in the mantle due to mantle metasomatism and later during their ascent in kimberlite magma. Diamonds, enclosed inside mantle xenoliths during the whole duration of ascent in kimberlite magma, can preserve their pre-kimberlite surface features, which record the last diamond-destructive metasomatic event to have occurred in the mantle. The geometry of diamond dissolution features acquired during mantle storage can provide information about diamond-destructive metasomatic events in the mantle. Diamond samples recovered from inside mantle xenoliths are extremely rare and mostly limited to eclogitic lithology, which suggests that variable resistance of different mantle lithologies to disintegration in kimberlite magma may affect representativity of these sample. Here we use whole diamond populations from exploration parcels and apply our earlier developed set of criteria to distinguish between kimberlitic and pre-kimberlitic surface features on diamonds. The study used diamonds (<1 to 4.5 mm size) from eight kimberlites in three localities: Orapa cluster, Botswana (BK1, AK15, and AK1 kimberlites), Ekati Mine, Northwest Territories, Canada (Grizzly, Leslie, Koala, and Misery kimberlites), and Snap Lake kimberlite dyke, Northwest Territories, Canada. The host kimberlites cover seven different volcaniclastic and coherent kimberlite lithologies, and our previous studies demonstrated a correlation between the style of kimberlitic resorption on diamonds and the host kimberlite lithology for these samples. From the total of 3256 studied diamonds, we identified 534 diamonds with pre-kimberlite surface textures. These pre-kimberlite surface textures display six distinct types, which are present in all the studied diamond parcels regardless of their geographic locality and host kimberlite lithology. The relative proportions of these types depend on the geographic locality showing linkage to a specific mantle source. We examined the relationship between the surface features on diamonds, their growth patterns revealed in cathodoluminescence (CL) images, the content and aggregation of nitrogen defects using Fourier transform infrared spectroscopy (FTIR), and nitrogen content in specific growth zones of diamonds obtained using secondary ion mass spectrometry (SIMS) for 82 Ekati diamonds. Our data show that growth step-faces develop on diamonds with complex multi-crystal cores, whereas flat-faced octahedra with simple oscillatory-zoned growth patterns derive from single growth events. Initial stages of dissolution affecting only outer growth zones develop simple serrate laminae on diamonds, while more extensive dissolution exposes more complex growth zones developing various shapes of laminae and etch features (trigons and irregular asperities). The effect of internal growth patterns on dissolution features is more profound during pre-kimberlitic than kimberlite-related resorption likely due to the greater role of defects in diamond dissolution at mantle conditions. Comparison with the results of diamond dissolution experiments shows that metasomatism by C-O-H fluid is not destructive to diamond, while carbonate-silicate melt-driven metasomatism causes diamond dissolution. Continuous change in the silicate content of silicate?carbonate melts and temperature variations within 200 °C can explain all pre-kimberlite dissolution features observed in this study. Similar pre-kimberlite dissolution features on diamonds from both the Zimbabwe and Slave cratons suggests that these metasomatic processes are widespread and affected the mantle below the eight studied kimberlites.
Geochemical Perspectives Letters, Vol. 17, pp. 11-15. pdf
Mantle
carbonatites
Abstract: Carbonatite, an unusual carbonate-rich igneous rock, is known to be sourced from the mantle which provides insights into mantle-to-crust carbon transfer. To constrain further the Ca isotopic composition of carbonatites, investigate the behaviour of Ca isotopes during their evolution, and constrain whether recycled carbonates are involved in their source regions, we report ?44/42Ca for 47 worldwide carbonatite and associated silicate rocks using a refined analytical protocol. Our results show that primary carbonatite and associated silicate rocks are rather homogeneous in Ca isotope compositions that are comparable to ?44/42Ca values of basalts, while non-primary carbonatites show detectable ?44/42Ca variations that are correlated to ?13C values. Our finding suggests that Ca isotopes fractionate during late stages of carbonatite evolution, making it a useful tool in the study of carbonatite evolution. The finding also implies that carbonatite is sourced from a mantle source without requiring the involvement of recycled carbonates.
Geological Society of London Special Publication Supercontinent Cycles through Earth History., Vol. 424, pp. 1-14.
Mantle
Supercontinents
Abstract: The supercontinent-cycle hypothesis attributes planetary-scale episodic tectonic events to an intrinsic self-organizing mode of mantle convection, governed by the buoyancy of continental lithosphere that resists subduction during closure of old ocean basins, and consequent reorganization of mantle convection cells leading to opening of new ocean basins. Characteristic timescales of the cycle are typically 500-700 myr. Proposed spatial patterns of cyclicity range from hemispheric (introversion) to antipodal (extroversion), to precisely between those end-members (orthoversion). Advances in our understanding can arise from theoretical or numerical modelling, primary data acquisition relevant to continental reconstructions, and spatiotemporal correlations between plate kinematics, geodynamic events and palaeoenvironmental history. The palaeogeographic record of supercontinental tectonics on Earth is still under development. The contributions in this special publication provide snap-shots in time of these investigations and indicate that Earth's palaeogeographic record incorporates elements of all three endmember spatial patterns.
Abstract: The supercontinent cycle of episodic assembly and breakup of almost all continents on Earth is commonly considered the longest period variation to affect mantle convection. However, global zircon Hf isotopic signatures and seawater Sr isotope ratios suggest the existence of a longer-term variation trend that is twice the duration of the supercontinent cycle. Here we propose that since ?2 billion years ago the superocean surrounding a supercontinent, as well as the circum-supercontinent subduction girdle, survive every second supercontinent cycle. This interpretation is in agreement with global palaeogeography and is supported by variations in passive margin, orogen, and mineral deposit records that each exhibits both ?500-700 million years periodic signal and a 1000-1500 million years variation trend. We suggest that the supercontinent cycle is modulated by an assembly that alternates between dominantly extroversion after a more complete breakup, and dominantly introversion after an incomplete breakup of the previous supercontinent.
Geochemistry, Geophysics, Geosystems, in press available pdf 20p.
Global
geodynamics
Abstract: The relative significance of various geodynamic mechanisms that drive supercontinent breakup is unclear. A previous analysis of extensional stress during supercontinent breakup demonstrated the importance of the plume?push force relative to the dragging force of subduction retreat. Here, we extend the analysis to basal traction (shear stress) and cross?lithosphere integrations of both extensional and shear stresses, aiming to understand more clearly the relevant importance of these mechanisms in supercontinent breakup. More importantly, we evaluate the effect of preexisting orogens (mobile belts) in the lithosphere on supercontinent breakup process. Our analysis suggests that a homogeneous supercontinent has extensional stress of 20-50 MPa in its interior (<40° from the central point). When orogens are introduced, the extensional stress in the continents focuses on the top 80?km of the lithosphere with an average magnitude of ~160 MPa, whereas at the margin of the supercontinent the extensional stress is 5-50 MPa. In both homogeneous and orogeny?embedded cases, the subsupercontinent mantle upwellings act as the controlling factor on the normal stress field in the supercontinent interior. Compared with the extensional stress, shear stress at the bottom of the supercontinent is 1-2 order of magnitude smaller (0-5 MPa). In our two end?member models, the breakup of a supercontinent with orogens can be achieved after the first extensional stress surge, whereas for a hypothetical supercontinent without orogens it starts with more diffused local thinning of the continental lithospheric before the breakup, suggesting that weak orogens play a critical role in the dispersal of supercontinents.
Abstract: Numerical experiments are used to investigate the thermo-mechanical controls for inducing flat subduction and why flat subduction is rare relative to normal/steep subduction. Our modeling results demonstrate that flat subduction is an end-member of a steady state subduction geometry and is characterized by a curved slab with a nearly-horizontal slab section. Intermediate cases between normal/steep and flat subduction appear to be transient in origin and evolve toward one of the stable end-members. Physical parameters inducing flat subduction can be classified into four categories: buoyancy of the subducting oceanic lithosphere (e.g., slab age, oceanic crustal thickness), viscous coupling between the overriding and downgoing plates (e.g., initial subduction angle), external kinematic conditions, and rheological properties of the subduction zone. On the basis of parameter sensitivity tests and the main characteristics of present-day flat subduction zones, positive buoyancy from either the young slab or the thickened oceanic crust is considered as the primary controlling parameter. Our results show that the possibility of flat subduction is directly proportional to oceanic crustal thickness and inversely proportional to the slab age. Furthermore, oceanic crust must be thicker than 8 km to induce flat subduction, when the slab is older than 30 Ma with an initial subduction angle of ? 20° and without absolute trenchward motion of the overriding plate. The lower the initial subduction angle or the thicker the overriding continental lithosphere, the more likelihood for flat subduction. The initial subduction angle is more influential for the development of flat subduction than the overriding lithospheric thickness, and a thick overriding lithosphere induces flat subduction only under the condition of an initial subduction angle of ? 25°, with a slab age of ? 30 Ma and without absolute trenchward motion of the overriding plate. However, when the initial subduction angle is increased to > 25°, no flat subduction is predicted. All the parameters are evaluated within the constraints of a mechanical framework in which the slab geometry is regarded as a result of a balance between the gravitational and hydrodynamic torques. Any factor that can sufficiently reduce gravitational torque or increase hydrodynamic torque will exert a strong effect on flat subduction development. Our results are consistent with the observations of modern flat subduction zones on Earth.
Abstract: The dynamics of formation and exhumation of high-pressure (HP) and ultra-high pressure (UHP) metamorphic orogens in double subduction-collision zones remain enigmatic. Here we employ two-dimensional thermo-mechanical numerical models to gain insights on the exhumation of HP-UHP metamorphic rocks, as well as their deformation during the collision of a micro-continent with pro- and retro-continental margins along two subduction zones. A three-stage collisional process with different convergence velocities is tested. In the initial collisional stage, a fold-and-thrust belt and locally rootless superimposed folds are developed in the micro-continent and subduction channel, respectively. In the second (exhumation) stage of HP-UHP rocks, a faster convergence model results in upwelling of the asthenosphere, which further leads to a detachment between the crust and lithospheric mantle of the micro-continent. A slower convergence model results in rapid exhumation of HP-UHP rocks along the north subduction channel and a typical piggy-back thrusting structure in the micro-continent. A non-convergence model produces a slab tear-off, leading to the rebound of residual lithosphere of the micro-continent. In the third and final stage, a series of back and ramp thrusts are formed in the micro-continent with the pro-continent re-subducted. Based on an analogy of our numerical results with the Western Dabie Orogen (WDO), we suggest that: (1) slab tear-off results in a rebound of residual lithosphere, which controls the two-stage syn-collisional exhumation process of HP-UHP rocks in the WDO; and (2) in contrast to the single subduction-collision system, the exhumation range of the partially molten rocks with lower viscosity and density is restricted to a specific region of the micro-continent by the Mianlue and Shangdan subduction zones, which generated the complex deformation features in the WDO.
Abstract: The dynamical evolution and exhumation mechanisms of oceanic?derived eclogites are controversial conundrums of oceanic subduction zones. The previous studies indicated that density is the primary factor controlling the exhumation of oceanic rocks. To explore their density evolution, we systematically investigate the phase relations and densities of different rock types in oceanic crust, including mid ocean ridge basalt (MORB), serpentinite, and global subducting sediments (GLOSS). According to the density of eclogites, these currently exposed natural eclogites can be classified into two categories: the self?exhumation of eclogites (?MORB < ?Mantle) and the carried exhumation of eclogites (?MORB > ?Mantle). The depth limit for an exhumation of oceanic?derived eclogites solely driven by their own buoyancies is 100-110 km, and it increases with the lithospheric thickness of the overriding plate. The parameters of carried?exhumation, that is, KGLOSS and KSerp, are defined in order to quantitatively evaluate the assistance ability of GLOSS and serpentinites for carrying the denser eclogites. KGLOSS is mainly controlled by pressure, whereas KSerp is dominantly affected by temperature. Using 2?D thermomechanical models, we demonstrate that the presences of low?density, low?viscosity GLOSS and seafloor serpentinites are the prerequisites for the exhumation of oceanic?derived eclogites. Our results show that oceanic?derived eclogites should be stalled and exhumed slowly at the Moho and Conrad discontinuities (named Moho/Conrad stagnation). We propose that oceanic?derived eclogites should undergo a two?stage exhumation generally, that is, early fast exhumation driven by buoyancy at mantle levels, and final exposure to surface actuated by tectonic exhumation facilitated by divergence between upper plate and accretionary wedge or by rollback of lower plate.
Abstract: Removal and thinning of cratonic lithosphere is believed to have occurred under different tectonic settings, for example, near subduction zones and above mantle plumes. Subduction-induced cratonic modification has been widely discussed; however, the mechanisms and dynamic processes of plume-induced lithospheric removal remain elusive and require further systematic investigation. In this study, we conduct a series of 2-D thermo-mechanical models to explore the dynamics of the removal and thinning of cratonic lithosphere due to the interaction between a mantle plume and a weak mid-lithosphere discontinuity (MLD) layer. Our modeling results suggest that the interaction between a mantle plume and weak MLD layer can lead to a large-scale removal of the cratonic lithosphere as long as the connection between the hot upwelling and weak MLD layer is satisfied. The presence of a vertical lithospheric weak zone and its closeness to the plume center play critical roles in creating a connection between the weak MLD and hot plume/asthenosphere. Furthermore, delamination of cratonic lithosphere is favored by a larger plume radius/volume, a higher plume temperature anomaly, and a lower viscosity of the MLD layer. A systematic comparison between subduction-induced and plume-induced lithospheric thinning patterns is further conducted. We summarize their significant differences on the origin and migration of melt generation, the water content in melts, and topographic evolution. The combination of numerical models and geological/geophysical observations indicates that mantle plume-MLD interaction may have played a crucial role in lithospheric removal beneath South Indian, South American and North Siberian Cratons.
Li, ZX., Lee, C-T.A, Peslier, A.H., Lenardic, A., Mackwell, S.J.
Water contents in mantle xeonoliths from the Colorado Plateau and vicinity: implications for the mantle rheology and hydration induced thinking of lithosphere
Journal of Geophysical Research, Vol. 113, B9, B09210.
Abstract: The Archean Yilgarn Craton in Western Australia is intruded by numerous mafic dykes of varying orientations, which are poorly exposed but discernible in aeromagnetic maps. Previous studies have identified two craton-wide dyke swarms, the 2408?Ma Widgiemooltha and the 1210?Ma Marnda Moorn Large Igneous Provinces (LIP), as well as limited occurrences of the 1075?Ma Warakurna LIP in the northern part of the craton. We report here a newly identified NW-trending mafic dyke swarm in southwestern Yilgarn Craton dated at 1888?±?9?Ma with ID-TIMS U-Pb method on baddeleyite from a single dyke and at 1858?±?54?Ma, 1881?±?37 and 1911?±?42?Ma with in situ SHRIMP U-Pb on baddeleyite from three dykes. Preliminary interpretation of aeromagnetic data indicates that the dykes form a linear swarm several hundred kilometers long, truncated by the Darling Fault in the west. This newly named Boonadgin dyke swarm is synchronous with post-orogenic extension and deposition of granular iron formations in the Earaheedy basin in the Capricorn Orogen and its emplacement may be associated with far field stresses. Emplacement of the dykes may also be related to initial stages of rifting and formation of the intracratonic Barren Basin in the Albany-Fraser Orogen, where the regional extensional setting prevailed for the following 300?million years. Recent studies and new paleomagnetic evidence raise the possibility that the dykes could be part of the coeval 1890?Ma Bastar-Cuddapah LIP in India. Globally, the Boonadgin dyke swarm is synchronous with a major orogenic episode and records of intracratonic mafic magmatism on many other Precambrian cratons.
Abstract: The Archean Yilgarn Craton in Western Australia hosts at least five generations of Proterozoic mafic dykes, the oldest previously identified dykes belonging to the ca. 2408-2401?Ma Widgiemooltha Supersuite. We report here the first known Archean mafic dyke dated at 2615?±?6?Ma by the ID-TIMS U-Pb method on baddeleyite and at 2610?±?25?Ma using in situ SHRIMP U-Pb dating of baddeleyite. Aeromagnetic data suggest that the dyke is part of a series of NE-trending intrusions that potentially extend hundreds of kilometres in the southwestern part of the craton, here named the Yandinilling dyke swarm. Mafic magmatism at 2615?Ma was possibly related to delamination of the lower crust during the final stages of assembly and cratonisation, and was coeval with the formation of late-stage gold deposit at Boddington. Paleogeographic reconstructions suggest that the Yilgarn and Zimbabwe cratons may have been neighbours from ca. 2690?Ma to 2401?Ma and if the Zimbabwe and Kaapvaal cratons amalgamated at 2660-2610?Ma, the 2615?Ma mafic magmatism in the southwestern Yilgarn Craton may be associated with the same tectonic event that produced the ca. 2607-2604?Ma Stockford dykes in the Central Zone of the Limpopo Belt. Paleomagnetic evidence and a similar tectonothermal evolution, including coeval low-pressure high-temperature metamorphism, voluminous magmatism, and emplacement of mafic dykes, support a configuration where the northern part of the Zimbabwe Craton was adjacent to the western margin of the Yilgarn Craton during the Neoarchean. Worldwide, reliably dated mafic dykes of this age have so far been reported from the Yilgarn Craton, the Limpopo Belt and the São Francisco Craton.
Abstract: Accurately mapping plate boundary types and locations through time is essential for understanding the evolution of the plate-mantle system and the exchange of material between the solid Earth and surface environments. However, the complexity of the Earth system and the cryptic nature of the geological record make it difficult to discriminate tectonic environments through deep time. Here we present a new method for identifying tectonic paleo-environments on Earth through a data mining approach using global geochemical data. We first fingerprint a variety of present-day tectonic environments utilising up to 136 geochemical data attributes in any available combination. A total of 38301 geochemical analyses from basalts aged from 5-0 Ma together with a well-established plate reconstruction model are used to construct a suite of discriminatory models for the first order tectonic environments of subduction and mid-ocean ridge as distinct from intraplate hotspot oceanic environments, identifying 41, 35, and 39 key discriminatory geochemical attributes, respectively. After training and validation, our model is applied to a global geochemical database of 1547 basalt samples of unknown tectonic origin aged between 1000-410 Ma, a relatively ill-constrained period of Earth's evolution following the breakup of the Rodinia supercontinent, producing 56 unique global tectonic environment predictions throughout the Neoproterozoic and Early Paleozoic. Predictions are used to discriminate between three alternative published Rodinia configuration models, identifying the model demonstrating the closest spatio-temporal consistency with the basalt record, and emphasizing the importance of integrating geochemical data into plate reconstructions. Our approach offers an extensible framework for constructing full-plate, deep-time reconstructions capable of assimilating a broad range of geochemical and geological observations, enabling next generation Earth system models.
Nature Communications, Vol 10, 1, doi.org/10.1038 /s41467-019-13300 8p. Pdf
Mantle
plumes, hotspots
Abstract: Plate tectonics and mantle plumes are two of the most fundamental solid-Earth processes that have operated through much of Earth history. For the past 300 million years, mantle plumes are known to derive mostly from two large low shear velocity provinces (LLSVPs) above the core-mantle boundary, referred to as the African and Pacific superplumes, but their possible connection with plate tectonics is debated. Here, we demonstrate that transition elements (Ni, Cr, and Fe/Mn) in basaltic rocks can be used to trace plume-related magmatism through Earth history. Our analysis indicates the presence of a direct relationship between the intensity of plume magmatism and the supercontinent cycle, suggesting a possible dynamic coupling between supercontinent and superplume events. In addition, our analysis shows a consistent sudden drop in MgO, Ni and Cr at ~3.2-3.0 billion years ago, possibly indicating an abrupt change in mantle temperature at the start of global plate tectonics.
Abstract: Understanding the dominant force responsible for supercontinent breakup is crucial for establishing Earth's geodynamic evolution that includes supercontinent cycles and plate tectonics. Conventionally, two forces have been considered: the push by mantle plumes from the sub-continental mantle which is called the active force for breakup, and the dragging force from oceanic subduction retreat which is called the passive force for breakup. However, the relative importance of these two forces is unclear. Here we model the supercontinent breakup coupled with global mantle convection in order to address this question. Our global model features a spherical harmonic degree-2 structure, which includes a major subduction girdle and two large upwelling (superplume) systems. Based on this global mantle structure, we examine the distribution of extensional stress applied to the supercontinent by both sub-supercontinent mantle upwellings and subduction retreat at the supercontinent peripheral. Our results show that: (1) at the center half of the supercontinent, plume push stress is ?3 times larger than the stress induced by subduction retreat; (2) an average hot anomaly of no higher than 50 K beneath the supercontinent can produce a push force strong enough to cause the initialization of supercontinent breakup; (3) the extensional stress induced by subduction retreat concentrates on a ?600 km wide zone on the boundary of the supercontinent, but has far less impact to the interior of the supercontinent. We therefore conclude that although circum-supercontinent subduction retreat assists supercontinent breakup, sub-supercontinent mantle upwelling is the essential force.
Abstract: The most dominant features in the present-day lower mantle are the two antipodal African and Pacific large low-shear-velocity provinces (LLSVPs). How and when these two structures formed, and whether they are fixed and long lived through Earth history or dynamic and linked to the supercontinent cycles, remain first-order geodynamic questions. Hotspots and large igneous provinces (LIPs) are mostly generated above LLSVPs, and it is widely accepted that the African LLSVP existed by at least ca. 200 Ma beneath the supercontinent Pangea. Whereas the continental LIP record has been used to decipher the spatial and temporal variations of plume activity under the continents, plume records of the oceanic realm before ca. 170 Ma are mostly missing due to oceanic subduction. Here, we present the first compilation of an Oceanic Large Igneous Provinces database (O-LIPdb), which represents the preserved oceanic LIP and oceanic island basalt occurrences preserved in ophiolites. Using this database, we are able to reconstruct and compare the record of mantle plume activity in both the continental and oceanic realms for the past 2 b.y., spanning three supercontinent cycles. Time-series analysis reveals hints of similar cyclicity of the plume activity in the continent and oceanic realms, both exhibiting a periodicity of ?500 m.y. that is comparable to the supercontinent cycle, albeit with a slight phase delay. Our results argue for dynamic LLSVPs where the supercontinent cycle and global subduction geometry control the formation and locations of the plumes.
Abstract: Accurately mapping plate boundary types and locations through time is essential for understanding the evolution of the plate-mantle system and the exchange of material between the solid Earth and surface environments. However, the complexity of the Earth system and the cryptic nature of the geological record make it difficult to discriminate tectonic environments through deep time. Here we present a new method for identifying tectonic paleo-environments on Earth through a data mining approach using global geochemical data. We first fingerprint a variety of present-day tectonic environments utilising up to 136 geochemical data attributes in any available combination. A total of 38301 geochemical analyses from basalts aged from 5-0 Ma together with a well-established plate reconstruction model are used to construct a suite of discriminatory models for the first order tectonic environments of subduction and mid-ocean ridge as distinct from intraplate hotspot oceanic environments, identifying 41, 35, and 39 key discriminatory geochemical attributes, respectively. After training and validation, our model is applied to a global geochemical database of 1547 basalt samples of unknown tectonic origin aged between 1000-410 Ma, a relatively ill-constrained period of Earth's evolution following the breakup of the Rodinia supercontinent, producing 56 unique global tectonic environment predictions throughout the Neoproterozoic and Early Paleozoic. Predictions are used to discriminate between three alternative published Rodinia configuration models, identifying the model demonstrating the closest spatio-temporal consistency with the basalt record, and emphasizing the importance of integrating geochemical data into plate reconstructions. Our approach offers an extensible framework for constructing full-plate, deep-time reconstructions capable of assimilating a broad range of geochemical and geological observations, enabling next generation Earth system models.
Abstract: Progressive mantle melting during the Earth’s earliest evolution led to the formation of a depleted mantle and a continental crust enriched in highly incompatible elements. Re-enrichment of Earth’s mantle can occur when continental crustal materials begin to founder into the mantle by either subduction or, to a lesser degree, by delamination processes, profoundly affecting the mantle’s trace element and volatile compositions. Deciphering when mantle re-enrichment/refertilization became a global-scale process would reveal the onset of efficient mass transfer of crust to the mantle and potentially when plate tectonic processes became operative on a global-scale. Here we document the onset of mantle re-enrichment/refertilization by comparing the abundances of petrogenetically significant isotopic values and key ratios of highly incompatible elements compared to lithophile elements in Archean to Early-Proterozoic mantle-derived melts (i.e., basalts and komatiites). Basalts and komatiites both record a rapid-change in mantle chemistry around 3.2 billion years ago (Ga) signifying a fundamental change in Earth geodynamics. This rapid-change is recorded in Nd isotopes and in key trace element ratios that reflect a fundamental shift in the balance between fluid-mobile and incompatible elements (i.e., Ba/La, Ba/Nb, U/Nb, Pb/Nd and Pb/Ce) in basaltic and komatiitic rocks. These geochemical proxies display a significant increase in magnitude and variability after ~3.2 Ga. We hypothesize that rapid increases in mantle heterogeneity indicate the recycling of supracrustal materials back into Earth’s mantle via subduction. Our new observations thus point to a???3.2 Ga onset of global subduction processes via plate tectonics.
Abstract: The Earth’s mantle is currently divided into the African and Pacific domains, separated by the circum-Pacific subduction girdle, and each domain features a large low shear-wave velocity province (LLSVP) in the lower mantle. However, it remains controversial as to whether the LLSVPs have been stationary through time or dynamic, changing in response to changes in global subduction geometry. Here we compile radiogenic isotope data on plume-induced basalts from ocean islands and oceanic plateaus above the two LLSVPs that show distinct lead, neodymium and strontium isotopic compositions for the two mantle domains. The African domain shows enrichment by subducted continental material during the assembly and breakup of the supercontinent Pangaea, whereas no such feature is found in the Pacific domain. This deep-mantle geochemical dichotomy reflects the different evolutionary histories of the two domains during the Rodinia and Pangaea supercontinent cycles and thus supports a dynamic relationship between plate tectonics and deep-mantle structures.
Abstract: The Earth’s mantle is currently divided into the African and Pacific domains, separated by the circum-Pacific subduction girdle, and each domain features a large low shear-wave velocity province (LLSVP) in the lower mantle. However, it remains controversial as to whether the LLSVPs have been stationary through time or dynamic, changing in response to changes in global subduction geometry. Here we compile radiogenic isotope data on plume-induced basalts from ocean islands and oceanic plateaus above the two LLSVPs that show distinct lead, neodymium and strontium isotopic compositions for the two mantle domains. The African domain shows enrichment by subducted continental material during the assembly and breakup of the supercontinent Pangaea, whereas no such feature is found in the Pacific domain. This deep-mantle geochemical dichotomy reflects the different evolutionary histories of the two domains during the Rodinia and Pangaea supercontinent cycles and thus supports a dynamic relationship between plate tectonics and deep-mantle structures.
Journal of Metamorphic Geology, Vol. 38, pp. 593-627.
Australia
geochronology
Abstract: The final assembly of the Mesoproterozoic supercontinent Nuna was marked by the collision of Laurentia and Australia at 1.60 Ga, which is recorded in the Georgetown Inlier of NE Australia. Here, we decipher the metamorphic evolution of this final Nuna collisional event using petrostructural analysis, major and trace element compositions of key minerals, thermodynamic modelling, and multi?method geochronology. The Georgetown Inlier is characterised by deformed and metamorphosed 1.70-1.62 Ga sedimentary and mafic rocks, which were intruded by c. 1.56 Ga old S?type granites. Garnet Lu-Hf and monazite U-Pb isotopic analyses distinguish two major metamorphic events (M1 at c. 1.60 Ga and M2 at c. 1.55 Ga), which allows at least two composite fabrics to be identified at the regional scale—c. 1.60 Ga S1 (consisting in fabrics S1a and S1b) and c. 1.55 Ga S2 (including fabrics S2a and S2b). Also, three tectono?metamorphic domains are distinguished: (a) the western domain, with S1 defined by low?P (LP) greenschist facies assemblages; (b) the central domain, where S1 fabric is preserved as medium?P (MP) amphibolite facies relicts, and locally as inclusion trails in garnet wrapped by the regionally dominant low?P amphibolite facies S2 fabric; and (c) the eastern domain dominated by upper amphibolite to granulite facies S2 foliation. In the central domain, 1.60 Ga MP-medium?T (MT) metamorphism (M1) developed within the staurolite-garnet stability field, with conditions ranging from 530-550°C at 6-7 kbar (garnet cores) to 620-650°C at 8-9 kbar (garnet rims), and it is associated with S1 fabric. The onset of 1.55 Ga LP-high?T (HT) metamorphism (M2) is marked by replacement of staurolite by andalusite (M2a/D2a), which was subsequently pseudomorphed by sillimanite (M2b/D2b) where granite and migmatite are abundant. P-T conditions ranged from 600 to 680°C and 4-6 kbar for the M2b sillimanite stage. 1.60 Ga garnet relicts within the S2 foliation highlight the progressive obliteration of the S1 fabric by regional S2 in the central zone during peak M2 metamorphism. In the eastern migmatitic complex, partial melting of paragneiss and amphibolite occurred syn? to post? S2, at 730-770°C and 6-8 kbar, and at 750-790°C and 6 kbar, respectively. The pressure-temperature-deformation-time paths reconstructed for the Georgetown Inlier suggest a c. 1.60 Ga M1/D1 event recorded under greenschist facies conditions in the western domain and under medium?P and medium?T conditions in the central domain. This event was followed by the regional 1.56-1.54 Ga low?P and high?T phase (M2/D2), extensively recorded in the central and eastern domains. Decompression between these two metamorphic events is ascribed to an episode of exhumation. The two?stage evolution supports the previous hypothesis that the Georgetown Inlier preserves continental collisional and subsequent thermal perturbation associated with granite emplacement.
Geological Society of America Bulletin, Vol. 133, 3/4, pp. 625-646. pdf
Global
Pangea
Abstract: The supercontinent Pangea formed by the subduction of the Iapetus and Rheic oceans between Gondwana, Laurentia, and Baltica during mid-to-late Paleozoic times. However, there remains much debate regarding how this amalgamation was achieved. Most paleogeographic models based on paleomagnetic data argue that the juxtaposition of Gondwana and Laurussia (Laurentia-Baltica) was achieved via long-lasting highly oblique convergence in the late Paleozoic. In contrast, many geology-based reconstructions suggest that the collision between the two continents was likely initiated via a Gondwanan promontory comprising the Iberian, Armorican, and Bohemian massifs, and parts of the basement units in the Alpine orogen during the Early Devonian. To help resolve this discrepancy, we present an updated compilation of high-quality paleopoles of mid-to-late Paleozoic ages (spanning Middle Ordovician and Carboniferous times) from Gondwana, Laurentia, and Baltica. These paleopoles were evaluated with the Van der Voo selection criteria, corrected for inclination error where necessary, and were used to revise their apparent polar wander (APW) paths. The revised APW paths were constructed using an innovative approach in which age errors, A95 ovals, and Q-factors of individual paleopoles are taken into account. By combining the resulting APW paths with existing geological data and field relationships in the European Variscides, we provide mid-to-late Paleozoic paleogeographic reconstructions which indicate that the formation of Pangea was likely initiated at 400 Ma via the collision between Laurussia and a ribbon-like Gondwanan promontory that was itself formed by a scissor-like opening of the Paleotethys Ocean, and that the amalgamation culminated in the mostly orthogonal convergence between Gondwana and Laurussia.
Abstract: A lack of precise age constraints for Neoproterozoic strata in the northwestern United States (Washington State), including the Buffalo Hump Formation (BHF), has resulted in conflicting interpretations of Rodinia amalgamation and breakup processes. Previous detrital zircon (DZ) studies identified a youngest ca. 1.1 Ga DZ age population in the BHF, interpreted to reflect mostly first-cycle sourcing of unidentified but proximal magmatic rocks intruded during the amalgamation of Rodinia at ca. 1.0 Ga. Alternatively, the ca. 1.1 Ga DZ population has been suggested to represent a distal source with deposition occurring during the early phases of Rodinia rifting, more than 250 m.y. after zircon crystallization. We combined conventional laser-ablation split-stream analyses of U-Pb/Lu-Hf isotopes in zircon with a method of rapid (8 s per spot) U-Pb analysis to evaluate these opposing models. Our study of ?2000 DZ grains from the BHF identified for the first time a minor (?1%) yet significant ca. 760 Ma population, which constrains the maximum depositional age. This new geochronology implies that the BHF records early rift deposition during the breakup of Rodinia and correlates with sedimentary rocks found in other late Tonian basins of southwestern Laurentia.
Abstract: Many Archean cratons exhibit Paleoproterozoic rifted margins, implying they were pieces of some ancestral landmass(es). The idea that such an ancient continental assembly represents an Archean supercontinent has been proposed but remains to be justified. Starkly contrasting geological records between different clans of cratons have inspired an alternative hypothesis where cratons were clustered in multiple, separate "supercratons". A new ca. 2.62 Ga paleomagnetic pole from the Yilgarn craton of Australia is compatible with either two successive but ephemeral supercontinents or two long-lived supercratons across the Archean-Proterozoic transition. Neither interpretation supports the existence of a single, long-lived supercontinent, suggesting that Archean geodynamics were fundamentally different from subsequent times (Proterozoic to present), which were influenced largely by supercontinent cycles.
Abstract: Rare oceanic diamonds are believed to have a mantle transition zone origin like super-deep continental diamonds. However, oceanic diamonds have a homogeneous and organic-like light carbon isotope signature (?13C ? 28 to ? 20‰) instead of the extremely variable organic to lithospheric mantle signature of super-deep continental diamonds (?13C ? 25‰ to?+?3.5‰). Here, we show that with rare exceptions, oceanic diamonds and the isotopically lighter cores of super-deep continental diamonds share a common organic ?13C composition reflecting carbon brought down to the transition zone by subduction, whereas the rims of such super-deep continental diamonds have the same ?13C as peridotitic diamonds from the lithospheric mantle. Like lithospheric continental diamonds, almost all the known occurrences of oceanic diamonds are linked to plume-induced large igneous provinces or ocean islands, suggesting a common connection to mantle plumes. We argue that mantle plumes bring the transition zone diamonds to shallower levels, where only those emplaced at the base of the continental lithosphere might grow rims with lithospheric mantle carbon isotope signatures.
Abstract: Rare oceanic diamonds are believed to have a mantle transition zone origin like super-deep continental diamonds. However, oceanic diamonds have a homogeneous and organic-like light carbon isotope signature (?13C ? 28 to ? 20‰) instead of the extremely variable organic to lithospheric mantle signature of super-deep continental diamonds (?13C ? 25‰ to?+?3.5‰). Here, we show that with rare exceptions, oceanic diamonds and the isotopically lighter cores of super-deep continental diamonds share a common organic ?13C composition reflecting carbon brought down to the transition zone by subduction, whereas the rims of such super-deep continental diamonds have the same ?13C as peridotitic diamonds from the lithospheric mantle. Like lithospheric continental diamonds, almost all the known occurrences of oceanic diamonds are linked to plume-induced large igneous provinces or ocean islands, suggesting a common connection to mantle plumes. We argue that mantle plumes bring the transition zone diamonds to shallower levels, where only those emplaced at the base of the continental lithosphere might grow rims with lithospheric mantle carbon isotope signatures.
Abstract: Carbonatite represents a major host rock for niobium (Nb) resources worldwide. Both magmatic and post-magmatic metasomatic processes are crucial for Nb mineralization in carbonatites. However, the roles of these metasomatic processes are difficult to be evaluated due to their multiple origins and complexity of the physico-chemical conditions. In this study, we present detailed mineralogical investigations of pyrochlore group minerals and chemical U-Th-Pb geochronology of uraninite within the Miaoya carbonatite complex, aiming to better characterize the role of post-magmatic metasomatic events. The Miaoya complex (ca. 420-440?Ma) hosts the second largest carbonatite-related Nb deposit in China, mainly in the form of pyrochlore group minerals, ferrocolumbite and Nb-bearing rutile. Primary pyrochlore group minerals evolved from pyrochlore to uranpyrochlore, and ultimately reaching the betafite end-member during the magmatic stage. They have then experienced an episode of metasomatic events at 235.4?±?4.1?Ma, as determined by U-Th-Pb chemical ages of secondary uraninite. Fluids activity for uranpyrochlore alteration was concomitant with the hydrothermal reworking of REE mineralization, which was probably related to tectono-thermal events that occurred during the Triassic closure of the ancient Mianlue Ocean. During this process, hydration and decomposition of uranpyrochlore were characterized by the leaching of Na, Ca and F from its structure, the incorporation of Fe, Si, Sr and Ba from the fluids, and the final in situ replacement by secondary ferrocolumbite, uraninite and Nb-bearing rutile. In addition, parts of Nb and U liberated from uranpyrochlore by metamictization were then transported over distances of several hundreds of microns in relatively reducing (Fe, Si, S, CO2)-bearing fluids under high temperature, and were ultimately re-precipitated in amorphous Fe-Si-U-Nb-bearing oxide veins and poorly crystallized Nb-Ti-Ca-Fe-rich oxides. The relatively weak fluids activity failed to efficiently promote the Nb re-enrichment.
In situ U Pb dating and Sr Nd isotopic analysis of perovskite: constraints on the age and petrogenesis of the Kuruman kimberlite province, Kaapvaal Craton, South Africa.
Geochemical evidence for Proterozoic continental arc and continental margin rift magmatism along the northern margin of the Yangtze craton, South China
Earth and planetary Science Letters, Vol. 579, 117343, 11p. Pdf
Australia
geophysics- seismics
Abstract: Plate tectonics, including rifting, subduction, and collision processes, was likely to have been different in the past due to the secular cooling of the Earth. The northeastern part of the West Australian Craton (WAC) has a complex Archean and Paleoproterozoic tectonic history; therefore, it provides an opportunity to study how subduction and collision processes evolved during the emergence of plate tectonics, particularly regarding the assembly of Earth's first supercontinent, Columbia. Because the northeastern boundary of the WAC and the southwestern boundary of the North Australian Craton (NAC) are covered by the Phanerozoic Canning Basin, the regional tectonic evolution has remained enigmatic, including how many tectonic elements were assembled and what may have driven rifting and subsequent collision events. Here, we use new passive-source seismic modeling to identify a seismically distinct segment of the lithosphere, the Percival Lakes Province, which lies east of the Pilbara Craton and is separated by two previously unknown southeast-trending lithosphere scale Paleoproterozoic sutures. We interpret that the northeastern suture, separates the Percival Lakes Province from the NAC, records the amalgamation of the WAC with the NAC. The southwestern suture separates the PLP from the reworked northeastern margin of the Pilbara Craton, including the East Pilbara Terrane and the Rudall Province. A significant upper mantle dipping structure was identified in the southwestern suture, and we interpret it to be a relic of subduction that records a previously unknown Paleoproterozoic collision that pre-dated the amalgamation of the WAC and NAC. By comparing our findings with previously documented dipping features, we show that the Paleoproterozoic collisions are seismically distinguishable from their Phanerozoic counterparts.
Geological Society of America Annual Meeting, Vol. 47, 7, p. 163. abstract
Europe, Turkey
Moissanite
Abstract: The Aladag ophiolite in the eastern Tauride belt, southern Turkey, is a well-preserved remnant of oceanic lithosphere. It consists of, in ascending order, harzburgitic to dunitic tectonites, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes and basaltic pillow lavas. Podiform chromitites are common in the mantle peridotites. Thus far, more than 200 grains of microdiamond and more than 100 grains of moissanite (SiC) have been separated from one sample of podiform chromitite. The microdiamonds occur mostly as subhedral to euhedral, colorless to pale yellow grains, about 50-300 ?m in size. Moissanite grains are generally subhedral, light blue to deep blue in color and variable in size. These grains of diamond and moissanite are very similar to in-situ grains in podiform chromitites of Tibet and the Polar Urals of Russia (Yang et al., 2014; 2015), indicating that they are natural minerals, not the result of natural or anthropogenic contamination. As reported elsewhere, the diamonds and moissanite are accompanied by a range of other minerals, including rutile, zircon, quartz and sulfides. The discovery of diamond, moissanite and other unusual minerals in the podiform chromitites of the Aladag massif provide additional evidence for the widespread occurrence of these minerals in ophiolites, indicating that they are related to global mantle processes.
Abstract: The Pozanti-Karsanti ophiolite situated in the eastern Tauride belt, southern Turkey, is a well-preserved oceanic lithosphere remnant comprising, in ascending order, mantle peridotite, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes, and basaltic pillow lavas. Two types of chromitites are observed in the Pozanti-Karsanti ophiolite. One type of chromitites occurs in the cumulate dunites around the Moho, and the other type of chromitites is hosted by the mantle harzburgites below the Moho. The second type of chromitites has massive, nodular, and disseminated textures. We have conducted the mineral separation work on the podiform chromitites hosted by harzburgites. So far, more than 100 grains of microdiamond and moissanite (SiC) have been recovered from the podiform chromitite. The diamonds and moissanite are accompanied by large amounts of rutile. Besides zircon, monazite and sulfide are also very common phases within the separated minerals. The discovery of diamond, moissanite, and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new evidences for the common occurrences of these unusual minerals in ophiolitic peridotites and chromitites. This discovery also suggests that deep mantle processes and materials have been involved in the formation of podiform chromitite.
Abstract: The Cameca 1280-HR large geometry SIMS instrument is a highly versatile analytical tool which can support a broad range of geochemical applications. Research using the Potsdam 1280 instrument focuses primarily on isotope ratio determinations in geomaterials. Optimized measurement protocols have already been established for ?18O determinations in zircon, and we are also working towards routine oxygen isotope determinations for quartz, calcite, mica, apatite and titanite. The primary challenge in developing such measurement systems are the identification and characterization of suitable reference materials (RMs), and this is made particularly challenging due to the matrix dependent ion yields of the SIMS ion source. Here we wish to report our progress towards establishing new analytical protocols for the determination of ?13C in both diamond and moissanite. In the case of diamond, our facility possesses three natural RMs with which we are able to produce data with a typical analytical repeatability of ?0.15 ‰ (1sd). An inter-comparison of our three diamond RMs demonstrates an overall data quality of better than 0.5‰ in terms of systematic offset between the various materials characterized using gas source mass spectrometry (Palot et al., 2012). A single such ?13C determination in diamond requires 80 s of data acquisition and involves a test portion mass of ?400 pg of material. In-house diamond reference materials for ?15N calibration allow us to measure this isotopic system to a total analytical uncertainty of ± 1.6 ‰ (1sd) at nitrogen concentrations reaching down to 250 ?g/g. Due to the relatively low abundance of nitrogen in diamonds, such isotope ratio determinations require around 9 minutes of data collection. With respect to ?13C determinations in moissanite, we use a kimberlitic SiC as calibrant (Mathez et al., 1995), on which we achieve a repeatability of ?0.2 ‰ (1sd) on a ?350 pg test portion mass. Total data acquisition time for such measurements is 80 s. We are currently in the process of developing a second moissanite RM based on a synthetic, coarse-grained powder. We will also investigate this new material for its ?30Si characteristics.
Abstract: In recent years diamonds and other unusual minerals (carbides, nitrides, metal alloys and native elements) have been recovered from mantle peridotites and chromitites (both high-Cr chromitites and high-Al chromitites) from a number of ophiolites of different ages and tectonic settings. Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite, west Albania. So far, more than 20 grains of microdiamonds and 30 grains of moissanites (SiC) have been separated from the podiform chromitite. The diamonds are mostly light yellow, transparent, euhedral crystals, 200-300 ?m across, with a range of morphologies; some are octahedral and cuboctahedron and others are elongate and irregular. Secondary electron images show that some grains have well-developed striations. All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at ?1325 cm?1. The moissanite grains recovered from the Skenderbeu chromitites are mainly light blue to dark blue, but some are yellow to light yellow. All the analyzed grains have typical Raman spectra with shifts at 766 cm?1, 787 cm?1, and 967 cm?1. The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon. This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys. Our new findings suggest that diamonds and moissanites are present, and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites.
Contributions to Mineralogy and Petrology, doi.org:10.1007/ s00410-018-1499-5 19p.
Europe, Turkey
diamond inclusions
Abstract: The Pozanti-Karsanti ophiolite (PKO) is one of the largest oceanic remnants in the Tauride belt, Turkey. Micro-diamonds were recovered from the podiform chromitites, and these diamonds were investigated based on morphology, color, cathodoluminescence, nitrogen content, carbon and nitrogen isotopes, internal structure and inclusions. The diamonds recovered from the PKO are mainly mixed-habit diamonds with sectors of different brightness under the cathodoluminescence images. The total ?13C range of the PKO diamonds varies between ? 18.8 and ? 28.4‰, with a principle ?13C mode at ? 25‰. Nitrogen contents of the diamonds range from 7 to 541 ppm with a mean value of 171 ppm, and the ?15N values range from ? 19.1 to 16.6‰, with a ?15N mode of ? 9‰. Stacking faults and partial dislocations are commonly observed in the Transmission Electron Microscopy foils whereas inclusions are rather rare. Combinations of ( Ca0.81Mn0.19)SiO3, NiMnCo-alloy and nanosized, quenched fluid phases were observed as inclusions in the PKO diamonds. We believe that the 13C-depleted carbon signature of the PKO diamonds derived from previously subducted crustal matter. These diamonds may have crystallized from C-saturated fluids in the asthenospheric mantle at depth below 250 km which were subsequently carried rapidly upward by asthenospheric melts.
Abstract: Geophysical investigations and laboratory experiments provide strong evidence for subduction of ancient oceanic crust, and geological and mineralogical observations suggest that subducted oceanic crust is recycled into the upper mantle. This model is supported by some direct petrologic and miner-alogical evidence, principally the recovery of super-deep diamonds from kimberlites and the presence of crustal materials in ophiolitic chromitites and peridotites, but many details are still unclear. Here we report the discovery of ophiolite-hosted diamonds in the podiform chromitites of the Skenderbeu massif of the Mirdita ophiolite in the western part of Neo-Tethys. The diamonds are characterized by exceedingly light C isotopes (?13CPDB ~ -25‰), which we interpret as evidence for subduction of organic carbon from Earth's surface. They are also characterized by an exceptionally large range in ? 15Nair (-12.9‰ to +25.5‰), accompanied by a low N aggregation state. Materials sparsely included in diamonds include amorphous material, Ni-Mn-Co alloy, nanocrystals (20 × 20 nm) of calcium silicate with an orthorhombic perovskite structure (Ca-Pv), and fluids. The fluids coexisting with the alloy and Ca-Pv provide clear evidence that the diamonds are natural rather than synthetic. We suggest that the Skenderbeu diamonds nucleated and grew from a C-saturated, NiMnCo-rich melt derived from a subducted slab of ocean crust and lithosphere in the deep mantle, at least in the diamond stability field, perhaps near the top of the mantle transition zone. The subsequent rapid upward transport in channeled networks related to slab rollback during subduction initiation may explain the formation and preservation of Skenderbeu diamonds. The discovery of diamonds from the Mirdita ophiolite not only provides new evidence of diamonds in these settings but also provides a valuable opportunity to understand deep cycling of subducted oceanic crust and mantle composition.
Abstract: Diamonds have been discovered in mantle peridotites and chromitites of six ophiolitic massifs along the 1300 km?long Yarlung?Zangbo suture (Bai et al., 1993; Yang et al., 2014; Xu et al., 2015), and in the Dongqiao and Dingqing mantle peridotites of the Bangong?Nujiang suture in the eastern Tethyan zone (Robinson et al., 2004; Xiong et al., 2018). Recently, in?situ diamond, coesite and other UHP mineral have also been reported in the Nidar ophiolite of the western Yarlung?Zangbo suture (Das et al., 2015, 2017). The above?mentioned diamond?bearing ophiolites represent remnants of the eastern Mesozoic Tethyan oceanic lithosphere. New publications show that diamonds also occur in chromitites in the Pozanti?Karsanti ophiolite of Turkey, and in the Mirdita ophiolite of Albania in the western Tethyan zone (Lian et al., 2017; Xiong et al., 2017; Wu et al., 2018). Similar diamonds and associated minerals have also reported from Paleozoic ophiolitic chromitites of Central Asian Orogenic Belt of China and the Ray?Iz ophiolite in the Polar Urals, Russia (Yang et al., 2015a, b; Tian et al., 2015; Huang et al, 2015). Importantly, in?situ diamonds have been recovered in chromitites of both the Luobusa ophiolite in Tbet and the Ray?Iz ophiolite in Russia (Yang et al., 2014, 2015a). The extensive occurrences of such ultra?high pressure (UHP) minerals in many ophiolites suggest formation by similar geological events in different oceans and orogenic belts of different ages. Compared to diamonds from kimberlites and UHP metamorphic belts, micro?diamonds from ophiolites present a new occurrence of diamond that requires significantly different physical and chemical conditions of formation in Earth's mantle. The forms of chromite and qingsongites (BN) indicate that ophiolitic chromitite may form at depths of >150?380 km or even deeper in the mantle (Yang et al., 2007; Dobrthinetskaya et al., 2009). The very light C isotope composition (?13C ?18 to ?28‰) of these ophiolitic diamonds and their Mn?bearing mineral inclusions, as well as coesite and clinopyroxene lamallae in chromite grains all indicate recycling of ancient continental or oceanic crustal materials into the deep mantle (>300 km) or down to the mantle transition zone via subduction (Yang et al., 2014, 2015a; Robinson et al., 2015; Moe et al., 2018). These new observations and new data strongly suggest that micro?diamonds and their host podiform chromitite may have formed near the transition zone in the deep mantle, and that they were then transported upward into shallow mantle depths by convection processes. The in?situ occurrence of micro?diamonds has been well?demonstrated by different groups of international researchers, along with other UHP minerals in podiform chromitites and ophiolitic peridotites clearly indicate their deep mantle origin and effectively address questions of possible contamination during sample processing and analytical work. The widespread occurrence of ophiolite?hosted diamonds and associated UHP mineral groups suggests that they may be a common feature of in?situ oceanic mantle. The fundamental scientific question to address here is how and where these micro?diamonds and UHP minerals first crystallized, how they were incorporated into ophiolitic chromitites and peridotites and how they were preserved during transport to the surface. Thus, diamonds and UHP minerals in ophiolites have raised new scientific problems and opened a new window for geologists to study recycling from crust to deep mantle and back to the surface.
International Symposium on Deep Earth Exploration and Practices, Beijing Oct. 24-26. 1 p. abstract
China
diamond genesis
Abstract: Diamonds have been discovered in mantle peridotites and chromitites of six ophiolitic massifs along the 1300 km?long Yarlung?Zangbo suture (Bai et al., 1993; Yang et al., 2014; Xu et al., 2015), and in the Dongqiao and Dingqing mantle peridotites of the Bangong?Nujiang suture in the eastern Tethyan zone (Robinson et al., 2004; Xiong et al., 2018). Recently, in?situ diamond, coesite and other UHP mineral have also been reported in the Nidar ophiolite of the western Yarlung?Zangbo suture (Das et al., 2015, 2017). The above?mentioned diamond?bearing ophiolites represent remnants of the eastern Mesozoic Tethyan oceanic lithosphere. New publications show that diamonds also occur in chromitites in the Pozanti?Karsanti ophiolite of Turkey, and in the Mirdita ophiolite of Albania in the western Tethyan zone (Lian et al., 2017; Xiong et al., 2017; Wu et al., 2018). Similar diamonds and associated minerals have also reported from Paleozoic ophiolitic chromitites of Central Asian Orogenic Belt of China and the Ray?Iz ophiolite in the Polar Urals, Russia (Yang et al., 2015a, b; Tian et al., 2015; Huang et al, 2015). Importantly, in?situ diamonds have been recovered in chromitites of both the Luobusa ophiolite in Tbet and the Ray?Iz ophiolite in Russia (Yang et al., 2014, 2015a). The extensive occurrences of such ultra?high pressure (UHP) minerals in many ophiolites suggest formation by similar geological events in different oceans and orogenic belts of different ages. Compared to diamonds from kimberlites and UHP metamorphic belts, micro?diamonds from ophiolites present a new occurrence of diamond that requires significantly different physical and chemical conditions of formation in Earth's mantle. The forms of chromite and qingsongites (BN) indicate that ophiolitic chromitite may form at depths of >150?380 km or even deeper in the mantle (Yang et al., 2007; Dobrthinetskaya et al., 2009). The very light C isotope composition (?13C ?18 to ?28‰) of these ophiolitic diamonds and their Mn?bearing mineral inclusions, as well as coesite and clinopyroxene lamallae in chromite grains all indicate recycling of ancient continental or oceanic crustal materials into the deep mantle (>300 km) or down to the mantle transition zone via subduction (Yang et al., 2014, 2015a; Robinson et al., 2015; Moe et al., 2018). These new observations and new data strongly suggest that micro?diamonds and their host podiform chromitite may have formed near the transition zone in the deep mantle, and that they were then transported upward into shallow mantle depths by convection processes. The in?situ occurrence of micro?diamonds has been well?demonstrated by different groups of international researchers, along with other UHP minerals in podiform chromitites and ophiolitic peridotites clearly indicate their deep mantle origin and effectively address questions of possible contamination during sample processing and analytical work. The widespread occurrence of ophiolite?hosted diamonds and associated UHP mineral groups suggests that they may be a common feature of in?situ oceanic mantle. The fundamental scientific question to address here is how and where these micro?diamonds and UHP minerals first crystallized, how they were incorporated into ophiolitic chromitites and peridotites and how they were preserved during transport to the surface. Thus, diamonds and UHP minerals in ophiolites have raised new scientific problems and opened a new window for geologists to study recycling from crust to deep mantle and back to the surface.
Abstract: As reported in our prior work, we have recovered microdiamonds and other unusual minerals, including pseudomorph stishovite, moissanite, qingsongite, native elements, metallic alloys, and some crustal minerals (i.e., zircon, quartz, amphibole, and rutile) from ophiolitic peridotites and chromitites. These ophiolite-hosted microdiamonds display different features than kimberlitic, metamorphic, and meteoritic diamonds in terms of isotopic values and mineral inclusions. The characteristic of their light carbon isotopic composition implies that the material source of ophiolite-hosted diamonds is surface-derived organic matter. Coesite inclusions coexisting with kyanite rimming an FeTi alloy from the Luobusa ophiolite show a polycrystalline nature and a prismatic habit, indicating their origin as a replacement of stishovite. The occurrence in kyanite and coesite with inclusions of qingsongite, a cubic boron nitride mineral, and a high-pressure polymorph of rutile (TiO2 II) point to formation pressures of 10-15?GPa at temperatures ?1300?°C, consistent with depths greater than 380?km, near the mantle transition zone (MTZ). Minerals such as moissanite, native elements, and metallic alloys in chromite grains indicate a highly reduced environment for ophiolitic peridotites and chromitites. Widespread occurrence of diamonds in ophiolitic peridotites and chromitites suggests that the oceanic mantle may be a more significant carbon reservoir than previously thought. These ophiolite-hosted diamonds have proved that surface carbon can be subducted into the deep mantle, and have provided us with a new window for probing deep carbon cycling.
Abstract: Geophysical investigations and laboratory experiments show evidence for possible subduction of ancient oceanic crust. Geological and mineralogical observations suggest that subducted oceanic crust is recycled into the upper mantle. The subduction is supported by the recovery of super?deep diamonds from kimberlites and the presence of crustal materials in ophiolitic chromitites and their host peridotites. What is the mechanism? Here we report the new discovery of ophiolite?hosted diamonds in the podiform chromitites within the Skenderbeu massif from the Mirdita ophiolite in the western part of Neo?Tethys (Fig. 1). The diamonds are characterized by exceedingly light C isotopes (?13CPDB ? ?25‰), which can be interpreted as evidence for subduction of organic carbon from Earth's surface. The diamonds are also characterized by an exceptionally large range in ?15Nair (?12.9‰ to +25.5‰), accompanied by a low N aggregation state (Fig. 2). On the other hand, materials sparsely included in diamonds include amorphous material, Ni?Mn?Co alloy, nanocrystals (20 nm×20 nm) of calcium silicate with an orthorhombic perovskite structure (Ca?Pv), and fluids (Fig. 3). We consider that the Skenderbeu diamonds nucleated and grew from a C?saturated, NiMnCo?rich melt derived from a subducted slab of ocean crust and lithosphere in the deep mantle environment. The environment is in the diamond stability field or near the top of the mantle transition zone. The new discovery of diamonds from the Mirdita ophiolite provides a valuable opportunity to understand deep cycling of subducted oceanic crust and mantle (i.e., composition and process).
Abstract: Limited field studies and sparse chronological constraints in the southwestern Great Slave Lake area creates uncertainties about the Laurentide Ice Sheet (LIS) flow history and deglacial chronology. Improved understanding of the western LIS ice-margin morphology and retreat history is required to refine larger ice-sheet interpretations and timing for northwest drainage of glacial Lake McConnell. Using new field observations and geochronology we establish ice-flow history and better constrain regional deglaciation. Paleo-ice flow indicators (n = 66) show an oldest southwestern flow (230°), an intermediate northwesterly flow (305°), and a youngest westerly flow (250°). Till samples bulk sediment and matrix properties (n = 160) allowed identification of two till units. A lower grey till sourced mainly from local Paleozoic sediments produced clast fabrics indicating a southwesterly flow direction, overlain by a brown till that contained an increased Canadian Shield content with lodged elongate boulders a-axes and boulder-top striation orientations indicating a west to northwest ice-flow direction. Ice-flow results show a clockwise shift in direction interpreted as evidence for ice-divide migration followed by topographically controlled deglacial westward flow influenced by the Mackenzie River valley. Minimum deglacial timing estimates were constrained through optical dating of fine-sand deposits in a well-developed strandline (n = 2) and seven aeolian dunes; ages range from 9.9 ± 0.6 to 10.8 ± 0.7 ka BP. These ages are from dunes located below glacial Lake McConnell maximum water level and may thus provide new local lake level age constraints. Ice retreat is informed by a newly-mapped segment of the Snake River moraine, which is an understudied feature in the region. New ice-flow history and ice-margin retreat interpretations will be integrated into the larger body of work on the western LIS providing more confident conclusions on ice-sheet evolution and meltwater drainage pathways, specifically in the southwestern Great Slave Lake area.
Liu, F.L., Gerdes, A., Liou, J.G., Xue, H.M., Liang, F.H.
SHRIMP U Pb zircon dating from Sulu Dabie dolomitic marble, eastern China: constraints on prograde, ultrahigh pressure and retrograde metamorphic ages.
Journal of Metamorphic Geology, Vol. 24, 7, Sept. pp. 569-589.
ISPRS Journal of Photogrammetry and Remote Sensing, Vol. 146, pp. 91-107.
Mantle
remote sensing
Abstract: Since Google Earth was first released in 2005, it has attracted hundreds of millions of users worldwide and made a profound impact on both academia and industry. It can be said that Google Earth epitomized the first-generation of Digital Earth prototypes. The functionalities and merits that have sustained Google Earth’s lasting influence are worth a retrospective review. In this paper, we take the liberty to conduct a bibliometric study of the applications of Google Earth during 2006-2016. We aim first to quantify the multifaceted impacts, and then to develop a structured understanding of the influence and contribution associated with Google Earth. To accomplish these objectives, we analyzed a total of 2115 Scopus publication records using scientometric methods and then proceed to discussion with a selected set of applications. The findings and conclusions can be summarized as follows: (1) the impact of Google Earth has been profound and persistent over the past decade. Google Earth was mentioned in an average of 229 publications per year since 2009. (2) Broadly, the impact of Google Earth has touched upon most scientific disciplines. Specifically, during 2006-2016, Google Earth has been mentioned in 2115 publications covering all of Scopus’s 26 subject areas; (3) the influence of Google Earth has largely concentrated in GIScience, remote sensing and geosciences. The extended influence of Google Earth has reached a wider range of audiences with a concentration in fields such as human geography, geoscience education and archaeology.
The American Mineralogist, in press available 59p. Pdf
South America, Guyana
diamond crystallography
Abstract: Diamonds have long been mined from alluvial terrace deposits within the rainforest of Guyana, South America. No primary kimberlite deposits have been discovered in Guyana, nor has there been previous studies on the mineralogy and origin of the diamonds. Paleoproterozoic terranes in Guyana are prospective to diamond occurrences because the most productive deposits are associated spatially with the eastern escarpment of the Paleoproterozoic Roraima Supergroup. Geographic proximity suggests that the diamonds are detrital grains eroding from the <1.98 Ga conglomerates, metamorphosed to zeolite and greenschist facies. The provenance and paragenesis of the alluvial diamonds are described using a suite of placer diamonds from different locations across the Guiana Shield. Guyanese diamonds are typically small, and those in our collection range from 0.3 to 2.7 mm in diameter; octahedral and dodecahedral, with lesser cubic and minor macle forms. The diamonds are further subdivided into those with abraded and non-abraded surfaces. Abraded diamonds show various colors in cathodoluminescence whereas most non-abraded diamonds appear blue. In all populations, diamonds are predominantly colorless, with lesser brown to yellow and very rare white. Diamonds are predominantly Type IaAB and preserve moderate nitrogen aggregation and total nitrogen concentrations ranging from trace to ~1971 ppm. The kinetics of nitrogen aggregation indicate mantle-derived residence temperatures of 1124 ± 100 ºC, assuming residence times of 1.3 Ga and 2.6 Ga for abraded and non-abraded diamonds respectively. The diamonds are largely sourced from the peridotitic to eclogitic lithospheric upper mantle based on both ?13C values of -5.82 ± 2.45‰ (VPDB-LSVEC) and inclusion suites predominantly comprised of forsterite, enstatite, Cr-pyrope, chromite, rutile, clinopyroxene, coesite, and almandine garnet. Detrital, accessory minerals are non-kimberlitic. Detrital zircon geochronology indicates diamondiferous deposits are predominantly sourced from Paleoproterozoic rocks of 2079 ± 88 Ma.
Geochemical Perspectives Letters, Vol. 10, pp. 51-55.
Mantle
redox
Abstract: Among redox sensitive elements, carbon is particularly important because it may have been a driver rather than a passive recorder of Earth’s redox evolution. The extent to which the isotopic composition of carbon records the redox processes that shaped the Earth is still debated. In particular, the highly reduced deep mantle may be metal-saturated, however, it is still unclear how the presence of metallic phases in?uences the carbon isotopic compositions of super-deep diamonds. Here we report ab initio results for the vibrational properties of carbon in carbonates, diamond, and Fe3C under pressure and temperature conditions relevant to super-deep diamond formation. Previous work on this question neglected the effect of pressure on the equilibrium carbon isotopic fractionation between diamond and Fe3C but our calculations show that this assumption overestimates the fractionation by a factor of ~1.3. Our calculated probability density functions for the carbon isotopic compositions of super-deep diamonds derived from metallic melt can readily explain the very light carbon isotopic compo- sitions observed in some super-deep diamonds. Our results therefore support the view that metallic phases are present during the formation of super-deep diamonds in the mantle below ~250 km.
The importance of crystal chemistry on REE partitioning between mantle minerals ( garnet, clinopyroxene, orthopyroxene, and olivine) and basaltic melts.
Contributions to Mineralogy and Petrology, 176, 19p. Pdf
Mantle
REE
Abstract: Clinopyroxene and orthopyroxene are the two major repositories of rare earth elements (REE) in spinel peridotites. Most geochemical studies of REE in mantle samples focus on clinopyroxene. Recent advances in in situ trace element analysis has made it possible to measure REE abundance in orthopyroxene. The purpose of this study is to determine what additional information one can learn about mantle processes from REE abundances in orthopyroxene coexisting with clinopyroxene in residual spinel peridotites. To address this question, we select a group of spinel peridotite xenoliths (9 samples) and a group of abyssal peridotites (12 samples) that are considered residues of mantle melting and that have major element and REE compositions in the two pyroxenes reported in the literature. We use a disequilibrium double-porosity melting model and the Markov chain Monte Carlo method to invert melting parameters from REE abundance in the bulk sample. We then use a subsolidus reequilibration model to calculate REE redistribution between cpx and opx at the extent of melting inferred from the bulk REE data and at the closure temperature of REE in the two pyroxenes. We compare the calculated results with those observed in clinopyroxene and orthopyroxene in the selected peridotitic samples. Results from our two-step melting followed by subsolidus reequilibration modeling show that it is more reliable to deduce melting parameters from REE abundance in the bulk peridotite than in clinopyroxene. We do not recommend the use of REE in clinopyroxene alone to infer the degree of melting experienced by the mantle xenolith, as HREE in clinopyroxene in the xenolith are reset by subsolidus reequilibration. In general, LREE in orthopyroxene and HREE in clinopyroxene are more susceptible to subsolidus redistribution. The extent of redistribution depends on the modes of clinopyroxene and orthopyroxene in the sample and thermal history experienced by the peridotite. By modeling subsolidus redistribution of REE between orthopyroxene and clinopyroxene after melting, we show that it is possible to discriminate mineral mode of the starting mantle and cooling rate experienced by the peridotitic sample. We conclude that endmembers of the depleted MORB mantle and the primitive mantle are not homogeneous in mineral mode. A modally heterogeneous peridotitic starting mantle provides a simple explanation for the large variations of mineral mode observed in mantle xenoliths and abyssal peridotites. Finally, by using different starting mantle compositions in our simulations, we show that composition of the primitive mantle is more suitable for modeling REE depletion in cratonic mantle xenoliths than the composition of the depleted MORB mantle.
Contribution to Mineralogy and Petrology, Vol. 177, 19 , 18p. Pdf
Global
garnet
Abstract: Garnet is a common U-bearing mineral in various magmatic and metamorphic rocks with a high U-Pb closure temperature (>?850 °C), rendering it a potentially valuable U-Pb geochronometer. However, a high U (>?10 ppm) garnet reference material that suits both quadrupole and/or multi-collector inductively coupled plasma mass spectrometry (ICP-MS) is yet to be established. This study evaluates a potential reference material for in situ garnet U-Pb analysis with anomalously high U content from the Prairie Lake alkaline complex, Canada. The PL57 garnet, occurring in a calcite ijolite, has high TiO2 (6.5-15.0 wt%, average 12.7 wt%) and Fe2O3 (17.1-21.3 wt%) contents and is a member of the andradite (26-66 mol.%)-morimotoite (18-41 mol.%)-schorlomite (16-35 mol.%) solid solution series. Four samples were dated by U-Pb ID-TIMS to assess reproducibility. Twelve TIMS analyses produced concordant, equivalent results. Garnet PL57 yielded a concordant age of 1156.2?±?1.2 Ma (2?, n?=?10, MSWD?=?1.0), based on ten analyses with two results discarded due to possible mineral inclusions (if included, the concordia age is 1156.6?±?1.8 Ma; n?=?12, MSWD?=?2.0). PL57 had 27-76 ppm (average 41 ppm) U with Th/U of 0.51-0.68 (average 0.63). The total common Pb content ranged from 0.4 to 3.9 pg (average 1.1 pg). Laser ablation coupled with ICP-MS and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging provide direct evidence that U is incorporated and homogeneously distributed within the garnet lattice rather than as defects or pore spaces. Published garnet samples and standards were then tested by calibrating the Willsboro, Mali, Qicun, and Tonglvshan garnet against PL57, which gave accurate ages within the recommended values. Case studies of garnet from the Archean Musselwhite orogenic gold deposit in Canada and the Cenozoic Changanchong and Habo skarn deposits in China yield reliable ages. This suggests that PL57 is a robust U-Pb isotope reference material. The limited variations of U and Pb isotopic ratios, together with the high U concentration and extremely low initial common Pb, make PL57 an ideal calibration and monitor reference material for in situ measurements.
Composition of phengites in eclogites and their retrogressive derivatives of Dabie shan region: implication for the applicability of phengite geobarometre....
Chinese Journal of Geochemistry, Vol. 21, 1, pp.52-56.
Abstract: The NNW trending tholeiitic Sonakhan mafic dyke swarm of the Northern Bastar Craton is comprised of basalt to basaltic andesite (SiO2?=?46.3?wt% to 55.3?wt%; Mg#?=?37 to 70) dykes. A single basaltic dyke yielded a weighted-mean 207Pb/206Pb baddeleyite age of 1851.1?±?2.6?Ma. The Sr and Nd isotopes (87Sr/86Sri?=?0.70396 to 0.70855; ?Nd(t)?=??5.7 to +2.0) are variable which is a consequence of crustal contamination. Trace element modeling suggests the dykes were likely derived by partial melting of a spinel-bearing mantle source. The Sonakhan dykes are 30 million years younger than the 1.88?Ga Bastar-Cuddapah dykes (Bastanar-Hampi swarm) of the southern and central Bastar Craton indicating they represent a distinct period of magmatism. However, much like the 1.88?Ga dykes, the Sonakhan dykes appear to be correlative with dykes from the Yilgarn Craton (Yalgoo dyke?=?1854?±?5?Ma) of Western Australia. The temporal and compositional similarity of the Sonakhan dykes with the Yalgoo dyke is evidence that they are petrologically related and may represent different branches of the same dyke swarm. The existence of two distinct Paleoproterozoic dyke swarms in the Bastar Craton that each have a correlative unit in the Yilgarn Craton is supportive of a link between India and Australia before 1.9?Ga. Moreover, it suggests that the break-up of India and Western Australia was protracted and lasted for at least 30 million years.
Abstract: The Indian Shield is cross-cut by a number of distinct Paleoproterozoic mafic dyke swarms. The density of dykes in the Dharwar and Bastar Cratons is amongst the highest on Earth. Globally, boninitic dyke swarms are rare compared to tholeiitic dyke swarms and yet they are common within the Southern Indian Shield. Geochronology and geochemistry are used to constrain the petrogenesis and relationship of the boninitic dykes (SiO2?=?51.5 to 55.7?wt%, MgO?=?5.8 to 18.7?wt%, and TiO2?=?0.30?wt% to 0.77?wt%) from the central Bastar Craton (Bhanupratappur) and the NE Dharwar Craton (Karimnagar). A single U-Pb baddeleyite age from a boninitic dyke near Bhanupratappur yielded a weighted-mean 207Pb/206Pb age of 2365.6?±?0.9?Ma that is within error of boninitic dykes from the Dharwar Craton near Karimnagar (2368.5?±?2.6?Ma) and farther south near Bangalore (2365.4?±?1.0?Ma to 2368.6?±?1.3?Ma). Rhyolite-MELTS modeling indicates that fractional crystallization is the likely cause of major element variability of the boninitic dykes from Bhanupratappur whereas trace element modeling indicates that the primary melt may be derived from a pyroxenite mantle source near the spinel-garnet transition zone. The Nd isotopes (?Nd(t)?=??6.4 to +4.5) of the Bhanupratappur dykes are more variable than the Karimnagar dykes (?Nd(t)?=??0.7 to +0.6) but they overlap. The variability of Sr-Nd isotopes may be related to crustal contamination during emplacement or is indicative of an isotopically heterogeneous mantle source. The chemical and temporal similarities of the Bhanupratappur dykes with the dykes of the Dharwar Craton (Karimnagar, Penukonda, Chennekottapalle) indicate they are members of the same giant radiating dyke swarm. Moreover, our results suggest that the Bastar and Dharwar Cratons were adjacent but likely had a different configuration at 2.37?Ga than the present day. It is possible that the 2.37Ga dyke swarm was related to a mantle plume that assisted in the break-up of an unknown or poorly constrained supercontinent.
Contribution to Mineralogy and Petrology, Vol. 177, 19 , 18p. Pdf
Global
garnet
Abstract: Garnet is a common U-bearing mineral in various magmatic and metamorphic rocks with a high U-Pb closure temperature (>?850 °C), rendering it a potentially valuable U-Pb geochronometer. However, a high U (>?10 ppm) garnet reference material that suits both quadrupole and/or multi-collector inductively coupled plasma mass spectrometry (ICP-MS) is yet to be established. This study evaluates a potential reference material for in situ garnet U-Pb analysis with anomalously high U content from the Prairie Lake alkaline complex, Canada. The PL57 garnet, occurring in a calcite ijolite, has high TiO2 (6.5-15.0 wt%, average 12.7 wt%) and Fe2O3 (17.1-21.3 wt%) contents and is a member of the andradite (26-66 mol.%)-morimotoite (18-41 mol.%)-schorlomite (16-35 mol.%) solid solution series. Four samples were dated by U-Pb ID-TIMS to assess reproducibility. Twelve TIMS analyses produced concordant, equivalent results. Garnet PL57 yielded a concordant age of 1156.2?±?1.2 Ma (2?, n?=?10, MSWD?=?1.0), based on ten analyses with two results discarded due to possible mineral inclusions (if included, the concordia age is 1156.6?±?1.8 Ma; n?=?12, MSWD?=?2.0). PL57 had 27-76 ppm (average 41 ppm) U with Th/U of 0.51-0.68 (average 0.63). The total common Pb content ranged from 0.4 to 3.9 pg (average 1.1 pg). Laser ablation coupled with ICP-MS and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging provide direct evidence that U is incorporated and homogeneously distributed within the garnet lattice rather than as defects or pore spaces. Published garnet samples and standards were then tested by calibrating the Willsboro, Mali, Qicun, and Tonglvshan garnet against PL57, which gave accurate ages within the recommended values. Case studies of garnet from the Archean Musselwhite orogenic gold deposit in Canada and the Cenozoic Changanchong and Habo skarn deposits in China yield reliable ages. This suggests that PL57 is a robust U-Pb isotope reference material. The limited variations of U and Pb isotopic ratios, together with the high U concentration and extremely low initial common Pb, make PL57 an ideal calibration and monitor reference material for in situ measurements.
Journal of Asian Earth Sciences, Vol. 214, 104772, 8p. Pdf
Mantle
carbon
Abstract: The temperature of the upper mantle was a principal factor controlling the style of plate tectonics and influencing magmatism and metamorphism on Earth over geological history. Recent studies emphasized that Earth’s tectonic style has transited into the modern plate tectonics since the late Neoproterozoic, which is characterized by a global network of plate boundaries with deep and cold oceanic plate subduction. However, the consequence of the establishment of modern plate tectonics to Earth’s mantle temperature and deep carbon cycle has not been fully understood. Here we apply statistical analysis on the geochemical data of continental igneous rocks and identify an increased magnitude of nephelinitic volcanism at the end of the Ediacaran. Nephelinitic rocks, a silica-undersaturated high-alkaline rock group, are mostly formed by low-degree melting of carbonated mantle sources. We link their widespread emergence with an enhanced mantle cooling event and a dramatically increased flux of crustal carbonates transporting to the mantle. The rapid cooling of the mantle was ascribed to the onset of modern-style plate tectonics with global-scale cold oceanic and continental subduction since the late Neoproterozoic. The declined upper-mantle temperature could not only favor the low-degree melting but also allow the subduction of carbonates into the deep mantle without decarbonation at shallow depth. Considering the high oxygen fugacity feature of the nephelinitic rocks and some other high-alkaline volcanism, the establishment of modern plate tectonics and thereafter enhanced mantle cooling and deep carbon cycle might contribute to the high-level atmospheric oxygen content during the Phanerozoic.
Abstract: At the Davdar mine in Xinjiang, north-western China, emeralds are hosted mainly by carbonate, quartz-carbonate and quartz veins cutting metasedimentary rocks, and are associated with minerals such as hematite, dolomite, quartz, orthoclase and albite. Sixteen rough emeralds obtained during the authors’ visit to the mining area in 2019 were studied by standard gemmolog-ical techniques and various spectroscopic methods (FTIR, Raman, UV-Vis-NIR and EPR), as well as LA-ICP-MS chemical analysis. The analysed samples were mostly coloured by Cr, and showed a wide range of Fe, V, Mg and alkali contents, along with relatively low Cs, Rb and Sc. UV-Vis-NIR spectra showed features at 370 nm (Fe3+), 430 nm (Cr3+ with contributions from V3+ and possibly Fe3+), 580-630 nm (Cr3+ and V3+), 638 and 683 nm (Cr3+), and 850 nm (Fe2+ and possibly Fe2+-Fe3+interactions). In addition, the more V-rich emeralds displayed a distinct V3+ absorption band at about 385-395 nm. Notably, the chemical composition of Davdar emeralds shows significant overlap with those from Panjshir, Afghanistan.
The timing of mantle and crustal events in South Namibia, as defined by SHRIMP dating of zircon domains from a garnet peridotite xenolith of the Gideon Kimberlite province.
Journal of African Earth Sciences, Vol. 39, 3-5, pp. 147-157.
The timing of mantle and crustal events in South Namibia as defined by SHRIMP dating of zircon domains from a garnet peridotite xenolith of the Gibeon kimberlite province.
Journal of African Earth Sciences, Vol. 39, 3-5, June pp. 147-157.
Physics of the Earth and Planetary Interiora, 13p. Pdf
Mantle
melting
Abstract: Physical properties of silicate melts play a key role for global planetary dynamics, controlling for example volcanic eruption styles, mantle convection and elemental cycling in the deep Earth. They are significantly modified by structural changes at the atomic scale due to external parameters such as pressure and temperature or due to chemistry. Structural rearrangements such as 4- to 6-fold coordination change of Si with increasing depth may profoundly influence melt properties, but have so far mostly been studied at ambient temperature due to experimental difficulties. In order to investigate the structural properties of silicate melts and their densification mechanisms at conditions relevant to the deep Earth's interior, we studied haplo basaltic glasses and melts (albite-diopside composition) at high pressure and temperature conditions in resistively and laser-heated diamond anvil cells using X-ray absorption near edge structure spectroscopy. Samples were doped with 10 wt of Ge, which is accessible with this experimental technique and which commonly serves as a structural analogue for the network forming cation Si. We acquired spectra on the Ge K edge up to 48 GPa and 5000 K and derived the average Ge-O coordination number , and bond distance as functions of pressure. Our results demonstrate a continuous transformation from tetrahedral to octahedral coordination between ca. 5 and 30 GPa at ambient temperature. Above 1600 K the data reveal a reduction of the pressure needed to complete conversion to octahedral coordination by ca. 30 . The results allow us to determine the influence of temperature on the Si coordination number changes in natural melts in the Earth's interior. We propose that the complete transition to octahedral coordination in basaltic melts is reached at about 40 GPa, corresponding to a depth of ca. 1200 km in the uppermost lower mantle. At the core-mantle boundary (2900 km, 130 GPa, 3000 K) the existence of non-buoyant melts has been proposed to explain observed low seismic wave velocity features. Our results highlight that the melt composition can affect the melt density at such extreme conditions and may strongly influence the structural response.
Earth and Planetary Science Letters, Vol. 479, pp. 170-178.
Canada, Nunavut, Baffin Island
geophysics - seismics
Abstract: The northern Hudson Bay region in Canada comprises several Archean cratonic nuclei, assembled by a number of Paleoproterozoic orogenies including the Trans-Hudson Orogen (THO) and the Rinkian-Nagssugtoqidian Orogen. Recent debate has focused on the extent to which these orogens have modern analogues such as the Himalayan-Karakoram-Tibet Orogen. Further, the structure of the lithospheric mantle beneath the Hudson Strait and southern Baffin Island is potentially indicative of Paleoproterozoic underthrusting of the Superior plate beneath the Churchill collage. Also in question is whether the Laurentian cratonic root is stratified, with a fast, depleted, Archean core underlain by a slower, younger, thermally-accreted layer. Plate-scale process that create structures such as these are expected to manifest as measurable fossil seismic anisotropic fabrics. We investigate these problems via shear wave splitting, and present the most comprehensive study to date of mantle seismic anisotropy in northern Laurentia. Strong evidence is presented for multiple layers of anisotropy beneath Archean zones, consistent with the episodic development model of stratified cratonic keels. We also show that southern Baffin Island is underlain by dipping anisotropic fabric, where underthrusting of the Superior plate beneath the Churchill has previously been interpreted. This provides direct evidence of subduction-related deformation at 1.8 Ga, implying that the THO developed with modern plate-tectonic style interactions.
Journal of Geophysical Research, Vol. 123, 7, pp. 5690-5709.
Canada, Nunavut
Geophysics - seismic
Abstract: The geology of northern Hudson Bay, Canada, documents more than 2 billion years of history including the assembly of Precambrian and Archean terranes during several Paleoproterozoic orogenies, culminating in the Trans?Hudson Orogen (THO) ?1.8 Ga. The THO has been hypothesized to be similar in scale and nature to the ongoing Himalaya?Karakoram?Tibetan orogen, but the nature of lithospheric terrane boundaries, including potential plate?scale underthrusting, is poorly understood. To address this problem, we present new P and S wave tomographic models of the mantle seismic structure using data from recent seismograph networks stretching from northern Ontario to Nunavut (60-100?W and 50-80?N). The large size of our network requires careful mitigation of the influence of source side structure that contaminates our relative arrival time residuals. Our tomographic models reveal a complicated internal structure in the Archean Churchill plate. However, no seismic wave speed distinction is observed across the Snowbird Tectonic Zone, which bisects the Churchill. The mantle lithosphere in the central region of Hudson Bay is distinct from the THO, indicating potential boundaries of microcontinents and lithospheric blocks between the principal colliders. Slow wave speeds underlie southern Baffin Island, the leading edge of the generally high wave speed Churchill plate. This is interpreted to be Paleoproterozoic material underthrust beneath Baffin Island in a modern?style subduction zone setting.
Abstract: The Canadian High Arctic preserves a long and complex tectonic history, including craton formation, multiple periods of orogenesis, extension and basin formation, and the development of a passive continental margin. We investigate the possible preservation of deformational structures throughout the High Arctic subcontinental lithosphere using measurements of seismic anisotropy from shear wave splitting at 11 seismograph stations across the region, including a N-S transect along Ellesmere Island. The majority of measurements indicate a fast-polarisation orientation that parallels tectonic trends and boundaries, suggesting that lithospheric deformation is the dominant source of seismic anisotropy in the High Arctic; however, a sub-lithospheric contribution cannot be ruled out. Beneath Resolute in the central Canadian Arctic, distinct back-azimuthal variations in splitting parameters can be explained by two anisotropic layers. The upper layer is oriented E-W and correlates with tectonic trends and the inferred lithospheric deformation history of the region. The lower layer has a ?NNE-SSW orientation and may arise from present-day convective mantle flow beneath locally-thinned continental lithosphere. In addition to inferences of anisotropic structure beneath the Canadian High Arctic, measurements from the far north of our study region suggest the presence of an anisotropic zone in the lowermost mantle beneath northwest Alaska.
Abstract: Neoproterozoic kimberlite, ultramafic lamprophyre, and carbonatite magmatic activity was widespread across the Canadian-Greenland Shield. Models to explain the preponderance of this deeply-derived CO2-rich magmatism between 680-540 Ma range from impingement of multiple mantle plumes to rifting activity linked to the breakout of the Laurentian plate from the Rodinia supercontinent configuration. We add to the debate about the origin of kimberlite magmas and evaluate possible mantle sources of the 655 Ma ‘diamond-rich’ Renard (new SIMS U/Pb perovskite ages) and 629 Ma ‘barren’ Wemindji kimberlites on the eastern Superior craton in Quebec, Canada. Our Sr-Nd-Hf and carbon isotope data (87Sr/86Sri = 0.70241-0.70442; ?Ndi = + 0.2 to + 4.8; ?Hfi = + 0.3 to + 6.5; ?13C = ? 5.6 to ? 3.9‰) suggest a common and moderately depleted convecting upper mantle source region for both the Renard and Wemindji kimberlites, which occur 400 km apart in the interior of the Superior craton. In contrast, the low Os isotope ratios (187Os/188Osi = 0.11078-0.12620; ?Osi = ? 13.7 to ? 1.6) and unfractionated chondritic relative HSE abundances (Os, Ir, Ru, Pt, Pd, Re) indicate significant involvement of ancient refractory cratonic mantle material in kimberlite magma formation. Our model calculations suggest that for both the diamond-rich Renard and the barren Wemindji kimberlite magmas up to 30% of the Os was derived from refractory cratonic peridotites. This material might have been assimilated by originally more CO2-rich carbonated silicate melts derived from the asthenosphere. We also show that the geochemical and Sr-Nd-Hf-Os isotopic compositions of the Renard and Wemindji kimberlites do not require significant input from melts derived from olivine-poor cratonic mantle lithologies such as MARID-type veins and pyroxenites/eclogites. This contrasts with the petrogenesis of deeply-derived volatile-rich potassic magmas found along the peripheries of cratons (e.g., ultramafic lamprophyres, kamafugites, and olivine lamproites), a setting where abundant non-peridotitic components have been added to the lithospheric mantle over the course of continent evolution. Provided that CO2-rich melts, such as proto-kimberlites, occur near the solidus of volatile-fluxed peridotites, no excess mantle heat is required in their formation. This important but often overlooked constraint, together with the observation that there exist no spatial or temporal relationships between the Superior craton kimberlites and Large Igneous Provinces during the Late Neoproterozoic, suggests that kimberlite magmatic activity was tectonically controlled. In our preferred model, ubiquitous CO2-rich proto-kimberlite melts form during volatile-controlled redox melting processes at ambient mantle temperatures in a thermal boundary layer directly beneath thick cratonic lithosphere. The success rate of ‘evolving’ hybrid kimberlite magmas reaching Earth’s surface increases when tensile stresses propagate into the > 200 km thick keels of continental lithosphere. These conditions are frequently met during fast and changing plate motions associated with the assembly and breakup of supercontinents.
The new exhibit at the Harry Oppenheimer Diamond Museum: a meeting of past, present and future. Ancient jewelry from archeological excavations in Israel.
Earth and Planetary Science Letters, Vol. 412, pp. 42-51.
Mantle
Coesite
Abstract: Compressional and shear wave velocities of coesite have been measured using ultrasonic interferometry in a multi-anvil apparatus up to 12.6 GPa at room temperature for the first time. While the P wave velocity increases continuously with pressure, the S wave exhibits an anomalous softening and the velocity decreases continuously with pressure. Finite strain analysis of the data yielded KS0=103.6(4) GPaKS0=103.6(4) GPa, G0=61.6(2) GPaG0=61.6(2) GPa and View the MathML sourceK0?=2.9(1), View the MathML sourceG0?=0.3(1) for the bulk and shear moduli and their pressure derivatives, respectively. The anomalous elastic behavior of coesite results in large velocity and impedance contrasts across the coesite–stishovite transition, reaching ?39% and ?48% for P and S wave velocity contrasts, and ?70% and 78% for P and S wave impedance contrasts, respectively, at pressure ?8 GPa, with P and S wave velocity perturbations showing no apparent dependence on depths (i.e., View the MathML sourcedln?V(PorS)/dh?0) within 8–12 GPa. These unusually large contrasts and depth independent characteristics render the transition between the two silica polymorphs one of the most plausible candidates for the cause of the seismically observed X-discontinuity. The current P and S wave velocity perturbation dependences on the SiO2 content, d(ln?VP)/d(SiO2)?0.43 (wt%)?1d(ln?VP)/d(SiO2)?0.43 (wt%)?1 and d(ln?VS)/d(SiO2)?0.60 (wt%)?1d(ln?VS)/d(SiO2)?0.60 (wt%)?1, can serve as a geophysical probe to track ancient subducted eclogite materials to gain insights on the geodynamics of the mantle.
Abstract: Roots of continental cratons keep a long record of multiple metasomatic events, but their trace is complicated due to the mixed signals left by these events in the composition of mantle silicate minerals. Simple composition helps diamonds to provide a more robust record of the latest metasomatic events which they witnessed. Growth and dissolution features on the diamond surface are sensitive to the composition of the reacting media. In this study we use mantle-derived resorption features on natural diamonds to examine the nature of metasomatic events in diamondiferous mantle lithologies. We use experiments at mantle conditions to examine how the composition of fluids and melts affect diamond resorption. We then compare these results to the features of natural diamonds to determine which of the tested compositions could have acted as metasomatic agents in Earth’s cratonic roots. Diamond dissolution experiments conducted at 6 GPa, 1200 - 1500oC using synthetic MgO-CaO-SiO2-CO2-H2O system examined the effect of CHO fluid, silica-saturated CHO fluid, aqueous and “dry” silica-carbonate and carbonate melts. Results show that the main control of diamond resorption morphology is the state of the reacting media: fluid vs. melt. We compared the experimental results to diamonds with mantle-derived resorption features from two kimberlites from the Orapa kimberlite cluster (Botswana). We identified twelve mantle-derived resorption types, none of which resembled the products of resorption in fluids. Most of the observed resorption types could be produced by dissolution in mantle melts with variable proportions of carbonate and silicate components and in the range of temperatures. The most abundant resorption type resembles the product of diamond dissolution in carbonate melts at temperatures above 1450oC. Our results suggest that fluid-metasomatism is not destructive for diamonds while melt-metasomatism is. The lower hydrous carbonated solidus of lherzolite compared to harzburgite can result in the shift the process from diamond growth in fluids to diamond dissolution in melts due to metasomatic transformation of harzburgite into lherzolite.
Geophysical Research Letters, Vol. 45, 15, pp. 7434-7443. doi.org/10.1029/ 2018GLO79130
Mantle
nitrogen
Abstract: On the early Earth nitrogen was redistributed between three prevailing reservoirs: the core forming metal, the silicate magma ocean, and the atmosphere. To shed light on the behavior of N during core segregation, we have experimentally determined N solubilities in Fe?dominated metal melts at high temperatures and pressures (1200-1800 °C, 0.4-9.0 GPa) using high?pressure devices. Based on our experimental results a model has been developed to describe N solubility into metal melts as a function of pressure and temperature. The model suggests that core?forming metal melts can dissolve N quantities that are as high as the Earth's core density deficit. However, the N concentrations in the core?forming metal are dependent on the accretionary scenario and its partitioning with silicate magma ocean; our solubilities provide an upper limit for possible N concentrations within the Earth's core. Nevertheless, this study shows that N in the modern mantle will largely dissolve in its small metal fraction and not in the dominating silicates.
Earth and Planetary Science Letters, Vol. 506, pp. 493-506.
Mantle
metasomatism
Abstract: Most diamonds found in kimberlites show complex patterns of growth and dissolution (resorption) surface features. Populations of diamonds from within single kimberlite bodies commonly contain a large diversity of diamond surface forms, some of which are a result of dissolution in kimberlite magma and others are inherited from the mantle. Morphological studies of natural diamonds differentiated features produced during dissolution in kimberlite magma and during mantle metasomatism. The former features were experimentally reproduced at 1 3 GPa and used to infer the presence and composition of magmatic fluid in different kimberlites. However, the mantle-derived resorption features have not been reproduced experimentally and the composition and origins of their formative solvents are unknown. Here we report the results of diamond dissolution experiments conducted in a multi-anvil apparatus at 6 GPa and 1200 to 1500 °C in synthetic CaO MgO SiO2 CO2 H2O system. The experiments produced very different diamond resorption morphologies in COH fluid, in silicate-saturated fluid, and in silicate and carbonate melts. Dissolution in SiO2-free COH fluid developed rounded crystal forms with shallow negative trigons, striations and hillocks, which are commonly observed on natural diamonds and are similar in 6 GPa and in 1 3 GPa experiments. However, silicate-saturated fluid produced very different resorption features that are rarely observed on natural diamonds. This result confirms that natural, SiO2-poor fluid-induced resorption develops under the comparatively low-pressures of kimberlite ascent, because at mantle pressures the high content of SiO2 in fluids would produce features like those from the silicate-saturated experiments. Comparison of the experimental products from this study to natural diamond resorption features from the literature suggests that natural diamonds show no record of dissolution by fluids during mantle metasomatism. Diamond resorption morphologies developed in experiments with silicate carbonate melts closely resemble many of the mantle-derived resorption features of natural diamonds, whose diversity can result from variable SiO2 concentration in carbonatitic melts and temperature variation. The experimental results imply that metasomatism by fluids does not dissolve diamond, whereas metasomatism by melts is diamond-destructive. The repetitive growth-dissolution patterns of natural diamonds could be due to diamond growth from fluids in harzburgitic lithologies followed by its dissolution in partial melts.
Abstract: The terrestrial planets are believed to have been formed from primitive material sampling a broad region of the inner solar system. Several meteoritic mixing models attempting to reconcile isotopic characteristics of Mars and Earth have been proposed, but, because of the inherent non-uniqueness of these solutions, additional independent observations are required to resolve the question of the primary building blocks of the terrestrial planets. Here, we consider existing isotopic measurements of O, ?48Ca, ?50Ti, ?54Cr, ?62Ni, and ?84Sr for primitive chondrites and differentiated achondrites and mix these stochastically to reproduce the isotopic signatures of Mars and Earth. For both planets we observe ? 105 unique mixing solutions out of 108 random meteoritic mixtures, which are categorised into distinct clusters of mixtures using principal component analysis. The large number of solutions implies that isotopic data alone are insufficient to resolve the building blocks of the terrestrial planets. To further discriminate between isotopically valid mixtures, each mixture is converted into a core and mantle component via mass balance for which geophysical properties are computed and compared to observations. For Mars, the geophysical parameters include mean density, mean moment of inertia, and tidal response, whereas for Earth upper mantle Mg/(Mg+Fe) ratio and core size are employed. The results show that Mars requires an oxidised, FeO-rich differentiated object next to chondritic material as main building blocks. In contrast, Earth's origin remains enigmatic. From a redox perspective, it appears inescapable that enstatite chondrite-like matter constitutes a dominant proportion of the building blocks from which Earth is made. The apparent need for compositionally distinct building blocks for Mars and Earth suggests that dissimilar planetesimal reservoirs were maintained in the inner Solar System during accretion.
Earth and Planetary Science Letters, Vol. 510, pp. 186-197.
Mantle
nitrogen
Abstract: Nitrogen is a key constituent of our atmosphere and forms the basis of life, but its early distribution between Earth reservoirs is not well constrained. We investigate nitrogen partitioning between metal and silicate melts over a wide range of conditions relevant for core segregation during Earth accretion, i.e. 1250-2000 °C, 1.5-5.5 GPa and oxygen fugacities of ?IW-5.9 to ?IW-1.4 (in log units relative to the iron-wüstite buffer). At 1250 °C, 1.5 GPa, ranges from 14 ± 0.1 at ?IW-1.4 to 2.0 ± 0.2 at ?IW-5, N partitioning into the core forming metal. Increasing pressure has no effect on , while increasing temperature dramatically lowers to 0.5 ± 0.15 at ?IW-4. During early core formation N was hence mildly incompatible in the metal. The partitioning data are then parameterised as a function of temperature and oxygen fugacity and used to model the evolution of N within the two early prevailing reservoirs: the silicate magma ocean and the core. Depending on the oxidation state during accretion, N either behaves lithophile or siderophile. For the most widely favoured initially reduced Earth accretion scenario, N behaves lithophile with a bulk partition coefficient of 0.17 to 1.4, leading to 500-700 ppm N in closed-system core formation models. However, core formation from a magma ocean is very likely accompanied by magma ocean degassing, the core would thus contain ?100 ppm of N, and hence, does not constitute the missing N reservoir. Bulk Earth N would thus be 34-180 ppm in the absence of other suitable reservoirs, >98% N of the chondritic N have hence been lost during accretion.
Earth and Planetary Science Letters, Vol. 554, doi.org/10.1016/ j.epsl.2020. 116659 12p . Pdf
Mantle
carbon
Abstract: In the accreting Earth and planetesimals, carbon was distributed between a core forming metallic melt, a silicate melt, and a hot, potentially dense atmosphere. Metal melt droplets segregating gravitationally from the magma ocean equilibrated near its base. To understand the distribution of carbon, its partitioning between the two melts is experimentally investigated at 1.5-6.0 GPa, 1300-2000 °C at oxygen fugacities of ?0.9 to ?1.9 log units below the iron-wuestite reference buffer (IW). One set of experiments was performed in San Carlos olivine capsules to investigate the effect of melt depolymerization (NBO/T), a second set in graphite capsules to expand the data set to higher pressures and temperatures. Carbon concentrations were analyzed by secondary ionization mass spectrometry (SIMS) and Raman spectra were collected to identify C-species in the silicate melt. Partition coefficients are governed by the solubility of C in the silicate melt, which varies from 0.01 to 0.6 wt%, while metal melts contain ?7 wt% C in most samples. C solubility in the silicate melt correlates strongly with NBO/T, which, in olivine capsules, is mostly a function of temperature. Carbon partition coefficients DCmetal/silicate at 1.5 GPa, 1300-1750 °C decrease from 640(49) to 14(3) with NBO/T increasing from 1.04 to 3.11. For the NBO/T of the silicate Earth of 2.6, DCmetal/silicate is 34(9). Pressure and oxygen fugacity show no clear effect on carbon partitioning. The present results differ from those of most previous studies in that carbon concentrations in the silicate melt are comparatively higher, rendering C to be about an order of magnitude less siderophile, and the discrepancies may be attributed to differences in the experimental protocols. Applying the new data to a magma ocean scenario, and assuming present day mantle carbon mantle concentrations from 120 to 795 ppm, implies that the core may contain 0.4-2.6 wt% carbon, resulting in 0.14-0.9 wt% of this element for the bulk Earth. These values are upper limits, considering that some of the carbon in the modern silicate Earth has very likely been delivered by the late veneer.
Journal of African Earth Sciences, Vol. 145, pp. 274-283.
Africa, Chad
lineaments
Abstract: This work reports an analysis of the relationships existing between the structural lineaments and the Cenozoic volcanism of the Tibesti area (northern Chad). Shield volcanoes, cinder cones, structural lineaments, intersection points of lineaments and faults are mapped using the combination of Shuttle Radar Topography Mission (SRTM), Digital Elevation Models (DEMs) and Landsat satellite images of the Tibesti Volcanic Province. The interpretation of the distribution of these structural and morphological features allows constraining the structural/tectonic setting of the Tibesti. We show that the relationships between the lineaments and the volcanic centres of the Tibesti province can locally be explained as the result of the combination of two Riedel dextral tectonic systems, respectively oriented at N120°E and N30-35°E. Taking into account the geological features of the area, a geodynamical model is proposed: the emplacement of the Tibesti Volcanic Province results from the reactivation of inherited structures of the Saharan metacraton, characterized by relict rigid cratonic nuclei and metacratonic areas reworked during the Pan-African orogeny, among which is located the Tibesti. The contrasted behaviour of these rheologically different zones can explain the location and the evolution of the Tibesti swell and volcanism. The new data presented in this paper and their interpretation in terms of the emplacement of the Tibesti volcanic province in the Saharan metacraton bring a new and major information about the behaviour of the African plate within its collisional context with Europe.
Abstract: Cubic CaSiO3 perovskite is a major phase in subducted oceanic crust, where it forms at a depth of about 550?kilometres from majoritic garnet1,2,28. However, its rheological properties at temperatures and pressures typical of the lower mantle are poorly known. Here we measured the plastic strength of cubic CaSiO3 perovskite at pressure and temperature conditions typical for a subducting slab up to a depth of about 1,200?kilometres. In contrast to tetragonal CaSiO3, previously investigated at room temperature3,4, we find that cubic CaSiO3 perovskite is a comparably weak phase at the temperatures of the lower mantle. We find that its strength and viscosity are substantially lower than that of bridgmanite and ferropericlase, possibly making cubic CaSiO3 perovskite the weakest lower-mantle phase. Our findings suggest that cubic CaSiO3 perovskite governs the dynamics of subducting slabs. Weak CaSiO3 perovskite further provides a mechanism to separate subducted oceanic crust from the underlying mantle. Depending on the depth of the separation, basaltic crust could accumulate at the boundary between the upper and lower mantle, where cubic CaSiO3 perovskite may contribute to the seismically observed regions of low shear-wave velocities in the uppermost lower mantle5,6, or sink to the core-mantle boundary and explain the seismic anomalies associated with large low-shear-velocity provinces beneath Africa and the Pacific.
Nature , Vol. 603, pp. 276-279. 10.1038/s41586-021-04378-2
Mantle
perovskite
Abstract: Cubic CaSiO3 perovskite is a major phase in subducted oceanic crust, where it forms at a depth of about 550 kilometres from majoritic garnet1,2,28. However, its rheological properties at temperatures and pressures typical of the lower mantle are poorly known. Here we measured the plastic strength of cubic CaSiO3 perovskite at pressure and temperature conditions typical for a subducting slab up to a depth of about 1,200 kilometres. In contrast to tetragonal CaSiO3, previously investigated at room temperature3,4, we find that cubic CaSiO3 perovskite is a comparably weak phase at the temperatures of the lower mantle. We find that its strength and viscosity are substantially lower than that of bridgmanite and ferropericlase, possibly making cubic CaSiO3 perovskite the weakest lower-mantle phase. Our findings suggest that cubic CaSiO3 perovskite governs the dynamics of subducting slabs. Weak CaSiO3 perovskite further provides a mechanism to separate subducted oceanic crust from the underlying mantle. Depending on the depth of the separation, basaltic crust could accumulate at the boundary between the upper and lower mantle, where cubic CaSiO3 perovskite may contribute to the seismically observed regions of low shear-wave velocities in the uppermost lower mantle5,6, or sink to the core-mantle boundary and explain the seismic anomalies associated with large low-shear-velocity provinces beneath Africa and the Pacific7-9.
Noble gas isotopes in minerals from phoscorites and carbonatites in Kovdor and Seblyavr ultramafic alkaline complexes ( Kola alkaline province NW Russia).
Periodico di Mineralogia, (in english), Vol. LXX11, 1. April, pp. 135-146.
Shang, C.K., Satir, M., Siebel, W., Nsifa, E.N., Taubald, H., Ligeois, J.P., Tchoua, F.M.
TTG Magmatism in the Congo Craton: a view from major and trace element geochemistry, Rb Sr Sm Nd systematics: case of the Sangmelima region, Ntem Complex
Journal of African Earth Sciences, Vol. 39, 3-5, pp. 61-79.
Abstract: Kimberlite is the host rock of diamonds and varies widely in geological and mineralogical features as well as color, processing capability, and dewatering characteristics. This study investigated the dewatering behavior of problematic Angolan kimberlites. The presence of clay minerals in kimberlite causes difficulties in dewatering due to high flocculant demand, poor supernatant clarity, and low settling rates. Identifying critical parameters governing the settling behavior will assist in managing the settling behavior of different kimberlite slurries. The influence of particle size, pH of the kimberlite slurry, cation exchange capacity, exchangeable sodium percentage, and smectite content of the kimberlite on the settling rate were investigated for 18 different African kimberlite samples. The settling rate and slurry bed compaction during natural settling were also measured for the kimberlite slurries. Seventeen different Angolan clay-rich kimberlites and one South African clay-rich kimberlite were tested, and, except for two kimberlites, colloidal stability was experienced during natural settling. The pH values of the kimberlite slurries ranged between 9 and 11, which is similar to the pH band where colloidal stability was found during earlier research. The results indicate that colloidal stable slurries were experienced with kimberlites that had exchangeable sodium percentages as low as 0.7%. The cation exchange capacity of the various kimberlites differentiated more distinctly between colloidal stability and instability. A new model is proposed whereby clay-rich kimberlites with a cation exchange capacity of more than 10cmol/kg will experience colloidal stability if the pH of the solvent solution is within the prescribed pH range of 9-11.The Trans-Saharan Belt is one of the most important orogenic systems constitutive of the Pan-African cycle, which, at the end of the Neoproterozoic, led to the formation of the Gondwana Supercontinent. It is marked by the opening and closing of oceanic domains, collision of continental blocks and the deformation of thick synorogenic sedimentary basins. It extends from north to south over a distance of 3000?km in Africa, including the Nigerian Shield and the Tuareg Shield as well as their counterparts beneath the Phanerozoic oil-rich North- and South-Saharan sedimentary basins. In this study, we take advantage of potential field methods (magnetism and gravity) to analyze the crustal-scale structures of the Tuareg Shield terranes and to track these Pan-African structures below the sedimentary basins, offering a new, >1000?km extent. The map interpretations are based on the classical potential field transforms and two-dimensional forward modeling. We have identified geophysical units and first-order bounding lineaments essentially defined owing to magnetic and gravimetric anomaly signatures. In particular, we are able to highlight curved terminations, which in the Trans-Saharan context have been still poorly documented. We provide for the first time a rheological map showing a categorization of contrasted basement units from the south of the Tuareg Shield up to the Atlas Belt. These units highlight the contrasted rheological behavior of the Tuareg tectonostratigraphic terranes during (i) the northerly Pan-African tectonic escape characteristic of the Trans-Saharan Belt and (ii) the North Sahara basin development, especially during intraplate reworking tied to the Variscan event. The discovery of a relatively rigid E-W oriented unit to the south of the Atlas system, and on which the escaping Pan-African terranes were blocked, offers a new perspective on the structural framework of the north-Gondwana margin. It will help to understand how occurred the rendezvous of the N-S oriented Pan-African terranes and the E-W oriented Cadomian peri-Gondwanan terranes.
Maia, M., Sichel, S., Briais, A., Brunelli, D., Ligi, M., Ferreira, N., Campos, T., Mougel, B., Brehme, I., Hemond, C., Motoki, A., Moura, D., Scalabrin, C., Pessanha, I., Alves, E., Ayres, A., Oliveira, P.
Abstract: Mantle exhumation at slow-spreading ridges is favoured by extensional tectonics through low-angle detachment faults1, 2, 3, 4, and, along transforms, by transtension due to changes in ridge/transform geometry5, 6. Less common, exhumation by compressive stresses has been proposed for the large-offset transforms of the equatorial Atlantic7, 8. Here we show, using high-resolution bathymetry, seismic and gravity data, that the northern transform fault of the St Paul system has been controlled by compressive deformation since ~10?million years ago. The long-lived transpression resulted from ridge overlap due to the propagation of the northern Mid-Atlantic Ridge segment into the transform domain, which induced the migration and segmentation of the transform fault creating restraining stepovers. An anticlockwise change in plate motion at ~11?million years ago5 initially favoured extension in the left-stepping transform, triggering the formation of a transverse ridge, later uplifted through transpression, forming the St Peter and St Paul islets. Enhanced melt supply at the ridge axis due to the nearby Sierra Leone thermo chemical anomaly9 is responsible for the robust response of the northern Mid-Atlantic Ridge segment to the kinematic change. The long-lived process at the origin of the compressive stresses is directly linked to the nature of the underlying mantle and not to a change in the far-field stress regime.
Journal of Volcanology and Geothermal Research, in press available 34p. Pdf
Global
mantle plumes, hotspots
Abstract: The magmatic components of continental Large Igneous Provinces (LIPs) include flood basalts and their plumbing system of giant mafic dyke swarms (radiating, linear, and the recently discovered circumferential type), mafic sill provinces, a lower crustal magmatic underplate, mafic-ultramafic (M-UM) intrusions, associated silicic magmatism, and associated carbonatites and kimberlites. This paper proposes a new plumbing system framework for mantle plume-related continental LIPs that incorporates all of these components, and provides a context for addressing key thematic aspects such as tracking magma batches "upstream" and "downstream" and their geochemical evolution, assessing the setting of M-UM intrusions and their economic potential, interpreting deep magmatic component identified by geophysical signatures, and estimating magnitudes of extrusive and intrusive components with climate change implications. This plumbing system model, and its associated implications, needs to be tested against the rapidly improving LIP record.
Diamond & Related Materials, Vol. 91, pp. 207-212.
Russia, Siberia
Popigai
Abstract: We report the results of a study of the polycrystalline powder of the diamond-lonsdaleite from the Popigai crater (Siberia) using UV micro-Raman spectroscopy and high-resolution synchrotron X-ray diffraction. By subtracting two experimental Raman spectra of diamond-lonsdaleite samples with close amounts of diamond and lonsdaleite, we were able to identify the polytypic composition of impact diamonds in contrast to the method of X-ray diffraction. We have managed to get for the first time the spectrum of “pure” lonsdaleite. Its deconvolution has allowed us to identify all the three Raman - active vibrational modes E2g, A1g, and E1g whose positions agree well with the results of ab initio calculations.
Abstract: Very few zircon-bearing, kimberlite-hosted mantle eclogite xenoliths have been identified to date; however, the zircon they contain is crucial for our understanding of subcratonic lithospheric mantle evolution and eclogite genesis. In this study, we constrain the characteristics of zircon from mantle eclogite xenoliths based on existing mineralogical and geochemical data from zircons from different geological settings, and on the inferred origin of mantle eclogites. Given the likely origin and subsequent evolution of mantle eclogites, we infer that the xenoliths can contain zircons with magmatic, metamorphic and xenogenic (i.e. kimberlitic zircon) origins. Magmatic zircon can be inherited from low-pressure mafic oceanic crust precursors, or might form during direct crystallization of eclogites from primary mantle-derived melts at mantle pressures. Metamorphic zircon within mantle eclogites has a number of possible origins, ranging from low-pressure hydrothermal alteration of oceanic crustal protoliths to metasomatism related to kimberlite magmatism. This study outlines a possible approach for the identification of inherited magmatic zircon within subduction-related mantle eclogites as well as xenogenic kimberlitic zircon within all types of mantle eclogites. We demonstrate this approach using zircon grains from kimberlite-hosted eclogite xenoliths from the Kasai Craton, which reveals that most, if not all, of these zircons were most likely incorporated as a result of laboratory-based contamination.
Geochimica et Cosmochimica Acta, in press available 38p.
Mantle
carbonatite
Abstract: Carbon isotope exchange between carbon-bearing high temperature phases records carbon (re-) processing in the Earth's interior, where the vast majority of global carbon is stored. Redox reactions between carbonate phases and elemental carbon govern the mobility of carbon, which then can be traced by its isotopes. We determined the carbon isotope fractionation factor between graphite and a Na2CO3-CaCO3 melt at 900-1500 °C, 1 GPa using a piston-cylinder device. The failure to isotopically equilibrate preexisting graphite led us to synthesize graphite anew from organic material during the melting of the carbonate mixture. Graphite growth proceeds by (1) decomposition of organic material into globular amorphous carbon, (2) restructuring into nano-crystalline graphite, and (3) recrystallization into hexagonal graphite flakes. Each transition is accompanied by carbon isotope exchange with the carbonate melt. High-temperature (1200 - 1500 °C) equilibrium isotope fractionation with type (3) graphite can be described by (temperature T in K). As the experiments do not yield equilibrated graphite at lower temperatures, we combined the ?1200 °C experimental data with those derived from upper amphibolite and lower granulite facies carbonate-graphite pairs (Kitchen and Valley, 1995, Valley and O'Neil, 1981). This yields the general fractionation function usable as a geothermometer for solid or liquid carbonate at ? 600 °C. Similar to previous observations, lower-temperature experiments (?1100 °C) deviate from equilibrium. By comparing our results to diffusion and growth rates in graphite, we show that at ?1100 °C carbon diffusion is slower than graphite growth, hence equilibrium surface isotope effects govern isotope fractionation between graphite and carbonate melt and determine the isotopic composition of newly formed graphite. The competition between diffusive isotope exchange and growth rates requires a more careful interpretation of isotope zoning in graphite and diamond. Based on graphite crystallization rates and bulk isotope equilibration, a minimum diffusivity of Dgraphite = 2x10-17 m2s-1 for T >1150 °C is required. This value is significantly higher than calculated from experimental carbon self-diffusion constants (?1.6x10-29 m2s-1) but in good agreement with the value calculated for mono-vacancy migration (?2.8x10-16 m2s-1).
Geochimica et Cosmochimica Acta, Vol. 253, pp. 290-306.
Mantle
geothermometry
Abstract: Carbon isotope exchange between carbon-bearing high temperature phases records the carbon (re-) processing in the Earth's interior, where the vast majority of global carbon is stored. Redox reactions between carbonate phases and elemental carbon govern the mobility of carbon, which then can be traced by its isotopes. We determined the carbon isotope fractionation factor between graphite and a Na2CO3-CaCO3 melt at 900-1500?°C and 1?GPa; The failure to isotopically equilibrate preexisting graphite led us to synthesize graphite anew from organic material during the melting of the carbonate mixture. Graphite growth proceeds by (1) decomposition of organic material into globular amorphous carbon, (2) restructuring into nano-crystalline graphite, and (3) recrystallization into hexagonal graphite flakes. Each transition is accompanied by carbon isotope exchange with the carbonate melt. High-temperature (1200-1500?°C) equilibrium isotope fractionation with type (3) graphite can be described by (temperature T in K). As the experiments do not yield equilibrated bulk graphite at lower temperatures, we combined the ?1200?°C experimental data with those derived from upper amphibolite and lower granulite facies carbonate-graphite pairs (Kitchen and Valley, 1995; Valley and O'Neil, 1981). This yields the general fractionation function usable as a geothermometer for solid or liquid carbonate at ?600?°C. Similar to previous observations, lower-temperature experiments (?1100?°C) deviate from equilibrium. By comparing our results to diffusion and growth rates in graphite, we show that at ?1100?°C carbon diffusion is slower than graphite growth, hence equilibrium surface isotope effects govern isotope fractionation between graphite and carbonate melt and determine the isotopic composition of newly formed graphite. The competition between diffusive isotope exchange and growth rates requires a more careful interpretation of isotope zoning in graphite and diamond. Based on graphite crystallization rates and bulk isotope equilibration, a minimum diffusivity of Dgraphite?=?2?×?10?17 m2s?1 for T?>?1150?°C is required. This value is significantly higher than calculated from experimental carbon self-diffusion constants (?1.6?×?10?29?m2?s?1) but in good agreement with the value calculated for mono-vacancy migration (?2.8?×?10?16?m2?s?1).
Earth and Planetary Science Letters, Vol. 529, 115848 12p. Pdf
Global
carbon
Abstract: At high temperatures, isotope partitioning is often assumed to proceed under equilibrium and trends in the carbon isotope composition within graphite and diamond are used to deduce the redox state of their fluid source. However, kinetic isotope fractionation modifies fluid- or melt-precipitated mineral compositions when growth rates exceed rates of diffusive mixing. As carbon self-diffusion in graphite and diamond is exceptionally slow, this fractionation should be preserved. We have hence performed time series experiments that precipitate graphitic carbon through progressive oxidization of an initially CH4-dominated fluid. Stearic acid was thermally decomposed at 800 °C and 2 kbar, yielding a reduced COH-fluid together with elemental carbon. Progressive hydrogen loss from the capsule caused CH4 to dissociate with time and elemental carbon to continuously precipitate. The newly formed C0, aggregating in globules, is constantly depleted by ‰ in 13C relative to the methane, which defines a temperature dependent kinetic graphite-methane 13C/12C fractionation factor. Equilibrium fractionation would instead yield graphite heavier than the methane. In dynamic environments, kinetic isotope fractionation may control the carbon isotope composition of graphite or diamond, and, extended to nitrogen, could explain the positive correlation of and sometimes observed in coherent diamond growth zones. 13C enrichment trends in diamonds are then consistent with reduced deep fluids oxidizing upon their rise into the subcontinental lithosphere, methane constituting the main source of carbon.
Abstract: The high amount of Fe-rich ferropericlase inclusions found in diamonds of a potential super-deep origin questions the bulk chemical model of the Earth [e.g., 1]. Although this might be due to a biased sampling of the lower mantle, it is worth to further address this discrepancy. A limiting factor of the Fe-content of the Earth´s deep mantle (TZ and lower mantle) is a correlation of the depths of the observed main mantle discontinuities with the (Fe,Mg)SiO4 phase diagram. In particular, the 520 kmdiscontinuity is related to the phase transformation of wadsleyite (assuming Fa10) to ringwoodite. The existing phase diagrams suggest a stability limit of wadsleyite ?Fa40 [e.g., 2,3], which limits the Fe-content of the Earth´s transition zone. Here we report on a discovery of Fe-rich wadsleyite grains (up to Fa56) in the high-pressure silicate melt droplets within Fe,Ni-metal in shock veins of the CB (Bencubbin-like) metal-rich carbonaceous chondrite QC 001 [4], which were identified using HR-EDX, nano-EBSD and TEM. Although the existence of such Fe-rich wadsleyite in shock veins may be due to the kinetic reasons, new theoretical and experimental studies of the stability of (Fe,Mg)SiO4 at high temperature (> 1800 K) are clearly needed. This may have significant impact on the temperature and chemical estimates of the Earth´s transition zone.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0607-6 16p.
Canada, Northwest Territories, South America, Brazil
deposit - Ekati, Grizzly, Kaola, Limpeza-18, Tres Ranchos-04, Kaalvallei, Samada, New Robinson
Abstract: Olivine in kimberlites can provide unique insights into magma petrogenesis, because it is the most abundant xenocrystic phase and a stable magmatic product over most of the liquid line of descent. In this study we examined the petrography and chemistry of olivine in kimberlites from different tectonic settings, including the Slave craton, Canada (Ekati: Grizzly, Koala), the Brasilia mobile belt (Limpeza-18, Tres Ranchos-04), and the Kaapvaal craton, South Africa (Kaalvallei: Samada, New Robinson). Olivine cores display a wide range of compositions (e.g., Mg#?=?78-95). The similarity in olivine composition, resorption of core zones and inclusions of mantle-derived phases, indicates that most olivine cores originated from the disaggregation of mantle peridotites, including kimberlite-metasomatised lithologies (i.e. sheared lherzolites and megacrysts). Olivine rims typically show a restricted range of Mg#, with decreasing Ni and increasing Mn and Ca contents, a characteristic of kimberlitic olivine worldwide. The rims host inclusions of groundmass minerals, which implies crystallisation just before and/or during emplacement. There is a direct correlation between olivine rim composition and groundmass mineralogy, whereby high Mg/Fe rims are associated with carbonate-rich kimberlites, and lower Mg/Fe rims are correlated with increased phlogopite and Fe-bearing oxide mineral abundances. There are no differences in olivine composition between explosive (Grizzly) and hypabyssal (Koala) kimberlites. Olivine in kimberlites also displays transitional zones and less common internal zones, between cores and rims. The diffuse transitional zones exhibit intermediate compositions between cores and rims, attributed to partial re-equilibration of xenocrystic cores with the ascending kimberlite melt. In contrast, internal zones form discrete layers with resorbed margins and restricted Mg# values, but variable Ni, Mn and Ca concentrations, which indicates a discrete crystallization event from precursor kimberlite melts at mantle depths. Overall, olivine exhibits broadly analogous zoning in kimberlites worldwide. Variable compositions for individual zones relate to different parental melt compositions rather than variations in tectonic setting or emplacement mechanism.
Abstract: Quantifying the compositional evolution of mantle-derived melts from source to surface is fundamental for constraining the nature of primary melts and deep Earth composition. Despite abundant evidence for interaction between carbonate-rich melts, including diamondiferous kimberlites, and mantle wall rocks en route to surface, the effects of this interaction on melt compositions are poorly constrained. Here, we demonstrate a robust linear correlation between the Mg/Si ratios of kimberlites and their entrained mantle components and between Mg/Fe ratios of mantle-derived olivine cores and magmatic olivine rims in kimberlites worldwide. Combined with numerical modeling, these findings indicate that kimberlite melts with highly variable composition were broadly similar before lithosphere assimilation. This implies that kimberlites worldwide originated by partial melting of compositionally similar convective mantle sources under comparable physical conditions. We conclude that mantle assimilation markedly alters the major element composition of carbonate-rich melts and is a major process in the evolution of mantle-derived magmas.
Geochemistry, Geophysics, Geosystems: G3, in press available
Technology
geophsyics - magnetics
Abstract: Remanent magnetization in geological samples may record the past intensity and direction of planetary magnetic fields. Traditionally, this magnetization is analyzed through measurements of the net magnetic moment of bulk millimeter to centimeter sized samples. However, geological samples are often mineralogically and texturally heterogeneous at submillimeter scales, with only a fraction of the ferromagnetic grains carrying the remanent magnetization of interest. Therefore, characterizing this magnetization in such cases requires a technique capable of imaging magnetic fields at fine spatial scales and with high sensitivity. To address this challenge, we developed a new instrument, based on nitrogenvacancy centers in diamond, which enables direct imaging of magnetic fields due to both remanent and induced magnetization, as well as optical imaging, of room-temperature geological samples with spatial resolution approaching the optical diffraction limit. We describe the operating principles of this device, which we call the quantum diamond microscope (QDM), and report its optimized image-area-normalized magnetic field sensitivity (20 µT?µm/Hz½), spatial resolution (5 µm), and field of view (4 mm), as well as trade-offs between these parameters. We also perform an absolute magnetic field calibration for the device in different modes of operation, including three-axis (vector) and single-axis (projective) magnetic field imaging. Finally, we use the QDM to obtain magnetic images of several terrestrial and meteoritic rock samples, demonstrating its ability to resolve spatially distinct populations of ferromagnetic carriers.
Proceedings National Academy of Science, Vol. 116, pp. 407-412.
Australia
paleomagnetism
Abstract: Zircon crystals from the Jack Hills, Western Australia, are one of the few surviving mineralogical records of Earth’s first 500 million years and have been proposed to contain a paleomagnetic record of the Hadean geodynamo. A prerequisite for the preservation of Hadean magnetization is the presence of primary magnetic inclusions within pristine igneous zircon. To date no images of the magnetic recorders within ancient zircon have been presented. Here we use high-resolution transmission electron microscopy to demonstrate that all observed inclusions are secondary features formed via two distinct mechanisms. Magnetite is produced via a pipe-diffusion mechanism whereby iron diffuses into radiation-damaged zircon along the cores of dislocations and is precipitated inside nanopores and also during low-temperature recrystallization of radiation-damaged zircon in the presence of an aqueous fluid. Although these magnetites can be recognized as secondary using transmission electron microscopy, they otherwise occur in regions that are indistinguishable from pristine igneous zircon and carry remanent magnetization that postdates the crystallization age by at least several hundred million years. Without microscopic evidence ruling out secondary magnetite, the paleomagnetic case for a Hadean-Eoarchean geodynamo cannot yet been made.
Abstract: The Indaiá-I and Indaiá-II intrusions are hypabyssal, small-sized ultrabasic bodies belonging to the Cretaceous magmatism of the Alto Paranaiba Alkaline Province (southeast-central western Brazil). While Indaiá-I is classified as an archetypal group-I kimberlite, Indaiá-II (its satellite intrusion) presents several petrographic and chemical distinctions: (1) an ultrapotassic composition (similar to kamafugites), (2) lower volumes of olivine macrocrysts, (3) diopside as the main matrix phase (in contrast with the presence of monticellite in Indaiá-I), (4) high amounts of phlogopite, and (5) abundant felsic boudinaged and stretched microenclaves and crustal xenoliths. Disequilibrium features, such as embayment and sieve textures in olivine and clinopyroxene grains, are indicative of open-system processes in Indaiá-II. Mineral reactions observed in Indaiá-II (e.g., diopside formed at the expense of monticellite and olivine; phlogopite nearby crustal enclaves and close to olivine macrocrysts) point to an increase in the silica activity of the kimberlite magma; otherwise partially melted crustal xenoliths present kalsilite, generated by desilification reactions. The high Contamination Index (2.12-2.25) and the large amounts of crustal xenoliths (most of them totally transformed or with evidence of partial melting) indicate a high degree of crustal assimilation in the Indaiá-II intrusion. Calculated melts (after removal of olivine xenocrysts) of Indaiá-II have higher amounts of SiO2, Al2O3, K2O, slightly higher Rb/Sr ratios, lower Ce/Pb and Gd/Lu ratios, higher 87Sr/86Sr, and lower 143Nd/144Nd than those calculated for Indaiá-I. Crustal contamination models were developed considering mixing between the calculated melts of Indaiá-I and partial melts modeled from the granitoid country rocks. Mixing-model curves using major and trace elements and isotopic compositions are consistent with crustal assimilation processes with amounts of crustal contribution of ca. 30%. We conclude that (1) Indaiá-II is representative of a highly contaminated kimberlitic intrusion, (2) this contamination occurred by the assimilation of anatectic melts from the main crustal country rocks of this area, and (3) Indaiá-I and Indaiá-II could have had the same parent melt, but with different degrees of crustal contamination. Our petrological model also indicates that Indaiá-II is a satellite blind pipe linked to the main occurrence of Indaiá-I.
Abstract: Phlogopite is widely accepted as a major mineral indicator of the modal metasomatism in the upper mantle within a very wide P-T range. The paper reviews data on various phlogopite-forming reactions in upper-mantle peridotites. The review includes both descriptions of naturally occurring reactions and results of experiments that model some of these reactions. Relations of phlogopite with other potassic phases, such as K-richterite, sanidine and K-titanates, are discussed. These data are taken as a basis for thermodynamic modeling of the phlogopite-forming reactions for specific mantle rocks in terms of log(aH2O) ? log(aK2O) diagrams (pseudosections) using the Gibbs free energy minimization. These diagrams allow estimation of potassium-water activity relations during metasomatic transformations of mantle rocks, prediction sequences of mineral assemblages with respect to these parameters and comparison of metasomatic processes in the rocks of different composition. This approach is illustrated by examples from peridotite xenoliths from kimberlites.
Abstract: The salt components of aqueous and aqueous-carbonic fluids are very important agents of metasomatism and partial melting of crustal and mantle rocks. The paper presents examples and synthesized data on mineral associations in granulite- and amphibolite-facies rocks of various composition in the middle and lower crust and in upper-mantle eclogites and peridotites that provide evidence of reactions involving salt components of fluids. These data are analyzed together with results of model experiments that reproduce some of these associations and make it possible to more accurately determine their crystallization parameters.
Abstract: The results of experimental studies are presented for reactions in the orthopyroxene-garnet-phlogopite system in the presence of H2O-KCl fluid at 3-5 GPa and 900-1000°C, which model the processes of phlogopite formation in garnet peridotites and pyroxenites during alkaline metasomatism of the upper mantle. The experiments demonstrated regular variations in the composition of garnet, pyroxenes, and phlogopite depending on the KCl content of the fluid. With increasing KCl content of the fluid, enstatite and garnet become unstable, the Al2O3 content of enstatite decreases, and the amount of grossular and knorringite components in garnet are maximum at a KCl content of ~10 mol %. Our results illustrate well the regular variations in the compositions of the coexisting minerals and their zoning in phlogopite-bearing peridotites of the lithospheric mantle.
Abstract: This study focuses on the causes, modalities and obstacles of sediment transfer in the longest cell of littoral sand drift documented on Earth so far. Sand derived from the Orange River is dragged by swell waves and persistent southerly winds to accumulate in four successive dunefields in coastal Namibia to Angola. All four dunefields are terminated by river valleys, where aeolian sand is flushed back to the ocean; and yet sediment transport continues at sea, tracing an 1800 km long submarine sand highway. Sand drift would extend northward to beyond the Congo if the shelf did not become progressively narrower in southern Angola, where drifting sand is funnelled towards oceanic depths via canyon heads connected to river mouths. Garnet-magnetite placers are widespread along this coastal stretch, indicating systematic loss of the low-density feldspatho-quartzose fraction to the deep ocean. More than half of Moçamedes Desert sand is derived from the Orange River, and the rest in similar proportions from the Cunene River and from the Swakop and other rivers draining the Damara Orogen in Namibia. The Orange fingerprint, characterized by basaltic rock fragments, clinopyroxene grains and bimodal zircon-age spectra with peaks at ca 0•5 Ga and ca 1•0 Ga, is lost abruptly at Namibe, and beach sands further north have abundant feldspar, amphibole-epidote suites and unimodal zircon-age spectra with a peak at ca 2•0 Ga, documenting local provenance from Palaeoproterozoic basement. Along with this oblique-rifted continental margin, beach placers are dominated by Fe-Ti-Cr oxides with more monazite than garnet and thus have a geochemical signature sharply different from beach placers found all the way along the Orange littoral cell. High-resolution mineralogical studies allow us to trace sediment dispersal over distances of thousands of kilometres, providing essential information for the correct reconstruction of ‘source to sink’ relationships in hydrocarbon exploration and to predict the long-term impact of man-made infrastructures on coastal sediment budgets.
Earth and Planetary Science Letters, Vol. 479, pp. 43-49.
Mantle
perovskite
Abstract: Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal conductivity at relevant pressure-temperature conditions of Earth's core-mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core-mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg0.9Fe0.1SiO3 post-perovskite (?1.1 W/m/K) is almost two times smaller than that of bridgmanite (?2.0 W/m/K) at the base of the mantle. By combining this result with the present-day core-mantle heat flow and available estimations on the lattice thermal conductivity we conclude that post-perovskite is at least as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth.
Physics of the Earth and Planetary Interiors, Vol. 247, pp. 11-16.
Mantle
Experimental Petrology
Abstract: Lattice thermal conductivity of ferropericlase and radiative thermal conductivity of iron bearing magnesium silicate perovskite (bridgmanite) - the major mineral of Earth’s lower mantle- have been measured at room temperature up to 30 and 46 GPa, respectively, using time-domain thermoreflectance and optical spectroscopy techniques in diamond anvil cells. The results provide new constraints for the pressure dependencies of the thermal conductivities of Fe bearing minerals. The lattice thermal conductivity of ferropericlase Mg0.9Fe0.1O is 5.7(6) W/(m * K) at ambient conditions, which is almost 10 times smaller than that of pure MgO; however, it increases with pressure much faster (6.1(7)%/GPa vs 3.6(1)%/GPa). The radiative conductivity of a Mg0.94Fe0.06SiO3 bridgmanite single crystal agrees with previously determined values for powder samples at ambient pressure; it is almost pressure-independent in the investigated pressure range. Our results confirm the reduced radiative conductivity scenario for the Earth’s lower mantle, while the assessment of the heat flow through the core-mantle boundary still requires in situ measurements at the relevant pressure-temperature conditions.
Geophysical Research Letters, Vol. 45, 10, pp. 4725-4732.
Mantle
bridgmanite
Abstract: Seismic heterogeneities in the Earth's lower mantle have been attributed to thermal and/or chemical variations of constituent minerals. Bridgmanite is the most abundant lower?mantle mineral and contains Fe and Al in its structure. Knowing the effect of Fe on compressional and shear wave velocities (VP, VS) and density of bridgmanite at relevant pressure?temperature conditions can help to understand seismic heterogeneities in the region. However, experimental studies on both VP and VS of Fe?bearing bridgmanite have been limited to pressures below 40 GPa. In this study, VP and VS of Fe?bearing bridgmanite were measured up to 70 GPa in the diamond anvil cell. We observed drastic softening of VP by ~6(±1)% at 42.6-58 GPa and increased VS at pressures above 40 GPa. We interpret these observations as due to a spin transition of Fe3+. These observations are different to previous views on the effect of Fe on seismic velocities of bridgmanite. We propose that the abnormal sound velocities of Fe?bearing bridgmanite could help to explain the seismically observed low correlation between VP and VS in the mid?lower mantle. Our results challenge existing models of Fe enrichment to explain the origin of Large Low Shear Velocity provinces in the lowermost mantle.
Geochemical Perspectives Letters, Vol. 10, pp. 51-55.
Mantle
redox
Abstract: Among redox sensitive elements, carbon is particularly important because it may have been a driver rather than a passive recorder of Earth’s redox evolution. The extent to which the isotopic composition of carbon records the redox processes that shaped the Earth is still debated. In particular, the highly reduced deep mantle may be metal-saturated, however, it is still unclear how the presence of metallic phases in?uences the carbon isotopic compositions of super-deep diamonds. Here we report ab initio results for the vibrational properties of carbon in carbonates, diamond, and Fe3C under pressure and temperature conditions relevant to super-deep diamond formation. Previous work on this question neglected the effect of pressure on the equilibrium carbon isotopic fractionation between diamond and Fe3C but our calculations show that this assumption overestimates the fractionation by a factor of ~1.3. Our calculated probability density functions for the carbon isotopic compositions of super-deep diamonds derived from metallic melt can readily explain the very light carbon isotopic compo- sitions observed in some super-deep diamonds. Our results therefore support the view that metallic phases are present during the formation of super-deep diamonds in the mantle below ~250 km.
Abstract: Diamonds are key messenger from the deep Earth because someare sourced from the longest isolated and deepest accessible regions of the Earth’s mantle. They are prime recorders of the carbon isotopic compositionof the Earth. The C isotope composition (d13C) of natural diamonds showsa widevariationfrom -41‰ to +3‰ with the primary mode at -5 ± 3‰ [1]. In comparison, the d13C values of chondrites and other planetary bodies range between -26‰ and -15‰ [2]. It is possible that some of the low d13C values were inherited from the Earth’s building blocks,but this is unlikely to be the sole explanation for all low d13C values that can reach as low as -41‰. Organic matter at the Earth’s surface that has low d13C values[3] has been regarded as a possible origin for low d13C values. However, organic carbon is usually accompanied by carbonate with higher d13C values (~0 ‰),and it is not clear why this d13C value does not appear frequently in diamonds. Low d13C diamonds were also formed by deposition from C-O-H fluids,but the equilibrium fractionationinvolved between diamonds and fluids issmall at mantle temperatures [1] and the low d13C values of diamonds can only be achieved after extensive Rayleigh distillation. One unique feature of the Earth isactive plate tectonics driven by mantle convection. Relatively oxidized iron and carbon species at the surface, such as carbonate, Fe2+-and Fe3+-bearing silicatesand oxides, are transported to the deep mantle by subducted slabs and strongly involved inthe redox reactions that generatediamonds [4]. The extent to which the isotopic compositionof C duringdiamond formation recordsredox processes that shaped the Earth is still controversial. Here we report onvibration properties of C andFe at high pressure in carbonates, diamond and Fe3C,based on nuclear resonant inelastic X-ray scattering measurements and density functional theory calculationsand further calculate equilibrium C isotope fractionations among these C-bearing species. Our results demonstrate that redox reactions in subducted slabs could generate eclogitic diamonds with d13C values as low as -41‰ if C in diamonds was sourced from the oxidation of a Fe-C liquid. The large C isotopic fractionation and potentially fast separation between diamonds and a Fe-C melt could enable diamond formation as high as 2%with d13C lower than -40‰.
Abstract: The heat flux across the core-mantle boundary (QCMB) is the key parameter to understand the Earth/s thermal history and evolution. Mineralogical constraints of the QCMB require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (krad) of a realistic lower mantle (pyrolite) in situ using an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to ~3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from ~0.8 W/m/K at 1000 km to ~0.35 W/m/K at the CMB, the latter being ~30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of ~8.5 TW, at odds with present estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo.
Journal of Geophysical Research: Solid Earth, doi:10.1029/ 2019JB017792
Mantle
Core boundary
Abstract: Seismic observations show a reduced P wave velocity gradient layer at the bottom ~280 km of the outer core and a hemispherical dichotomy at the top ~50-200 km of the inner core compared to the one?dimensional Preliminary reference Earth model (PREM). These seismic features manifest physical and chemical phenomena linked to thermal evolution and formation processes of the inner core. We have developed a physical model to explain these seismic features. At the inner?outer boundary, the crystallization of Fe alloy co?exists with the residue melt producing a “snowing” slurry layer (F layer), consistent with observed seismic velocity gradient. Solid Fe alloy crystals accumulate and eventually compact at the top of the inner core, and may exhibit lateral variations in thickness between the east?west hemispheres. Our model can explain the east?west asymmetry observed in the seismic velocity. Our model uses mineral physics and seismological results to provide a holistic view of the physical and chemical processes for the inner?core growth over geological time.
Geochimica et Cosmochimica Acta, Vol. 294, pp. 215-231. pdf
Mantle
bridgmanite
Abstract: The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth’s lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (?-factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its ?-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller ?-factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰.
Trap, P., Faure, P., Lin, M., Bruguier, O., Monie, P.
Contrasted tectonic styles for the Paleoproterozoic evolution of the North Chin a Craton: evidence for a 2.1 Ga thermal and tectonic event in the Fuping Massif.
Journal of Structural Geology, Vol. 30, 9, pp. 1109-1125.
From subcalcic pyropes to uvarovites: experimental crystalllization of Cr-rich garnets in ultramafic systems with presence of Ca bearing hydrous fluid.
Prograde pressure temperature path of jadeite bearing eclogites and associated high pressure low temperature rocks from western Tianshan, northwest China.
The architecture of the HP-UHP Dabie Massif: new insights from geothermobarometry of eclogites and implication for the continental exhumation processes.
Journal of Asian Earth Sciences, Vol. 86, pp. 38-58.
The architecture of the HP-UHP Dabie massif: new insights from geothermobarometry of eclogites, and implication for the continental exhumation processes.
Journal of Asian Earth Sciences, Vol. 86, I, pp. 38-58.
Abstract: Harzburgites with high modal orthopyroxene (generally >23?vol%) in Archean craton, mantle wedge and oceanic lithospheric mantle are considered to be produced by the interaction between Si-rich liquids and rocks. However, the absence of samples from continental margin hinders the recognition whether this process is prevalent. Mantle xenoliths entrained in Miocene basalts from the Thrace Basin, the margin of Eurasian continent, are dominated by harzburgites with anomalously high orthopyroxene modes. These orthopyroxene grains closely associate with spinel and occasionally with clinopyroxene. In these orthopyroxene-spinel associations, orthopyroxene grains can be up to 1?cm in diameter and display high Al2O3 contents (1.41-4.61?wt%) and Mg# values (89.6-92.4), while spinel crystals are anhedral and bud-shaped and are commonly foliated, with a wide variation in Cr# values ranging from 7.8 to 52.7. The Fe2+/Fe3+ vs. TiO2 diagram shows lots of these spinels are “magmatic” (i.e. spinel crystallized from melts). The orthopyroxene grains have LREE diverging from the modelled melting trends, indicating possible metasomatism following partial melting. They are present in elongated shape, cutting across olivine grains and also replacing olivine as surrounding rims. Fine-grained olivine is occasionally enclosed in the orthopyroxene-spinel association. We, therefore, propose that the association of orthopyroxene and spinel developed from the melt/fluid-rock interaction. These features indicate mineral phase transformation from olivine to orthopyroxene, which can be expressed by the equation: ‘Mg2SiO4 (Ol)?+?SiO2?=?Mg2Si2O6 (Opx)’. The observed Al-rich rim of spinel and bud-shaped Al-spinel, suggest sufficient amount of Al in the Si-rich liquids. The mechanism involved here is the consumption of olivine to produce orthopyroxene and spinel as in the equation: ‘Mg2SiO4 (Ol)?+?Al2O3?=?MgSiO3 (Opx)?+?MgAl2O4 (Sp)’. The Si and Al were enriched in the percolating liquids. Both the high-Cr# and low-Cr# spinels with ‘magmatic’ features imply the percolating liquids were multi-staged or inhomogeneous Cr contents in the liquids. This melt/fluid-rock interaction may account for the formation of abundant harzburgites with high orthopyroxene modes in the Eurasian continental margin. Thus, it indicates the reacting harzburgites are prevalent in the lithospheric mantle beneath oceanic crust, Archean craton and mantle wedge, as well as in the continental margin.
Abstract: Harzburgites with high modal orthopyroxene (generally >23?vol%) in Archean craton, mantle wedge and oceanic lithospheric mantle are considered to be produced by the interaction between Si-rich liquids and rocks. However, the absence of samples from continental margin hinders the recognition whether this process is prevalent. Mantle xenoliths entrained in Miocene basalts from the Thrace Basin, the margin of Eurasian continent, are dominated by harzburgites with anomalously high orthopyroxene modes. These orthopyroxene grains closely associate with spinel and occasionally with clinopyroxene. In these orthopyroxene-spinel associations, orthopyroxene grains can be up to 1?cm in diameter and display high Al2O3 contents (1.41-4.61?wt%) and Mg# values (89.6-92.4), while spinel crystals are anhedral and bud-shaped and are commonly foliated, with a wide variation in Cr# values ranging from 7.8 to 52.7. The Fe2+/Fe3+ vs. TiO2 diagram shows lots of these spinels are “magmatic” (i.e. spinel crystallized from melts). The orthopyroxene grains have LREE diverging from the modelled melting trends, indicating possible metasomatism following partial melting. They are present in elongated shape, cutting across olivine grains and also replacing olivine as surrounding rims. Fine-grained olivine is occasionally enclosed in the orthopyroxene-spinel association. We, therefore, propose that the association of orthopyroxene and spinel developed from the melt/fluid-rock interaction. These features indicate mineral phase transformation from olivine to orthopyroxene, which can be expressed by the equation: ‘Mg2SiO4 (Ol)?+?SiO2?=?Mg2Si2O6 (Opx)’. The observed Al-rich rim of spinel and bud-shaped Al-spinel, suggest sufficient amount of Al in the Si-rich liquids. The mechanism involved here is the consumption of olivine to produce orthopyroxene and spinel as in the equation: ‘Mg2SiO4 (Ol)?+?Al2O3?=?MgSiO3 (Opx)?+?MgAl2O4 (Sp)’. The Si and Al were enriched in the percolating liquids. Both the high-Cr# and low-Cr# spinels with ‘magmatic’ features imply the percolating liquids were multi-staged or inhomogeneous Cr contents in the liquids. This melt/fluid-rock interaction may account for the formation of abundant harzburgites with high orthopyroxene modes in the Eurasian continental margin. Thus, it indicates the reacting harzburgites are prevalent in the lithospheric mantle beneath oceanic crust, Archean craton and mantle wedge, as well as in the continental margin.
Proceedings of the National Academy of Sciences, Vol. 78, 10.1073/pnas2110427118 6p. Pdf
Cosmos
melting
Abstract: Refractory oxygen bound to cations is a key component of the interior of rocky exoplanets. Its abundance controls planetary properties including metallic core fraction, core composition, and mantle and crust mineralogy. Interior oxygen abundance, quantified with the oxygen fugacity (fO2), also determines the speciation of volatile species during planetary outgassing, affecting the composition of the atmosphere. Although melting drives planetary differentiation into core, mantle, crust, and atmosphere, the effect of fO2 on rock melting has not been studied directly to date, with prior efforts focusing on fO2-induced changes in the valence ratio of transition metals (particularly iron) in minerals and magma. Here, melting experiments were performed using a synthetic iron-free basalt at oxygen levels representing reducing (log fO2 = ?11.5 and ?7) and oxidizing (log fO2 = ?0.7) interior conditions observed in our solar system. Results show that the liquidus of iron-free basalt at a pressure of 1 atm is lowered by 105 ± 10?°C over an 11 log fO2 units increase in oxygen abundance. This effect is comparable in size to the well-known enhanced melting of rocks by the addition of H2O or CO2. This implies that refractory oxygen abundance can directly control exoplanetary differentiation dynamics by affecting the conditions under which magmatism occurs, even in the absence of iron or volatiles. Exoplanets with a high refractory oxygen abundance exhibit more extensive and longer duration magmatic activity, leading to more efficient and more massive volcanic outgassing of more oxidized gas species than comparable exoplanets with a lower rock fO2.
Doklady Earth Sciences, Vol. 489, 2, pp. 1409-1412. pdf
Russia, Siberia
diamond alluvials
Abstract: Based on a study of diamond grains from placers of the northeastern Siberian Platform, it is shown that certain types of diamonds (rounded dodecahedroids, diamonds of the II and V?VII varieties, according to the classification by Yu.L. Orlov) could have originated from Precambrian sources. “Ancient” diamonds also differ in terms of their sedimentological history: those of varieties V?VII, despite the maximum abrasion resistance, have the maximum degree of rounding, reflecting their more long-term sedimentological history, and, therefore, their ore bodies were likely the most ancient.
Doklady Earth Sciences, Vol. 489, 2, pp. 1449-1452 .pdf
Russia
diamond morphology, CLIPPIR
Abstract: The primary results are presented on the dissolution of plane-faced diamond crystals of octahedral habit in a Fe-Ni-S melt under 3.5 GPa and 1400°C. It was found that the dissolution resulted in the transformation of plane-faced into curve-faced individuals of morphological features characteristic for kimberlite diamonds. It was concluded that the diamond forms as such might have formed in reduced domains of the Earth’s mantle before becoming involved in the kimberlite magma.
Geology of Ore Deposits, Vol. 62, 6, pp. 497-507. pdf
Russia, Yakutia
deposit Yubileinaya
Abstract: An experimental study was carried out on the dissolution of natural octahedral diamonds from the Internatsionalnaya and Yubileinaya kimberlite pipes (Yakutia) in an Fe-S melt at 4 GPa and 1450-1500°C with different sulfur contents (10-25 wt %). It was found that with an increase in sulfur content in the iron melt, the degree of diamond dissolution sharply decreases. The stationary (final) shape of diamond crystal dissolution under the achieved conditions corresponds to an octahedroid with trigonal etching layers, which is confirmed by photogoniometry. Diamonds with similar morphology are common in kimberlite pipes, especially in mantle xenoliths from kimberlites. It was concluded that diamonds with this shape did not undergo natural dissolution in a kimberlite magma, but, similar to flat-faced octahedra, were probably isolated from it in xenoliths. Therefore, the higher the content of octahedroid-shaped diamonds with trigonal layers in a deposit, the smaller the direct influence of an aggressive kimberlite magma on the diamond content.
Koehm, D., Lindenfeld, M., Rumpker, G., Aanyu, K., Haines, S., Passchier, C.W., Sachu, T.
Active transgression faults in rift transfer zones: evidence for complex stress fields and implications for crustal fragmentation processes in the western branch
International Journal of Earth Sciences, Vol. 99, 7, pp. 1633-1642.
Abstract: The Earth’s cratons are traditionally regarded as tectonically stable cores that were episodically buried by thin sedimentary covers. Cratonic crust in southern Finland holds seven post-1.7 Ga tiered unconformities, with remnants of former sedimentary covers. We use the geometries of the tiered unconformities, along with previously dated impact structures and kimberlite and carbonatite pipes, to reconstruct the erosion and burial history of the craton and to derive estimates of depths of erosion in basement and former sedimentary rocks. The close vertical spacing (<200 m) of the unconformities and the survival of small (D ? 5 km) Neoproterozoic and Early Palaeozoic impact structures indicate minor later erosion. Average erosion rates (<2.5 m/Ma) in basement and cover are amongst the lowest reported on Earth. Ultra-slow erosion has allowed the persistence in basement fractures of Phanerozoic fracture coatings and Palaeogene groundwater and microbiomes. Maximum thicknesses of foreland basin sediments in Finland during the Sveconorwegian and Caledonide orogenies are estimated as ~1.0 km and <0.68-1.0 km, respectively. Estimated losses of sedimentary cover derived from apatite fission track thermochronology are higher by factors of at least 2 to 4. A dynamic epeirogenic history of the craton in Finland, with kilometre-scale burial and exhumation, proposed in recent thermochronological models is not supported by other geological proxies. Ultra-slow erosion rates in southern Finland reflect long term tectonic stability and burial of the craton surface for a total of ~1.0 Ga beneath generally thin sedimentary cover.
Abstract: Scientists at ANU have produced the best estimate of Earth's elemental composition which will help them understand how the Earth formed 4.6 billion years ago. The Solar System began as a dense blob in a molecular cloud of hydrogen gas and dust that collapsed under its own gravity, forming the early Sun, Earth and other planets. Co-researcher Associate Professor Charley Lineweaver said the Earth's chemical composition was set at that early stage of formation. "The four most abundant elements - iron, oxygen, silicon and magnesium - make up more than 90 per cent of the Earth's mass, but working out exactly what the Earth is made of is tricky," said Dr Lineweaver from the Research School of Earth Sciences and the Research School of Astronomy and Astrophysics at ANU. "Seismological studies of earthquakes inform us about the Earth's core, mantle and crust, but it's hard to convert this information into an elemental composition. "Our deepest drilling has only scratched the surface down to 10 kilometres of our 6,400 kilometre radius planet. Rocks at the surface only come from as deep as the upper mantle." The research is published in the international journal Icarus and is available here. Lead author ANU PhD scholar Haiyang Wang said the team made the most comprehensive estimates of the Earth's composition based on a meta-analysis of previous estimates of the mantle and core, and a new estimate of the core's mass. "Our work focused on getting realistic uncertainties so that our reference model can be used in future comparisons of the Earth with the Sun, or with Mars or with any other body in the Solar System," said Mr Wang from the ANU Research School of Astronomy and Astrophysics. Co-researcher Professor Trevor Ireland from the ANU Research School of Earth Sciences said planetary scientists would find many uses for this new composition record. "This will have far-reaching importance, not only for planetary bodies in our Solar System but also other star systems in the universe," he said.
Proceedings of the Royal Society B: Biological sciences, Vol. 284. no 1856, pp.
Mantle
pangea
Abstract: The break-up of the supercontinent Pangaea around 180 Ma has left its imprint on the global distribution of species and resulted in vicariance-driven speciation. Here, we test the idea that the molecular clock dates, for the divergences of species whose geographical ranges were divided, should agree with the palaeomagnetic dates for the continental separations. Our analysis of recently available phylogenetic divergence dates of 42 pairs of vertebrate taxa, selected for their reduced ability to disperse, demonstrates that the divergence dates in phylogenetic trees of continent-bound terrestrial and freshwater vertebrates are consistent with the palaeomagnetic dates of continental separation.
Abstract: The Bayan Obo ore deposit in Inner Mongolia, North China, the largest-known rare earth element (REE) deposit in the world, is closely associated with carbonatite dykes. Scarce zircon grains, with a wide range of ages and diverse origins, have been extracted from the Wu dyke, a REE-enriched calcitic carbonatite dyke 2?km from the East Ore Body of the Bayan Obo deposit. Three zircon populations were identified based on ages and trace element compositions: 1) Captured zircons with Paleoproterozoic and Archean ages. These zircons have REE patterns and moderate Th/U ratios similar to zircon with silicate inclusions from basement igneous rocks, which have been recognized as contaminants from wall rocks. 2) Carbonatite magmatic zircons with Mesoproterozoic ages. These zircons have high to extremely high Th/U ratios (13-1600), a characteristic signature of the Bayan Obo deposit. Two zircon grains yielded concordant 206Pb/238U ages (1.27?±?0.11?Ga???1.42?±?0.18?Ga) and 208Pb/232Th age (1.26?±?0.20?Ga) with calcite inclusions, indicating that the Wu dyke was emplaced at ca. 1.34?Ga, which coincides with a worldwide generation of Mesoproterozoic kimberlites, lamprophyres, carbonatites, and anorogenic magmatism. 3) Hydrothermal zircons with Caledonian and Triassic ages. The Caledonian zircon has 206Pb/238U age of 381?±?4?Ma and 208Pb/232Th age of 367?±?14?Ma with dolomite inclusion. These evidences are consistent with multiple stages of mineralization, Mesoproterozoic calcite carbonatite magmatism interacted by protracted fluxing of subduction-released Caledonian fluids during the closure of the Palaeo-Asian Ocean, coupled with interaction with the mantle wedge and metasomatism of overlying sedimentary carbonate.
Abstract: The Bayan Obo ore deposit in Inner Mongolia, North China, the largest-known rare earth element (REE) deposit in the world, is closely associated with carbonatite dykes. Scarce zircon grains, with a wide range of ages and diverse origins, have been extracted from the Wu dyke, a REE-enriched calcitic carbonatite dyke 2?km from the East Ore Body of the Bayan Obo deposit. Three zircon populations were identified based on ages and trace element compositions: 1) Captured zircons with Paleoproterozoic and Archean ages. These zircons have REE patterns and moderate Th/U ratios similar to zircon with silicate inclusions from basement igneous rocks, which have been recognized as contaminants from wall rocks. 2) Carbonatite magmatic zircons with Mesoproterozoic ages. These zircons have high to extremely high Th/U ratios (13-1600), a characteristic signature of the Bayan Obo deposit. Two zircon grains yielded concordant 206Pb/238U ages (1.27?±?0.11?Ga???1.42?±?0.18?Ga) and 208Pb/232Th age (1.26?±?0.20?Ga) with calcite inclusions, indicating that the Wu dyke was emplaced at ca. 1.34?Ga, which coincides with a worldwide generation of Mesoproterozoic kimberlites, lamprophyres, carbonatites, and anorogenic magmatism. 3) Hydrothermal zircons with Caledonian and Triassic ages. The Caledonian zircon has 206Pb/238U age of 381?±?4?Ma and 208Pb/232Th age of 367?±?14?Ma with dolomite inclusion. These evidences are consistent with multiple stages of mineralization, Mesoproterozoic calcite carbonatite magmatism interacted by protracted fluxing of subduction-released Caledonian fluids during the closure of the Palaeo-Asian Ocean, coupled with interaction with the mantle wedge and metasomatism of overlying sedimentary carbonate.
Abstract: The oxygen fugacity of the upper mantle is 3-4 orders of magnitude higher than that of the lower mantle and this has been attributed to Fe2 + disproportionating into Fe3 + plus Fe0 at pressures > 24 GPa. The upper mantle might therefore have been expected to have evolved to more oxidizing compositions through geological time, but it appears that the oxygen fugacity of the upper mantle has remained constant for the last 3.5 billion years. Thus, it indicates that the mantle has been actively buffered from the accumulation of Fe3 +, and that this is linked to oxidation of diamond to carbonate coupled with reduction of Fe3 + to Fe2 +. When subducted plates penetrate into the lower mantle, compensational upwelling transports bridgmanite into the transition zone, where it breaks down to ringwoodite and majorite, releasing the ferric iron. The system returns to equilibrium through oxidation of diamond. Early in Earth history, diamond may have been enriched at the base of the transition zone in the Magma Ocean, because it is denser than peridotite melts at depths shallower than 660 km, and it is more buoyant below. Ongoing oxidation of diamond forms carbonate, leading to relatively high carbonate concentrations in the source of ocean island basalts.
Abstract: Carbonatitic magmatism plays a significant role in Earth's carbon cycle, which is also a lithoprobe of crust-mantle interaction, mantle metasomatism and partial melting. Due to different mineral assemblages and geochemical compositions, and diverse tectonic settings, the origin of carbonatite has long been debated. At subduction zones, sediments (including carbonates) are subducted into the mantle with the downgoing oceanic slab. However, the detailed mechanism of how subducted carbonates contribute to carbonatitic magmatism remains unclear. Here we present geochronological, geochemical and isotopic study on the Taohuala Mountain carbonatite at the southern margin of the Alxa Block, North China Craton. The classification of carbonatite from the Taohuala Mountain relies strongly on the observations of obvious intrusion contact relationships and flow structures in field outcrop. The Taohuala Mountain carbonatite has SiO2 ranging from 2.37 wt.% to 11.45 wt%, high CaO (45.93-53.86 wt%) and low MgO (0.51-4.39 wt%), and is characterized by enrichment of LILE (Ba, Sr), depletion of HFSE (Nb, Ta, Zr, Hf), and slightly negative Ce and Eu anomalies. Carbonates in the samples have high 87Sr/86Sr (0.70686-0.70694) and low 143Nd/144Nd (0.511635-0.511924). Remarkably, the highly fractionated ?18OVSMOW (11.83-25.92‰) indicates components of both sedimentary and mantle origin. Detailed zircon in situ U-Pb dating and oxygen isotope analysis exhibit contrast ages and ?18OVSMOW from core to rim, i.e., old ages (mainly > 800 Ma), high Th/U (mainly > 0.5) and low ?18OVSMOW (6.37-11.44‰) in cores (inherited), whereas young ages (~ 400 Ma), low Th/U (mainly < 0.01) and high ?18OVSMOW (20.04-24.54‰) in rims, suggesting that the Taohuala Mountain carbonatite may have been generated from melting of subducted sedimentary carbonates. Considering all these evidences, and that the collision along Qilian Mountains was older than the carbonatite, we propose that a large volume of sedimentary carbonates subducted and remained in the lithospheric mantle under the Alxa block during the closure of the Paleo-Qilian Ocean. Subsequently, the carbonatite was formed by melting of carbonates with minor contributions from the mantle during the breakoff or rollback of the Paleo-Asian oceanic slab.
Abstract:
Carbonatitic magmatism plays a significant role in outgassing carbon from mantle and the formation of rare earth element (REE), rare metal (e.g., Nb and Th) and other types of deposits. The mechanism of REE mineralization associated with carbonatite have been widely studied. However, it is hard to understand U-Nb mineralization without Th enrichment associated with carbonatite. Here we report a carbonatite-hosted U-Nb deposit in Huayangchuan, located in the north Qinling Orogenic Belt. Field observation, mineralogy and geochemical analysis on a suite of drillhole samples were conducted to decipher the mineralization mechanism and its relationship with carbonatite. Huayangchuan carbonatite samples mainly consist of calcite and augite with small volume of accessory minerals (e.g., allanite, fluorapatite, barite and celestite). Betafite [(Ca,U)2(Ti,Nb,Ta)2O6(OH)] is the major ore-bearing mineral in Huayangchuan deposit. The carbonatite shows high CaO, low MgO and alkali contents, which should be products to be differentiated from primary carbonatite (high MgO and alkali contents). The immiscibility and crystallization processes could explain the high CaO/(CaO + MgO + FeO) ratios and the enrichment of LILE. Numerical modeling also indicates positive ?18OSMOW (7.29 to 15.53‰) and negative ?13CPDB (?5.26 to ?10.08‰) shifts are induced by reduced sediments assimilation from source consistent with there being enriched Sr-Nd and low Mg isotopic compositions. LA-ICP-MS zircon U-Pb dating of Huayangchuan carbonatite yielded Triassic ages of 229 ± 3 Ma, which corresponds to the post-collision stage of Qinling Orogen during the middle-late Triassic. We then proposed that the recycling of subducted sediments and later re-melting of those materials in shallow mantle generated the Huayangchuan carbonatite and subsequently formed the Huayangchuan deposit. Fluorine concentration decrease, caused by fluorapatite crystallization, ultimately resulted in betafite mineralization.
Geochimica et Cosmochimica Acta, Vol. 197, pp. 356-377.
China
Picrite
Abstract: Iron-rich silicate melts in the Earth’s deep mantle have been seismologically and geochemically inferred in recent years. The origin of local enrichments in iron and low-velocity seismic anomalies that have been detected in dense mantle domains are critical to understanding the mantle’s evolution, which has been canonically explained by long-term chemical reactions between the Earth’s silicate mantle and its liquid iron outer core. However, the Pleistocene alkaline ferropicrites (?0.73 Ma) from Wudi, North China, show chemical and Sr-Nd-Os isotopic features that suggest derivation from the preferential melting of silica-deficient eclogite, a lithology of delaminated mafic lower continental crust that had stagnated at mid-upper mantle depths during the Mesozoic decratonization of the North China block. These rocks are characterized by substantial enrichment in iron (14.9-15.2 wt% Fe2O3), relative depletion in silica (40-41 wt% SiO2) and decoupled Y and heavy rare earth element (HREE) compositions. These ferropicrites have particularly higher Y/Yb ratios than the other Cenozoic basalts from North China. The pressure-dependent compatibility of Fe, Y and Yb in eclogitic garnet can adequately explain the Fe-enrichment and Y-HREE decoupling of the Wudi ferropicrites and indicates that the eclogites were melted at pressures of 5-8 GPa, as also constrained by previous high-P-T experiments. This melting depth ties together a seismically imaged high-velocity anomaly that extends from 150 km to 350 km in depth under the study area, which has been commonly interpreted as evidence for the stagnation of the missing, delaminated continental lithosphere. Our findings provide an alternative mechanism to produce an extremely iron-rich mantle reservoir in addition to core-mantle interaction. Iron-rich silicate melts that form by this process are likely to be denser than the ambient mantle peridotite (and therefore drive flow downward) and may play a more significant role in the deep-mantle geophysical and geochemical diversities than previously considered.
Abstract: Carbonatites are few but significant to understand carbon recycling of the earth, the crust-mantle interactions, deep mantle magmatism and regional continental evolution. The Lijiahe carbonatite intrusion, located at the Micangshan Mountains along the NW margin of the Yangtze block, South China was emplaced into the Paleoproterozoic strata, but the timing of the igneous event has long been unknown. Dating by U-Pb apatite was carried out by this work, and it gives an age of 766 ± 11 Ma (MSWD=0.15) for the carbonatitic magmatism of the region. The carbonatite comprises mainly of calcite, magnitite and apatite with minor minerals of salite, biotite, tremolite, hornblende and muscovite and accessary minerals of pyrrhotite, silver marcasite, niobite, spinel and zircon. Its spatial distribution was obviously controlled by regional tectonics. Besides, ultra-alkaline silicate intrusive complex in the region has been reported by us and other works, and mostly consists of iolite, urtite and jacupirangite with ages of ~890-875 Ma. Furthermore, a large number of gabbro and diorite plutons are found in the Micangshan Mountains and dated at ~780-760 Ma. NW margin of the Yangtze block is connected with the South Qinling orogenic belt generally thought having an affinity of the Yangtze block during the Neoproterozoic. Our works revealed that the South Qinling is discriminated from the NW Yangtze by intensive ~680 Ma igneous activities which are poorly reported in the interior of South China. Given that a ~815 Ma collision between the South Qinling ribbon and NW Yangtze margin is recognized by our recent work, the regional massive mantle-derived magmatism including the Lijiahe carbonatitic pluton is explained to indicate a drifting of South Qinling terrane from the NW margin of Yangtze block along previous weak-tectonic zones during the Rodinia breakup caused by continental rifting.
Abstract: A lack of precise age constraints for Neoproterozoic strata in the northwestern United States (Washington State), including the Buffalo Hump Formation (BHF), has resulted in conflicting interpretations of Rodinia amalgamation and breakup processes. Previous detrital zircon (DZ) studies identified a youngest ca. 1.1 Ga DZ age population in the BHF, interpreted to reflect mostly first-cycle sourcing of unidentified but proximal magmatic rocks intruded during the amalgamation of Rodinia at ca. 1.0 Ga. Alternatively, the ca. 1.1 Ga DZ population has been suggested to represent a distal source with deposition occurring during the early phases of Rodinia rifting, more than 250 m.y. after zircon crystallization. We combined conventional laser-ablation split-stream analyses of U-Pb/Lu-Hf isotopes in zircon with a method of rapid (8 s per spot) U-Pb analysis to evaluate these opposing models. Our study of ?2000 DZ grains from the BHF identified for the first time a minor (?1%) yet significant ca. 760 Ma population, which constrains the maximum depositional age. This new geochronology implies that the BHF records early rift deposition during the breakup of Rodinia and correlates with sedimentary rocks found in other late Tonian basins of southwestern Laurentia.
The continental crustal age distribution: methods of determining mantle seperation ages from samarium-neodymium (Sm-Nd) isotopic dat a and application to the southwest UnitedStates
Journal of Geophysical Research, Pt. B., Vol. 96, No. B2, February 10, pp. 2071-2088
The provenance of northern Kalahari Basin sediments and growth history of the southern Congo Craton reconstructed by U-Pb ages of zircons from recent river sands.
International Journal of Earth Sciences, Vol. 103, 2, pp. 579-595.
Hofmann, M., Linnemann, U., Hoffmann, K-H., Gerdes, A., Eckelmann, K., Gartner, A.
The Namuskluft and Dreigratberg sections in southern Namibia ( Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana.
International Journal of Earth Sciences, Vol. 103, pp. 1187-1202.
Journal of Asian Earth Science, Vol. 200, 104432, 18p. Pdf
India
deposit - Munnar
Abstract: Metacarbonate assemblages in high-grade metamorphic terranes often pose challenges when trying to distinguish between mantle-derived carbonatite and sedimentary carbonate protoliths. We present a study of granulite-facies metacarbonate samples of the putative Munnar carbonatite described as decimeter-thick dikes and veins, and layers of a meter-thick metacarbonate and calc-silicate assemblage, respectively. Thin sections of the metacarbonate dike samples show absence of pyrochlore and ubiquitous scapolite, titanite, wollastonite, and detrital zircons are compatible with impure limestone protoliths. Nd and Sr isotope compositions indicate protoliths with Paleoproterozoic crustal residence times which contrast the mantle sources of Indian and global carbonatites. Trace-element patterns display the characteristics of upper crust, and Ce- and Y-anomalies in a number of samples suggest protolith formation under marine conditions. Carbon and oxygen isotope compositions of the metacarbonate samples interlayered with calc-silicate rocks are similar to those in marine limestone. The metacarbonate dikes, however, show mantle-like compositions which are interpreted as reflecting equilibration with mantle-derived CO2 during granulite-facies metamorphism. The dikes yielded a U-Pb zircon crystallization age of 1020 ± 70 Ma and a cross-cutting quartz syenite, thought to be cogenetic, a magmatic age of 620 ± 35 Ma; the hosting gneiss provided a magmatic age of 2452 ± 14 Ma. We conclude that the layered metacarbonate and calc-silicate rocks represent a former marine limestone and marl sequence and the metacarbonate dikes and veins small-volume melts of crust-derived carbonate-rich sediment.
Geochemistry: Exploration, Environment, Analysis, Vol. 16, 3-4, pp. 193-212.
Canada, Ontario
Alkalic
Abstract: Mineral exploration is increasingly taking advantage of real time techniques that dramatically reduce the costs and time taken to obtain results compared to traditional analytical methods. Portable X-ray fluorescence (pXRF) is now a well-established technique that is used to acquire lithogeochemical data. To date, however, benchtop scanning electron microscopes, equipped with energy dispersive systems (bSEM-EDS) have received little attention as a possible mineral exploration tool. This study examines the utility of combining pXRF and bSEM-EDS to characterize the igneous stratigraphy and its relationship to Cu-Pd mineralization in a drill hole at the Four Dams occurrence, located within the Eastern Gabbro assemblage of the Coldwell Alkaline Complex, Canada. The first part of this study compares field portable and laboratory techniques. Seventy-two powdered samples analysed by pXRF are compared with traditional major elements analysed by inductively coupled atomic emission spectroscopy (ICP-AES) and trace elements by inductively coupled plasma spectrometry (ICP-MS), and the compositions of 128 olivine, clinopyroxene and plagioclase grains analysed by bSEM-EDS are compared with traditional electron microprobe data. Our results show that each portable technique yields results similar to their lab-based counterparts within the analytical capabilities and precisions of the respective instruments. The second part presents a case study for the application of pXRF and bSEM-EDS to resolve questions related to igneous stratigraphy as an aid to mineral exploration in a complicated geological setting. A major problem for Cu-Pd exploration in the Coldwell Complex of NW Ontario is that the oxide-rich units that host Cu-Pd mineralization in the Marathon Series are petrographically similar to the barren oxide-rich units in the Layered Series. However, the mineralized units are geochemically distinctive. Our results show that the mineralized Marathon Series can be distinguished from the barren Layered Series, including oxide-rich units of both, by combinations of P2O5, Ba, Zr and V/Ti values, determined by pXRF, combined with plagioclase, olivine or clinopyroxene compositions measured by bSEM-EDS. The combination of pXRF and bSEM-EDS thus shows considerable promise as an exploration technique.
Abstract: Magnetite (mag)-ilmenite (ilm) intergrowths are more common than mag-ulvöspinel (usp) intergrowths in mafic-ultramafic Ni-Cu-PGE systems, yet the former has no known solid solution. The most accepted model for the formation of mag-ilm intergrowths in terrestrial environments is fluid-induced oxidation of mag-usp assemblages by oxygen in water. In this study, we re-examine this model in light of the fact that crustal fluids have very low pO2 and that mag-ilm intergrowths commonly occur in rocks that show little or no evidence of hydrothermal alteration. We also characterize the chemical changes that occurred during the formation of mag-ilm intergrowths and how they affect the use of Fe-Ti oxide chemistry for petrogenesis and mineral exploration. In the Eastern Gabbro, Coldwell Complex, a continuum of Fe-Ti oxide intergrowths occur ranging from cloth (mag-usp) to trellis (mag-ilm) types. Trellis-textured intergrowths have higher bulk Fe3+:Fe2+ ratios and are predominantly enriched not only in some multivalent (Ge, Mo, W, Sn) elements, but also in Cu and Ga, consistent with their formation via oxidation by a metal-rich fluid. These compositional changes are significant relative to typical elemental abundances in Fe-Ti oxides and could potentially lead to erroneous interpretations regarding primary magmatic processes if they are not taken into consideration. The irregular distribution of the intergrowths throughout the Eastern Gabbro suggests that different rock series and mineralized zones experienced variable degrees of fluid-induced oxidation. It is proposed that C in CO2 rather than O2 in water could potentially be an important oxidizing agent in mafic systems:
9Fe2+2TiO4+0.75CO2+1.5H2O?9Fe2+TiO3+3Fe3+2Fe2+O4+0.75CH4. The applicability of this model is supported by the common occurrence of CO2 and CH4 in fluid inclusions in mafic rocks.
Abstract: The Eastern Gabbro of the alkaline Coldwell Complex, Canada, represents a Ni-poor conduit-type system that comprises two rock series, the Layered Series and Marathon Series, which intruded into a metabasalt package. Based on distinct variations in magnetite compatible (e.g., Ni, Cr) and incompatible (e.g., Sn, Nb) elements in Fe-Ti oxide intergrowths, the metabasalts, Layered Series, and Marathon Series must have crystallized from magmas that originated from compositionally distinct sources. Of these rock units, the metabasalts crystallized from a more primitive melt than the Layered Series as Fe-Ti oxides in the former have higher concentrations of magnetite-compatible elements. Unlike the metabasalts and Layered Series, the Marathon Series crystallized from multiple, compositionally distinct magmas as Fe-Ti oxides in this series exhibit large variations in both magnetite compatible and incompatible elements. Accordingly, the various rock types of the Marathon Series cannot be related by fractional crystallization of a single batch of magma. Rather, the magmas from which the rock types crystallized had to have interacted to variable degrees with a late input of more primitive melt. The degree of this magma interaction was likely controlled by the geometry of the conduit and the location of emplacement given that Fe-Ti oxides in the oxide-rich rocks occur in pod-like bodies and exhibit no compositional evidence for magma mixing. Mirrored variations in magnetite compatible and incompatible elements in Fe-Ti oxides in the Footwall Zone, Main Zone, and W Horizon of the Marathon Cu-PGE deposit indicate that these zones could not have formed from a single, evolving magma, but rather multiple batches of compositionally distinct magmas. Fe-Ti oxides exhibit no compositional difference between those hosted by barren and mineralized rock. This is likely because sulfide liquated at depth in all of the magmas from which the Marathon Series crystallized. The composition of Fe-Ti oxides in the Eastern Gabbro fall outside of the compositional fields for Ni-Cu mineralization defined by Dupuis and Beaudoin (Mineral Deposita 46:319-335, 2011) and Ward et al. (J Geochem Explor 188:172-184, 2018) demonstrating that their discrimination diagrams can distinguish between Ni-rich and Ni-poor systems that contain disseminated and massive sulfides.
Abstract: Apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated (>1000?m above sea level) but low-relief Tanzanian Craton. Coeval episodes of sedimentation documented within adjacent Paleozoic to Mesozoic basins of southern Tanzania and the Congo basin of the Democratic Republic of Congo indicate that most of the cooling in the basement rocks in Tanzania was linked to erosion. Basement samples were from an exploration borehole located within the craton and up to 2200?m below surface. Surface samples were also analyzed. AFT dates range between 317?±?33?Ma and 188?±?44?Ma. Alpha (Ft)-corrected AHe dates are between 433?±?24?Ma and 154?±?20?Ma. Modeling of the data reveals two important periods of cooling within the craton: one during the Carboniferous-Triassic (340-220?Ma) and a later, less well constrained episode, during the late Cretaceous. The later exhumation is well detected proximal to the East African Rift (70?Ma). Thermal histories combined with the estimated geothermal gradient of 9°C/km constrained by the AFT and AHe data from the craton and a mean surface temperature of 20°C indicate removal of up to 9?±?2?km of overburden since the end of Paleozoic. The correlation of erosion of the craton and sedimentation and subsidence within the Congo basin in the Paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far-field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes.
Fluids in deeply subducted continental crust: petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu Orogen, eastern China.
Geochimica et Cosmochimica Acta, Vol. 72, 13, July 1, pp. 3200-3228.
Low temperature eclogites and eclogitic schists in Mn-rich metabasites in Ward Creek, California: Mn and iron effects on the transition blueschist andeclogites
Journal of Petrology, Vol. 32, No. 2, April pp. 275-302
Limiting effect of UHP metamorphism on length scales of oxygen, hydrogen and argon isotope exchange: an example from the Qinglongshan UHP eclogites, Sulu Terrain.
International Geology Review, Vol. 47, 7, pp. 716-749.
Epidote rich talc kyanite phengite eclogites, Sulu terrane, eastern China: P T fo2 estimates and the significance of epidote talc assemblage in eclogite.
Song, S., Zhang, L., Chen, J., Liou, J.G., Niu, Y.
Sodic amphibole exsolutions in garnet from garnet-peridotite, North Qaidam UHP belt, NW China: implications for ultradeep origin and hydroxyl defects in mantle garnets.
Liu, F.L., Gerdes, A., Liou, J.G., Xue, H.M., Liang, F.H.
SHRIMP U Pb zircon dating from Sulu Dabie dolomitic marble, eastern China: constraints on prograde, ultrahigh pressure and retrograde metamorphic ages.
Journal of Metamorphic Geology, Vol. 24, 7, Sept. pp. 569-589.
SHRIMP U Pb zircon dating of the Rongcheng eclogite and associated peridotite: new constraints for UHP metamorphism of mantle derived mafic ultramafic bodies
Geological Society of America Special Paper, No. 403, pp. 115-126.
Identification of UHP and non-UHP orthogneisses in the Sulu UHP terrane, eastern China: evidence from SHRIMP U-Pb dating of mineral inclusion bearing zircons.
International Geology Review, Vol. 48, 12, pp. 1067-1086.
Ecologite and carpholite bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Paleozoic cold oceanic subduction and water transport into
Journal of Metamorphic Geology, Vol. 25, 5, pp. 547-563.
Mineral REE ad Lu Hf isotope geochemistry of zircon in the mantle - derived eclogite from Donghai the Sulu UHP terrane: new constraints for the origin of eclogite.
Frontiers in Mineral Sciences 2007, Joint Meeting of Mineralogical societies Held June 26-28, Cambridge, Abstract Volume p. 250.
Mineral REE ad Lu Hf isotope geochemistry of zircon in the mantle - derived eclogite from Donghai the Sulu UHP terrane: new constraints for the origin of eclogite.
Frontiers in Mineral Sciences 2007, Joint Meeting of Mineralogical societies Held June 26-28, Cambridge, Abstract Volume p. 250.
Chemical composition and ultrahigh P metamorphism of garnet peridotites from the Sulu UHP terrane, China: investigation of major, trace elements and Hf isotopes
Chemical composition and ultrahigh P metamorphism of garnet peridotites from the Sulu UHP terrane, China: investigation of major trace elements and Hf isotopes.
Chemical Geology, Vol. 255, 1-2, Sept. 30, pp. 250-264.
Zhang, Z.M., Schertl, H.P., Wang, J.L., Shen, K., Liou, J.G.
Source of coesite inclusions within inherited magmatic zircon from Sulu UHP rocks, eastern China, and their bearing for fluid rock interaction and SHRIMP dating.
Journal of Metamorphic Geology, Vol. 27, 4, pp. 317-333.
An AEM study of garnet clinopyroxenite from the Sulu ultrahigh pressure terrane: formation mechanisms of oriented ilmenite, spinel, magnetite, amphibole and
Contributions to Mineralogy and Petrology, in press available, 14p.
Zircon as the best mineral for P-T time history of UHP metamorphism: a review on mineral inclusions and U-Pb SHRIMP ages of zircons from the Dabie Sulu UHP rocks.
Journal of Asian Earth Sciences, Vol. 40, 1, pp. 1-39.
An AEM study of garnet clinopyroxenite from the Sulu ultrahigh pressure terrane: formation mechanisms of oriented ilmenite, spinel, magnetite, amphibole garnet
Contributions to Mineralogy and Petrology, Vol. 161, 6, pp. 901-920.
Zhang, Z.M., Dong, X., Liou, J.G., Liu, F., Wang, W., Yui, F.
Metasomatism of garnet periodite from Jiangzhuang, Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of cont. lithosphere
Journal of Metamorphic Geology, in press available
Zhang, Z.M., Dong, X., Liou, J.G., Liu, F., Wang, W., Yui, F.
Metasomatism of garnet peridotite from Jiangzhuang, southern Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of continental
Journal of Metamorphic Geology, Vol. 29, 9, pp. 917-937.
Liou, J.G., Zhang, R., Liu, F., Zhang, Z., Ernst, W.G.
Mineralogy, petrology, U-Pb geochronology, and geologic evolution of the Dabie Sulu classic ultrahigh pressure metamorphic terrane, east-central China.
American Mineralogist, Vol. 97, no. 10, pp. 1533-1543.
Russian Geology and Geophysics, Vol. 63, 3, pp. 245-254.
Russia
deposit - Nakyn
Abstract: The paper presents the results of studies of diamonds from Early Jurassic sediments making up the Nyurbinskoe buried placer of the Nakyn kimberlite field, unique in diamond reserves. The main task is to identify diamond distribution patterns in the deposits of the Dyakhtar Stratum (lower deposit) and the Ukugut Suite (upper deposit) within the placer. A comparative analysis of the typomorphic features of diamonds from the upper and lower deposits of the placer was carried out. Variations in the contents of crystals with certain properties that form the image of a diamond-bearing geologic object have been revealed. The zonal distribution of diamonds by characteristics in sedimentary deposits, regardless of their age, has been established. The properties of diamonds and their associations change within the placer, which is due to their redeposition during the Early Jurassic sedimentation.
Mineralogical and palynological substantiation of the existence of remedial carboniferous intermediate reservoirs of kimberlite minerals in the northMalo-Botuobinsk
Doklady Academy of Sciences Akademy Nauk SSSR, (Russian), Vol. 288, No. 6, pp. 1453-1456
Mineralogic and playnologic proof of the Pre-middle Carboniferous age of intermediate kimberlitic mineral reservoirs in the northern part of the Malaya
Doklady Academy of Science USSR, Earth Science Section, Vol. 288, No. 1-6, pp. 102-105
Abstract: The metamorphic core of the Betic-Rif orogenic chain (Alboran Domain) is made up of lower crustal rocks forming the envelope of the Ronda (Spain) and Beni Bousera (Morocco) peridotites. The deepest sections of the crustal envelopes are made of migmatitic granulites associated with diffuse acidic magmatic products, making these exposure and ideal site to investigate the textural and petrological connection between crustal anatexis and granite magmatism in the contintental crust. However, still debated is the timing of intracrustal emplacement of the peridotite bodies, with models proposing either Alpine (early Miocene) or Hercynian ages, and still uncertain is the linkage between peridotite emplacement and crustal anatexis. In this study, by combining rock textures with whole-rock geochemistry, metamorphic thermobarometry, the U-Pb zircon geochronology and the analysis of the garnet and zircon REE chemistry, we document the P-T-t evolution of the granulite facies migmatites that form the immediate envelope of the Beni Bousera peridotites of the Rif belt. A main episode of Permo-Carboniferous (ca. 300-290?Ma) deep crustal anatexis, melt extraction and migration is documented that we link to the crustal emplacement of the Beni Bousera peridotites during collapse of the Hercynian orogen. Correlation at a regional scale suggests that the Beni-Bousera section can be tentatively correlated with the pre-Alpine (Permo-Carboniferous) basement units tectonically interleaved within the orogenic structure of the Alpine chain. The results of this study provide ultimate constraints to reconstruct the tectono-metamorphic evolution of the Alboran Domain in the Western Mediterranean and impose re-assessment of the modes and rates through which Alpine orogenic construction and collapse occurred and operated in the region.
Abstract: Geochemical and petrological characteristics of lamprophyre dykes at Kalagalla intruded into the auriferous schistose rocks of the Ramagiri- Penakacherla Schist Belt, Anantapur district, Andhra Pradesh, India are presented here. The Kalagalla lamprophyre (KGL) is a melanocratic rock exhibiting typical knobby or pustular texture on the surface. The microtextures and mineralogy typical of lamprophyres are obscured by metamorphism; however, it exhibits porphyritic, nemato-granoblastic texture representative of greenschist facies of metamorphism. The rock is sheared and possesses several globules formed by polycrystalline aggregates of calcite rimmed by coronitic subhedral plagioclase and biotite, evidencing its mantle-magmatic origin. The mineral assemblages noticed in thin-sections include amphibole, plagioclase, biotite, phlogopite and calcite ocelli as essential while apatite, zircon, magnetite, ilmenite, Ni-bearing chalcopyrite and pyrite as accessory phases. The SEM-EDS investigation on the accessory minerals revealed accessory sulphide and silicate phases like As-free pyrite, haematitised Ni-bearing chalcopyrite and Ni-As-Co- minerals indicative of sulphidation associated with greenstone auriferous lodes, along with silicates like LREE-bearing titanite partially transformed into leucoxene and oxide phases like magnetite altered to goethite at places. Based on mineral chemistry, whole rock geochemistry, presence of amphibole and dominance of plagioclase, the KGL is classified as a calc-alkaline variety in general and as spessartite in particular possessing shoshonitic affinity. No anomalous chemical composition is noticed in the ocellar calcite. The LREE-bearing titanite appears to be the contributor of LREE enrichment. The high Mg# (77- 79), Ni (153-162 ppm) and Cr (380-470 ppm) support a mantle source. The absence of Eu anomaly reflects lack of plagioclase fractionation. The high Zr/Hf ratio (163-202) indicates absence of crustal contamination and contribution of magmatic carbonate at the source to form ocelli as product of late-stage liquid silicate-carbonate immiscibility of segregation mechanism. The trace and REE patterns (?REE: 326-343 ppm, LREE>HREE) indicate involvement of residual garnet at the source presumably enriched in phlogopite in a ‘subduction-related’ environment.
Abstract: The Earth’s mantle is heterogeneous as a result of early planetary differentiation and subsequent crustal recycling during plate tectonics. Radiogenic isotope signatures of mid-ocean ridge basalts have been used for decades to map mantle composition, defining the depleted mantle endmember. These lavas, however, homogenize via magma mixing and may not capture the full chemical variability of their mantle source. Here, we show that the depleted mantle is significantly more heterogeneous than previously inferred from the compositions of lavas at the surface, extending to highly enriched compositions. We perform high-spatial-resolution isotopic analyses on clinopyroxene and plagioclase from lower crustal gabbros drilled on a depleted ridge segment of the northern Mid-Atlantic Ridge. These primitive cumulate minerals record nearly the full heterogeneity observed along the northern Mid-Atlantic Ridge, including hotspots. Our results demonstrate that substantial mantle heterogeneity is concealed in the lower oceanic crust and that melts derived from distinct mantle components can be delivered to the lower crust on a centimetre scale. These findings provide a starting point for re-evaluation of models of plate recycling, mantle convection and melt transport in the mantle and the crust.
American Geophysical Union, Fall Meeting. , V23A-05 1p.
Mantle
geophysics
Abstract: Variations in radiogenic isotopes in mid-ocean ridge basalts (MORB) are interpreted to reflect the presence of enriched and depleted mantle components in their source regions and have been used to infer the abundance and time scales of crustal recycling. However, MORB are homogenized via magma mixing prior to eruption and may not capture the full heterogeneity of melts generated in their upper mantle source. Here we show that primitive cumulate minerals, formed by crystallization of mantle melts in the lower crust, retain the signature of the recycled material. We performed high spatial resolution Nd and Sr isotopic analyses on clinopyroxene and plagioclase of gabbroic cumulates from the Atlantis massif, located on a depleted ridge segment on the northern Mid-Atlantic Ridge, and compared these data with whole rock isotopic compositions of diabase and microgabbros collected on the same core, associated basalts flows, and MORB data from the literature. We find that cumulate minerals: (1) are significantly more isotopically heterogeneous than the associated diabase and lavas, exceeding the range of 143Nd/144Nd in MORB by a factor of seven; and (2) contain the full Nd isotopic heterogeneity of all of North Atlantic MORB. Furthermore, we find that isotopic heterogeneity occurs down to the sample scale, with plagioclase and clinopyroxene from individual samples commonly not in isotopic equilibrium. We further demonstrate that the MORB and cumulate mineral data can be reconciled with constant high magnitude, small length scale heterogeneity through the North Atlantic upper mantle, with limited magma mixing in the mantle and extensive mixing in the oceanic crust.The isotopic heterogeneity revealed in the lower oceanic crust provides strong evidence that MORB is not an accurate representation of the heterogeneity of its mantle source. Hence, the true isotopic variation of the upper mantle requires rigorous further examination, and models of convective thinning and stretching and melt migration must be re-evaluated to account for greater local variation.
Abstract: Plutons associated with a 1.4 Ga magmatic event intrude across southwestern Laurentia. The tectonic setting of this major magmatic province is poorly understood. Proposed melting models include anorogenic heating from the mantle, continental arc or transpressive orogeny, and anatexis from radiogenic heat buildup in thickened crust. Re-Os analyses of refractory mantle xenoliths from the Navajo volcanic field (NVF; central Colorado Plateau) yield Re depletion ages of 2.1–1.7 Ga, consistent with the age of the overlying Yavapai and Mazatzal crust. However, new Sm-Nd isotope data from clinopyroxene in peridotite xenoliths from NVF diatremes show a subset of xenoliths that plot on a ca. 1.4 Ga isochron, which likely reflects mantle melt production and isotopic resetting at 1.4 Ga. This suggests that Paleoproterozoic subcontinental lithospheric mantle was involved in the 1.4 Ga magmatic event. Our constraints support a subduction model for the generation of the 1.4 Ga granites but are inconsistent with rifting and anorogenic anatexis models, both of which would require removal of ancient lithosphere.
Abstract: Southern Africa is characterised by unusually elevated topography and abnormal heat flow. This can be explained by thermal perturbation of the mantle, but the origin of this is unclear. Geophysics has not detected a thermal anomaly in the upper mantle and there is no geochemical evidence of an asthenosphere mantle contribution to the Cenozoic volcanic record of the region. Here we show that natural CO2 seeps along the Ntlakwe-Bongwan fault within KwaZulu-Natal, South Africa, have C-He isotope systematics that support an origin from degassing mantle melts. Neon isotopes indicate that the melts originate from a deep mantle source that is similar to the mantle plume beneath Réunion, rather than the convecting upper mantle or sub-continental lithosphere. This confirms the existence of the Quathlamba mantle plume and importantly provides the first evidence in support of upwelling deep mantle beneath Southern Africa, helping to explain the regions elevation and abnormal heat flow.
Sano, A., Ohtani, E., Litasov, K., Kubo, T., Hosoya, T., Funakoshi, K., Kikegawa, T.
In situ x-ray diffraction study of the effect of water on the garnet perovksite transformation in MORB and implications for the penetration of oceanic crust...
Physics of the Earth and Planetary Interiors, Vol. 159, 1-2, pp. 118-126.
Dymshits, A., Litasov, K., Sharygin, I., Shatskiy, A., Ohtani, E.
Mineral physics of high pressure garnets.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Kolesnichenko, M., Zedgenizov, D., Ragozin, A., Litasov, K.
Water content in olivines of mantle xenoliths from Udachnaya kimberlite pipe, Yakutia.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Martirosyan, N., Yoshino, T., Shatskiy, A., Chanyshev, A., Litasov, K.
Kenetic study of Ca- carbonate - iron interaction. ( global geodynamic processes - diamond formation)
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept 22-26, 2p. Abstract
Martirosyan, N., Yoshino, T., Shatskiy, A., Chanyshev, A., Litasov, K.
Effect of water on the stability of magnesite in the mantle under reduced conditions.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Generation of hydrous plumes in the mantle transition zone linked to the tectonic erosion of continental crust.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 3p. Abstract
Sharygin, I., Litasov, K., Shatskiy, A., Golovin, A., Ohtani, E., Pokhilenko, N.
Is kimberlite magma ascent fuelled by CO2 degassing via orthopyroxene assimilation?
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Sharygin, I., Litasov, K., Shatskiy, A., Golovin, A., Ohtani, E., Pokhilenko, N.
Melting phase relations of the Udachnaya East Group 1 kimberlite at 3.0-6.5 GPA: experimental evidence for alkali-carbonatite composition of primary kimberlite melt.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Shatskiy, A., Litasov, K., Palyanov, Y.N., Ohtaini, E.
Phase relationships on the K2CO3 MgCOs join at 6 Gpa and 900-1400C: implications for incipient melting in carbonated mantle domains.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Yelisseyev, A., Khrenov, A., Afanasiev, V., Pustavarov, V., Gromilov, S., Panchenko, A., Poikilenko, N., Litasov, K.
Luminesence of impact diamonds from the Popigai astrobleme.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Diamond and Related Materials, Vol. 58, pp. 69-77.
Russia
Deposit - Popigai
Abstract: Impact diamonds (IDs) from the Popigai crater are aggregates of nanoparticulate graphite and cubic and hexagonal diamonds. IDs demonstrate broad-band emissions at 3.05, 2.8, 2.3 and 2.0 eV, which are associated with structural defects and are similar to those in detonation ultra-dispersed diamonds and CVD diamond films. A doublet with components at 1.7856 and 1.7892 eV in some ID samples is related to R1,2 lines of Cr3 + ions in corundum inclusions. The presence of N3, H3, NV0 and NV? vibronic systems in some of the ID samples shows that (i) there is nitrogen impurity and (ii) samples underwent high temperature annealing that promoted vacancies and nitrogen diffusion and defect aggregation. The luminescence decay fits with a sum of two exponential components: lifetime of the fast one is in the 5 to 9 ns range. Parameters of the traps responsible for broad thermoluminescence peaks at 148, 180, 276 and 383 K were estimated.
Earth and Planetary Science Letters, Vol. 581, 11p.
Mantle
metasomatism
Abstract: It is generally accepted that carbonatite metasomatism in the subcontinental lithospheric mantle (SCLM) inevitably causes wehrlitization of the primary lherzolite substrate. However, the K-rich carbonatite inclusions in kimberlitic diamonds containing orthopyroxene indicate that this is not always the case. In the present study, we equilibrated natural garnet lherzolite with carbonate melts containing 33-38 wt% K2O with various Ca# = 10, 20, 30, and 40 at 6 GPa and 1200-1500 °C, where Ca# = 100?Ca/(Ca+Mg+Fe). The original ratio of peridotite to carbonate was 58 to 42 by weight. In the studied temperature range, the melt retains essentially carbonate composition with silica content increasing from 1 to 11-12 wt%. The melt with Ca# 10 alters lherzolite to harzburgite, replacing clinopyroxene by orthopyroxene and decreasing CaO content in garnet below 4 wt%. The melts with Ca# 20-30 also consume clinopyroxene; although CaO content in garnet remains in the range of lherzolitic compositions. The melt with Ca# 40 yields wehrlitization, consuming orthopyroxene, increasing clinopyroxene fraction, and increasing CaO content in garnet above 6 wt%. After the interaction, the Ca# of the melt changes as follows 10 ? 16-28, 20 ? 20-33, 30 ? 27-34, and 40 ? 30-34. The olivine + orthopyroxene + clinopyroxene + garnet assemblage was found in equilibrium with carbonatite melt with Ca# 34 at 1200 °C and Ca# 30 at 1400 °C. Thus, K-rich (26-35 wt% K2O) carbonatite melts with Ca# = 30-34 can appear in equilibrium with garnet lherzolite, while the melts with Ca# < 30 and > 34 can be in equilibrium with harzburgite and wehrlite, respectively, at 6 GPa and 1200-1400 °C. Considering that Ca-Mg-Fe carbonates do not melt at the geothermal conditions of the SCLM, while sodic, dolomitic melt causes wehrlitization, high-Mg (Ca# < 35) K-rich dolomitic melt is the only possible carbonatite fluids that are thermodynamically stable in equilibrium with garnet harzburgites and lherzolites in the SCLM at a depth of about 200 km. At higher temperatures corresponding to the underlying asthenosphere, the high alkalinity ceases to be a requirement for the stability of the carbonate melt. Nevertheless, the regularities established here for the K-rich melts remain valid for less alkaline (4-15 wt% Na2O+K2O) primary kimberlite (i.e., mantle carbonatite) melts in the sublithospheric mantle.
Phase and melting relations of peridotite H2O and basalt H2O systems to the top of the lower mantle and implications for slab subduction and hot mantle plume.
18th. International Mineralogical Association Sept. 1-6, Edinburgh, abstract p.75.
Self consistent pressure scales based on the equations of state for ruby, diamond, MgO, B2-NaCl, as well as Au, Pt and other metals to 4 Mbar and 3000K.
Russian Geology and Geophysics, Vol. 54, pp. 181-199.
Sharygin, I.S., Litasov, K.D., Shatskiy, A., Golovin, A.V., Ohtani, E., Pokhilenko, N.P.
Melting phase relations of the Udachnaya-East Group 1 kimberlite at 3.0-6.5GPa: experimental evidence for alkali-carbonatite composition of primary kimberlite melts and implications for mantle plumes.
The equations of state of forsterite, wadsleyite, ringwoodite, akimotoite, Mg2SiO4 perovskite and post perovskite and phase diagram for the Mg2SiO4 system at pressures of up to 130 Gpa.
Russian Geology and Geophysics, Vol. 56, 1-2, pp. 172-189.
Abstract: Experiments on the origin of the Udachnaya-East kimberlite (UEK) have been performed using a Kawai-type multianvil apparatus at 3-6.5GPa and 900-1500°C. The studied composition represents exceptionally fresh Group-I kimberlite containing (wt.%): SiO2=25.9, TiO2=1.8, Al2O3=2.8, FeO=9.0, MgO=30.1, CaO=12.7, Na2O=3.4, K2O=1.3, P2O5=1.0, Cl=0.9, CO2=9.9, and H2O=0.5. The super-solidus assemblage consists of melt, olivine (Ol), Ca-rich (26.0-30.2wt.% CaO) garnet (Gt), Al-spinel (Sp), perovskite (Pv), a CaCO3 phase (calcite or aragonite), and apatite. The low pressure assemblage (3-4GPa) also includes clinopyroxene. The apparent solidus was established between 900 and 1000°C at 6.5GPa. At 6.5GPa and 900°C Na-Ca carbonate with molar ratio of (Na+K)/Ca?0.44 was observed. The UEK did not achieve complete melting even at 1500°C and 6.5GPa, due to excess xenogenic Ol in the starting material. In the studied P-T range, the melt has a Ca-carbonatite composition (Ca#=molar Ca/(Ca+Mg) ratio=0.62-0.84) with high alkali and Cl contents (7.3-11.4wt.% Na2O, 2.8-6.7wt.% K2O, 1.6-3.4wt.% Cl). The K, Na and Cl contents and Ca# decrease with temperature. It is argued that the primary kimberlite melt at depths>200km was an essentially carbonatitic (<5wt.% SiO2), but evolved toward a carbonate-silicate composition (up to 15-20wt.% SiO2) during ascent. The absence of orthopyroxene among the run products indicates that xenogenic orthopyroxene was preferentially dissolved into the kimberlite melt. The obtained subliquidus phase assemblage (Ol+Sp+Pv+Ca-rich Gt) at P-T conditions of the UEK source region, i.e. where melt was in the last equilibrium with source rock before magma ascent, differs from the Opx-bearing peridotitic mineral assemblage of the UEK source region. This difference can be ascribed to the loss of substantial amounts of CO2 from the kimberlite magma at shallow depths, as indicated by both petrological and experimental data. Our study implies that alkali-carbonatite melt would be a liquid phase within mantle plumes generated at the core-mantle boundary or shallower levels of the mantle, enhancing the ascent velocity of the plumes. We conclude that the long-term activity of a rising hot mantle plume and associated carbonatite melt (i.e. kimberlite melt) causes thermo-mechanical erosion of the subcontinental lithosphere mantle (SCLM) roots and creates hot and deformed metasomatic regions in the lower parts of the SCLM, which corresponds to depths constrained by P-T estimates of sheared Gt-peridotite xenoliths. The sheared Gt-peridotites undoubtedly represent samples of these regions.
Doklady Earth Sciences, Vol. 466, 2, Feb. pp. 173-176.
Russia, Yakutia
Deposit - International
Abstract: The results of study of rutile inclusions in pyrope from the Internatsionalnaya kimberlite pipe are presented. Rutile is characterized by unusually high contents of impurities (up to 25 wt %). The presence of Cr2O3 (up to 9.75 wt %) and Nb2O5 (up to 15.57 wt %) are most typical. Rutile inclusions often occur in assemblage with Ti-rich oxides: picroilmenite and crichtonite group minerals. The Cr-pyropes with inclusions of rutile, picroilmenite, and crichtonite group minerals were formed in the lithospheric mantle beneath the Mirnyi field during their joint crystallization from melts enriched in Fe, Ti, and other incompatible elements as a result of metasomatic enrichment of the depleted lithospheric mantle.
Abstract: Although kimberlite magma carries large amounts of mantle-derived xenocrysts and xenoliths (with sizes up to meters), this magma ascends from the Earth's mantle (> 150-250 km) to the surface in a matter of hours or days, which enables diamonds to survive. The recently proposed assimilation-fuelled buoyancy model for kimberlite magma ascent emphasizes the importance of fluid CO2 that is produced via the reactive dissolution of mantle-derived orthopyroxene xenocrysts into kimberlite melt, which initially has carbonatitic composition. Here, we use a series of high-pressure experiments to test this model by studying the interaction of orthopyroxene (Opx) with an alkali-dolomitic melt (simplified to 0.7Na2CO3 + 0.3K2CO3 + 2CaMg(CO3)2), which is close to the melt that is produced by the partial melting of a kimberlite source, at P = 3.1-6.5 GPa and T = 1200-1600 °C, i.e., up to pressures that correspond to depths (~ 200 km) from where the ascent of kimberlite magma would start. During the first set of experiments, we study the reaction between powdered Opx and model carbonate melt in a homogeneous mixture. During the second set of experiments, we investigate the mechanism and kinetics of the dissolution of Opx crystals in alkali-dolomitic melt. Depending on the P-T conditions, Opx dissolves in the alkali-dolomitic melt (CL) either congruently or incongruently via the following reactions: Mg2Si2O6 (Opx) + CaMg(CO3)2 (CL) = CaMgSi2O6 (clinopyroxene) + 2MgCO3 (CL) and Mg2Si2O6 (Opx) = Mg2SiO4 (olivine) + SiO2 (CL). The experiments confirm that the dissolution of Opx causes gradual SiO2 enrichment in the initial carbonate melt, as previously suggested. However, the assimilation of Opx by carbonate melt does not produce fluid CO2 in the experiments because the CO2 is totally dissolved in the evolved melt. Thus, our results clearly demonstrate the absence of exsolved CO2 fluid at 3.1-6.5 GPa in ascending kimberlite magma and disprove the assimilation-fuelled buoyancy model for kimberlite magma ascent in the lithospheric mantle. We alternatively suggest that the extreme buoyancy of kimberlite magma at depths of 100-250 km is an exclusive consequence of the unique physical properties (i.e., low density, ultra-low viscosity and, thus, high mobility) of the kimberlite melt, which are dictated by its carbonatitic composition.
Abstract: Mineralogical studies and U-Pb dating have been carried out on rutile included in peridotitic and eclogitic garnets from the Internatsionalnaya pipe, Mirny field, Siberian craton. We also describe a unique peridotitic paragenesis (rutile + forsterite + enstatite + Cr-diopside + Cr-pyrope) preserved in diamond from the Mir pipe, Mirny field. Compositions of rutile from the heavy mineral concentrates of the Internatsionalnaya pipe and rutile inclusions in crustal almandine-rich garnets from the Mayskaya pipe (Nakyn field), as well as from a range of different lithologies, are presented for comparison. Rutile from cratonic mantle peridotites shows characteristic enrichment in Cr, in contrast to lower-Cr rutile from crustal rocks and off-craton mantle. Rutile with Cr2O3 > 1.7 wt% is commonly derived from cratonic mantle, while rutiles with lower Cr2O3 may be both of cratonic and off-cratonic origin. New analytical developments and availability of standards have made rutile accessible to in situ U-Pb dating by laser ablation ICP-MS. A U-Pb age of 369 ± 10 Ma for 9 rutile grains in 6 garnets from the Internatsionalnaya pipe is consistent with the accepted eruption age of the pipe (360 Ma). The equilibrium temperatures of pyropes with rutile inclusions calculated using Ni-in-Gar thermometer range between ~ 725 and 1030 °C, corresponding to a depth range of ca ~ 100-165 km. At the time of entrainment in the kimberlite, garnets with Cr-rich rutile inclusions resided at temperatures well above the closure temperature for Pb in rutile, and thus U-Pb ages on mantle-derived rutile most likely record the emplacement age of the kimberlites. The synthesis of distinctive rutile compositions and U-Pb dating opens new perspectives for using rutile in diamond exploration in cratonic areas.
Abstract: The paper presents new petrographic, major element and Fourier transform infrared (FTIR) spectroscopy data and PT-estimates of whole-rock samples and minerals of a collection of 19 relatively fresh peridotite xenoliths from the Udachnaya kimberlite pipe, which were recovered from its deeper levels. The xenoliths are non-deformed (granular), medium-deformed and highly deformed (porphyroclastic, mosaic-porphyroclastic, mylonitic) lherzolites, harzburgite and dunite. The lherzolites yielded equilibration temperatures (T) and pressures (P) ranging from 913 to 1324 °C and from 4.6 to 6.3 GPa, respectively. The non-deformed and medium-deformed peridotites match the 35 mW/m2 conductive continental geotherm, whereas the highly deformed varieties match the 45 mW/m2 geotherm. The content of water spans 2 ± 1-95 ± 52 ppm in olivine, 1 ± 0.5-61 ± 9 ppm in orthopyroxene, and 7 ± 2-71 ± 30 ppm in clinopyroxene. The amount of water in garnets is negligible. Based on the modal proportions of mineral phases in the xenoliths, the water contents in peridotites were estimated to vary over a wide range from < 1 to 64 ppm. The amount of water in the mantle xenoliths is well correlated with the deformation degree: highly deformed peridotites show highest water contents (64 ppm) and those medium-deformed and non-deformed contain ca. 1 ppm of H2O. The high water contents in the deformed peridotites could be linked to metasomatism of relatively dry diamondiferous cratonic roots by hydrous and carbonatitic agents (fluids/melts), which may cause hydration and carbonation of peridotite and oxidation and dissolution of diamonds. The heterogeneous distribution of water in the cratonic mantle beneath the Udachnaya pipe is consistent with the models of mantle plume or veined mantle structures proposed based on a trace element study of similar xenolithic suits. Mantle metasomatism beneath the Siberian Craton and its triggered kimberlite magmatism could be induced by mantle enrichment in volatiles (H2O, CO2) supplied by numerous subduction zones which surrounded the Siberian continent in Neoproterozoic-Cambrian time.
Earth and Planetary Science Letters, Vol. 465, pp. 208-227.
Russia
Deposit - Udachnaya-East
Abstract: The critical issue in the study of kimberlites, known as principal host rocks of diamonds, is the reconstruction of their primary melt composition, which is poorly constrained due to contamination by xenogenic materials, significant loss of volatiles during eruption, and post-magmatic alteration. It is generally accepted that the last equilibration of primary kimberlite melt with surrounding mantle (garnet lherzolite) occurred beneath cratons at 5-7 GPa (150-230 km depths). However, the subliquidus mineral assemblages obtained in kimberlite melting experiments at mantle pressures differ from lherzolite, probably owing to unaccounted loss of CO2. Here we present experiments at 6.5 GPa and 1200-1600 °C on unaltered kimberlite with an addition of 2-22 mol% CO2 over its natural abundance in the rock (13 mol%), but keeping proportions of other components identical to those in an exceptionally fresh anhydrous kimberlite from Udachnaya-East pipe in Siberia. We found that the partial melt achieves equilibrium with garnet lherzolite at 1500 °C and 19-23 mol% CO2 in the system. Under these conditions this melt contains (mol%): SiO2 = 9, FeO = 6-7, MgO = 23-26, CaO = 16, Na2O = 4, K2O = 1, and CO2 = 30-35. We propose, therefore, the alkali-rich carbonatitic composition of primary kimberlite melt and loss of 34-45 mol% (34-46 wt%) CO2 during ascent of the kimberlite magma to the surface.
Abstract: The paper presents new petrographic, major element and Fourier transform infrared (FTIR) spectroscopy data and PT-estimates of whole-rock samples and minerals of a collection of 19 relatively fresh peridotite xenoliths from the Udachnaya kimberlite pipe, which were recovered from its deeper levels. The xenoliths are non-deformed (granular), medium-deformed and highly deformed (porphyroclastic, mosaic-porphyroclastic, mylonitic) lherzolites, harzburgite and dunite. The lherzolites yielded equilibration temperatures (T) and pressures (P) ranging from 913 to 1324 °C and from 4.6 to 6.3 GPa, respectively. The non-deformed and medium-deformed peridotites match the 35 mW/m2 conductive continental geotherm, whereas the highly deformed varieties match the 45 mW/m2 geotherm. The content of water spans 2 ± 1-95 ± 52 ppm in olivine, 1 ± 0.5-61 ± 9 ppm in orthopyroxene, and 7 ± 2-71 ± 30 ppm in clinopyroxene. The amount of water in garnets is negligible. Based on the modal proportions of mineral phases in the xenoliths, the water contents in peridotites were estimated to vary over a wide range from < 1 to 64 ppm. The amount of water in the mantle xenoliths is well correlated with the deformation degree: highly deformed peridotites show highest water contents (64 ppm) and those medium-deformed and non-deformed contain ca. 1 ppm of H2O. The high water contents in the deformed peridotites could be linked to metasomatism of relatively dry diamondiferous cratonic roots by hydrous and carbonatitic agents (fluids/melts), which may cause hydration and carbonation of peridotite and oxidation and dissolution of diamonds. The heterogeneous distribution of water in the cratonic mantle beneath the Udachnaya pipe is consistent with the models of mantle plume or veined mantle structures proposed based on a trace element study of similar xenolithic suits. Mantle metasomatism beneath the Siberian Craton and its triggered kimberlite magmatism could be induced by mantle enrichment in volatiles (H2O, CO2) supplied by numerous subduction zones which surrounded the Siberian continent in Neoproterozoic-Cambrian time.
Abstract: Although kimberlite magma carries large amounts of mantle-derived xenocrysts and xenoliths (with sizes up to meters), this magma ascends from the Earth's mantle (> 150–250 km) to the surface in a matter of hours or days, which enables diamonds to survive. The recently proposed assimilation-fuelled buoyancy model for kimberlite magma ascent emphasizes the importance of fluid CO2 that is produced via the reactive dissolution of mantle-derived orthopyroxene xenocrysts into kimberlite melt, which initially has carbonatitic composition. Here, we use a series of high-pressure experiments to test this model by studying the interaction of orthopyroxene (Opx) with an alkali-dolomitic melt (simplified to 0.7Na2CO3 + 0.3K2CO3 + 2CaMg(CO3)2), which is close to the melt that is produced by the partial melting of a kimberlite source, at P = 3.1–6.5 GPa and T = 1200–1600 °C, i.e., up to pressures that correspond to depths (~ 200 km) from where the ascent of kimberlite magma would start. During the first set of experiments, we study the reaction between powdered Opx and model carbonate melt in a homogeneous mixture. During the second set of experiments, we investigate the mechanism and kinetics of the dissolution of Opx crystals in alkali-dolomitic melt. Depending on the P-T conditions, Opx dissolves in the alkali-dolomitic melt (CL) either congruently or incongruently via the following reactions: Mg2Si2O6 (Opx) + CaMg(CO3)2 (CL) = CaMgSi2O6 (clinopyroxene) + 2MgCO3 (CL) and Mg2Si2O6 (Opx) = Mg2SiO4 (olivine) + SiO2 (CL). The experiments confirm that the dissolution of Opx causes gradual SiO2 enrichment in the initial carbonate melt, as previously suggested. However, the assimilation of Opx by carbonate melt does not produce fluid CO2 in the experiments because the CO2 is totally dissolved in the evolved melt. Thus, our results clearly demonstrate the absence of exsolved CO2 fluid at 3.1–6.5 GPa in ascending kimberlite magma and disprove the assimilation-fuelled buoyancy model for kimberlite magma ascent in the lithospheric mantle. We alternatively suggest that the extreme buoyancy of kimberlite magma at depths of 100–250 km is an exclusive consequence of the unique physical properties (i.e., low density, ultra-low viscosity and, thus, high mobility) of the kimberlite melt, which are dictated by its carbonatitic composition.
Contribution to Mineralogy and Petrology, Vol. 173, 22p.
Mantle
carbonatite
Abstract: We performed an experimental study, designed to reproduce the formation of an unusual merwinite?+?olivine-bearing mantle assemblage recently described as a part of a Ca-rich suite of inclusions in sublithospheric diamonds, through the interaction of peridotite with an alkali-rich Ca-carbonatite melt, derived from deeply subducted oceanic crust. In the first set of experiments, we studied the reaction between powdered Mg-silicates, olivine and orthopyroxene, and a model Ca-carbonate melt (molar Na:K:Ca?=?1:1:2), in a homogeneous mixture, at 3.1 and 6.5 GPa. In these equilibration experiments, we observed the formation of a merwinite?+?olivine-bearing assemblage at 3.1 GPa and 1200 °C and at 6.5 GPa and 1300-1400 °C. The melts coexisting with this assemblage have a low Si and high Ca content (Ca#?=?molar 100?×?Ca/(Ca?+?Mg)?>?0.57). In the second set of experiments, we investigated reaction rims produced by interaction of the same Ca-carbonate melt (molar Na:K:Ca?=?1:1:2) with Mg-silicate, olivine and orthopyroxene, single crystals at 3.1 GPa and 1300 °C and at 6.5 GPa and 1400 °C. The interaction of the Ca-carbonate melt with olivine leads to merwinite formation through the expected reaction: 2Mg2SiO4 (olivine)?+?6CaCO3 (liquid)?=?Ca3MgSi2O8 (merwinite)?+?3CaMg(CO3)2 (liquid). Thus, our experiments confirm the idea that merwinite in the upper mantle may originate via interaction of peridotite with Ca-rich carbonatite melt, and that diamonds hosting merwinite may have a metasomatic origin. It is remarkable that the interaction of the Ca-carbonate melt with orthopyroxene crystals does not produce merwinite both at 3.1 and 6.5 GPa. This indicates that olivine grain boundaries are preferable for merwinite formation in the upper mantle.
Russian Geology and Geophysics, Vol. 59, 7, pp. 763-779.
Russia, Siberia
deposit - Udachnaya
Abstract: A comprehensive study of 26 mafic mantle xenoliths from the Udachnaya kimberlite pipe was carried out. The contents of major and trace elements, equilibrium temperature parameters, and water content in the rock-forming minerals were determined. The temperatures of formation of the studied rocks are estimated at 800-1300 °C. According to IR spectroscopy data, the water content in clinopyroxenes from the studied eclogites varies from values below the detection limit to 99 ppm. The IR spectra of garnets lack bands of water. The water content in clinopyroxene and orthopyroxene from garnet websterite is 72 and 8 ppm, respectively. The water content in the average rock, calculated from the ratio of the rock-forming minerals, varies from a few to 55 ppm. No relationship among the water content, equilibrium temperatures, and rock composition is established. The low water contents in the eclogites are close to the earlier determined water contents in peridotites from the same pipe and are, most likely, due to the re-equilibration of the eclogites with the rocks of the peridotitic lithospheric mantle. The dehydration of the protolith during its subduction and the partial melting of eclogites before their removal by kimberlitic magma to the surface might be an additional cause of the low water contents in the mantle eclogite xenoliths.
Abstract: The subduction of carbonates beyond 250-300?km, where redox conditions favour the presence of metallic iron (Fe), will result in redox reactions with the Fe dispersed in the silicate rocks. Here, we studied the effect of water on the carbonate-Fe interaction in the hydromagnesite-Fe system at 6, 8 and 16?GPa and the peridotite-CO2-H2O-Fe system at 8?GPa, using a multianvil apparatus. In all of the studied samples, we observed the formation of magnesiowüstite, graphite and carbide. Additionally, in the peridotite-CO2-H2O-Fe system, magnesiowüstite reacted with pyroxenes, resulting in olivine enrichment. Kinetic calculations performed at 8?GPa showed that, at the pressure-temperature (P-T) parameters of the ‘hot’, ‘medium’ and ‘cold’ subduction, about 40, 12 and 4?vol% of carbonates, respectively, would be reduced in the hydrous system within 1 Myr, assuming direct contact with Fe. Based on the present results, it is suggested that carbonates will largely be consumed during the characteristic subduction time to the mantle transition zone by reaction with the reduced mantle in the presence of hydrous fluid.
Abstract: The phase relations in the system Na2CO3?CaCO3?MgCO3 have been studied at 3?GPa and 700-1285?°C using a Kawai-type multianvil press. At 700?°C, the system has five intermediate compounds: dolomite, Mg-bearing Na2Ca4(CO3)5 burbankite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and eitelite. As temperature increases to 800?°C, the system is complicated by an appearance of Ca-dolomite and Mg-bearing shortite, while Na2Ca4(CO3)5 disappears. At 850?°C, Na4Ca(CO3)3 decomposes to produce Na carbonate and nyerereite. The latter melts incongruently at 875?±?25?°C to form Na2Ca3(CO3)4. Incongruent melting of eitelite to magnesite and liquid, occurs at 925?±?25 °C. Mg-bearing shortite melts incongruently at 950?±?50?°C, producing Na2Ca3(CO3)4 and liquid. Na2Ca3(CO3)4 disappears at 1000?°C via incongruent melting to calcite and liquid. The liquidus projection of the studied ternary system has seven primary solidification phase regions for magnesite, dolomite-calcite solid solutions, Na2Ca3(CO3)4, Mg-bearing shortite, nyerereite, eitelite, and Na carbonate. The primary solidification regions are separated by five peritectic and three cotectic monovariant lines. The system has six ternary peritectic points and one minimum on the liquidus at 850?°C and 52Na2CO3?48(Ca0.62Mg0.38)CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, a liquid transforms into three solid phases: shortite, Na carbonate, and eitelite. Since the system has a single eutectic at 3?GPa, there is no thermal barrier preventing continuous liquid fractionation from Na-poor toward Na-rich dolomitic compositions more alkaline than eitelite and nyerereite. Considering the present results and previous data, a range of Na-Ca-Mg double carbonates changes in the following sequence upon pressure and temperature increase: Na2Ca2(CO3)3 (Amm2) shortite, Na2Ca(CO3)2 (P21ca) nyerereite, Na2Mg(CO3)2 () eitelite (0.1?GPa)???Na2(Ca0.97-0.98Mg0.02-0.03)4(CO3)5 (P63mc), Na2(Ca?0.91Mg?0.09)3(CO3)4 (P1n1), Na2(Ca???0.81?Mg0?0.19)(CO3)2 () nyerereite, Na2(Ca0.77-0.93Mg0.07-0.23)2(CO3)3 (Amm2) shortite, Na4(Ca0.90-0.98Mg0.02-0.10)(CO3)3 (Ia3d), Na2(Mg?0.9Ca0?0.1)(CO3)2 (P21ca) eitelite (3?GPa)???Na2(Ca?0.87Mg0?0.13)4(CO3)5 (P63mc), Na2(Ca?0.89Mg?0.11)3(CO3)4 (P1n1), Na4(Ca???0.7?Mg0?0.3)(CO3)3 (Ia3d), Na2(Mg?0.92Ca0?0.08)(CO3)2 (P21ca) eitelite (6?GPa). Using the present results at 3?GPa and previous data at 6?GPa in the Na2CO3?CaCO3?MgCO3 system, we constrained isopleths of the Na2CO3 content in melt coexisting with Ca-Mg carbonates. We found that the cratonic geotherms cross the isopleths so that the carbonatite melt percolating upward via the continental mantle lithosphere should become progressively enriched in Na, evolving from alkali-poor dolomitic composition at depths exceeding 200?km toward sodic dolomitic with the ~52?mol% Na2CO3 at 80-120?km depths.
Abstract: The phase relations in the system Na2CO3?CaCO3?MgCO3 have been studied at 3?GPa and 700-1285?°C using a Kawai-type multianvil press. At 700?°C, the system has five intermediate compounds: dolomite, Mg-bearing Na2Ca4(CO3)5 burbankite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and eitelite. As temperature increases to 800?°C, the system is complicated by an appearance of Ca-dolomite and Mg-bearing shortite, while Na2Ca4(CO3)5 disappears. At 850?°C, Na4Ca(CO3)3 decomposes to produce Na carbonate and nyerereite. The latter melts incongruently at 875?±?25?°C to form Na2Ca3(CO3)4. Incongruent melting of eitelite to magnesite and liquid, occurs at 925?±?25 °C. Mg-bearing shortite melts incongruently at 950?±?50?°C, producing Na2Ca3(CO3)4 and liquid. Na2Ca3(CO3)4 disappears at 1000?°C via incongruent melting to calcite and liquid. The liquidus projection of the studied ternary system has seven primary solidification phase regions for magnesite, dolomite-calcite solid solutions, Na2Ca3(CO3)4, Mg-bearing shortite, nyerereite, eitelite, and Na carbonate. The primary solidification regions are separated by five peritectic and three cotectic monovariant lines. The system has six ternary peritectic points and one minimum on the liquidus at 850?°C and 52Na2CO3?48(Ca0.62Mg0.38)CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, a liquid transforms into three solid phases: shortite, Na carbonate, and eitelite. Since the system has a single eutectic at 3?GPa, there is no thermal barrier preventing continuous liquid fractionation from Na-poor toward Na-rich dolomitic compositions more alkaline than eitelite and nyerereite. Considering the present results and previous data, a range of Na-Ca-Mg double carbonates changes in the following sequence upon pressure and temperature increase: Na2Ca2(CO3)3 (Amm2) shortite, Na2Ca(CO3)2 (P21ca) nyerereite, Na2Mg(CO3)2 () eitelite (0.1?GPa)???Na2(Ca0.97-0.98Mg0.02-0.03)4(CO3)5 (P63mc), Na2(Ca?0.91Mg?0.09)3(CO3)4 (P1n1), Na2(Ca???0.81?Mg0?0.19)(CO3)2 () nyerereite, Na2(Ca0.77-0.93Mg0.07-0.23)2(CO3)3 (Amm2) shortite, Na4(Ca0.90-0.98Mg0.02-0.10)(CO3)3 (Ia3d), Na2(Mg?0.9Ca0?0.1)(CO3)2 (P21ca) eitelite (3?GPa)???Na2(Ca?0.87Mg0?0.13)4(CO3)5 (P63mc), Na2(Ca?0.89Mg?0.11)3(CO3)4 (P1n1), Na4(Ca???0.7?Mg0?0.3)(CO3)3 (Ia3d), Na2(Mg?0.92Ca0?0.08)(CO3)2 (P21ca) eitelite (6?GPa). Using the present results at 3?GPa and previous data at 6?GPa in the Na2CO3?CaCO3?MgCO3 system, we constrained isopleths of the Na2CO3 content in melt coexisting with Ca-Mg carbonates. We found that the cratonic geotherms cross the isopleths so that the carbonatite melt percolating upward via the continental mantle lithosphere should become progressively enriched in Na, evolving from alkali-poor dolomitic composition at depths exceeding 200?km toward sodic dolomitic with the ~52?mol% Na2CO3 at 80-120?km depths.
Abstract: The enigmatic appearance of cuboctahedral diamonds in ophiolitic and arc volcanic rocks with morphology and infrared characteristics similar to synthetic diamonds that were grown from metal solvent requires a critical reappraisal. We have studied 15 diamond crystals and fragments from Tolbachik volcano lava flows, using Fourier transform infrared spectrometry (FTIR), transmission electron microscopy (TEM), synchrotron X-ray fluorescence (SRXRF) and laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS). FTIR spectra of Tolbachik diamonds correspond to typical type Ib patterns of synthetic diamonds. In TEM films prepared using focused ion beam technique, we find Mn-Ni and Mn-Si inclusions in Tolbachik diamonds. SRXRF spectra indicate the presence of Fe-Ni and Fe-Ni-Mn inclusions with Cr, Ti, Cu, and Zn impurities. LA-ICP-MS data show variable but significantly elevated concentrations of Mn, Fe, Ni, and Cu reaching up to 70?ppm. These transition metal concentration levels are comparable with those determined by LA-ICP-MS for similar diamonds from Tibetan ophiolites. Mn-Ni (+Fe) solvent was widely used to produce industrial synthetic diamonds in the former USSR and Russia with very similar proportions of these metals. Hence, it appears highly probable that the cuboctahedral diamonds recovered from Kamchatka arc volcanic rocks represent contamination and are likely derived from drilling tools or other hard instruments. Kinetic data on diamond dissolution in basaltic magma or in fluid phase demonstrate that diamond does not form under the pressures and temperature conditions prevalent within the magmatic system beneath the modern-day Klyuchevskoy group of arc volcanoes. We also considered reference data for inclusions in ophiolitic diamonds and compared them with the composition of solvent used in industrial diamond synthesis in China. The similar inclusion chemistry close to Ni70Mn25Co5 for ophiolitic and synthetic Chinese diamonds scrutinized here suggests that most diamonds recovered from Tibetan and other ophiolites are not natural but instead have a synthetic origin. In order to mitigate further dubious reports of diamonds from unconventional tectonic settings and source rocks, we propose a set of discrimination criteria to better distinguish natural cuboctahedral diamonds from those produced synthetically in industrial environments and found as contaminants in mantle- and crust-derived rocks.
Abstract: The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO3-Fe0 system at 70-150 GPa and 800-2600 K in a laser-heated diamond anvil cell. Using in situ synchrotron X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by: 6MgCO3 + 19Fe = 8FeO +10(Mg0.6Fe0.4)O + Fe7C3 + 3C. The presented results suggest that the interaction of carbonates with Fe0 or Fe0-bearing rocks can produce Fe-carbide and diamond, which can accumulate in the D’’ region, depending on its carbon to Fe ratio. Due to the sluggish kinetics of the transformation, diamond can remain metastable at the core-mantle boundary (CMB) unless it is in a direct contact with Fe-metal. In addition, it can be remobilized by redox melting accompanying the generation of mantle plumes.
Abstract: Melt inclusions in kimberlitic and metamorphic diamonds worldwide range in composition from potassic aluminosilicate to alkali-rich carbonatitic and their low-temperature derivative, a saline high-density fluid (HDF). The discovery of CO2 inclusions in diamonds containing eclogitic minerals are also essential. These melts and HDFs may be responsible for diamond formation and metasomatic alteration of mantle rocks since the late Archean to Phanerozoic. Although a genetic link between these melts and fluids was suggested, their origin is still highly uncertain. Here we present experimental results on melting phase relations in a carbonated pelite at 6?GPa and 900-1500?°C. We found that just below solidus K2O enters potassium feldspar or K2TiSi3O9 wadeite coexisting with clinopyroxene, garnet, kyanite, coesite, and dolomite. The potassium phases react with dolomite to produce garnet, kyanite, coesite, and potassic dolomitic melt, 40(K0.90Na0.10)2CO3•60Ca0.55Mg0.24Fe0.21CO3?+?1.9?mol% SiO2?+?0.7?mol% TiO2?+?1.4?mol% Al2O3 at the solidus established near 1000?°C. Molecular CO2 liberates at 1100?°C. Potassic aluminosilicate melt appears in addition to carbonatite melt at 1200?°C. This melt contains (mol/wt%): SiO2?=?57.0/52.4, TiO2?=?1.8/2.3, Al2O3?=?8.5/13.0, FeO?=?1.4/1.6, MgO?=?1.9/1.2, CaO?=?3.8/3.2, Na2O?=?3.2/3.0, K2O?=?10.5/15.2, CO2?=?12.0/8.0, while carbonatite melt can be approximated as 24(K0.81Na0.19)2CO3•76Ca0.59Mg0.21Fe0.20CO3?+?3.0?mol% SiO2?+?1.6?mol% TiO2?+?1.4?mol% Al2O3. Both melts remain stable to at least 1500?°C coexisting with CO2 fluid and residual eclogite assemblage consisting of K-rich omphacite (0.4-1.5?wt% K2O), almandine-pyrope-grossular garnet, kyanite, and coesite. The obtained immiscible alkali?carbonatitic and potassic aluminosilicate melts resemble compositions of melt inclusions in diamonds worldwide. Thus, these melts entrapped by diamonds could be derived by partial melting of the carbonated material of the continental crust subducted down to 180-200?km depths. Given the high solubility of chlorides and water in both carbonate and aluminosilicate melts inferred in previous experiments, the saline end-member, brine, could evolve from potassic carbonatitic and/or silicic melts by fractionation of Ca-Mg carbonates/eclogitic minerals and accumulation of alkalis, chlorine and water in the residual low-temperature supercritical fluid. Direct extraction from the hydrated marine sediments under conditions of cold subduction would be another possibility for the brine formation.
Abstract: Melt inclusions in kimberlitic and metamorphic diamonds worldwide range in composition from potassic aluminosilicate to alkali-rich carbonatitic and their low-temperature derivative, a saline high-density fluid (HDF). The discovery of CO2 inclusions in diamonds containing eclogitic minerals are also essential. These melts and HDFs may be responsible for diamond formation and metasomatic alteration of mantle rocks since the late Archean to Phanerozoic. Although a genetic link between these melts and fluids was suggested, their origin is still highly uncertain. Here we present experimental results on melting phase relations in a carbonated pelite at 6?GPa and 900-1500?°C. We found that just below solidus K2O enters potassium feldspar or K2TiSi3O9 wadeite coexisting with clinopyroxene, garnet, kyanite, coesite, and dolomite. The potassium phases react with dolomite to produce garnet, kyanite, coesite, and potassic dolomitic melt, 40(K0.90Na0.10)2CO3•60Ca0.55Mg0.24Fe0.21CO3?+?1.9?mol% SiO2?+?0.7?mol% TiO2?+?1.4?mol% Al2O3 at the solidus established near 1000?°C. Molecular CO2 liberates at 1100?°C. Potassic aluminosilicate melt appears in addition to carbonatite melt at 1200?°C. This melt contains (mol/wt%): SiO2?=?57.0/52.4, TiO2?=?1.8/2.3, Al2O3?=?8.5/13.0, FeO?=?1.4/1.6, MgO?=?1.9/1.2, CaO?=?3.8/3.2, Na2O?=?3.2/3.0, K2O?=?10.5/15.2, CO2?=?12.0/8.0, while carbonatite melt can be approximated as 24(K0.81Na0.19)2CO3•76Ca0.59Mg0.21Fe0.20CO3?+?3.0?mol% SiO2?+?1.6?mol% TiO2?+?1.4?mol% Al2O3. Both melts remain stable to at least 1500?°C coexisting with CO2 fluid and residual eclogite assemblage consisting of K-rich omphacite (0.4-1.5?wt% K2O), almandine-pyrope-grossular garnet, kyanite, and coesite. The obtained immiscible alkali?carbonatitic and potassic aluminosilicate melts resemble compositions of melt inclusions in diamonds worldwide. Thus, these melts entrapped by diamonds could be derived by partial melting of the carbonated material of the continental crust subducted down to 180-200?km depths. Given the high solubility of chlorides and water in both carbonate and aluminosilicate melts inferred in previous experiments, the saline end-member, brine, could evolve from potassic carbonatitic and/or silicic melts by fractionation of Ca-Mg carbonates/eclogitic minerals and accumulation of alkalis, chlorine and water in the residual low-temperature supercritical fluid. Direct extraction from the hydrated marine sediments under conditions of cold subduction would be another possibility for the brine formation.
Abstract: A novel structure of calcium orthocarbonate Ca2CO4-Pnma has been discovered using ab-initio crystal structure prediction methods (AIRSS and USPEX) based on the density functional theory. This phase appears above 13 GPa and remains stable to at least 50 GPa and 2000 K according to the calculations within quasi-harmonic approximation. Thus, the discovered phase can be stable at Earth's transition zone and lower mantle P-T conditions. The carbon atoms in the discovered phase are presented in 4-fold coordination, and its structure is similar to the high-pressure and high-temperature ?'H-Ca2SiO4 phase.
Abstract: A novel structure of calcium orthocarbonate Ca2CO4-Pnma has been discovered using ab-initio crystal structure prediction methods (AIRSS and USPEX) based on the density functional theory. This phase appears above 13 GPa and remains stable to at least 50 GPa and 2000 K according to the calculations within quasi-harmonic approximation. Thus, the discovered phase can be stable at Earth's transition zone and lower mantle P-T conditions. The carbon atoms in the discovered phase are presented in 4-fold coordination, and its structure is similar to the high-pressure and high-temperature ?'H-Ca2SiO4 phase.
Abstract: To evaluate the effect of Na on the carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments along some specific joins of the system KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 at 6 GPa. Melting in all studied joins begins at 1000-1050 °C. The melting in the Kfs + Dol system is controlled by the reaction 6 KAlSi3O8 (K-feldspar) + 6 CaMg(CO3)2 (dolomite) = 2 (Can,Mg1-n)3Al2Si3O12 (garnet) + Al2SiO5 (kyanite) + 11 SiO2 (coesite) + 3 K2(Ca1-n,Mgn)2(CO3)3 (carbonatitic melt) + 3 CO2 (fluid), where n ~ 0.3-0.4. A temperature increasing to 1300 °C yields an appearance of the silicic immiscible melt in addition to carbonatitic melt via the reaction K2CO3 (carbonatitic melt) + Al2SiO5 (kyanite) + 5 SiO2 (coesite) = 2 KAlSi3O8 (silicic melt) + CO2 (fluid or solute in melts). The silicic melt composition is close to KAlSi3O8 with dissolved CaMg(CO3)2 and molecular CO2. An addition of NaAlSi2O6 or Na2CO3 to the system results in partial decomposition of K-feldspar and formation of K-bearing carbonates, (K, Na)2Mg(CO3)2 and (K, Na)2Ca3(CO3)4. Their melting produces carbonatite melt with the approximate composition of 4(K, Na)2CO3•6Ca0.6Mg0.4CO3 and magnesite. Besides, the presence of NaAlSi2O6 in the studied system shifts the lower-temperature limit of immiscibility to 1500°?, while the presence of Na2CO3 eliminates the appearance of silicic melt by the following reaction: 2 KAlSi3O8 (in the silicic melt) + Na2CO3 = 2 NaAlSi2O6 (in clinopyroxene) + K2CO3 (in the carbonatitic melt) + SiO2 (coesite). Thus, an increase of the Na2O content in the system Na2O-K2O-CaO-MgO-Al2O3-SiO2-CO2 consumes Al2O3 and SiO2 from silicic melt to form clinopyroxene. We found that grossular-pyrope and diopside-jadeite solid solutions can coexist with CO2 fluid at 900-1500 °C and 6 GPa. Thus, CO2 fluid is stable in the eclogitic suite in the diamond stability field under temperature conditions of the continental lithosphere and subducting slabs. Variations in the Na2O content observed in carbonatitic melts trapped by natural in diamonds exceed those derived by the pelite melting. The present experiments show that an addition of NaAlSi2O6 to the Kfs + Dol system does not cause an increase of the Na2O content in the carbonatitic melt, whereas the addition of Na2CO3 at Na2O/Al2O3 > 1 yields the formation of the melts with the Na2O contents covering the entire range of natural compositions. Thus, only the presence of additional salt components can explain the elevated Na2O content in the melts trapped in lithospheric diamonds. In addition to carbonates, sodium can be hosted by chlorides, sulfates, etc.
Abstract: The range of carbonatite melts in equilibrium with the subcontinental lithospheric mantle (SCLM) under geothermal conditions is limited by alkali-rich near-eutectic compositions. Therefore, here we employed eutectic Na/K-Ca-Mg-Fe carbonate mixtures to model the interaction of a metasomatic carbonatite melt with natural garnet lherzolite. The experiments were performed at 1100 and 1200 °C and 6 GPa in graphite capsules using a multianvil press. The run duration was 111 and 86 h, respectively. To verify achieving an equilibrium, a synthetic mixture identical to natural lherzolite was also employed. We have found that both Na- and K-bearing carbonatite melts cause wehrlitization accompanying by the elimination of orthopyroxene and an increase of CaO in garnet at a constant Cr2O3. Interaction with the K?carbonatite melt alters clinopyroxene composition toward lower Na2O (0.2-0.3 wt%), and higher K2O (0.5-1.0 wt%), whereas the Na?carbonatite melt revealed the opposite effect. The resulting melts have a following approximate composition [40(Na, K)2CO3?60Ca0.5Mg0.4Fe0.1CO2 + 0.6-1.4 wt% SiO2] displaying a decrease in Ca# at a nearly constant alkali content relative to the initial composition, where Ca# = 100?Ca/(Ca + Mg + Fe). We have also found that alkali-poor (? 20 mol% (Na, K)2CO3) carbonate mixtures do not melt completely but yield magnesite and alkali- and Ca-rich melts like those in the systems with eutectic mixtures. Under SCLM P-T conditions the range of carbonatite melt compositions is restricted by the full melting field of alkali-rich carbonates in the corresponding Na/K-Ca-Mg carbonate systems. Infiltration of less alkaline higher-temperature carbonatite melt in SCLM and its subsequent cooling to the ambient mantle temperature, 1100-1200 °C at 6 GPa, should cause crystallization of magnesite and shift the melt composition to [30(Na, K)2CO3?70Ca0.6Mg0.3Fe0.1CO3]. Owing to its high Ca#, this melt is not stable in equilibrium with orthopyroxene yielding its disappearance by CaMg exchange reaction producing clinopyroxene, magnesite, and shifting the melt composition toward higher alkali content. The melts containing 40-45 mol% of alkaline carbonates have no limitation in Ca# because the corresponding binary NaMg and KMg carbonate eutectics are located near 1200 °C. Therefore, these melts can achieve Ca# ? 30-40 and, be in equilibrium with garnet lherzolites and harzburgites under the geothermal condition of SCLM. Considering the present results and previous experimental data the following ranges of carbonatite melt compositions can be expected in equilibrium with garnet peridotites at the base of SCLM: Ca# < 30 and > 30 mol% (K, Na)2CO3 in equilibrium with harzburgite; Ca# 30-40, >25 mol% (K, Na)2CO3 in equilibrium with lherzolite; and Ca# 40-60 and >20 mol% (K, Na)2CO3 in equilibrium with wehrlite.
The American Mineralogist, Vol. 106. pp. 44-53. pdf
Russia
deposit - Kamchatka
Abstract: Taking into account recent publications, we provide additional comprehensive evidence that type Ib cuboctahedral diamonds and some other microcrystalline diamonds from Kamchatka volcanic rocks and alluvial placers cannot be natural and undoubtedly represent synthetic materials, which appear in the natural rocks by anthropogenic contamination. The major arguments provided in favor of the natural origin of those diamonds can be easily disproved. They include the coexistence of diamond and deltalumite from Koryaksky volcano; coexistence with super-reduced corundum and moissanite, Mn-Ni silicide inclusions, F-Cl enrichment and F/Cl ratios, and carbon and nitrogen isotopes in Tolbachik diamonds, as well as microtwinning, Mn-Ni silicides, and other inclusions in microcrystalline diamond aggregates from other Kamchatka placers. We emphasize the importance of careful comparison of unusual minerals found in nature, which include type Ib cuboctahedral diamonds and super-reduced phase assemblages resembling industrial slags, with synthetic analogs. The cavitation model proposed for the origin of Tolbachik diamonds is also unreliable since cavitation has only been shown to cause the formation of nanosized diamonds only.
Abstract: To evaluate the effect of Na on the carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments along some specific joins of the system KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 at 6 GPa. Melting in all studied joins begins at 1000-1050 °C. The melting in the Kfs + Dol system is controlled by the reaction 6 KAlSi3O8 (K-feldspar) + 6 CaMg(CO3)2 (dolomite) = 2 (Can,Mg1-n)3Al2Si3O12 (garnet) + Al2SiO5 (kyanite) + 11 SiO2 (coesite) + 3 K2(Ca1-n,Mgn)2(CO3)3 (carbonatitic melt) + 3 CO2 (fluid), where n ~ 0.3-0.4. A temperature increasing to 1300 °C yields an appearance of the silicic immiscible melt in addition to carbonatitic melt via the reaction K2CO3 (carbonatitic melt) + Al2SiO5 (kyanite) + 5 SiO2 (coesite) = 2 KAlSi3O8 (silicic melt) + CO2 (fluid or solute in melts). The silicic melt composition is close to KAlSi3O8 with dissolved CaMg(CO3)2 and molecular CO2. An addition of NaAlSi2O6 or Na2CO3 to the system results in partial decomposition of K-feldspar and formation of K-bearing carbonates, (K, Na)2Mg(CO3)2 and (K, Na)2Ca3(CO3)4. Their melting produces carbonatite melt with the approximate composition of 4(K, Na)2CO3•6Ca0.6Mg0.4CO3 and magnesite. Besides, the presence of NaAlSi2O6 in the studied system shifts the lower-temperature limit of immiscibility to 1500°?, while the presence of Na2CO3 eliminates the appearance of silicic melt by the following reaction: 2 KAlSi3O8 (in the silicic melt) + Na2CO3 = 2 NaAlSi2O6 (in clinopyroxene) + K2CO3 (in the carbonatitic melt) + SiO2 (coesite). Thus, an increase of the Na2O content in the system Na2O-K2O-CaO-MgO-Al2O3-SiO2-CO2 consumes Al2O3 and SiO2 from silicic melt to form clinopyroxene. We found that grossular-pyrope and diopside-jadeite solid solutions can coexist with CO2 fluid at 900-1500 °C and 6 GPa. Thus, CO2 fluid is stable in the eclogitic suite in the diamond stability field under temperature conditions of the continental lithosphere and subducting slabs. Variations in the Na2O content observed in carbonatitic melts trapped by natural in diamonds exceed those derived by the pelite melting. The present experiments show that an addition of NaAlSi2O6 to the Kfs + Dol system does not cause an increase of the Na2O content in the carbonatitic melt, whereas the addition of Na2CO3 at Na2O/Al2O3 > 1 yields the formation of the melts with the Na2O contents covering the entire range of natural compositions. Thus, only the presence of additional salt components can explain the elevated Na2O content in the melts trapped in lithospheric diamonds. In addition to carbonates, sodium can be hosted by chlorides, sulfates, etc.
Contributions to Mineralogy and Petrology, Vol. 176, 34 21p. Pdf
Mantle
carbonatites
Abstract: The reactions between pyroxenes and carbonates have been studied in the CaMgSi2O6 + MgCO3 (Di + 2Mgs), CaMgSi2O6 + NaAlSi2O6 + 2MgCO3 (Di + Jd + 2Mgs), CaMgSi2O6 + Na2Mg(CO3)2 (Di + Eit), and CaMgSi2O6 + K2Mg(CO3)2 (Di + K2Mg) systems at pressures of 3.0 and 4.5 GPa in the temperature range 850-1300 °C and compared with those established previously at 6.0 GPa. The Di + 2Mgs solidus locates at 1220 °C / 3 GPa and 1400 °C / 6 GPa. Near-solidus melt is carbonatitic with SiO2 < 4 wt% and Ca# 56. The Di + Jd + 2Mgs solidus locates near 1050 °C at 3 GPa, rises to 1200 °C at 4.5 GPa, and 1350 °C at 6 GPa. The solidus is controlled by the reaction: 4NaAlSi2O6.2CaMgSi2O6 (clinopyroxene) + 12MgCO3 (magnesite) = 2MgAl2SiO6.5Mg2Si2O6 (clinopyroxene) + 2[Na2CO3.CaCO3.MgCO3] (liquid) + 6CO2. As pressure increases, the composition of solidus melt evolves from 26Na2CO3?74Ca0.58Mg0.42CO3 at 3 GPa to 10Na2CO3?90Ca0.50Mg0.50CO3 at 6 GPa. Melting in the Di + Eit and Di + K2Mg systems is controlled by the reactions: CaMgSi2O6 (clinopyroxene) + 2(Na or K)2 Mg(CO3)2 (eitelite) = Mg2Si2O6 (orthopyroxene) + 2[(Na or K)2CO3?Ca0.5Mg0.5CO3] (liquid). The Di + Eit solidus locates at 925 °C / 3 GPa and 1100 °C / 6 GPa, whereas the Di + K2Mg solidus is located at 50 °C lower. The resulting melts have alkali-rich carbonate compositions, (Na or K)2CO3?Ca0.4Mg0.6CO3. The obtained results suggest that most carbonates belong to the ultramafic suite would survive during subduction into the deep mantle and experience partial melting involving alkaline carbonates, eitelite or K2Mg(CO3)2, under geothermal conditions of the subcontinental lithospheric mantle (35-40 mW/m2). On the other hand, the jadeite component in clinopyroxene would be an important fluxing agent responsible for the partial melting of carbonated rocks under the rift margin geotherm (60 mW/m2) at a depth of about 100 km, yielding the formation of Na-carbonatite melt.
Geochimica et Cosmochimica Acta, doi.org/10.1016/j.gca.2021.06.022 31p. Pdf
Global
meteorite
Abstract: The occurrence of shock-induced diamonds in ureilite meteorites is common and is used to constrain the history of the ureilite parent bodies. We have investigated a fragment of the Kenna ureilite by micro-X-ray diffraction, micro-Raman spectroscopy and scanning electron microscopy to characterize its carbon phases. In addition to olivine and pigeonite, within the carbon-bearing areas, we identified microdiamonds (up to about 10 ?m in size), nanographite and magnetite. The shock features observed in the silicate minerals and the presence of microdiamonds and nanographite indicate that Kenna underwent a shock event with a peak pressure of at least 15 GPa. Temperatures estimated using a graphite geothermometer are close to 1180 °C. Thus, Kenna is a medium-shocked ureilite, yet it contains microdiamonds, which are typically found in highly shocked carbon-bearing meteorites, instead of the more common nanodiamonds. This can be explained by a relatively long shock event duration (in the order of 4-5 s) and/or by the catalytic effect of Fe-Ni alloys known to favour the crystallization of diamonds. For the first time in a ureilite, carletonmooreite with formula Ni3Si and grain size near 4-7 nm, was found. The presence of nanocrystalline carletonmooreite provides further evidence to support the hypothesis of the catalytic involvement of Fe-Ni bearing phases into the growth process of diamond from graphite during shock events in the ureilite parent body, enabling the formation of micrometer-sized diamond crystals.
International Journal of High Pressure Research, Vol. 39, 3, pp. 480-488.
Mantle
diamond morphology
Abstract: Here we present trace element compositions of synthetic diamonds, which show spectacular similarity with the compositions of metallic inclusions in type Ib cuboctahedral diamonds in ophiolitic chromitites and peridotites. The compositions of inclusions in synthetic and ophiolite diamonds closely correspond to Ni:Mn:Co?=?70:25:5 in wt.%, which is the most widely used catalyst for HPHT growth of synthetic diamonds in China. Thus, we claim for thorough reconsideration of diamonds in ophiolitic rocks and emphasize that most of them appear by anthropogenic contamination.
Abstract: It is well known that water significantly lowers mantle solidi. But it turns out this paradigm is not always true. Here, we studied the interaction of K-rich carbonate melts with the natural garnet lherzolite from the Udachnaya kimberlite (Russia) in the presence of water at 3.0-6.5 GPa, corresponding to depths of 100-200 km. We found that at ? 1100 °C, the metasomatic interaction consumes garnet, orthopyroxene, and melt to produce phlogopite ± K-richterite + magnesite ± dolomite. Besides, primary clinopyroxene is replaced by one with a lower amount of jadeite component. Thus, the addition of water to the K-rich carbonate melt, infiltrating the subcontinental lithospheric mantle, should yield its partial or complete disappearance accompanied by phlogopitization and carbonation. The studied systems have H2O/K2O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite peridotite under fluid-absent conditions. At 4.0-6.5 GPa, the solidus of carbonated phlogopite peridotite is controlled by the following reaction: phlogopite + clinopyroxene + magnesite = garnet + orthopyroxene + olivine + hydrous K-carbonatite melt, which is bracketed between 1100 and 1200 °C. At 3 GPa, the solidus temperature decreases to about 1050 °C presumably owing to the Ca-Mg exchange reaction, clinopyroxene + magnesite = orthopyroxene + dolomite, which stabilizes dolomite reacting with phlogopite at a lower temperature than magnesite. Our results suggest that the phlogopite- and carbonate-rich metasomatic vein networks, weakening rigid lithosphere and promoting continental rifting, could be formed as a result of infiltration of hydrous K-carbonatite melt at the base of subcontinental lithospheric mantle. Stretching and thinning of the cratonic lithosphere make geotherms warmer and shifts their intersections with the solidus of carbonated phlogopite peridotite to shallower depths. Consequently, the successive erosion of the continental lithosphere and ascent of the lithosphere-asthenosphere boundary during continental rifting should be accompanied by remelting of phlogopite-carbonate metasomes, upward percolation of K-rich melt, and its solidification at the front of the magmatic-metasomatic mantle system.
Abstract: The interaction of natural eclogite (Ecl) with synthetic hydrous carbonate melts with Na:K = 0:1 (KH2) and 1:1 (NKH2) was studied in multianvil experiments at 3-6 GPa and 850-1250 °C. The interaction with KH2 consumes garnet and clinopyroxene producing phlogopite and calcite-dolomite solid solution. Besides, the interaction yields a decrease in the jadeite component of clinopyroxene, evolving eclogite toward pyroxenite. This is consistent with a metasomatic alteration of eclogite xenoliths, manifested as Na-poor “spongy” clinopyroxene, replacing primary omphacite, and kelyphitic rims around garnet, containing phlogopite and carbonates. The interaction with NKH2 also produces phlogopite and carbonate, but the latter is more magnesian and represented by magnesite, above the solidus, and magnesite + dolomite below the solidus. The interaction with NKH2 increases the jadeite component in clinopyroxene and grossular component in garnet, evolving eclogite Group A to eclogite Group B. The studied systems have H2O/K2O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite eclogite under fluid-absent conditions. Based on the obtained results its solidus is situated near 1050 °C at 3 GPa and decreases to 950 °C at 6 GPa. Thus, hydrous K- and Na-K-carbonatite melts can coexist with eclogite in SCLM at depths exceeding 120-170 km, and solidify as temperature decreases below 950-1050 °C according to the following solidus reactions: pyrope + diopside + melt ? phlogopite + dolomite, below 6 GPa, and pyrope + diopside + melt ? phlogopite + magnesite + grossular, at 6 GPa. The melting reaction, involving phlogopite and dolomite, suggests the partial melting at the peak of ultrahigh-pressure metamorphism (UHPM) during continent-continent plate collision. The prograde P-T path of UHPM crosses the solidus of clinopyroxene + garnet + phlogopite + dolomite assemblage at 4.7-5.2 GPa and 970-990 °C and yields the formation of hydrous K-carbonatite melt-fluid in situ. This melt could be responsible for the formation of K-bearing clinopyroxenes and microdiamonds in the UHPM marbles in the Kokchetav massif, Kazakhstan. The retrograde P-T path intersects the solidus that has a negative Clapeyron slope in the diamond stability field. Thus, the hydrous K-carbonatite melt should disappear soon after the peak of metamorphism reacting with garnet to produce Ca-Mg carbonates and phlogopite.
Abstract: The viscosity structure of Earth’s deep mantle affects the thermal evolution of Earth, the ascent of mantle plumes, settling of subducted oceanic lithosphere, and the mixing of compositional heterogeneities in the mantle. Based on a reanalysis of the long-wavelength nonhydrostatic geoid, we infer viscous layering of the mantle using a method that allows us to avoid a priori assumptions about its variation with depth. We detect an increase in viscosity at 800- to 1200-kilometers depth, far greater than the depth of the mineral phase transformations that define the mantle transition zone. The viscosity increase is coincident in depth with regions where seismic tomography has imaged slab stagnation, plume deflection, and changes in large-scale structure and offers a simple explanation of these phenomena.
Earth and Planetary Science Letters, Vol. 435, 1, pp. 55-63.
Mantle
Plume
Abstract: In order to link the geochemical signature of hot spot basalts to Earth's deep interior, it is first necessary to understand how plumes sample different regions of the mantle. Here, we investigate the relative amounts of deep and shallow mantle material that are entrained by an ascending plume and constrain its source region. The plumes are generated in a viscous syrup using an isolated heater for a range of Rayleigh numbers. The velocity fields are measured using stereoscopic Particle-Image Velocimetry, and the concept of the ‘vortex ring bubble’ is used to provide an objective definition of the plume geometry. Using this plume geometry, the plume composition can be analysed in terms of the proportion of material that has been entrained from different depths. We show that the plume composition can be well described using a simple empirical relationship, which depends only on a single parameter, the sampling coefficient, scsc. High-scsc plumes are composed of material which originated from very deep in the fluid domain, while low-scsc plumes contain material entrained from a range of depths. The analysis is also used to show that the geometry of the plume can be described using a similarity solution, in agreement with previous studies. Finally, numerical simulations are used to vary both the Rayleigh number and viscosity contrast independently. The simulations allow us to predict the value of the sampling coefficient for mantle plumes; we find that as a plume reaches the lithosphere, 90% of its composition has been derived from the lowermost 260–750 km in the mantle, and negligible amounts are derived from the shallow half of the lower mantle. This result implies that isotope geochemistry cannot provide direct information about this unsampled region, and that the various known geochemical reservoirs must lie in the deepest few hundred kilometres of the mantle.
Geological Society of London Special Publication: Magmatic Rifting and Active Volcanism, Vol. 420, pp. 11-30.
Africa
Global finite element model
Abstract: Continental rifting has a fundamental role in the tectonic behaviour of the Earth, shaping the surface we live on. Although there is not yet a consensus about the dominant mechanism for rifting, there is a general agreement that the stresses required to rift the continental lithosphere are not readily available. Here we use a global finite element model of the lithosphere to calculate the stresses acting on Africa. We consider the stresses induced by mantle flow, crustal structure and topography in two types of models: one in which flow is exclusively driven by the subducting slabs and one in which it is derived from a shear wave tomographic model. The latter predicts much larger stresses and a more realistic dynamic topography. It is therefore clear that the mantle structure beneath Africa plays a key part in providing the radial and horizontal tractions, dynamic topography and gravitational potential energy necessary for rifting. Nevertheless, the total available stress (c. 100 MPa) is much less than that needed to break thick, cold continental lithosphere. Instead, we appeal to a model of magma-assisted rifting along pre-existing weaknesses, where the strain is localized in a narrow axial region and the strength of the plate is reduced significantly. Mounting geological and geophysical observations support such a model.
Geological Society, London Magmatic Rifting and Active Volcanism, Eds. Wright, T.J., Ayele, A., Ferguson, D.J., Kidane, T., Vye-Brown, X. publ. 2016, SP 420, pp. 11-30. from http://sp.lyell collection.org
Africa
tectonics, geodynamics
Abstract: Continental rifting has a fundamental role in the tectonic behaviour of the Earth, shaping the surface we live on. Although there is not yet a consensus about the dominant mechanism for rifting, there is a general agreement that the stresses required to rift the continental lithosphere are not readily available. Here we use a global finite element model of the lithosphere to calculate the stresses acting on Africa. We consider the stresses induced by mantle flow, crustal structure and topography in two types of models: one in which flow is exclusively driven by the subducting slabs and one in which it is derived from a shear wave tomographic model. The latter predicts much larger stresses and a more realistic dynamic topography. It is therefore clear that the mantle structure beneath Africa plays a key part in providing the radial and horizontal tractions, dynamic topography and gravitational potential energy necessary for rifting. Nevertheless, the total available stress (c. 100 MPa) is much less than that needed to break thick, cold continental lithosphere. Instead, we appeal to a model of magma-assisted rifting along pre-existing weaknesses, where the strain is localized in a narrow axial region and the strength of the plate is reduced significantly. Mounting geological and geophysical observations support such a model.
Abstract: The sinking remnant of a surface plate crosses and interacts with multiple boundaries in Earth's interior. Here, we specifically investigate the prominent dynamic interaction of the sinking plate portion with the upper-mantle transition zone and its corresponding surface elevation signal. We unravel, for the first time, that the collision of the sinking slab with the transition zone induces a sudden, dramatic downward tilt of the upper plate towards the subduction trench. Unraveling this crucial interaction was only possible thanks to state-of-the-art numerical modelling and post-processing. The new model that is introduced here to study the dynamically self-consistent temporal evolution of subduction features accurate subduction-zone topography, robust single-sided plate sinking, stronger plates close to laboratory values, an upper-mantle phase transition, and simple continents at a free surface. To distinguish the impact of the new physical model features, three different setups are used: the simplest model setup includes a basic high-viscosity lower mantle, the second adds a 660-km phase transition, and the third includes, additionally, a continental upper plate. Common to all models is the clear topographic signal upon slab-transition-zone interaction: the upper plate tilts abruptly towards the subduction trench by about 0.05° and over around 10 Ma. This dramatic increase in upper-plate tilt can be related to the slab-induced excitation of the high-viscosity lower mantle, which introduces a wider flow pattern. A large change in horizontal extent of inundation of up to 900 km is observed as a direct consequence of the upper-plate tilting. Such an abrupt variation in surface topography and inundation extent should be clearly visible in temporal records of large-scale surface elevation and might explain continental tilting as observed in Australia since the Eocene and North America during the Phanerozoic.
Journal of Geophysical Research: Solid Earth, https://doi,org/ 10.1029/2018JB016482
Mantle
anistropy
Abstract: Several theoretical studies indicate that a substantial fraction of the measured seismic anisotropy could be interpreted as extrinsic anisotropy associated with compositional layering in rocks, reducing the significance of strain?induced intrinsic anisotropy. Here we quantify the potential contribution of grain?scale and rock?scale compositional anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties and (ii) measuring velocities of synthetic seismic waves propagating through modeled strain?induced microstructures. It is shown that for typical mantle and oceanic crust subsolidus compositions, rock?scale compositional layering does not generate any substantial extrinsic anisotropy (<1%) because of the limited contrast in isotropic elastic moduli among different rocks. Quasi?laminated structures observed in subducting slabs using P and S wave scattering are often invoked as a source of extrinsic anisotropy, but our calculations show that they only generate minor seismic anisotropy (<0.1-0.2% of Vp and Vs radial anisotropy). More generally, rock?scale compositional layering, when present, cannot be detected with seismic anisotropy studies but mainly with wave scattering. In contrast, when grain?scale layering is present, significant extrinsic anisotropy could exist in vertically limited levels of the mantle such as in a mid?ocean ridge basalt?rich lower transition zone or in the uppermost lower mantle where foliated basalts and pyrolites display up to 2-3% Vp and 3-6% Vs radial anisotropy. Thus, seismic anisotropy observed around the 660?km discontinuity could be possibly related to grain?scale shape?preferred orientation. Extrinsic anisotropy can form also in a compositionally homogeneous mantle, where velocity variations associated with major phase transitions can generate up to 1% of positive radial anisotropy.
GSL SP 420 ( Lyell release May 11, 2020), Vol. 420, pp. 11-30. pdf
Africa
Tectonics
Abstract: Continental rifting has a fundamental role in the tectonic behaviour of the Earth, shaping the surface we live on. Although there is not yet a consensus about the dominant mechanism for rifting, there is a general agreement that the stresses required to rift the continental lithosphere are not readily available. Here we use a global finite element model of the lithosphere to calculate the stresses acting on Africa. We consider the stresses induced by mantle flow, crustal structure and topography in two types of models: one in which flow is exclusively driven by the subducting slabs and one in which it is derived from a shear wave tomographic model. The latter predicts much larger stresses and a more realistic dynamic topography. It is therefore clear that the mantle structure beneath Africa plays a key part in providing the radial and horizontal tractions, dynamic topography and gravitational potential energy necessary for rifting. Nevertheless, the total available stress (c. 100 MPa) is much less than that needed to break thick, cold continental lithosphere. Instead, we appeal to a model of magma-assisted rifting along pre-existing weaknesses, where the strain is localized in a narrow axial region and the strength of the plate is reduced significantly. Mounting geological and geophysical observations support such a model.
Abstract: At the Earth's surface, the majority of water resides in the oceans, while in the interior, major rock?forming minerals can incorporate significant amounts of water as hydroxyl groups (OH), likely forming another reservoir of water inside the planet. The amount of water that can be dissolved in Earth's mantle minerals, called its water storage capacity, generally decreases at higher temperatures. Over billion?year timescales, the exchange of water between Earth's interior and surface may control the surface oceans' volume change. Here, we calculated the water storage capacity in Earth's solid mantle as a function of mantle temperature. We find that water storage capacity in a hot, early mantle may have been smaller than the amount of water Earth's mantle currently holds, so the additional water in the mantle today would have resided on the surface of the early Earth and formed bigger oceans. Our results suggest that the long?held assumption that the surface oceans' volume remained nearly constant through geologic time may need to be reassessed.
Abstract: Volcanic hotspots are thought to be fed by hot, active upwellings from the deep mantle, with excess temperatures (Tex) ~100° to 300°C higher than those of mid-ocean ridges. However, Tex estimates are limited in geographical coverage and often inconsistent for individual hotspots. We infer the temperature of oceanic hotspots and ridges simultaneously by converting seismic velocity to temperature. We show that while ~45% of plume-fed hotspots are hot (Tex ? 155°C), ~15% are cold (Tex ? 36°C) and ~40% are not hot enough to actively upwell (50°C ? Tex ? 136°C). Hot hotspots have an extremely high helium-3/helium-4 ratio and buoyancy flux, but cold hotspots do not. The latter may originate at upper mantle depths. Alternatively, the deep plumes that feed them may be entrained and cooled by small-scale convection.
On the Content of the Nickel, Chromium, Titanium, Niobium and Some Other elements in Kimberlites and the Possibility of Geochemical Prospecting for Kimberlite Bodies.
Safonov, O.G., Perchuk, L.L., Litrvin, Y.A., Bindi, L.
Phase relations in the Ca Mg Si2O6 K Al Si308 join at 6 and s.5 GPa as a model for formation of some potassium bearing deep seated mineral assemblages.
Contributions to Mineralogy and Petrology, Vol. 149, 3, pp. 316-337.
Abstract: Inherited structural, compositional, thermal, and mechanical properties from previous tectonic phases can affect the deformation style of lithosphere entering a new stage of the Wilson cycle. When continental crust jams a subduction zone, the transition from subduction to extension can occur rapidly, as is the case following slab breakoff of the leading subducted oceanic slab. This study explores the extent to which geometric and physical properties of the subduction phase affect the subsequent deformation style and surface morphology of post subduction extensional systems. We focus on regions that transition rapidly from subduction to extension, retaining lithospheric heterogeneities and cold thermal structure inherited from subduction. We present numerical models suggesting that following failed subduction of continental crust (with or without slab breakoff), the extensional deformation style depends on the strength and dip of the preexisting subduction thrust. Our models predict three distinct extensional modes based on these inherited properties: (1) reactivation of the subduction thrust and development of a rolling?hinge detachment that exhumes deep crustal material in a domal structure prior to onset of an asymmetric rift; (2) partial reactivation of a low?angle subduction thrust, which is eventually abandoned as high?angle, “domino”?style normal faults cut and extend the crust above the inherited thrust; and (3) no reactivation of the subduction fault but instead localized rifting above the previous subduction margin as new rift?bounding, high?angle normal faults form. We propose that the first mode is well exemplified by the young, rapidly exhumed Dayman?Suckling metamorphic core complex that is exhuming today in Papua New Guinea.
Geophysical Research Letters, Vol. 46, 20, pp. 11065-110670.
Mantle
geothermometry
Abstract: Earth's magnetic field is produced by a dynamo in the core that requires motion of the fluid Fe alloy. Both thermal convection, arising from the transport of heat in excess of conducted heat, and compositional convection, arising from light element exsolution at the freezing inner core boundary, are suggested as energy sources. The contribution of thermal convection (possibly ranging from nothing to significant) depends on thermal conductivity of the outer core. Our experimental measurements of electrical resistivity of solid and liquid Fe at high pressures show that resistivity is constant along the pressure?dependent melting boundary of Fe. Using our derived thermal conductivity value at the inner core (freezing) boundary, we calculate the heat conducted in the liquid outer core and find that thermal convection is needed to carry additional heat through the outer core to match the heat extracted through the core?mantle boundary.
Diamond & Related Materials, Vol. 116, 108386 10p. Pdf
Global
spectroscopy
Abstract: One of the most important parameters affecting the value of natural colorless diamonds is its light transparency, defined as its color grade. The regular range of color grades in the trade is denoted by alphabet letters in the range D-M, where D represents the best commercial quality. The color grade of diamonds is largely influenced by their nitrogen content (when nitrogen atoms substitute carbon atoms in the crystal) and can be determined from this property. Diamonds absorb electromagnetic radiation in the UV-visible as well as in the Infrared spectral range and therefore, their color grade is measured via spectroscopic light absorption in these frequency range. The electromagnetic properties of different polished diamonds having several nitrogen concentrations in the frequency range of 100-110 GHz (W band) have been studied. The results indicate that there is a good correlation between the amount of nitrogen impurities and the Free Spectral Range (FSR) parameter of a reflection signal, S11, in the antenna. From the study It is concluded that measuring the diamonds dielectric properties via spectroscopic analysis in the millimeter wavelength range, can determine the color grading. In addition, the FSR measurements were correlated well with the FTIR measurements. The methodology of the new color determination mode and a novel color estimate, based on the FSR vs the nitrogen correlation, has been tested on 26 diamonds with a success rate higher than 70%.
Physics and Chemistry of Minerals, DOI 10.1007/ s00269-015-0765-6
Mantle
Melting
Abstract: Carbonatic components of parental melts of the deeper mantle diamonds are inferred from their primary inclusions of (Mg, Fe, Ca, Na)-carbonate minerals trapped at PT conditions of the Earth’s transition zone and lower mantle. PT phase diagrams of MgCO3-FeCO3-CaCO3-Na2CO3 system and its ternary MgCO3-FeCO3-Na2CO3 boundary join were studied at pressures between 12 and 24 GPa and high temperatures. Experimental data point to eutectic solidus phase relations and indicate liquidus boundaries for completely miscible (Mg, Fe, Ca, Na)- and (Mg, Fe, Ca)-carbonate melts. PT fields for partial carbonate melts associated with (Mg, Fe)-, (Ca, Fe, Na)-, and (Na2Ca, Na2Fe)-carbonate solid solution phases are determined. Effective nucleation and mass crystallization of deeper mantle diamonds are realized in multicomponent (Mg, Fe, Ca, Na)-carbonatite-carbon melts at 18 and 26 GPa. The multicomponent carbonate systems were melted at temperatures that are lower than the geothermal ones. This gives an evidence for generation of diamond-parental carbonatite melts and formation of diamonds at the PT conditions of transition zone and lower mantle.
Physics and Chemistry of Minerals, Vol. 42, pp. 817-824.
Mantle
Carbonatite, diamond genesis
Abstract: Carbonatic components of parental melts of the deeper mantle diamonds are inferred from their primary inclusions of (Mg, Fe, Ca, Na)-carbonate minerals trapped at PT conditions of the Earth’s transition zone and lower mantle. PT phase diagrams of MgCO3-FeCO3-CaCO3-Na2CO3 system and its ternary MgCO3-FeCO3-Na2CO3 boundary join were studied at pressures between 12 and 24 GPa and high temperatures. Experimental data point to eutectic solidus phase relations and indicate liquidus boundaries for completely miscible (Mg, Fe, Ca, Na)- and (Mg, Fe, Ca)-carbonate melts. PT fields for partial carbonate melts associated with (Mg, Fe)-, (Ca, Fe, Na)-, and (Na2Ca, Na2Fe)-carbonate solid solution phases are determined. Effective nucleation and mass crystallization of deeper mantle diamonds are realized in multicomponent (Mg, Fe, Ca, Na)-carbonatite-carbon melts at 18 and 26 GPa. The multicomponent carbonate systems were melted at temperatures that are lower than the geothermal ones. This gives an evidence for generation of diamond-parental carbonatite melts and formation of diamonds at the PT conditions of transition zone and lower mantle.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 4773 Abstract
Mantle
Melting
Abstract: Ultrabasic peridotites and pyroxenites together with basic eclogites are the upper-mantle in situ rocks among xenoliths in kimberlites. Occasionally their diamond-bearing varieties have revealed within the xenoliths. Therewith the compositions of rock-forming minerals demonstrate features characteristic for primary diamond-included minerals of peridotite and eclogite parageneses (the elevated contents of Cr-component in peridotitic garnets and Na-jadeitic component in eclogitic clinopyroxenes). High-pressure experimental study of melting equilibria on the multicomponent peridotie-pyroxenite system olivine Ol - orthopyroxene Opx - clinopyroxene Cpx - garnet Grt showed that Opx disappeared in the peritectic reaction Opx+L?Cpx (Litvin, 1991). As a result, the invariant peritectic equilibrium Ol+Opx+Cpx+Grt+L of the ultrabasic system was found to transform into the univariant cotectic assemblage Ol+Cpx+Grt+L. Further experimental investigation showed that olivine reacts with jadeitic component (Jd) with formation of garnet at higher 4.5 GPa (Gasparik, Litvin, 1997). Study of melting relations in the multicomponent system Ol - Cpx - Jd permits to discover the peritectic point Ol+Omph+Grt+L (where Omph - omphacitic clinopyroxene) at concentration 3-4 wt.% Jd-component in the system. The reactionary loss of Opx and Ol makes it possible to transform the 4-phase garnet lherzolite ultrabasic association into the bimineral eclogite assemblage. The regime of fractional Ol, Cpx and Grt crystallization must be accompanied by increasing content of jadeitic component in residual melts that causes the complete "garnetization of olivine". In the subsequent evolution, the melts would have to fractionate for basic SiO2-saturated compositions responsible for petrogenesis of eclogite varieties marked with accessory corundum Crn, kyanite Ky and coesite Coe. Both the peritectic mechanisms occur in regime of fractional crystallization. The sequence of the upper-mantle fractional ultrabasic-basic magmatic evolution and petrogenesis may be controlled by the following melting relations: from Ol, Opx, L field to cotectic curve Ol, Opx, Cpx, L, peritectic point Ol, Opx, Cpx, Grt, L (loss of Opx), cotectic curve Ol, (Cpx+Jd), Grt, L, peritectic point Ol, (Cpx?Omph), Grt, L (loss of Ol), divariant field Omph,Grt,L, cotectic curve Ky, Omph, Grt, L, eutectic point Ky,Coe,Omph, Grt,L, subsolidus assemblage Ky,Coe,Omph, Grt. The fractional ultrabasic-basic evolution of the upper-mantle silicate-carbonate-carbon melts-solutions, which are responsible for genesis of diamond-and-inclusions associations and diamond-bearing peridotites and eclogites, follows the similar physico-chemical mechanisms (Litvin et al., 2016). This is illustrated by fractional syngenesis diagram for diamonds and associated minerals which construction is based on evidence from high pressure experiments. References Gasparik T., Litvin Yu.A (1997). Stability of Na2Mg2Si2O7 and melting relations on the forsterite - jadeite join at pressures up to 22 GPa.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 4785 Abstract
Mantle
Melting
Abstract: Melting relations of the lower-mantle magmatic system MgO - FeO - CaO - SiO2 are characterized by peritectic reaction of bridgmanite (Mg,Fe)SiO3 and melt with formation of Fe-rich phases of periclase-wustite solid solutions (MgO•FeO)ss and stishovite SiO2. The reaction proceeds also in melts-solutions of lower-mantle diamond-parental system MgO - FeO - CaO - SiO2 - (Mg-Fe-Ca-Na-carbonate) - C. Xenoliths of lower mantle rocks were never found among the deep mantle derived materials. Estimation of lower-mantle mineralogy as ferropericlase+ bridgmanite+ Ca-perovskite association is inferred from high-pressure subsolidus experiments with ultrabasic pyrolite composition (Akaogi, 2007). The paradoxical in situ paragenesis of stishovite and ferropericlase as primary inclusions in lower-mantle diamonds (Kaminsky, 2012) takes its explanation from the bridgmanite peritectic reaction (effect of "stishovite paradox") (Litvin et al., 2014). Based on the data for inclusions, physico-chemical study on syngenesis of diamonds and primary inclusions has experimentally revealed the ferropericlase-bridgmanite-Ca-perovskite-stishovite-magnesiowustite-(Mg-Fe-Ca-Na-carbonate)-carbon compositions of the lower-mantle diamond-forming system .(Litvin et al., 2016). The generalized diagram of diamong-forming media characterizes the variable compositions of growths melts for diamonds and paragenetic phases and their genetic relationships with lower mantle matter, and it is the reason for genetic classifying primary inclusions. Fractional ultrabasic-basic evolution and continuous paragenetic transition from ultrabasic bridgmanite-ferropericlase to basic stishovite-magnesiowustite assemblages in the of lower-mantle diamond-parental melts-solutions are providing by the physico-chemical mechanism of stishovite paradox. References Akaogi M. (2007). Phase transformations of minerals in the transition zone and upper part of the lower mantle.
Melting equilibration temperatures of the CaMgSiO3 Mg3Al2Si3O12 K2 Ca (Co2) system modelling a source composition of carbonate - silicate diamond bearing rocks Kokchetav
18th. International Mineralogical Association Sept. 1-6, Edinburgh, abstract p.242. (poster)
Melting relations in the chloride carbonate silicate systems at high pressure and model for formation of alkalic diamond forming liquids in the upper mantle.
Earth and Planetary Science Letters, in press available
Experimental evidence for liquid immiscibility in the model system CaCO3 pyrope pyrrhotite at 7.0 GPa: role of carbonatite and sulfide melts in diamond genesis
Abstract: Experimental studies of phase relations in the oxide-silicate system MgO-FeO-SiO2 at 24 GPa show that the peritectic reaction of bridgmanite controls the formation of stishovite as a primary in situ mineral of the lower mantle and as an effect of the stishovite paradox. The stishovite paradox is registered in the diamond-forming system MgO-FeO-SiO2-(Mg-Fe-Ca-Na carbonate)-carbon in experiments at 26 GPa as well. The physicochemical mechanisms of the ultrabasic-basic evolution of deep magmas and diamondforming media, as well as their role in the origin of the lower mantle minerals and genesis of ultradeep diamonds, are studied.
Abstract: Experimental studies of phase relations in the oxide–silicate system MgO–FeO–SiO2 at 24 GPa show that the peritectic reaction of bridgmanite controls the formation of stishovite as a primary in situ mineral of the lower mantle and as an effect of the stishovite paradox. The stishovite paradox is registered in the diamond-forming system MgO–FeO–SiO2–(Mg–Fe–Ca–Na carbonate)–carbon in experiments at 26 GPa as well. The physicochemical mechanisms of the ultrabasic–basic evolution of deep magmas and diamondforming media, as well as their role in the origin of the lower mantle minerals and genesis of ultradeep diamonds, are studied.
Abstract: This book sheds valuable new light on the genetic mineralogy of lower-mantle diamonds and syngenetic minerals. It presents groundbreaking experimental results revealing the melting relations of ultrabasic and basic associations and a physicochemical peritectic mechanism of their evolution. The experimental investigations included here reveal the key multicomponent, multiphase oxide-silicate-carbonate-carbon parental media for lower-mantle diamonds and syngenetic minerals. Consequently, readers will find extensive information on the diamond-parental oxide-silicate-carbonate-carbon melts-solutions that supplement the general features of lower-mantle diamond genesis and the most efficient ultrabasic-basic evolution. The experimental results on physicochemical aspects, combined with analytical mineralogy data, make it possible to create a generalized composition diagram of the diamond-parental melts-solutions, there by completing the mantle-carbonatite concept for the genesis of lower-mantle diamonds and syngenetic minerals. This book addresses the needs of all researchers studying the Earth’s deepest structure, super-deep mineral formation including diamonds, and magmatic evolution.
Peridotite eclogite carbonatite systems at 7.0-8.5 GPa: concentration barrier of diamond nucleation and syngenesis of the silicate and carbonate inclusions.
Russian Geology and Geophysics, Vol. 50, 12, pp. 1221-1233.
Experimental study of partition of rare elements between minerals and melts of diamond forming eclogite carbonatite and peridotite carbonatites systems.
Interface partition coefficients of trace elements in carbonate-silicate parental media for diamonds and paragenetic inclusions ( experiments at 7.0-8.5 Gpa)
Russian Geology and Geophysics, Vol. 56, 1-2, pp. 221-231.
Abstract: Physical-chemical experimental studies at 12-23 GPa of phase relationships within four-members carbonate system MgCO3-FeCO3-CaCO3-Na2CO3 and its marginal system MgCO3-FeCO3-Na2CO3 were carried out. The systems are quite representative for a set of carbonate phases from inclusions in diamonds within transitional zone and lower mantle. PT-phase diagrams of multicomponent carbonate systems are suggested. PT parameters of boundaries of their eutectic melting (solidus), complete melting (liquids) are established. These boundaries define area of partial melting. Carbonate melts are stable, completely mixable, and effective solvents of elemental carbon thus defining the possibility of ultra-deep diamonds generation.
Abstract: Experimental studies of melting relations in the system ilmenite-K-Na-Mg-Fe-Ca carbonatite-carbon at 8 GPa and 1600°C provide evidence for the effect of liquid immiscibility between ilmenite and carbonatite melts. It is shown that the solubility of ilmenite in carbonatitic melts is negligible and does not depend on its concentration in experimental samples within 25-75 wt %. However, carbonatite-carbon melts are characterized by a high diamond-forming efficiency. This means that the correlation between the concentration of TiO2 and diamond content is problematic for mantle chambers and requires further, more complex, experimental studies.
Geochemistry International, Vol. 56, 9, pp. 881-900.
Russia
deposit - Nyurbinskaya
Abstract: Interaction between a melt of kimberlite from the Nyurbinskaya pipe (Yakutia) and natural monocrystalline diamonds was studied experimentally at 0.15 GPa and 1200-1250°C in high-pressure and high-temperature Ar gas “bombs.” The loss of diamond weight with slight surface dissolution of diamonds in a Ca carbonate-bearing kimberlite melt over the course of 2 h (the period of kimberlite transport from upper-mantle diamond-forming chambers to the crustal cumulative centers) is 3-4.5%. In 4 and 7-8 days (under the conditions of crustal cumulative centers), the weight of diamond decreases with remarkable bulk dissolution by 13.5 and 24.5-27.5%, respectively. In the run at 0.15 GPa and 1200°C kimberlite and ilmenite (added) melts interact to produce perovskite melt. Both of the melts, rich in titanium minerals, are immiscible with kimberlite melt and therefore cannot influence the diamond dissolution kinetics in the kimberlite melt. The experimental results suggest that precisely the dissolution processes for thermodynamically metastable diamonds in silicate-carbonate kimberlitic magmas are responsible for the effective decrease in the diamond potential of kimberlite deposits. The paper discusses the physicochemical reasons for the decrease in the kimberlite diamond potential during the chemically active history of diamond genesis: from upper-mantle chambers to the explosive release of diamonds and kimberlite material from cumulative centers to the Earth’s surface. The data on experimental physicochemical studies of the origin, analytical mineralogy of inclusions, and isotope geochemistry of diamonds are correlated.
Geochemistry International, Vol. 57, 9, pp. 1000-1007.
Mantle
diamond genesis
Abstract: The peritectic reaction of ringwoodite (Mg,Fe)2SiO4 and silicate-carbonate melt with formation of magnesiowustite (Fe,Mg)O, stishovite SiO2, and Mg, Na, Ca, K-carbonates is revealed by experimental study at 20 GPa of phase relations in the multicomponent diamond-forming MgO-FeO-SiO2-Na2CO3-CaCO3-K2CO3 system of the Earth mantle transition zone. An interaction of CaCO3 and SiO2 with a formation of Ca-perovskite CaSiO3 is also detected. It is shown that the peritectic reaction of ringwoodite and melt with the formation of stishovite controls physicochemically the fractional ultrabasic-basic evolution of both magmatic and diamond-forming systems of deep horizons of the transition zone up to its boundary with the Earth lower mantle.
Geochemistry International, Vol. 66, 7, pp. 607-629. pdf
Mantle
plumes
Abstract: The Western Pacific Triangular Zone (WPTZ) is the frontier of a future supercontinent to be formed at 250 Ma after present. The WPTZ is characterized by double-sided subduction zones to the east and south, and is a region dominated by extensive refrigeration and water supply into the mantle wedge since at least 200 Ma. Long stagnant slabs extending over 1200 km are present in the mid-Mantle Boundary Layer (MBL, 410-660 km) under the WPTZ, whereas on the Core-Mantle Boundary (CMB, 2700-2900 km depth), there is a thick high-V anomaly, presumably representing a slab graveyard. To explain the D? layer cold anomaly, catastrophic collapse of once stagnant slabs in MBL is necessary, which could have occurred at 30-20 Ma, acting as a trigger to open a series of back-arc basins, hot regions, small ocean basins, and presumably formation of a series of microplates in both ocean and continent. These events were the result of replacement of upper mantle by hotter and more fertile materials from the lower mantle.
Delamination and ultra deep subduction of continental crust: constraints from elastic wave velocity and density measurement in ultra high pressure metamorphic rocks
Journal of Metamorphic Geology, Vol. 29, 7, pp. 781-801.
Abstract: The genesis of Earth’s largest rare earth element (REE) deposit, Bayan Obo (China), has been intensely debated, in particular whether the host dolomite marble is of sedimentary or igneous origin. The protracted (Mesoproterozoic to Paleozoic) and intricate (magmatic to metasomatic) geological processes complicate geochemical interpretations. In this study, we present a comprehensive petrographic and in situ, high-spatial resolution Sr-Pb isotopic and geochemical investigation of the host dolomite from the Bayan Obo marble. Based on petrographic evidence, the dolomite marble is divided into three facies including coarse-grained (CM), fine-grained (FM), and heterogeneous marble (HM). All carbonates are ferroan dolomite with high SrO and MnO contents (>?0.15 wt.%), consistent with an igneous origin. Trace element compositions of these dolomites are highly variable both among and within individual samples, with CM dolomite displaying the strongest LREE enrichment. In situ 206Pb/204Pb and 207Pb/204Pb ratios of the dolomite are generally consistent with mantle values. However, initial 208Pb/204Pb ratios define a large range from 35.45 to 39.75, which may result from the incorporation of radiogenic Pb released from decomposition of monazite and/or bastnäsite during Early Paleozoic metasomatism. Moreover, in situ Sr isotope compositions of dolomite indicate a large range (87Sr/86Sr?=?0.70292-0.71363). CM dolomite is characterized by a relatively consistent, unradiogenic Sr isotope composition (87Sr/86Sr?=?0.70295-0.70314), which is typical for Mesoproterozoic mantle. The variation of 87Sr/86Sr ratios together with radiogenic 206Pb/204Pb signatures for dolomite within FM and HM possibly represents recrystallization during Early Paleozoic metasomatism with the contribution of radiogenic Sr and Pb from surrounding host rocks. Therefore, our in situ geochemical data support a Mesoproterozoic igneous origin for the ore-bearing dolomite marble in the Bayan Obo deposit, which subsequently underwent intensive metasomatism during the Early Paleozoic.
Earth and Planetary Science Letters, Vol. 580, 11p. Pdf
Mantle
eclogites
Abstract: Zinc isotopic compositions (ZnJMC-Lyon) of low-MgO (<13 wt.%) and high-MgO (>16 wt.%) eclogites from the Koidu kimberlite complex, Sierra Leone, West African Craton, help constrain the origins of cratonic eclogites. The Zn of low-MgO eclogites range from MORB-like to significantly higher values (0.21‰ to 0.75‰), and correlate inversely with Zn concentrations. Since marine carbonates are characterized by higher Zn and lower Zn concentration than basaltic rocks, the low-MgO eclogites are suggested to originate from altered oceanic crustal protoliths that underwent isotopic exchange with carbonates within the crust during subduction. Compared to low-MgO eclogites, all but one of the high-MgO eclogites also have high Zn (0.35‰ to 0.95‰), but they have lower Zn concentrations and Zn/Fe ratios, both of which are negatively correlated with MgO contents. These features point to formation of high-MgO eclogites via metasomatic overprinting of low-MgO eclogites through addition of secondary clinopyroxenes crystallized from infiltrating ultramafic melts. Thus, both low-MgO and high-MgO eclogites bear the imprint of subducted carbonate-bearing oceanic crust. Our study shows that the distinctively high-Zn signatures of marine carbonates can be retained in deeply subducted oceanic crust that may contribute to mantle sources of intraplate alkali basalts with elevated Zn and Zn/Fe. Therefore, Zn isotopes provide a viable means to trace carbonate recycling in the mantle.
Abstract: Aillikites are carbonate-rich ultramafic lamprophyres often associated with carbonatites. Despite their common field relationships, the petrogenetic links, if any, between aillikites and carbonatites remain controversial. To address this question, this study reports the results of a detailed geochemical and isotopic examination of the Permian Wajilitag aillikites in the northwestern Tarim large igneous province, including bulk-rock major-, trace-element and Sr-Nd isotope compositions, olivine major- and trace-element and (in-situ secondary ion mass spectrometry) oxygen isotope compositions, oxygen isotope data for clinopyroxene separates, and bulk-carbonate C-O isotopic analyses. Olivine in the aillikites occurs in two textural types: (i) microcrysts, 0.3-5?mm; and (ii) macrocrysts, 0.5-2.5?cm. The microcrysts exhibit well-defined linear correlations between Fo (79-89), minor and trace elements (e.g., Ni?=?1304-3764??g/g and Mn?=?1363-3042??g/g). In contrast, the olivine macrocrysts show low Fo79-81, Ni (5.3-442??g/g) and Ca (477-1018??g/g) and very high Mn (3418-5123??g/g) contents, and are displaced from the compositional trend of the microcrysts. The microcrysts are phenocrysts crystallized from the host aillikite magmas. Conversely, the lack of mantle-derived xenoliths in these aillikites suggests that the macrocrysts probably represent cognate crystals (i.e., antecrysts) that formed from earlier, evolved aillikite melts. Olivine phenocrysts in the more primitive aillikite dykes (Dyke 1) have relatively higher Fo82-89 and mantle-like oxygen isotope values, whereas those in the more evolved dykes (Dyke 2 and 3) exhibit lower Fo79-86 and oxygen isotope values that trend toward lower than mantle ?18O values. The decreasing ?13C values of carbonate from Dyke 1 through to Dyke 2 and 3, coupled with the indistinguishable Sr-Nd isotopes of these dykes, suggest that the low ?18O values of olivine phenocrysts in Dyke 2 and 3 resulted from carbonate melt/fluid exsolution from a common progenitor melt. These lines of evidence combined with the overlapping emplacement ages and Sr-Nd isotope compositions of the aillikites and carbonatites in this area suggest that these exsolved carbonate melts probably contributed to the formation of the Tarim carbonatites thus supporting a close petrogenetic relationship between aillikites and carbonatites.
Contributions to Mineralogy and Petrology, 176, 19p. Pdf
Mantle
REE
Abstract: Clinopyroxene and orthopyroxene are the two major repositories of rare earth elements (REE) in spinel peridotites. Most geochemical studies of REE in mantle samples focus on clinopyroxene. Recent advances in in situ trace element analysis has made it possible to measure REE abundance in orthopyroxene. The purpose of this study is to determine what additional information one can learn about mantle processes from REE abundances in orthopyroxene coexisting with clinopyroxene in residual spinel peridotites. To address this question, we select a group of spinel peridotite xenoliths (9 samples) and a group of abyssal peridotites (12 samples) that are considered residues of mantle melting and that have major element and REE compositions in the two pyroxenes reported in the literature. We use a disequilibrium double-porosity melting model and the Markov chain Monte Carlo method to invert melting parameters from REE abundance in the bulk sample. We then use a subsolidus reequilibration model to calculate REE redistribution between cpx and opx at the extent of melting inferred from the bulk REE data and at the closure temperature of REE in the two pyroxenes. We compare the calculated results with those observed in clinopyroxene and orthopyroxene in the selected peridotitic samples. Results from our two-step melting followed by subsolidus reequilibration modeling show that it is more reliable to deduce melting parameters from REE abundance in the bulk peridotite than in clinopyroxene. We do not recommend the use of REE in clinopyroxene alone to infer the degree of melting experienced by the mantle xenolith, as HREE in clinopyroxene in the xenolith are reset by subsolidus reequilibration. In general, LREE in orthopyroxene and HREE in clinopyroxene are more susceptible to subsolidus redistribution. The extent of redistribution depends on the modes of clinopyroxene and orthopyroxene in the sample and thermal history experienced by the peridotite. By modeling subsolidus redistribution of REE between orthopyroxene and clinopyroxene after melting, we show that it is possible to discriminate mineral mode of the starting mantle and cooling rate experienced by the peridotitic sample. We conclude that endmembers of the depleted MORB mantle and the primitive mantle are not homogeneous in mineral mode. A modally heterogeneous peridotitic starting mantle provides a simple explanation for the large variations of mineral mode observed in mantle xenoliths and abyssal peridotites. Finally, by using different starting mantle compositions in our simulations, we show that composition of the primitive mantle is more suitable for modeling REE depletion in cratonic mantle xenoliths than the composition of the depleted MORB mantle.
Abstract: Aillikites are carbonate-rich ultramafic lamprophyres, and although they are volumetrically minor components of large igneous province (LIP), these rocks provide important clues to melting and meta-somatism in the deep mantle domain during the initial stages of LIPs. In this study, we investigate the Wajilitag “kimberlites” in the northwestern part of the Tarim LIP that we redefine as hypabyssal aillikites based on the following features: (1) micro-phenocrystic clinopyroxene and Ti-rich andradite garnet occurring in abundance in the carbonate-rich matrix; (2) Cr-spinel exhibiting typical Fe-Ti enrichment trend also known as titanomagnetite trend; and (3) olivine showing dominantly low Mg values (Fo < 90). To constrain the magma source and evolution, the major, minor, and trace element abundance in olivine grains from these rocks were analyzed using electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry. Olivine in the aillikites occurs as two textural types: (1) groundmass olivines, as sub-rounded grains in matrix, and (2) macrocrysts, as euhedral-anhedral crystals in nodules. The groundmass olivines show varying Mg (Fo89-80) with high-Ni (1606-3418 ppm) and Mn (1424-2860 ppm) and low-Ca (571-896 ppm) contents. In contrast, the macrocrysts exhibit a restricted Fo range but a wide range in Ni and Mn. The former occurs as phenocrysts, whereas the latter are cognate cumulates that formed from earlier, evolved aillikite melt. The two olivine populations can be further divided into sub-groups, indicating a multi-stage crystallization history of the aillikite melt. The crystallization temperatures of groundmass olivines and macrocrysts in dunite nodules as computed from the spinel-olivine thermometers are 1005-1136 and 906-1041 °C, respectively. The coupled enrichment of Ca and Ti and lack of correlation between Ni and Sc and Co in the olivine grains suggest a carbonate-silicate metasomatized mantle source. Moreover, the high 100•Mn/Fe (average 1.67) at high Ni (up to 3418 ppm), overlapping with OIB olivine, and the 100•Ni/Mg (~1) of primitive Mg-Ni-rich groundmass olivines suggest a mixed source that involved phlogopite- and carbonate-rich metasomatic veins within mantle peridotite.
Geochimica et Cosmochimica Acta, in press available 45p.
Africa, Russia
Geochronology
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243‰ to ?0.204‰ with an average of ?0.225 ± 0.037‰ (2?, n = 19), and ?56Fe from ?0.038‰ to 0.060‰ with an average of ?0.003 ± 0.068‰ (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (?±0.05‰ for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Geochimica et Cosmochimica Acta, Vol. 200, pp. 167-185.
Africa, South Africa, Russia
Metasomatism
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243 to ?0.204 with an average of ?0.225 ± 0.037 (2?, n = 19), and ?56Fe from ?0.038‰ o 0.060 with an average of ?0.003 ± 0.068 (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (±0.05 for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Geochimica et Cosmochimica Acta, Vol. 200, pp. 167-185.
Africa, South Africa, Russia
Craton, Peridotite
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243‰ to ?0.204‰ with an average of ?0.225 ± 0.037‰ (2?, n = 19), and ?56Fe from ?0.038‰ 0.060 with an average of ?0.003 ± 0.068‰ (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (?±0.05‰ for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Abstract: A fundamental goal of mineralogy and petrology is the deep understanding of mineral phase relationships and the consequent spatial and temporal patterns of mineral coexistence in rocks, ore bodies, sediments, meteorites, and other natural polycrystalline materials. The multi-dimensional chemical complexity of such mineral assemblages has traditionally led to experimental and theoretical consideration of 2-, 3-, or n-component systems that represent simplified approximations of natural systems. Network analysis provides a dynamic, quantitative, and predictive visualization framework for employing “big data” to explore complex and otherwise hidden higher-dimensional patterns of diversity and distribution in such mineral systems. We introduce and explore applications of mineral network analysis, in which mineral species are represented by nodes, while coexistence of minerals is indicated by lines between nodes. This approach provides a dynamic visualization platform for higher-dimensional analysis of phase relationships, because topologies of equilibrium phase assemblages and pathways of mineral reaction series are embedded within the networks. Mineral networks also facilitate quantitative comparison of lithologies from different planets and moons, the analysis of coexistence patterns simultaneously among hundreds of mineral species and their localities, the exploration of varied paragenetic modes of mineral groups, and investigation of changing patterns of mineral occurrence through deep time. Mineral network analysis, furthermore, represents an effective visual approach to teaching and learning in mineralogy and petrology.
Abstract: Porphyritic olivine kimberlitic breccia, discovered in the Dörbed Banner of Inner Mongolia, Western China, is referred to as Longtou Shan Kimberlite in our study. This kimberlite occurs as a pipe in the Halahuogete Formation of Bayan Obo Group. Zircon U-Pb ages of Longtou Shan Kimberlite reveals a Mesoproterozoic age of ~1,552 Ma, constraining the deposition age of Halahuogete Formation to the Mesoproterozoic. Compared with Mesoproterozoic kimberlite of the ancient landmass, it can be inferred that the North China Craton is a member of the Ur ancient continent of the Columbia supercontinent. Furthermore, according to the tectonic background of the Bayan Obo Group, we raise this possibility that “Bayan Obo Aulacogen” should be renamed the “Bayan Obo Continental Rift.”
Abstract: It has been long observed that the amalgamation of supercontinents, including Rodinia, is coeval with peaks of UPb ages of global detrital zircons. However, our new compilation of global geochemical, mineralogical, and ore geologic records shows that the assembly of Rodinia stands out from others, in terms of whole-rock trace element geochemistry, as well as records of mineralogy and ore deposits. During the assembly of Rodinia, Nb, Y, and Zr concentrations were enriched in igneous rocks, with prolific formation of zircon and minerals bearing Th, Nb or Y, and formation of NYF-type pegmatites and REE ore deposits. At the same time, many types of ore deposits are relatively poorly represented in Rodinin terranes, including deposits of orogenic gold, porphyry copper, and volcanic hosted massive sulfide deposits, with a corresponding paucity of many minerals (e.g., minerals bearing Au, Sb, Ni) associated with these deposits. We interpret these records as indicating the prevalence of ‘non-arc’ magmatism and a relative lack of subduction-related arc magma preserved in the surviving pieces of the Rodinia supercontinent, distinct from other episodes of supercontinent assembly. We further attribute the prevalence of ‘non-arc’ magmatism to enhanced asthenosphere-lithosphere interactions in the Mesoproterozoic, and speculate that the lack of ‘arc-collisional’ magma may be related to enhanced erosion of Rodinia orogenic belts.
Abstract: Porphyritic olivine kimberlitic breccia, discovered in the Dörbed Banner of Inner Mongolia, Western China, is referred to as Longtou Shan Kimberlite in our study. This kimberlite occurs as a pipe in the Halahuogete Formation of Bayan Obo Group. Zircon U–Pb ages of Longtou Shan Kimberlite reveals a Mesoproterozoic age of ~1,552 Ma, constraining the deposition age of Halahuogete Formation to the Mesoproterozoic. Compared with Mesoproterozoic kimberlite of the ancient landmass, it can be inferred that the North China Craton is a member of the Ur ancient continent of the Columbia supercontinent. Furthermore, according to the tectonic background of the Bayan Obo Group, we raise this possibility that “Bayan Obo Aulacogen” should be renamed the “Bayan Obo Continental Rift.”
Repeated kimberlite magmatism beneath Yakutia and its relationship to Siberian flood volcanism: insights from in situ U-Pb and Sr-Nd perovskite isotope analysis.
Earth and Planetary Science Letters, Vol. 404, Oct. pp. 283-295.
Abstract: Neoproterozoic kimberlite, ultramafic lamprophyre, and carbonatite magmatic activity was widespread across the Canadian-Greenland Shield. Models to explain the preponderance of this deeply-derived CO2-rich magmatism between 680-540 Ma range from impingement of multiple mantle plumes to rifting activity linked to the breakout of the Laurentian plate from the Rodinia supercontinent configuration. We add to the debate about the origin of kimberlite magmas and evaluate possible mantle sources of the 655 Ma ‘diamond-rich’ Renard (new SIMS U/Pb perovskite ages) and 629 Ma ‘barren’ Wemindji kimberlites on the eastern Superior craton in Quebec, Canada. Our Sr-Nd-Hf and carbon isotope data (87Sr/86Sri = 0.70241-0.70442; ?Ndi = + 0.2 to + 4.8; ?Hfi = + 0.3 to + 6.5; ?13C = ? 5.6 to ? 3.9‰) suggest a common and moderately depleted convecting upper mantle source region for both the Renard and Wemindji kimberlites, which occur 400-km apart in the interior of the Superior craton. In contrast, the low Os isotope ratios (187Os/188Osi = 0.11078-0.12620; ?Osi = ? 13.7 to ? 1.6) and unfractionated chondritic relative HSE abundances (Os, Ir, Ru, Pt, Pd, Re) indicate significant involvement of ancient refractory cratonic mantle material in kimberlite magma formation. Our model calculations suggest that for both the diamond-rich Renard and the barren Wemindji kimberlite magmas up to 30% of the Os was derived from refractory cratonic peridotites. This material might have been assimilated by originally more CO2-rich carbonated silicate melts derived from the asthenosphere. We also show that the geochemical and Sr-Nd-Hf-Os isotopic compositions of the Renard and Wemindji kimberlites do not require significant input from melts derived from olivine-poor cratonic mantle lithologies such as MARID-type veins and pyroxenites/eclogites. This contrasts with the petrogenesis of deeply-derived volatile-rich potassic magmas found along the peripheries of cratons (e.g., ultramafic lamprophyres, kamafugites, and olivine lamproites), a setting where abundant non-peridotitic components have been added to the lithospheric mantle over the course of continent evolution. Provided that CO2-rich melts, such as proto-kimberlites, occur near the solidus of volatile-fluxed peridotites, no excess mantle heat is required in their formation. This important but often overlooked constraint, together with the observation that there exist no spatial or temporal relationships between the Superior craton kimberlites and Large Igneous Provinces during the Late Neoproterozoic, suggests that kimberlite magmatic activity was tectonically controlled. In our preferred model, ubiquitous CO2-rich proto-kimberlite melts form during volatile-controlled redox melting processes at ambient mantle temperatures in a thermal boundary layer directly beneath thick cratonic lithosphere. The success rate of ‘evolving’ hybrid kimberlite magmas reaching Earth’s surface increases when tensile stresses propagate into the > 200 km thick keels of continental lithosphere. These conditions are frequently met during fast and changing plate motions associated with the assembly and breakup of supercontinents.
Abstract: Neoproterozoic kimberlite, ultramafic lamprophyre, and carbonatite magmatic activity was widespread across the Canadian-Greenland Shield. Models to explain the preponderance of this deeply-derived CO2-rich magmatism between 680–540 Ma range from impingement of multiple mantle plumes to rifting activity linked to the breakout of the Laurentian plate from the Rodinia supercontinent configuration. We add to the debate about the origin of kimberlite magmas and evaluate possible mantle sources of the 655 Ma ‘diamond-rich’ Renard (new SIMS U/Pb perovskite ages) and 629 Ma ‘barren’ Wemindji kimberlites on the eastern Superior craton in Quebec, Canada. Our Sr-Nd-Hf and carbon isotope data (87Sr/86Sri = 0.70241–0.70442; ?Ndi = + 0.2 to + 4.8; ?Hfi = + 0.3 to + 6.5; ?13C = ? 5.6 to ? 3.9‰) suggest a common and moderately depleted convecting upper mantle source region for both the Renard and Wemindji kimberlites, which occur 400 km apart in the interior of the Superior craton. In contrast, the low Os isotope ratios (187Os/188Osi = 0.11078–0.12620; ?Osi = ? 13.7 to ? 1.6) and unfractionated chondritic relative HSE abundances (Os, Ir, Ru, Pt, Pd, Re) indicate significant involvement of ancient refractory cratonic mantle material in kimberlite magma formation. Our model calculations suggest that for both the diamond-rich Renard and the barren Wemindji kimberlite magmas up to 30% of the Os was derived from refractory cratonic peridotites. This material might have been assimilated by originally more CO2-rich carbonated silicate melts derived from the asthenosphere. We also show that the geochemical and Sr-Nd-Hf-Os isotopic compositions of the Renard and Wemindji kimberlites do not require significant input from melts derived from olivine-poor cratonic mantle lithologies such as MARID-type veins and pyroxenites/eclogites. This contrasts with the petrogenesis of deeply-derived volatile-rich potassic magmas found along the peripheries of cratons (e.g., ultramafic lamprophyres, kamafugites, and olivine lamproites), a setting where abundant non-peridotitic components have been added to the lithospheric mantle over the course of continent evolution. Provided that CO2-rich melts, such as proto-kimberlites, occur near the solidus of volatile-fluxed peridotites, no excess mantle heat is required in their formation. This important but often overlooked constraint, together with the observation that there exist no spatial or temporal relationships between the Superior craton kimberlites and Large Igneous Provinces during the Late Neoproterozoic, suggests that kimberlite magmatic activity was tectonically controlled. In our preferred model, ubiquitous CO2-rich proto-kimberlite melts form during volatile-controlled redox melting processes at ambient mantle temperatures in a thermal boundary layer directly beneath thick cratonic lithosphere. The success rate of ‘evolving’ hybrid kimberlite magmas reaching Earth’s surface increases when tensile stresses propagate into the > 200 km thick keels of continental lithosphere. These conditions are frequently met during fast and changing plate motions associated with the assembly and breakup of supercontinents.
Abstract: The characteristics of the sub-continental lithospheric mantle (SCLM) post-date the Siberian plume event (250 Ma) is still unclear; nearly all published data for mantle xenoliths are from a single kimberlite erupt before he Siberian plume (Udachnaya). We report major elements of the whole rock, trace elements data of clinopyroxene and Re-Os isotope and PGE concentration of mantle xenoliths from the Obnazhennaya kimberlite pipe (160 Ma). The Obnazhennaya mantle xenoliths, including spinel harzburgites, spinel dunites, spinel lherzolites, spinel-garnet lherzolite. The spinel harzburgites and dunites have refractory compositions, with 0.23-1.35 wt.% Al2O3, 0.41-3.11 wt.% CaO and 0.00-0.09 wt.% TiO2. Clinopyroxenes in harzburgites and dunites have lower Na2O but higher Cr2O3 contents. Modeling of the Y and Yb contents in clinopyroxenes indicates that the spinel harzburgites and dunites have been subjected to ca. 12-17% degrees of partial melting. The spinel harzburgites and dunites have 187Os/188Os of 0.11227-0.11637, giving a TRD age of 1.6-2.2 Ga. This suggests that old cratonic mantle still existed beneath the Obnazhennaya. In contrast, the lherzolites (both spinel- and spinel-garnet-) have more fertile compositions, containing 2.16-6.55 wt.% Al2O3, 2.91-7.55 wt.% CaO and 0.04-0.15 wt.% TiO2. Both spinel and spinelgarnet lherzolites have more radiogenic 187Os/188Os ratios (0.11931-0.17627), enriched P-PGEs. The higher Al2O3 and Os content and depleted IPGE character of these lherzolites suggest that they were not juvenile mantle accreted by Siberian mantle plume but the refertilized ancient mantle. Therefore, our result suggest that the cratonic mantle beneath the Obnazhennaya has not been replaced by juvenile mantle during the Siberian mantle plume.
Abstract: The characteristics of the sub-continental lithospheric mantle (SCLM) post-date the Siberian plume event (250 Ma) is still unclear; nearly all published data for mantle xenoliths are from a single kimberlite erupt before he Siberian plume (Udachnaya). We report major elements of the whole rock, trace elements data of clinopyroxene and Re-Os isotope and PGE concentration of mantle xenoliths from the Obnazhennaya kimberlite pipe (160 Ma). The Obnazhennaya mantle xenoliths, including spinel harzburgites, spinel dunites, spinel lherzolites, spinel-garnet lherzolite. The spinel harzburgites and dunites have refractory compositions, with 0.23-1.35 wt.% Al2O3, 0.41-3.11 wt.% CaO and 0.00-0.09 wt.% TiO2. Clinopyroxenes in harzburgites and dunites have lower Na2O but higher Cr2O3 contents. Modeling of the Y and Yb contents in clinopyroxenes indicates that the spinel harzburgites and dunites have been subjected to ca. 12-17% degrees of partial melting. The spinel harzburgites and dunites have 187Os/188Os of 0.11227-0.11637, giving a TRD age of 1.6-2.2 Ga. This suggests that old cratonic mantle still existed beneath the Obnazhennaya. In contrast, the lherzolites (both spinel- and spinel-garnet-) have more fertile compositions, containing 2.16-6.55 wt.% Al2O3, 2.91-7.55 wt.% CaO and 0.04-0.15 wt.% TiO2. Both spinel and spinelgarnet lherzolites have more radiogenic 187Os/188Os ratios (0.11931-0.17627), enriched P-PGEs. The higher Al2O3 and Os content and depleted IPGE character of these lherzolites suggest that they were not juvenile mantle accreted by Siberian mantle plume but the refertilized ancient mantle. Therefore, our result suggest that the cratonic mantle beneath the Obnazhennaya has not been replaced by juvenile mantle during the Siberian mantle plume.
Abstract: A comprehensive, internally consistent U-Pb and Lu-Hf isotope data set for 93 mantle-derived zircons from the Yakutian kimberlite province confirms and further refines the four major episodes of kimberlite magmatism on the Siberian craton: 421-409?Ma (Late Silurian-Early Devonian), 358-353?Ma (Late Devonian-Early Carboniferous), 226-218?Ma (Late Triassic), and 161-144?Ma (Middle-Late Jurassic). The relatively narrow, constant range of ?Hf values between +2 and +10 for both the Paleozoic and Mesozoic mantle-derived zircons (and by inference kimberlites) suggests that the volatile-rich magmas were repeatedly sourced from the convecting upper mantle beneath the Siberian craton. This finding is in keeping with the narrow and constant range of ?Nd values for groundmass perovskites from the Yakutian kimberlite province between +1.8 and +5.5 between 420 and 150?Ma. Our preferred model implies that the convecting upper mantle beneath the Yakutian kimberlite province ‘recovered’ rapidly back to ambient conditions shortly after the giant plume-related flood volcanic event that produced the Siberian Traps at 250?Ma. Although close spatial relationships exist between kimberlites and flood basalts on the Siberian craton during both the Paleozoic and Mesozoic, exact timing of the igneous events and the isotopic compositions of the diverse deep-sourced melting products rule out any direct genetic links.Besides the highly economic kimberlite-hosted diamond deposits of Late Devonian age (e.g., Mir and Udachnaya), the Siberian craton also contains significant Mesozoic placer diamond deposits (e.g., along the Anabar river), for which lamproite sources have been suggested recently. Our study shows that mantle-derived zircon megacryst fragments from the Ebelyakh placer deposit have Late Triassic ages of ca. 224?Ma. Their long-term depleted Hf isotopic compositions (+8.5 ?Hf) suggest that the alluvial diamonds were sourced from asthenosphere-derived Triassic kimberlites rather than from lithospheric mantle derived isotopically enriched lamproites.
Abstract: A comprehensive, internally consistent U-Pb and Lu-Hf isotope data set for 93 mantle-derived zircons from the Yakutian kimberlite province confirms and further refines the four major episodes of kimberlite magmatism on the Siberian craton: 421-409?Ma (Late Silurian-Early Devonian), 358-353?Ma (Late Devonian-Early Carboniferous), 226-218?Ma (Late Triassic), and 161-144?Ma (Middle-Late Jurassic). The relatively narrow, constant range of ?Hf values between +2 and +10 for both the Paleozoic and Mesozoic mantle-derived zircons (and by inference kimberlites) suggests that the volatile-rich magmas were repeatedly sourced from the convecting upper mantle beneath the Siberian craton. This finding is in keeping with the narrow and constant range of ?Nd values for groundmass perovskites from the Yakutian kimberlite province between +1.8 and +5.5 between 420 and 150?Ma. Our preferred model implies that the convecting upper mantle beneath the Yakutian kimberlite province ‘recovered’ rapidly back to ambient conditions shortly after the giant plume-related flood volcanic event that produced the Siberian Traps at 250?Ma. Although close spatial relationships exist between kimberlites and flood basalts on the Siberian craton during both the Paleozoic and Mesozoic, exact timing of the igneous events and the isotopic compositions of the diverse deep-sourced melting products rule out any direct genetic links. Besides the highly economic kimberlite-hosted diamond deposits of Late Devonian age (e.g., Mir and Udachnaya), the Siberian craton also contains significant Mesozoic placer diamond deposits (e.g., along the Anabar river), for which lamproite sources have been suggested recently. Our study shows that mantle-derived zircon megacryst fragments from the Ebelyakh placer deposit have Late Triassic ages of ca. 224?Ma. Their long-term depleted Hf isotopic compositions (+8.5 ?Hf) suggest that the alluvial diamonds were sourced from asthenosphere-derived Triassic kimberlites rather than from lithospheric mantle derived isotopically enriched lamproites.
Abstract: Volcanic pipes, or maar-diatreme volcanoes, form during explosive eruptions of mantle-derived magmas near Earth's surface. Impressive examples are the carrot-shaped, downward tapering structures formed by kimberlite magmas. Kimberlites originate from >150 km depth within Earth's mantle beneath thick continental roots, away from tectonic plate margins. Kimberlite pipes can be significant diamond deposits, and the complex architecture revealed during exploration and mining is ascribed to repeated magma injections leading to multiple eruptions. Repeated magmatic pulses cause diatremes to widen and grow downward, forming kilometer-sized subterranean structures. However, the time-resolved evolution of kimberlite pipe systems is largely unknown. We present the first U/Pb perovskite ages for newly discovered kimberlite dikes (1139.8 ± 4.8 Ma) that cut through the volcaniclastic infill of the Premier kimberlite pipe (1153.3 ± 5.3 Ma) at Cullinan Diamond Mine, South Africa. The ages reveal that renewed kimberlite volcanic activity occurred, at a minimum, 3 m.y. after the main pipe formation. This finding suggests that the largest kimberlite pipes, and maar-diatreme volcanoes in general, may be magmatically active for several millions of years, which conflicts with this volcanism being described as 'monogenetic' at millennia time scales. Exemplified by Tier-1 diamond deposits on the Kaapvaal craton, long-lasting kimberlite volcanic activity may be an important factor in growing large diatremes, plus enabling effective transport of mantle cargo from the diamond stability field to Earth's surface.
Abstract: A comprehensive, internally consistent U-Pb and Lu-Hf isotope data set for 93 mantle-derived zircons from the Yakutian kimberlite province confirms and further refines the four major episodes of kimberlite magmatism on the Siberian craton: 421-409?Ma (Late Silurian-Early Devonian), 358-353?Ma (Late Devonian-Early Carboniferous), 226-218?Ma (Late Triassic), and 161-144?Ma (Middle-Late Jurassic). The relatively narrow, constant range of ?Hf values between +2 and +10 for both the Paleozoic and Mesozoic mantle-derived zircons (and by inference kimberlites) suggests that the volatile-rich magmas were repeatedly sourced from the convecting upper mantle beneath the Siberian craton. This finding is in keeping with the narrow and constant range of ?Nd values for groundmass perovskites from the Yakutian kimberlite province between +1.8 and +5.5 between 420 and 150?Ma. Our preferred model implies that the convecting upper mantle beneath the Yakutian kimberlite province ‘recovered’ rapidly back to ambient conditions shortly after the giant plume-related flood volcanic event that produced the Siberian Traps at 250?Ma. Although close spatial relationships exist between kimberlites and flood basalts on the Siberian craton during both the Paleozoic and Mesozoic, exact timing of the igneous events and the isotopic compositions of the diverse deep-sourced melting products rule out any direct genetic links. Besides the highly economic kimberlite-hosted diamond deposits of Late Devonian age (e.g., Mir and Udachnaya), the Siberian craton also contains significant Mesozoic placer diamond deposits (e.g., along the Anabar river), for which lamproite sources have been suggested recently. Our study shows that mantle-derived zircon megacryst fragments from the Ebelyakh placer deposit have Late Triassic ages of ca. 224?Ma. Their long-term depleted Hf isotopic compositions (+8.5 ?Hf) suggest that the alluvial diamonds were sourced from asthenosphere-derived Triassic kimberlites rather than from lithospheric mantle derived isotopically enriched lamproites.
Song, S., Zhang, L., Niu, Y., Su, L., Jian, P., Liu, D.
Geochronology of diamond bearing zircons from garnet peridotite in the North Qaidam UHPM belt, Northern Tibetan Plateau: a record of lithospheric subduction.
Earth and Planetary Science Letters, Vol. 234, 1-2, pp. 99-118.
Evolution from oceanic subduction to continental collision: a case study from the northern Tibetan Plateau based on geochemical and geochronological data.
Abstract: The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2?Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66-3.20?Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2?Ga granulite-facies assemblage in a metagabbro of the AGC of central Swaziland and relate this to a major thermo-magmatic event that not only affected the AGC but also the neighbouring Barberton granitoid-greenstone terrane. Some previous models have related the 3.2?Ga event in the eastern Kaapvaal craton to subduction processes, but we see no evidence for long, narrow belts and metamorphic facies changes reflecting lithospheric suture zones, and there is no unidirectional asymmetry in the thermal structure across the entire region from Swaziland to the southern Barberton granite-greenstone terrane as is typical of Phanerozoic and Proterozoic belts. Instead, we consider an underplating event at ca. 3.2?Ga, giving rise to melting in the lower crust and mixing with mantle-derived under- and intraplated mafic magma to generate the voluminous granitoid assemblages now observed in the AGC and the southern Barberton terrane. This is compatible with large-scale crustal reworking during a major thermo-magmatic event and the apparent lack of a mafic lower crust in the Kaapvaal craton as shown by seismic data.
Earth & Environment Communications, 10.1038/s43247-021-00145-3 16p. Pdf
United States, Hawaii
magmatism
Abstract: Volcanoes represent one of the largest natural sources of metals to the Earth’s surface. Emissions of these metals can have important impacts on the biosphere as pollutants or nutrients. Here we use ground- and drone-based direct measurements to compare the gas and particulate chemistry of the magmatic and lava-seawater interaction (laze) plumes from the 2018 eruption of K?lauea, Hawai’i. We find that the magmatic plume contains abundant volatile metals and metalloids whereas the laze plume is further enriched in copper and seawater components, like chlorine, with volatile metals also elevated above seawater concentrations. Speciation modelling of magmatic gas mixtures highlights the importance of the S2? ligand in highly volatile metal/metalloid degassing at the magmatic vent. In contrast, volatile metal enrichments in the laze plume can be explained by affinity for chloride complexation during late-stage degassing of distal lavas, which is potentially facilitated by the HCl gas formed as seawater boils.
Identification of UHP and non-UHP orthogneisses in the Sulu UHP terrane, eastern China: evidence from SHRIMP U-Pb dating of mineral inclusion bearing zircons.
International Geology Review, Vol. 48, 12, pp. 1067-1086.
Zhang, Z.M., Dong, X., Liou, J.G., Liu, F., Wang, W., Yui, F.
Metasomatism of garnet periodite from Jiangzhuang, Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of cont. lithosphere
Journal of Metamorphic Geology, in press available
Zhang, Z.M., Dong, X., Liou, J.G., Liu, F., Wang, W., Yui, F.
Metasomatism of garnet peridotite from Jiangzhuang, southern Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of continental
Journal of Metamorphic Geology, Vol. 29, 9, pp. 917-937.
Liou, J.G., Zhang, R., Liu, F., Zhang, Z., Ernst, W.G.
Mineralogy, petrology, U-Pb geochronology, and geologic evolution of the Dabie Sulu classic ultrahigh pressure metamorphic terrane, east-central China.
American Mineralogist, Vol. 97, no. 10, pp. 1533-1543.
Geological Society of America Annual Meeting, Vol. 47, 7, p. 163. abstract
Europe, Turkey
Moissanite
Abstract: The Aladag ophiolite in the eastern Tauride belt, southern Turkey, is a well-preserved remnant of oceanic lithosphere. It consists of, in ascending order, harzburgitic to dunitic tectonites, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes and basaltic pillow lavas. Podiform chromitites are common in the mantle peridotites. Thus far, more than 200 grains of microdiamond and more than 100 grains of moissanite (SiC) have been separated from one sample of podiform chromitite. The microdiamonds occur mostly as subhedral to euhedral, colorless to pale yellow grains, about 50-300 ?m in size. Moissanite grains are generally subhedral, light blue to deep blue in color and variable in size. These grains of diamond and moissanite are very similar to in-situ grains in podiform chromitites of Tibet and the Polar Urals of Russia (Yang et al., 2014; 2015), indicating that they are natural minerals, not the result of natural or anthropogenic contamination. As reported elsewhere, the diamonds and moissanite are accompanied by a range of other minerals, including rutile, zircon, quartz and sulfides. The discovery of diamond, moissanite and other unusual minerals in the podiform chromitites of the Aladag massif provide additional evidence for the widespread occurrence of these minerals in ophiolites, indicating that they are related to global mantle processes.
Abstract: The ophiolites that crop out discontinuously along the ?2000 km Yarlung Zangbo Suture zone (YZSZ) between the Nanga Parbat and Namche Barwa syntaxes in southern Tibet represent the remnants of Neotethyan oceanic lithosphere (Fig. 1a). We have investigated the internal structure and the geochemical makeup of mafic-ultramafic rock assemblages that are exposed in the westernmost segment of the YZSZ where the suture zone architecture displays two distinct sub-belts of ophiolitic and mélange units separated by a continental Zhongba terrane (Fig. 1b). These two sub-belts include the Daba – Xiugugabu in the south (Southern sub-belt, SSB) and the Dajiweng – Saga in the north (Northern sub-belt, NSB). We present new structural, geochemical, geochronological data from upper mantle peridotites and mafic dike intrusions occurring in these two sub-belts and discuss their tectonomagmatic origin. In-situ analysis of zircon grains obtained from mafic dikes within the Baer, Cuobuzha and Jianabeng massifs in the NSB, and within the Dongbo, Purang, Xiugugabu, Zhaga and Zhongba in the SSB have yielded crystallization ages ranging between130 and 122 Ma. Dike rocks in both sub-belts show N-MORB REE patterns and negative Nb, Ta and Ti anomalies, reminiscent of those documented from SSZ ophiolites. Harzburgitic host rocks of the mafic dike intrusions mainly display geochemical compositions of abyssal peridotites (Fig. 2), with the exception of the Dajiweng harzburgites, which show the geochemical signatures of forearc peridotites (Lian et al., 2016). Extrusive rocks that are spatially associated with these peridotite massifs in both sub-belts also have varying compositional and geochemical features. Tithonian to Valanginian (150 – 135 Ma) basaltic rocks in the Dongbo massif have OIB-like geochemistry and 138 Ma basaltic lavas in the Purang massif have EMORB-like geochemistry (Liu et al., 2015). Tuffaceous rocks in the Dajiweng massif are 140 Ma in age and show OIB-like geochemistry. We interpret these age and geochemical data to reflect a rifted continental margin origin of the extrusive rock units in both sub-belts. These data and structural observations show that the western Yarluang Zangbo ophiolites represent fragments of an Ocean-Continent Transition (OCT) peridotites altered by fluids in an initial supersubduction setting. We infer that mafic-ultramafic rock assemblages exposed in the SSB and NSB initially formed in an ocean – continent transition zone (OCTZ) during the late Jurassic, and that they were subsequently emplaced in the forearc setting of an intraoceanic subduction zone within a Neotethyan seaway during 130 to 122 Ma. The NSB and SSB are hence part of a single, S-directed nappe sheet derived from a Neotethyan seaway located north of the Zhongba terrane.
Abstract: The Pozanti-Karsanti ophiolite situated in the eastern Tauride belt, southern Turkey, is a well-preserved oceanic lithosphere remnant comprising, in ascending order, mantle peridotite, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes, and basaltic pillow lavas. Two types of chromitites are observed in the Pozanti-Karsanti ophiolite. One type of chromitites occurs in the cumulate dunites around the Moho, and the other type of chromitites is hosted by the mantle harzburgites below the Moho. The second type of chromitites has massive, nodular, and disseminated textures. We have conducted the mineral separation work on the podiform chromitites hosted by harzburgites. So far, more than 100 grains of microdiamond and moissanite (SiC) have been recovered from the podiform chromitite. The diamonds and moissanite are accompanied by large amounts of rutile. Besides zircon, monazite and sulfide are also very common phases within the separated minerals. The discovery of diamond, moissanite, and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new evidences for the common occurrences of these unusual minerals in ophiolitic peridotites and chromitites. This discovery also suggests that deep mantle processes and materials have been involved in the formation of podiform chromitite.
Earth and Planetary Science Letters, Vol. 474, pp. 128-137.
Mantle
peridotites
Abstract: To better constrain the Ca isotopic composition of the Bulk Silicate Earth (BSE) and explore the Ca isotope fractionation in the mantle, we determined the Ca isotopic composition of 28 peridotite xenoliths from Mongolia, southern Siberia and the Siberian craton. The samples are divided in three chemical groups: (1) fertile, unmetasomatized lherzolites (3.7-4.7 wt.% Al2O3); (2) moderately melt-depleted peridotites (1.3-3.0 wt.% Al2O3) with no or very limited metasomatism (LREE-depleted cpx); (3) strongly metasomatized peridotites (LREE-enriched cpx and bulk rock) further divided in subgroups 3a (harzburgites, 0.1-1.0% Al2O3) and 3b (fertile lherzolites, 3.9-4.3% Al2O3). In Group 1, ?44/40Ca of fertile spinel and garnet peridotites, which experienced little or no melting and metasomatism, show a limited variation from 0.90 to 0.99‰ (relative to SRM 915a) and an average of 0.94 ± 0.05‰ (2SD, ), which defines the Ca isotopic composition of the BSE. In Group 2, the ?44/40Ca is the highest for three rocks with the lowest Al2O3, i.e. the greatest melt extraction degrees (average ‰, i.e. ?0.1‰ heavier than the BSE estimate). Simple modeling of modal melting shows that partial melting of the BSE with ranging from 0.10 to 0.25 can explain the Group 2 data. By contrast, ?44/40Ca in eight out of nine metasomatized Group 3 peridotites are lower than the BSE estimate. The Group 3a harzburgites show the greatest ?44/40Ca variation range (0.25-0.96‰), with ?44/40Ca positively correlated with CaO and negatively correlated with Ce/Eu. Chemical evidence suggests that the residual, melt-depleted, low-Ca protoliths of the Group 3a harzburgites were metasomatized, likely by carbonate-rich melts/fluids. We argue that such fluids may have low (?0.25‰) ?44/40Ca either because they contain recycled crustal components or because Ca isotopes, similar to trace elements and their ratios, may be fractionated by kinetic and/or chromatographic effects of melt percolation in the mantle. The ?44/40Ca in Group 3b lherzolites (0.83-0.89‰) are lower than in the BSE as well, but the effects of metasomatism on ?44/40Ca are smaller, possibly because of the high Ca contents in their protoliths and/or smaller ?44/40Ca differences between the protoliths and metasomatic agents. The BSE estimates based on fertile peridotites in this study fall in the ?44/40Ca ranges for oceanic and continental basalts, various meteorites (achondrites; carbonaceous, ordinary and enstatite chondrites), Mars, and the Moon. These results provide benchmarks for the application of Ca isotopes to planet formation, mantle evolution, and crustal recycling.
Abstract:
We report new U-Pb zircon age data, zircon in situ oxygen isotope, mineral chemistry, whole-rock geochemistry and Sr-Nd isotopic compositions from the Early Devonian ultrapotassic Gucheng pluton in the North China Craton, and discuss its petrogenesis. The Gucheng pluton is exposed in the northern part of the North China Craton and forms a composite intrusion, consisting of K-feldspar-bearing clinopyroxenite, clinopyroxene-bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene (Wo43-48En19-35Fs18-38), sanidine (An0Ab3-11Or89-97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in situ oxygen isotopes (?18O = 5.2-6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the North China Craton. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the North China Craton facing the Palaeo-Asian Ocean.
Abstract: A modeling approach combining the entropy method and color difference formula is proposed in order to quantitatively evaluate diamond scintillation. The images of 66 diamonds were captured from 0° to 105° rotation at 15° intervals. The color difference of corresponding pixels in adjacent rotation angle images was calculated using a MatLab r2014a program, which indicated the diamond’s color change due to its scintillation. A threshold (10) was determined to divide the color difference into seven color difference intervals, the percentage of which indicated the color-change area. The color difference and the percentage were comprehensively analyzed using the entropy method to evaluate diamond scintillation objectively and quantitatively. Lightness was the main factor affecting the diamond scintillation while chroma and hue also significantly affected it.
Abstract: The Mengyin diamondiferous kimberlite cluster in Shandong province is one of the three major sources of natural diamond in China, where many brown diamonds are mined, but the genesis of their color is still controversial. In this paper, studies including microscopic examination, optical properties of orthogonal polarization, low temperature photoluminescence spectra, infrared spectra, Raman spectra, ultraviolet-visible absorption spectra, luminescence of cathodoluminescence, and transmission electron microscopy have been carried out on the uncut brown diamonds and their slice samples to constrain on the color genesis of brown diamond from the Mengyin deposit. The results show that the brown color is dominantly caused by plastic deformation, and some samples are also caused by non-deformation-related defects and inclusions.
Liu, F.L., Gerdes, A., Liou, J.G., Xue, H.M., Liang, F.H.
SHRIMP U Pb zircon dating from Sulu Dabie dolomitic marble, eastern China: constraints on prograde, ultrahigh pressure and retrograde metamorphic ages.
Journal of Metamorphic Geology, Vol. 24, 7, Sept. pp. 569-589.
Differential subduction and exhumation of crustal slices in the Sulu HP-UHP metamorphic terrane: insights from mineral inclusions, trace elements, U-Pb and Lu Hf isotope analyses of zircon in orthogneiss
Journal of Metamorphic Geology, Vol. 27, 9, pp. 805-825.
Zircon as the best mineral for P-T time history of UHP metamorphism: a review on mineral inclusions and U-Pb SHRIMP ages of zircons from the Dabie Sulu UHP rocks.
Journal of Asian Earth Sciences, Vol. 40, 1, pp. 1-39.
Earth and Planetary Science Letters, Vol. 496, pp. 80-88.
Mantle
perovskite, hotspots
Abstract: Mineralogical studies indicate that two major phase transitions occur near the depth of 660 km in the Earth's pyrolitic mantle: the ringwoodite (Rw) to perovskite (Pv) + magnesiowüstite (Mw) and the majorite (Mj) to perovskite (Pv) phase transitions. Seismological results also show a complicated phase boundary structure at this depth in plume regions. However, previous geodynamical modeling has mainly focused on the effects of the Rw-Pv+Mw phase transition on plume dynamics and has largely neglected the effects of the Mj-Pv phase transition. Here, we develop a 3-D regional spherical geodynamic model to study the combined influence of these two phase transitions on plume dynamics. Our results show the following: (1) A double phase boundary occurs in the high-temperature center of the plume, corresponding to the double reflections in seismic observations. Other plume regions feature a single, flat uplifted phase boundary, causing a gap of high seismic velocity anomalies. (2) Large amounts of relatively low-temperature plume materials can be trapped in the transition zone due to the combined effects of phase transitions, forming a complex truncated cone shape. (3) The Mj-Pv phase transition greatly enhances the plume penetration capability through 660-km phase boundary, which has a significant influence on the plume dynamics. Our results provide new insights which can be used to better constrain the 660-km discontinuity variations, seismic wave velocity structure and plume dynamics in the mantle transition zone. The model can also help to estimate the mantle temperature and Clapeyron slopes at the 660 km phase boundary.
Abstract: In October 2017, a natural diamond overgrown by a thick layer of CVD synthetic diamond was identified at the Beijing laboratory of the National Gemstone Testing Center (NGTC). The round-brilliant-cut sample was near-colourless and weighed 0.11 ct. No sign of the overgrowth was observed with magnification. However, DiamondView images showed a distinct boundary in the pavilion separating layers of different luminescence: The upper layer displayed red fluorescence with greenish blue phosphorescence, while the lower portion showed deep blue fluorescence and no phosphorescence. Infrared spectroscopy revealed that the upper layer was type IIa and the lower portion was type Ia. Ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy recorded an unusual co-existence of the N3 centre at 415 nm together with absorption due to [Si-V]-defects at 737 nm. The photoluminescence (PL) spectrum confirmed a high level of [Si-V]-defects. The approximate thickness of the CVD synthetic layer was ~740 µm, which is much thicker than previously reported for such overgrowths. The presence of the N3 centre in the natural diamond layer caused this sample to be passed as natural by various screening instruments. Luminescence imaging is key to identifying such overgrowths, and should be relied upon more heavily in the screening procedures used by gemmological laboratories in the future.
Abstract: Precise measurement of variations in the local gravitational acceleration is valuable for natural hazard forecasting, prospecting, and geophysical studies. Common issues of the present gravimetry technologies include their high cost, high mass, and large volume, which can potentially be solved by micro-electromechanical-system (MEMS) technology. However, the reported MEMS gravimeter does not have a high sensitivity and a large dynamic range comparable with those of the present commercial gravimeters, lowering its practicability and ruling out worldwide deployment. In this paper, we introduce a more practical MEMS gravimeter that has a higher sensitivity of 8??Gal/?Hz and a larger dynamic range of 8000 mGal by using an advanced suspension design and a customized optical displacement transducer. The proposed MEMS gravimeter has performed the co-site earth tides measurement with a commercial superconducting gravimeter GWR iGrav with the results showing a correlation coefficient of 0.91.
Earth-Science Reviews, in press available, 70p. Pdf
China
tectonics
Abstract: Contrasting models for internal and external locations of South China within the Nuna and Rodinia supercontinents can be resolved when the current lithotectonic associations of Mesoproterozoic and older rocks units that constitute the craton are redefined into four lithotectonic domains: Kongling, Kunming-Hainan, Wuyi, and Coastal. The Kongling and Kunming-Hainan domains are characterized by isolated Archean to early Paleoproterozoic rock units and events and crop out in northern and southern South China, respectively. The Kunming-Hainan Domain is preserved in three spatially separated regions at Kunming (southwestern South China), along the Ailaoshan shear zone, and within Hainan Island. Both domains were affected by late Paleoproterozoic tectonothermal events, indicating their likely juxtaposition by this time to form the proto-Yangtze Block. Late Paleoproterozoic and Mesoproterozoic sedimentary and igneous rock units developed on the proto-Yangtze Block, especially in its southern portions, and help link the rock units that formed along the shear zone at Ailaoshan and on Hainan Island into a single, spatially unified unit prior to Paleozoic to Cenozoic structural disaggregation and translation. The Wuyi Domain consists of late Paleoproterozoic rock units within a NE-SW trending, fault-bounded block in eastern South China. The Coastal Domain lies east of the Wuyi domain and is inferred to constitute a structurally separate block. Basement to the domain is not exposed, but zircon Hf model ages from Mesozoic granites suggest Mesoproterozoic basement at depth. The Archean to Paleoproterozoic tectonothermal record of the Kongling and Kunming-Hainan domains corresponds closely with that of NW Laurentia, suggesting all were linked, probably in association with assembly and subsequent partial fragmentation of the Nuna supercontinent. Furthermore, the age and character of Mesoproterozoic magmatism and detrital zircon signature of sedimentary rocks in the proto-Yangtze Block matches well with western Laurentia and eastern Australia-Antarctica. In particular, the detrital zircon signature of late Paleoproterozoic to early Mesoproterozoic sedimentary units in the block (e.g. Dongchuan Group) share a similar age spectrum with the Wernecke Supergroup of northwest Laurentia. This, together with similarities in the type and age of Fe-Cu mineralization in the domain with that in eastern Australia-Antarctica, especially northeast Australia, suggests a location adjacent to northwest Laurentia, southern Siberia, and northeast Australia within the Nuna supercontinent. The timing and character of late Paleoproterozoic magmatic activity in the Wuyi domain along with age of detrital zircons in associated sedimentary rocks matches the record of northern India. During rifting between Australia-Antarctica and Laurentia in the late Mesoproterozoic, the proto-Yangtze Block remained linked to northeast Australia. During accretionary orogenesis in the early Neoproterozoic, the proto-Yangtze Block assembled with the Wuyi Domain along the northern margin of India. The Coastal domain likely accreted at this time forming the South China Craton. Displacement of the Hainan and Ailaoshan assemblages from southwest of the Kunming assemblage likely occurred in the Cenozoic with the activation of the Ailaoshan-Red River fault system but could have begun in the early to mid-Paleozoic based on evidence for tectonothermal events in the Hainan assemblage.
Abstract: Gemmological and spectroscopic characteristics are reported for two type IIa blue CVD synthetic diamonds from Huzhou SinoC Semiconductor Science and Technology Co. Ltd, China. These are the first relatively large (1.76 and 2.63 ct) blue CVD synthetics examined in NGTC’s laboratories, and their colour was slightly brighter than other blue synthetic diamonds that we have encountered. In the DiamondView, they fluoresced blue (with purple-red in one sample), which is unusual for CVD synthetics. The mid- and near-IR absorption spectra of one sample showed no hydrogen-related features, while the other synthetic diamond showed a weak absorption at 6853 cm?1attributed to hydrogen. The spectra of both samples had a very weak line at 1332 cm?1 due to isolated nitrogen and a distinct band at 9282 cm-1 related to radiation. A very strong GR1 absorption feature was detected by UV-Vis-NIR spectroscopy. Photoluminescence spectra obtained at liquid-nitrogen temperature recorded emissions related to radiation (mainly in the 480-510 nm region), N-V and [Si-V]- centres, and several unassigned weak emissions. This combination of optical centres strongly suggests that these samples underwent post-growth treatment to improve their transparency before they were irradiated to produce blue colouration.
Post collisional, potassic and ultrapotassic magmatism of the northern Tibetan Plateau: constraints on characteristics of the mantle source, geodynamic uplift
Zhao, Z., Niu, Y., Christensen, N.I., Zhou, W., Hou, Q., Zhang, Z.M., Xie, H., Zhang, Z.C., Liu, J.
Delamination and ultradeep subduction of continental crust: constraints from elastic wave velocity and density measurement in ultrahigh pressure met. rocks
Journal of Metamorphic Geology, Vol. 29, 7, pp. 781-801.
Guo, S., Ye, K., Wu, Y., Chen, Y., Yang, Y., Zhang, L., Liu, J., Mao, Q., Ma, Y.
A potential method to confirm the previous existence of lawsonite in eclogite: the mass imbalance of Sr and LREEs in multi stage epidote ( Ganghe, Dabie UHP terrane).
Journal of Metamorphic Gology, Vol. 31, 4, pp. 415-435.
Abstract: Canada is host to at least six separate cratons that comprise a significant proportion of its crustal extent. Of these cratons, we possess knowledge of the cratonic lithospheric roots beneath only the Slave craton and, to a lesser extent, the Superior craton, despite the discovery of many new diamond-bearing kimberlites in Canada's North. Here we present the first age, composition and geothermal information for kimberlite-borne peridotite xenoliths from two localities within the central Rae craton: Pelly Bay and Repulse Bay. Our aim is to investigate the nature and evolution of the deep lithosphere in these regions and to examine how events recorded in the mantle may or may not correlate with the complex history of crustal evolution across the craton. Peridotite xenoliths are commonly altered by secondary processes including serpentinization, silicification and carbonation, which have variably affected the major element compositions. These secondary processes, as well as mantle metasomatism recorded in pristine silicate minerals, however, did not significantly modify the relative compositions of platinum-group elements (PGE) and Os isotope ratios in the majority of our samples from Pelly Bay and Repulse Bay, as indicated by the generally high absolute PGE concentrations and mantle-like melt-depleted PGE patterns. The observed PGE signatures are consistent with the low bulk Al2O3 contents (mostly lower than 2.5%) of the peridotites, as well as the compositions of the silicate and oxide minerals. Based on PGE patterns and Os model ages, the peridotites from both localities can be categorized into three age groups: Archean (3.0-2.6 Ga overall; 2.8-2.6 Ga for Pelly Bay and 3.0-2.7 Ga for Repulse Bay), Paleoproterozoic (2.1-1.7 Ga), and "Recent" (<1 Ga, with model ages similar to the ca. 546 Ma kimberlite eruption age). The Archean group provides the first direct evidence of depleted Archean lithospheric mantle forming coevally with the overlying Archean crustal basement, indicating cratonization of the Rae during the Archean. The subtle difference in Os model ages between Pelly Bay and Repulse Bay coincides with the age difference between crustal basement rocks beneath these two areas, supporting the suggestion that the Rae craton was assembled by collision of separate two Archean blocks at 2.7-2.6 Ga. The Paleoproterozoic peridotites are interpreted to represent newly formed lithospheric mantle, most likely associated with regional-scale underplating during the 1.77-1.70 Ga Kivalliq-Nueltin event via removal of the lower portion of Archean lithospheric mantle followed by replacement with juvenile Paleoproterozoic lithospheric mantle. The existence of multiple age clusters in the lithosphere at each locality is consistent with the observation of present-day seismic lithospheric discontinuities (0540 and 0545) that indicate two or more layers of fossil lithospheric mantle fabric beneath this region. Our data define a shallow mantle lithosphere layer dominated by Archean depletion ages underlain by a layer of mixed Archean and Paleoproterozoic ages. This lithospheric mantle structure is probably a response to complex tectonic displacement of portions of the lithospheric mantle during Paleoproterozoic orogeny/underplating. The best equilibrated Archean and Paleoproterozoic peridotites at both Pelly Bay and Repulse Bay define a typical cratonic geotherm at the time of kimberlite eruption, with a ?200 km thick lithospheric root extending well into the diamond stability field, in keeping with the diamondiferous nature of the kimberlites. Such thick lithosphere remains in place to the present day as suggested by seismic and magnetotelluric studies (0540, 0545 and 0550). The metasomatically disturbed peridotites in the Rae lithospheric mantle, yielding model ages indistinguishable from kimberlite eruption, may represent parts of the Rae craton mantle root that show anomalous magnetotelluric signatures.
Abstract: Canada is host to at least six separate cratons that comprise a significant proportion of its crustal extent. Of these cratons, we possess knowledge of the cratonic lithospheric roots beneath only the Slave craton and, to a lesser extent, the Superior craton, despite the discovery of many new diamond-bearing kimberlites in Canada's North. Here we present the first age, composition and geothermal information for kimberlite-borne peridotite xenoliths from two localities within the central Rae craton: Pelly Bay and Repulse Bay. Our aim is to investigate the nature and evolution of the deep lithosphere in these regions and to examine how events recorded in the mantle may or may not correlate with the complex history of crustal evolution across the craton. Peridotite xenoliths are commonly altered by secondary processes including serpentinization, silicification and carbonation, which have variably affected the major element compositions. These secondary processes, as well as mantle metasomatism recorded in pristine silicate minerals, however, did not significantly modify the relative compositions of platinum-group elements (PGE) and Os isotope ratios in the majority of our samples from Pelly Bay and Repulse Bay, as indicated by the generally high absolute PGE concentrations and mantle-like melt-depleted PGE patterns. The observed PGE signatures are consistent with the low bulk Al2O3 contents (mostly lower than 2.5%) of the peridotites, as well as the compositions of the silicate and oxide minerals. Based on PGE patterns and Os model ages, the peridotites from both localities can be categorized into three age groups: Archean (3.0-2.6 Ga overall; 2.8-2.6 Ga for Pelly Bay and 3.0-2.7 Ga for Repulse Bay), Paleoproterozoic (2.1-1.7 Ga), and “Recent” (<1 Ga, with model ages similar to the ca. 546 Ma kimberlite eruption age). The Archean group provides the first direct evidence of depleted Archean lithospheric mantle forming coevally with the overlying Archean crustal basement, indicating cratonization of the Rae during the Archean. The subtle difference in Os model ages between Pelly Bay and Repulse Bay coincides with the age difference between crustal basement rocks beneath these two areas, supporting the suggestion that the Rae craton was assembled by collision of separate two Archean blocks at 2.7-2.6 Ga. The Paleoproterozoic peridotites are interpreted to represent newly formed lithospheric mantle, most likely associated with regional-scale underplating during the 1.77-1.70 Ga Kivalliq-Nueltin event via removal of the lower portion of Archean lithospheric mantle followed by replacement with juvenile Paleoproterozoic lithospheric mantle. The existence of multiple age clusters in the lithosphere at each locality is consistent with the observation of present-day seismic lithospheric discontinuities (0540 and 0545) that indicate two or more layers of fossil lithospheric mantle fabric beneath this region. Our data define a shallow mantle lithosphere layer dominated by Archean depletion ages underlain by a layer of mixed Archean and Paleoproterozoic ages. This lithospheric mantle structure is probably a response to complex tectonic displacement of portions of the lithospheric mantle during Paleoproterozoic orogeny/underplating. The best equilibrated Archean and Paleoproterozoic peridotites at both Pelly Bay and Repulse Bay define a typical cratonic geotherm at the time of kimberlite eruption, with a ?200 km thick lithospheric root extending well into the diamond stability field, in keeping with the diamondiferous nature of the kimberlites. Such thick lithosphere remains in place to the present day as suggested by seismic and magnetotelluric studies (0540, 0545 and 0550). The metasomatically disturbed peridotites in the Rae lithospheric mantle, yielding model ages indistinguishable from kimberlite eruption, may represent parts of the Rae craton mantle root that show anomalous magnetotelluric signatures.
Abstract: Although trace element concentrations in clinopyroxene serve as a useful tool for assessing the depletion and enrichment history of mantle peridotites, this is not applicable for peridotites in which the clinopyroxene component has been consumed (~ 25% partial melting). Orthopyroxene persists in mantle residues until ~ 40% melting and it is therefore this mineral that offers petrological insights into the evolution of refractory peridotites. Major and trace element concentrations in orthopyroxene ± clinopyroxene from two spinel facies harzburgitic xenolith suites from New Zealand are examined. Samples from Cape L'Evique (CLEV) on Chatham Island contain traces of clinopyroxene (< 2 modal %) but a suite from Lake Moana (MOA) in the South Island is devoid of this mineral. When compared with modelled orthopyroxene trace element budgets, which are constructed from a review of published source modes, melting modes and element/melt partition co-efficients, the measured orthopyroxene rare earth element data in both suites generally indicate minimums of 25-30% partial melting. These results are consistent with co-existing elevated Mg# in olivine (mostly 91.4 to 93.0) and orthopyroxene (mostly 91.3 to 93.6), high spinel Cr# (commonly > 45) and low orthopyroxene Al2O3 (generally < 3.1 wt%). However, comparison of modelled and measured orthopyroxene compositions shows that all samples, even the most refractory, have undergone metasomatism by small volume light rare earth element-bearing agents. Measured orthopyroxene Ti concentrations show that the metasomatic agent that affected the CLEV suite carried Ti, but that the MOA suite metasomatiser was Ti-poor. Orthopyroxene trace elements in the inspected rocks are therefore partly decoupled from the major element abundances, with the results demonstrating that even highly refractory peridotites can record evidence for mantle metasomatism.
Abstract: Both mantle-derived clinopyroxene and orthopyroxene are generally homogeneous in water concentration, while water content in the coexisting olivine is affected by partial or complete loss during the ascent of the hosting magma. Here, we report the first record of water content profiles (higher water in the cores than in the rims) in natural orthopyroxene grains in peridotite xenoliths hosted by Cenozoic alkali basalts in Tianchang volcano, eastern China. The water contents of the coexisting clinopyroxene grains are homogeneous and are twice that measured in the cores of orthopyroxene grains, confirming previous chemical equilibrium between the two pyroxenes. The olivines (ol) are nearly dry (?0 ppm). These observations demonstrate that H diffusion in mantle orthopyroxene (opx) is faster than in clinopyroxene (cpx), and the relative mobility of H in each mineral phase could be quantified as: Graphic (where is the chemical diffusion coefficient of hydrogen). Combining this with experimental diffusion coefficients from the literature, we infer that (1) the xenoliths remained in contact with the magma below 900 °C for several months, and (2) clinopyroxene remains the more reliable recorder of water from depth, and orthopyroxene should be used more cautiously but can be considered with olivine for tracing slow transport and cooling of magma.
Abstract: Carbonates are common rock-forming minerals in the Earth’s crust and act as sinks of atmospheric carbon dioxide. Subduction of hydrothermally altered oceanic lithosphere returns carbon to the interior, where more than three quarters of Earth’s carbon is stored. The contribution of subducted carbonates to the Earth's long-term deep carbon cycle is uncertain and has recently emerged as a topic of intense debate [1]. Moreover, mantle-slab interaction has been proposed as a mechanism to produce super-deep diamonds, thus questioning the use of certain mineral inclusions to infer lower-mantle origin [2]. Here we report new data on the chemical stability and reaction kinetics of carbonates in the mantle from multianvil and diamond-anvil-cell experiments. Our results suggest that carbon can be sequestered into deep Earth through reaction freezing and that the index minerals for super-deep diamonds are not reliable indicators for their formation depths.
45th. Annual Yellowknife Geoscience Forum, p. 33 abstract
Canada, Northwest Territories
deposit - Darby
Abstract: New geological and geophysical research on Canada’s Rae craton are providing an increasingly good baseline for diamond exploration. This study uses mantle xenoliths and xenocrysts from the Darby property, located ~200 km southwest of the community of Kugaaruk, Nunavut, to provide new information on the lithospheric mantle and diamond potential of the western portion of the central Rae. Peridotite xenoliths containing enough fresh olivine have a median Mg# value of 92.5, indistinguishable from the median value of 92.6 typical of cratonic peridotites world-wide. Only of the 14 peridotitic xenoliths contain fresh garnet. Of these, garnet in one sample is classified as harzburgitic (G10), giving a minimum pressure of 4.7 GPa using the P38 geobarometer (38 mW/m2 model geothermal gradient), while garnets from three peridotites are classified as lherzolitic (G9). 52 garnets picked from concentrate have lherzolitic affinities. Lherzolitic diopsides from kimberlite heavy mineral concentrate yield a lithospheric thickness of ~ 200 km. The four garnet peridotite xenoliths and 49 peridotitic garnets from concentrate yield two distinct modes in mantle sampling depths using Ni thermometry, when projected to the Cpx geotherm. A cluster of samples from the higher Ca/Cr lherzolitic garnets equilibrated at 765 to 920 °C with a group of peridotitic garnets (50 % of xenoliths and 28 % of concentrate) from the lower Ca/Cr lherzolitic garnets with anomalously high Ti concentrations yielding super-adiabatic TNi values The aluminum-in-olivine thermometer applied to olivines filtered to be “garnet facies yielded a mantle sampling portion of the mantle cargo from the diamond stability field. A suite of pyroxenitic xenoliths are a feature of each Darby kimberlite target. New screening techniques indicate that these rocks likely originate close to the crust mantle boundary. Osmium isotope analyses of the Darby peridotites reveal whole-rock Re-depletion ages ranging from Mesoarchean to Paleoproterozoic. The pyroxenite xenoliths have very radiogenic Os isotope compositions and provide the first age information from pyroxenites/“eclogites” beneath the Rae craton. Their resulting Archean whole rock TMA ages are consistent with a Mesoarchean age of the western Central Rae lithosphere older than the lithosphere beneath the Repulse Bay block in the East section of the Rae craton (Liu et al., 2016. Precambrian Research 272). The highly depleted olivine compositions, thick cold lithosphere, and Archean ages of the Darby peridotite xenoliths clearly indicate the presence of 200 km thick cold cratonic lithospheric mantle beneath the western segment of the central Rae craton circa 540 Ma. The Archean model ages of most of the pyroxenites support this, notwithstanding the fact that some of these rocks could be sampling either crust or mantle lithologies very close to the crust-mantle boundary. Mantle sampling took place well into the diamond stability field at Darby.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0609-4 14p.
Canada, Northwest Territories
deposit - Darby
Abstract: New geological and geophysical research on Canada’s Rae craton are providing an increasingly good baseline for diamond exploration. This study uses mantle xenoliths and xenocrysts from the Darby property, located ~200 km southwest of the community of Kugaaruk, Nunavut, to provide new information on the lithospheric mantle and diamond potential of the western portion of the central Rae. Peridotite xenoliths containing enough fresh olivine have a median Mg# value of 92.5, indistinguishable from the median value of 92.6 typical of cratonic peridotites world-wide. Only of the 14 peridotitic xenoliths contain fresh garnet. Of these, garnet in one sample is classified as harzburgitic (G10), giving a minimum pressure of 4.7 GPa using the P38 geobarometer (38 mW/m2 model geothermal gradient), while garnets from three peridotites are classified as lherzolitic (G9). 52 garnets picked from concentrate have lherzolitic affinities. Lherzolitic diopsides from kimberlite heavy mineral concentrate yield a lithospheric thickness of ~ 200 km. The four garnet peridotite xenoliths and 49 peridotitic garnets from concentrate yield two distinct modes in mantle sampling depths using Ni thermometry, when projected to the Cpx geotherm. A cluster of samples from the higher Ca/Cr lherzolitic garnets equilibrated at 765 to 920 °C with a group of peridotitic garnets (50 % of xenoliths and 28 % of concentrate) from the lower Ca/Cr lherzolitic garnets with anomalously high Ti concentrations yielding super-adiabatic TNi values The aluminum-in-olivine thermometer applied to olivines filtered to be “garnet facies yielded a mantle sampling portion of the mantle cargo from the diamond stability field. A suite of pyroxenitic xenoliths are a feature of each Darby kimberlite target. New screening techniques indicate that these rocks likely originate close to the crust mantle boundary. Osmium isotope analyses of the Darby peridotites reveal whole-rock Re-depletion ages ranging from Mesoarchean to Paleoproterozoic. The pyroxenite xenoliths have very radiogenic Os isotope compositions and provide the first age information from pyroxenites/“eclogites” beneath the Rae craton. Their resulting Archean whole rock TMA ages are consistent with a Mesoarchean age of the western Central Rae lithosphere older than the lithosphere beneath the Repulse Bay block in the East section of the Rae craton (Liu et al., 2016. Precambrian Research 272). The highly depleted olivine compositions, thick cold lithosphere, and Archean ages of the Darby peridotite xenoliths clearly indicate the presence of 200 km thick cold cratonic lithospheric mantle beneath the western segment of the central Rae craton circa 540 Ma. The Archean model ages of most of the pyroxenites support this, notwithstanding the fact that some of these rocks could be sampling either crust or mantle lithologies very close to the crust-mantle boundary. Mantle sampling took place well into the diamond stability field at Darby.
Geochimica et Cosmochimica Acta, doi.org/10.1016/j.gca.2018.08.010 78p.
Canada, Nunavut, Parry Peninsula. Central Victoria Island
xenoliths
Abstract: While the mantle roots directly beneath Archean cratons have been relatively well studied because of their economic importance, much less is known about the genesis, age, composition and thickness of the mantle lithosphere beneath the regions that surround the cratons. Despite this knowledge gap, it is fundamentally important to establish the nature of relationships between this circum-cratonic mantle and that beneath the cratons, including the diamond potential of circum-cratonic regions. Here we present mineral and bulk elemental and isotopic compositions for kimberlite-borne mantle xenoliths from the Parry Peninsula and Central Victoria Island, Arctic Canada. These xenoliths provide key windows into the lithospheric mantle underpinning regions to the North and Northwest of the Archean Slave craton, where the presence of cratonic material has been proposed. The mantle xenolith data are supplemented by mineral concentrate data obtained during diamond exploration. The mineral and whole rock chemistry of peridotites from both localities is indistinguishable from that of typical cratonic mantle lithosphere. The cool mantle paleogeotherms defined by mineral thermobarometry reveal that the lithospheric mantle beneath the Parry Peninsula and Central Victoria Island terranes extended well into the diamond stability field at the time of kimberlite eruption, and this is consistent with the recovery of diamonds from both kimberlite fields. Bulk xenolith Se and Te contents, and highly siderophile element (including Os, Ir, Pt, Pd and Re) abundance systematics, plus corresponding depletion ages derived from Re-Os isotope data suggest that the mantle beneath these parts of Arctic Canada formed in the Paleoproterozoic Era, at ?2?Ga, rather than in the Archean. The presence of a diamondiferous Paleoproterozoic mantle root is part of the growing body of global evidence for diamond generation in mantle roots that stabilized well after the Archean. In the context of regional tectonics, we interpret the highly depleted mantle compositions beneath both studied regions as formed by mantle melting associated with hydrous metasomatism in the major Paleoproterozoic Wopmay-Great Bear-Hottah arc systems. These ?2?Ga arc systems were subsequently accreted along the margin of the Slave craton to form a craton-like thick lithosphere with diamond potential thereby demonstrating the importance of subduction accretion in building up Earth’s long-lived continental terranes.
Abstract: The Re-Os isotope system is well suited to constraining the timing of melt depletion of Archean mantle peridotites. In contrast, the variability inherent in post-Archean mantle Os isotope evolution leads to increasing uncertainty in Re-Os model ages. The Lu-Hf isotopic system has shown some potential for dating peridotite formation ages, providing valuable ages that are complementary to the Re-Os system. For post-Archean mantle peridotites, the key target in the Lu-Hf isotopic work is clinopyroxene (Cpx), because of its high Lu and Hf concentrations and the typical absence of garnet in these rocks. However, orthopyroxene (Opx) can contrain 20% or more of the Hf budget of spinel peridotites and somethimes over 40% of the Lu budget, with Lu/Hf ratios 3-4 times those of Cpx. Thus, Opx Lu-Hf isotopic compositions cannot be ignored or simply calculated, as the equilibrium temperatures of mantle peridotites prior to eruption could be lower or higher than the Hf closure temperature (Tc(Hf)~900ºC). Here we explore Lu-Hf partitioning in spinel peridotite xenoliths from the Cameroon Volcanic Line in additin to WR Re-Os analyses. The Hf isotopic composition of Opx in these rocks is equal to or higher than that of Cpx, consistent with some samples having equilibrium temperatures close to Tc(Hf). Combining Cpx and Opx, the constructed WR Lu-Hf isochron yields an age of 2.01±0.36 Ga (2?; MSWD = 11.4; ?Hfi = -0.8±19.2), which is in accordance with the oldest of the variable Re-Os model ages. The continental sector of the Cameroon Line runs close to the edge of the Congo craton. The Hf-Os data indicate that the lithosphere underpinning this region formed in the Paleoproterozoic (~2Ga) most likely during the Paleoproterzoic assembly between the Congo and West African Cratons. We emphasize that Opx and Cpx should be combined together to construct the WR isochron in order to obtain the precise age and initial Hf isotope compositions of post-Archean spinel peridotites.
Abstract: The mantle roots directly beneath Archean cratons have been relatively well studied because of their economic importance, yet much less is known about the genesis, age, composition and thickness of the mantle lithosphere beneath the regions surrounding these cratons. However, it is critically important to establish the nature of the relationship between this circum-cratonic mantle and that beneath the cratons, including the diamond potential of circum-cratonic regions. Here we present mineral and bulk elemental and isotopic compositions for kimberlite-borne mantle xenoliths from the Parry Peninsula (PP) and Central Victoria Island (CVI), Arctic Canada. These xenoliths provide key windows into the lithospheric mantle underpinning regions to the North and Northwest of the Slave craton, where the presence of cratonic mantle has been proposed. The mineral and whole rock chemistry of peridotites from both localities is indistinguishable from that of typical cratonic mantle lithosphere. The cool mantle geotherms defined by mineral thermobarometry reveal that the lithospheric mantle beneath the PP and CVI terranes extended well into the diamond stability field at the time of kimberlite eruption, consistent with the recovery of diamonds from both kimberlite fields. Bulk Se, Te, and highly siderophile element abundance systematics, plus Re-Os isotope age data suggest that the mantle beneath these parts of Arctic Canada formed at ~2 Ga, rather than in the Archean. The presence of a diamondiferous Paleoproterozoic mantle root is part of the growing body of evidence for peridotitic diamond generation in mantle roots that stabilized well after the Archean. In the context of regional tectonics, the highly depleted mantle compositions beneath both regions developed during mantle melting associated with hydrous metasomatism in the major Paleoproterozoic Wopmay- Great Bear-Hottah arc systems. These terranes were subsequently accreted along the margin of the Slave craton to form a craton-like thick lithosphere with significant diamond potential.
Abstract: How water cycles through the Earth's interior, presumably is of fundamental importance for understanding the evolution of our planet. The presence of even trace amounts of water (or hydrogen) can dramatically affect many physical and chemical properties of Earth materials, such as phase stability conditions, viscosity, thermal conductivity, etc. Here, we report that the reaction between water and iron to form a pyrite-structured hydrogen-bearing iron peroxide, FeO2Hx (with x = 0 to 1), under the pressure-temperature conditions relevant to the Earth’s deep lower mantle. Combined with theoretical calculations and high-pressure experiments using laser-heated diamond anvil cells coupled with a suite of insitu and characterization techniques (e.g. nuclear resonant inelastic X-ray scattering spectroscopy, X-ray absorption spectroscopy, and X-ray diffraction), we find that this extremely oxygen-rich form of iron peroxide has properties consistent with ultralow velocity zones that are seismically observed at the core-mantle boundary. This phase may also have implications for deep volatile cycling and mantle redox.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 403-424.
Earth and Planetary Science Letters, Vol. 507, pp. 175-186.
Mantle
peridotite
Abstract: To examine how the mantle lithosphere stabilises continents, we present a synthesis of the mantle beneath Zealandia in the SW Pacific Ocean. Zealandia, Earth's “8th continent”, occurs over 4.9 M km2 and comprises a fore-arc, arc and back-arc fragment rifted from the Australia-Antarctica Gondwana margin 85 Myr ago. The oldest extant crust is ?500 Ma and the majority is Permian-Jurassic. Peridotitic rocks from most known locations reveal the underpinning mantle to comprise regional domains varying from refractory (Al2O3 < 1 wt%, olivine Mg# > 92, spinel Cr# up to 80, Pt/Ir < 1) to moderately depleted (Al2O3 = 2-4 wt%, olivine Mg# ?90.5, spinel Cr# < ?60). There is no systematic distribution of these domains relative to the former arc configuration and some refractory domains underlie crust that is largely devoid of magmatic rocks. Re-depletion Os model ages have no correlation with depletion indices but do have a distribution that is very similar to global convecting mantle. Whole rock, mineral and isotopic data are interpreted to show that the Zealandia mantle lithosphere was constructed from isotopically heterogeneous convecting mantle fragments swept into the sub-arc environment, amalgamated, and variably re-melted under low-P hydrous conditions. The paucity of mafic melt volumes in most of the overlying crust that could relate to the depleted domains requires melting to have been followed by lateral accretion either during subduction or slab rollback. Recent Australia-Pacific convergence has thickened portions of the Zealandia mantle to >160 km. Zealandia shows that the generation of refractory and/or thick continental lithosphere is not restricted to the Archean. Since Archean cratons also commonly display crust-mantle age decoupling, contain spinel peridotites with extreme Cr# numbers that require low-P hydrous melting, and often have a paucity of mafic melts relative to the extreme depletion indicated by their peridotitic roots, they too may - in part - be compilations of peridotite shallowly melted and then laterally accreted at subduction margins.
Geophysical Research Letters, Vol. 46, 4, pp. 1984-1992.
Mantle
diamond genesis
Abstract: Superdeep diamonds originate from great depths inside Earth, carrying samples from inaccessible mantle to the surface. The reaction between carbonate and iron may be an important mechanism to form diamond through interactions between subducting slabs and surrounding mantle. Interestingly, most superdeep diamonds formed in two narrow zones, at 250-450 and 600-800 km depths within the ~2,700?km?deep mantle. No satisfactory hypothesis explains these preferred depths of diamond formation. We measured the rate of a diamond forming reaction between magnesite and iron. Our data show that high temperature promotes the reaction, while high pressure does the opposite. Particularly, the reaction slows down drastically at about 475(±55) km depth, which may explain the rarity of diamond formation below 450 km depth. The only exception is the second zone at 600-800 km, where carbonate accumulates and warms up due to the stagnation of subducting slabs at the top of lower mantle, providing more reactants and higher temperature for diamond formation. Our study demonstrates that the depth distribution of superdeep diamonds may be controlled by reaction rates.
Abstract: We studied the petrography, mineralogy, thermobarometry and whole rock chemistry of 120 peridotite and pyroxenite xenoliths collected from the 156 - 138 Ma Chidliak kimberlites CH-1, -6, -7 and -44. The xenoliths have higher CaO contents relative to Al2O3, and high Al for a given Mg/Si ratio compared to other cratonic peridotites. We assign the complex Ca-Al systematics of the Chidliak peridotites to repeated episodes of Ca-rich, Si-poor metasomatism, which introduced clinopyroxene and garnet, and later replaced orthopyroxene and clinopyroxene with secondary clinopyroxene and monticellite. This carbonatitic metasomatism, manifest in formation of wehrlites, acted upon the entire sampled mantle depth on a regional scale, including the proximal blocks of the North Atlantic Craton and the Chidliak mantle, where clinopyroxene and garnet modes are uniformly and heterogeneously high in the ~ 110 km deep mantle segment. Another, more recent type of mantle metasomatism, is expressed as elevated Ti in clinopyroxene and elevated Na and Ti in garnet, typical of sheared peridotites from CH-1, -7, and -44, but absent from CH-6 xenolith suite. The Ti-Na imprint is most intense in xenoliths derived from depths equivalent to 5.5 to 6.5 GPa, where it is associated with higher strain, the presence of sheared peridotites and higher temperatures varying isobarically by up to 200 °C. The horizontal scale of the thermal-metasomatic imprint is more ambiguous and could be as regional as 10's of kilometers or as local as < 1 km. The latter is constrained by the varied abundance of Ti-enriched garnets within a single kimberlite. The time-scale of this metasomatism relates to a conductive length-scale and could be as short as 100's ka, shortly predating the kimberlite formation. The Ti-Na, megacryst-like metasomatism may have resulted from a highly localized influx of hot hydrous proto-kimberlite fluids that weakened the mantle and triggered the formation of sheared peridotites.
Geochemical Perspectives Letters, Vol. 10, pp. 51-55.
Mantle
redox
Abstract: Among redox sensitive elements, carbon is particularly important because it may have been a driver rather than a passive recorder of Earth’s redox evolution. The extent to which the isotopic composition of carbon records the redox processes that shaped the Earth is still debated. In particular, the highly reduced deep mantle may be metal-saturated, however, it is still unclear how the presence of metallic phases in?uences the carbon isotopic compositions of super-deep diamonds. Here we report ab initio results for the vibrational properties of carbon in carbonates, diamond, and Fe3C under pressure and temperature conditions relevant to super-deep diamond formation. Previous work on this question neglected the effect of pressure on the equilibrium carbon isotopic fractionation between diamond and Fe3C but our calculations show that this assumption overestimates the fractionation by a factor of ~1.3. Our calculated probability density functions for the carbon isotopic compositions of super-deep diamonds derived from metallic melt can readily explain the very light carbon isotopic compo- sitions observed in some super-deep diamonds. Our results therefore support the view that metallic phases are present during the formation of super-deep diamonds in the mantle below ~250 km.
Abstract: It is widely accepted that the lithosphere beneath the eastern portion of the North China Craton (NCC) has suffered extensive thinning and destruction since the Mesozoic. The driving force for this transformation remains debated, although most models make a first-order link with the evolution of the Paleo-Pacific subduction and the effects of the Pacific slab subduction. In this review, we discuss the temporal and spatial relationships between the Paleo-Pacific and the Pacific slab subduction and the lithospheric thinning/destruction processes experienced by the NCC. We recognize four key stages: 1) an initial stage of low angle flat subduction of the Paleo-Pacific slab between ~170-145?Ma, 2) the sinking or rollback of the Paleo-Pacific slab and associated asthenosphere upwelling (145-110?Ma), 3) the disappearance of the Paleo-Pacific slab into lower mantle (110-55?Ma), and 4) the initiation of subduction of the present-day Pacific slab and associated formation of a Big Mantle Wedge (BMW) beneath East Asia (<55?Ma). The initial flat subduction of the Paleo-Pacific plate inhibited mantle-derived magmatism in the period between 170 and 145?Ma beneath the NCC. However, during this stage, intraplate deformation and crustal magmatism migrated westward from craton margin to interior. The cratonic subcontinental lithospheric mantle (SCLM) was further hydrated and metasomatized in addition to that caused by prior circum-cratonic orogenies/subductions. At ca. 155?Ma, the Paleo-Pacific plate began to sink or roll back, causing asthenosphere upwelling and triggering melting of the metasomatized SCLM to form arc-like basalts and low degree melts such as lamprophyres. Vigorous mantle flow/convection transported the metasomatically refertilized and weakened cratonic SCLM into the deep mantle and resulted in the thinning of the lithosphere. At the craton margins, where the lithosphere, thickened by collision, had lost a lower portion of the cratonic SCLM by mantle erosion, delamination of the eclogitic lower crust and underlying pre-thinned SCLM occurred. Upwelling asthenosphere replaced the detached lithosphere and then cooled by conduction to form new lithospheric mantle. This process may have continued to ca. 125?Ma when mantle-derived melts transitioned from arc-like to OIB-like basalts. Replacement of the mantle lithosphere by asthenosphere elevated the lithospheric geotherm and led to extensive crustal melting and the generation of massive volumes of felsic-intermediate magmatism in the eastern NCC until ~110?Ma. After the termination of lithosphere replacement, the speed of subduction of the Paleo-Pacific plate may have increased and by ca. 55?Ma, the whole slab vanished into the lower mantle. We suggest that the subsequent formation of present-day Pacific ocean lithosphere led to a new phase of low angle subduction of the Pacific plate margin. At ca. 35?Ma, the Pacific plate started to descend forming a BMW, accompanied by upwelling of asthenosphere and widespread eruption of alkali basalts across eastern China. The ongoing subduction of the Pacific plate may also lead to further lithospheric thinning.
Abstract: Diamonds are key messenger from the deep Earth because someare sourced from the longest isolated and deepest accessible regions of the Earth’s mantle. They are prime recorders of the carbon isotopic compositionof the Earth. The C isotope composition (d13C) of natural diamonds showsa widevariationfrom -41‰ to +3‰ with the primary mode at -5 ± 3‰ [1]. In comparison, the d13C values of chondrites and other planetary bodies range between -26‰ and -15‰ [2]. It is possible that some of the low d13C values were inherited from the Earth’s building blocks,but this is unlikely to be the sole explanation for all low d13C values that can reach as low as -41‰. Organic matter at the Earth’s surface that has low d13C values[3] has been regarded as a possible origin for low d13C values. However, organic carbon is usually accompanied by carbonate with higher d13C values (~0 ‰),and it is not clear why this d13C value does not appear frequently in diamonds. Low d13C diamonds were also formed by deposition from C-O-H fluids,but the equilibrium fractionationinvolved between diamonds and fluids issmall at mantle temperatures [1] and the low d13C values of diamonds can only be achieved after extensive Rayleigh distillation. One unique feature of the Earth isactive plate tectonics driven by mantle convection. Relatively oxidized iron and carbon species at the surface, such as carbonate, Fe2+-and Fe3+-bearing silicatesand oxides, are transported to the deep mantle by subducted slabs and strongly involved inthe redox reactions that generatediamonds [4]. The extent to which the isotopic compositionof C duringdiamond formation recordsredox processes that shaped the Earth is still controversial. Here we report onvibration properties of C andFe at high pressure in carbonates, diamond and Fe3C,based on nuclear resonant inelastic X-ray scattering measurements and density functional theory calculationsand further calculate equilibrium C isotope fractionations among these C-bearing species. Our results demonstrate that redox reactions in subducted slabs could generate eclogitic diamonds with d13C values as low as -41‰ if C in diamonds was sourced from the oxidation of a Fe-C liquid. The large C isotopic fractionation and potentially fast separation between diamonds and a Fe-C melt could enable diamond formation as high as 2%with d13C lower than -40‰.
Abstract: Accurately dating the formation and modification of Earth’s sub-cratonic mantle still faces many challenges, primarily due to the long and complex history of depletion and subsequent metasomatism of this reservoir. In an attempt to improve this, we carried out the first study on peridotites from the Kaapvaal craton (Finsch Mine) that integrates results from Re-Os, Lu-Hf, Sm-Nd and Sr-isotope systems together with analyses of major-, trace- and platinum-group elements. The Finsch peridotites are well-suited for such a study because certain compositional features reflect they were highly depleted residues of shallow melting (1.5?GPa) at ambient Archean mantle temperatures. Yet, many of them have overabundant orthopyroxene, garnet and clinopyroxene compared to expected modal amounts for residues from partial melting. Finsch peridotites exhibit a wide range of rhenium depletion ages (TRD) from present day to 2.7?Ga, with a prominent mode at 2.5?Ga. This age overlaps well with a Lu-Hf isochron of 2.64?Ga (?Hf (t)?=?+26) which records silico-carbonatitic metasomatism of the refractory residues. This late Archean metasomatism is manifested by positive correlations of Pt/Ir and Pd/Ir with 187Os/188Os ratios and good correlations of modal amounts of silicates, especially garnet, with Os isotope ratios. These correlations suggest that the Highly Siderophile Elements (HSE) and incompatible element reenrichment and modal metasomatism result from one single major metasomatic event at late Archean. Our detailed study of Finsch peridotites highlights the importance of using multiple isotopic systems, to constrain the ages of events defining the evolution of lithospheric mantle. The Re-Os isotope system is very effective in documenting the presence of Archean lithosphere, but only the oldest TRD ages may accurately date or closely approach the age of the last major partial melting event. For a meaningful interpretation of the Re-Os isotope systematics the data must be combined with HSE patterns, trace-element compositions and ideally other isotopic systems, e.g. Lu-Hf. This is highlighted by the widespread evidence in Finsch peridotites of Pt, Pd and Re enrichment through significant Base Metal Sulfide (BMS) addition (mainly in the range of 0.002-0.08?wt%) that systematically shifts the mode of TRD model ages to younger ages.
Geochemical Perspectives Letters, Vol. 11, pp. 18-22.
Mantle
nitrogen
Abstract: Nitrogen and carbon are essential elements for life, and their relative abundances in planetary bodies are important for understanding planetary evolution and habitability. The high C/N ratio in the bulk silicate Earth (BSE) relative to chondrites has been difficult to explain through partitioning during core formation and outgassing from molten silicate. Here we propose a new model that may have released nitrogen from the metallic cores of accreting bodies during impacts with the early Earth. Experimental observations of melting in the Fe-N-C system via synchrotron X-ray radiography of samples in a Paris-Edinburgh press reveal that above the liquidus, iron-rich melt and nitrogen-rich liquid coexist at pressures up to at least 6 GPa. The combined effects of N-rich supercritical fluid lost to Earth’s atmosphere and/or space as well as N-depleted alloy equilibrating with the magma ocean on its way to the core would increase the BSE C/N ratio to match current estimates.
Abstract: Cratons are the ancient landmasses that remain stable for billions of years on Earth but also have experienced episodic events of modification and rejuvenation throughout their history [1]. These alteration processes have modified the cratonic lithospheric mantle roots to different extents, e.g., ubiquitous cryptic/modal metasomatism, partial to entire loss of the mantle roots, to rifting apart of the craton. It remains unclear to what extent a cratonic mantle root can withstand modification and retain its integrity. We attempt to discuss this issue from the perspective of the Slave craton that has experienced the multiple impacts of major circum-cratonic Paleoproterozoic (1.93-1.84 Ga) orogenies and the intrusion of several 2.23-1.67 Proterozoic diabase dyke swarms. We use kimberlite-borne peridotite xenoliths to construct a N-S transect across the craton with an aim of probing the effects of these post-Archean events on the composition, age and depth of the lithospheric root. Chemically, all of these rocks are of typical cratonic refractory composition. P-T calculations and paleogeotherms show that they were derived from thick lithospheric mantle roots (>180 km), consistent with their diamondiferous nature. However, these peridotites exhibit variable N-S variation of modes in their Re-depletion Os model ages (TRD). Neoarchean TRD ages dominate in the Central and Southern Slave mantle. Progressing North there is a decreasing proportion of Archean TRD ages through Jericho to Artemisa in the Northern Slave craton. About 70% of the peridotites at Artemisia give TRD ages within error of the ~1.27 Ga Mackenzie LIP event, with the remaining (~ 30%) close to the Paleoproterozoic orogenic events. Combined with new data from regions to the N and NW of the Slave craton [2], the observed age spectrum in the far North of the craton indicates the likelihood of major new generation of lithospheric roots in both the Paleoproterozoic and Mesoproterozoic. Despite its complex history, the Northern Slave craton retains a ‘cratonic-like’ lithospheric root that allowed diamond mineralization.
Geochimica et Cosmochimica Acta, in press available. 13p.
Africa, Cameroon
peridotite
Abstract: Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to the increasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in dating post-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is important to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peridotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes, highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyos in the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized by low to moderate olivine Fo contents (88.9-91.2) and low spinel Cr# (8.4-19.3), together with moderate to high whole-rock Al2O3 contents (2.0-3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20% partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of melt depletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyroxene with 143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration, most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenes cannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recent metasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget of these rocks and has 176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rock Lu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron from ortho- and clinopyroxenes gives an age of 2.01?±?0.18?Ga (1?), and when olivine and spinel are considered, is 1.82?±?0.14?Ga (1?). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2-2.4?Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0?Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at ?2.0?Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that is complementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archean peridotites. The recognition of ?2.0?Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyos region was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleoproterozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin of the CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source of the CVL basalts.
Abstract: A drastically altered chemistry was recently discovered in the Fe-O-H system under deep Earth conditions, involving the formation of iron superoxide (FeO2Hx with x?=?0 to 1), but the puzzling crystal chemistry of this system at high pressures is largely unknown. Here we present evidence that despite the high O/Fe ratio in FeO2Hx, iron remains in the ferrous, spin-paired and non-magnetic state at 60-133?GPa, while the presence of hydrogen has minimal effects on the valence of iron. The reduced iron is accompanied by oxidized oxygen due to oxygen-oxygen interactions. The valence of oxygen is not -2 as in all other major mantle minerals, instead it varies around -1. This result indicates that like iron, oxygen may have multiple valence states in our planet’s interior. Our study suggests a possible change in the chemical paradigm of how oxygen, iron, and hydrogen behave under deep Earth conditions.
Abstract: The North China Craton (NCC) is an atypical ancient landmass that suffered lithospheric destruction. Previous studies suggest that the eastern part of the lithospheric mantle of the NCC has been thinned and modified in the Mesozoic. However, the initiation time and mechanism of the destruction remain controversial. Mafic magmatismcould provide a unique windowinto deciphering the lithospheric mantle composition and its evolution. Here we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan, located in the southeastern margin of the NCC. Zircon U-Pb dating yielded an average age of 174 ± 14 Ma, representing the first reported Jurassic basalts in the eastern NCC. The Lan'gan basalts are enriched in light rare earth elements (LREE) and large ion lithosphile elements (LILE). Sr-Nd-Pb-Hf isotopic compositions (87Sr/86Sr(t) = 0.70646-0.70925, ?Nd(t) = ?2.1 to ?4.9, 206Pb/204Pb(t) = 17.14-18.12, 207Pb/204Pb(t) = 15.28-15.61, 208Pb/204Pb(t) = 37.82-38.67, and zircon ?Hf(t) = ?17 to ?21) are enriched compared to depleted mantle. The presence of primary amphibole indicates that the magma source of the basalts was water enriched. These observations suggest that, the lithospheric mantle of the eastern NCC were significantly refertilized, likely by slab derived fluids/melts fromthe Paleo-Pacific subduction. Owing to the Paleo-Pacific subduction, the lithosphericmantle of the eastern NCCwere reduced in viscosity and intensity, and finally promoted partialmelting in a limited scale to generate the investigated alkaline basalts. Hence, the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick, and that large-scale destruction had not initiated in the early Jurassic beneath this region.
Abstract: Precise measurement of variations in the local gravitational acceleration is valuable for natural hazard forecasting, prospecting, and geophysical studies. Common issues of the present gravimetry technologies include their high cost, high mass, and large volume, which can potentially be solved by micro-electromechanical-system (MEMS) technology. However, the reported MEMS gravimeter does not have a high sensitivity and a large dynamic range comparable with those of the present commercial gravimeters, lowering its practicability and ruling out worldwide deployment. In this paper, we introduce a more practical MEMS gravimeter that has a higher sensitivity of 8??Gal/?Hz and a larger dynamic range of 8000 mGal by using an advanced suspension design and a customized optical displacement transducer. The proposed MEMS gravimeter has performed the co-site earth tides measurement with a commercial superconducting gravimeter GWR iGrav with the results showing a correlation coefficient of 0.91.
Abstract: The North China Craton (NCC) is an atypical ancient landmass that suffered lithospheric destruction. Previous studies suggest that the eastern part of the lithospheric mantle of the NCC has been thinned and refertilized in the Mesozoic. However, the initiation time and mechanism of the destruction remain controversial. Mafic magmatism could provide a unique window into deciphering the lithospheric mantle composition and its evolution. Here we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan, located in the southeastern margin of the NCC. Zircon UPb dating yielded an average age of 174?±?14?Ma, representing the first reported Jurassic basalts in the eastern NCC. The Lan'gan basalts are enriched in light rare earth elements (LREE) and large ion lithosphile elements (LILE). Sr-Nd-Pb-Hf isotopic compositions (87Sr/86Sr(t)?=?0.70646-0.70925, ?Nd(t)?=??2.1 to ?4.9, 206Pb/204Pb(t)?=?17.14-18.12, 207Pb/204Pb(t)?=?15.28-15.61, 208Pb/204Pb(t)?=?37.82-38.67, and zircon ?Hf(t)?=??17 to ?21) are slightly enriched compared to depleted mantle. The presence of primary amphibole indicates that the magma source of the basalts was water enriched. These observations suggest that, the lithospheric mantle of the eastern NCC were significantly refertilized, likely by slab derived fluids/melts from the Paleo-Pacific subduction. Owing to the Paleo-Pacific subduction, the lithospheric mantle of the eastern NCC were reduced in viscosity and intensity, and finally promoted partial melting in a limited scale to generate the investigated alkaline basalts. Hence, the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick, and that large-scale destruction had not initiated in the early Jurassic beneath this region.
Geochimica et Cosmochimica Acta, Vol. 278, pp. 177-198.
Africa, Cameroon
peridotites
Abstract: Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to the increasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in dating post-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is important to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peridotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes, highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyos in the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized by low to moderate olivine Fo contents (88.9-91.2) and low spinel Cr# (8.4-19.3), together with moderate to high whole-rock Al2O3 contents (2.0-3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20% partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of melt depletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyroxene with 143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration, most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenes cannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recent metasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget of these rocks and has 176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rock Lu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron from ortho- and clinopyroxenes gives an age of 2.01?±?0.18?Ga (1?), and when olivine and spinel are considered, is 1.82?±?0.14?Ga (1?). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2-2.4?Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0?Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at ?2.0?Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that is complementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archean peridotites. The recognition of ?2.0?Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyos region was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleoproterozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin of the CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source of the CVL basalts.
Dorfman, S.M., Potapkin, V., Lv, M., Greenberg, E., Kupenko, I., Chumakov, A.I., Bi, W., Alp, E.E., Liu, J., Magrez, A., Dutton, S.E., Cava, R.J., McCammon, C.A., Gillet, P.
American Mineralogist, Vol. 105, pp. 1030-1039. pdf
Mantle
redox
Abstract:
Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mössbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mössbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10-50% Fe/total cations, 0-25% Al/total cations, 12-100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mössbauer reference library for bridgmanite and demonstrate the effects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at ~70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mössbauer sites with center shift ~1 mm/s and quadrupole splitting 2.4-3.1 and 3.9 mm/s at ~70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the effects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to ~20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle.
Abstract: The accretionary mobile belts surrounding ancient cratonic cores are an important facet of the growth and preservation of continental landmasses. Peridotites from Nuominhe in the Xing'an Mongolia Orogenic Belt (XMOB) provide an additional opportunity to examine the age, structure and evolution of mantle lithosphere separating two of the largest existing ancient continental nuclei: the North China Craton and the Siberian Craton. This suite of mantle rocks comprises fertile to refractory garnet- and spinel-facies harzburgites and lherzolites. Their lithophile element systematics show that the peridotites were metasomatized to variable extent by silicate?carbonate melts. Despite this, the highly siderophile element and Os isotope systematics appear to have been largely undisturbed. The Nuominhe peridotites have Re-depletion Os model ages (TRD) that range from 0.5 Ga to 2.4 Ga, with three peaks/major ranges at ~2.0-2.4 Ga, ~1.4-1.5 Ga and ~ 0.8 Ga, of which the latter two are closely similar to those data from other XMOB localities reported in the literature. The only section of the mantle that appears to have ages which correlate with crust formation is the suite with Neoproterozoic (~0.8 Ga) depletion ages, while the older mantle domains document older episodes of mantle depletion. Given the lack of correlation between equilibrium temperatures and bulk composition or TRD ages, the Nuominhe peridotites were inter-mixed in the mantle column, most likely as a result of incorporation of recycled older continental mantle fragments into juvenile Neoproterozoic mantle during the orogenic processes responsible for new lithosphere formation. Geothermobarometry of the Nuominhe peridotites indicates a conductive geotherm of ~60 mWm?2 and therefore a lithosphere thickness of ~125 km, which is thicker than most Phanerozoic continental terranes, and even thicker than Proterozoic regions that comprise the larger cratonic unit of the Siberian craton. This thick Proterozoic lithosphere sandwiched between the converging North China and Siberian cratons was evidently partly constructed from recycled refractory continental mantle fragments, perhaps extant in the convecting mantle, or in-part derived from the surrounding cratons, leading to a composite nature of the mantle in this re-healed continental suture. Re-accretion of recycled refractory old continental mantle fragments plays a significant role in affecting mantle composition and controlling the thickness of circum-cratonic landmasses between cratonic blocks.
Geochimica et Cosmochimica Acta, Vol. 294, pp. 215-231. pdf
Mantle
bridgmanite
Abstract: The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth’s lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (?-factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its ?-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller ?-factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰.
Nature, doi.org/101038/ s41586-021-03395-5 5p. Pdf
Canada, Northwest Territories
craton
Abstract: Cratons are Earth’s ancient continental land masses that remain stable for billions of years. The mantle roots of cratons are renowned as being long-lived, stable features of Earth’s continents, but there is also evidence of their disruption in the recent1,2,3,4,5,6 and more distant7,8,9 past. Despite periods of lithospheric thinning during the Proterozoic and Phanerozoic eons, the lithosphere beneath many cratons seems to always ‘heal’, returning to a thickness of 150 to 200 kilometres10,11,12; similar lithospheric thicknesses are thought to have existed since Archaean times3,13,14,15. Although numerous studies have focused on the mechanism for lithospheric destruction2,5,13,16,17,18,19, the mechanisms that recratonize the lithosphere beneath cratons and thus sustain them are not well understood. Here we study kimberlite-borne mantle xenoliths and seismology across a transect of the cratonic lithosphere of Arctic Canada, which includes a region affected by the Mackenzie plume event 1.27 billion years ago20. We demonstrate the important role of plume upwelling in the destruction and recratonization of roughly 200-kilometre-thick cratonic lithospheric mantle in the northern portion of the Slave craton. Using numerical modelling, we show how new, buoyant melt residues produced by the Mackenzie plume event are captured in a region of thinned lithosphere between two thick cratonic blocks. Our results identify a process by which cratons heal and return to their original lithospheric thickness after substantial disruption of their roots. This process may be widespread in the history of cratons and may contribute to how cratonic mantle becomes a patchwork of mantle peridotites of different age and origin.
Abstract: The formation and preservation of cratons-the oldest parts of the continents, comprising over 60 per cent of the continental landmass-remains an enduring problem. Key to craton development is how and when the thick strong mantle roots that underlie these regions formed and evolved. Peridotite melting residues forming cratonic lithospheric roots mostly originated via relatively low-pressure melting and were subsequently transported to greater depth by thickening produced by lateral accretion and compression. The longest-lived cratons were assembled during Mesoarchean and Palaeoproterozoic times, creating the stable mantle roots 150 to 250 kilometres thick that are critical to preserving Earth’s early continents and central to defining the cratons, although we extend the definition of cratons to include extensive regions of long-stable Mesoproterozoic crust also underpinned by thick lithospheric roots. The production of widespread thick and strong lithosphere via the process of orogenic thickening, possibly in several cycles, was fundamental to the eventual emergence of extensive continental landmasses-the cratons.
Functional Diamond, Vol. 1, 1, pp. 63-82. doi.org/10.1080/ 26941112.2021.1877021
Global
nitrogen
Abstract: The burgeoning multi-field applications of diamond concurrently bring up a foremost consideration associated with nitrogen. Ubiquitous nitrogen in both natural and artificial diamond in most cases as disruptive impurity is undesirable for diamond material properties, eg deterioration in electrical performance. However, the feat of this most common element-nitrogen, can change diamond growth evolution, endow diamond fancy colors and even give quantum technology a solid boost. This perspective reviews the understanding and progress of nitrogen in diamond including natural occurring gemstones and their synthetic counterparts formed by high temperature high pressure (HPHT) and chemical vapor deposition (CVD) methods. The review paper covers a variety of topics ranging from the basis of physical state of nitrogen and its related defects as well as the resulting effects in diamond (including nitrogen termination on diamond surface), to precise control of nitrogen incorporation associated with selective post-treatments and finally to the practical utilization. Among the multitudinous potential nitrogen related centers, the nitrogen-vacancy (NV) defects in diamond have attracted particular interest and are still ceaselessly drawing extensive attentions for quantum frontiers advance.
Abstract: The term craton has a complex and confused etymology. Despite originally specifying only strength and stability - of the crust - the term craton, within the context of diamond exploration, has widely come to refer to a region characterised by crustal basement older than 2.5 Ga, despite the fact that some such “cratons” no longer possess their deep lithospheric root. This definition often precluded regions with deep lithospheric roots but basement younger than 2-2.5 Ga. Viscous, buoyant lithospheric mantle roots are key to the survival and stability of continental crust. Here we use a revised craton definition (Pearson et al., 2021, in press), that includes the requirement of a deep (~150 km or greater) and intact lithospheric root, to re-examine the link between cratons and diamonds. The revised definition has a nominal requirement for tectonic stability since ~ 1 Ga and recognises that some regions are “modified cratons” - having lost their deep roots, i.e., they may have behaved like cratons for an extended period but subsequently lost much of their stabilising mantle roots during major tectono-thermal events. In other words, despite being long-lived features, cratons are not all permanent. The 150 km lithospheric thickness cut-off provides an optimal match to crustal terranes with 1 Ga timescale stability. In terms of regional diamond exploration, for a given area, the crucial criterion is when a deep mantle root was extant, i.e., over what period was the lithospheric geotherm suitable for diamond formation, stability and sampling? A thick lithospheric root is key to the formation of deep-seated magmas such as olivine lamproites and to the evolution of sub-lithospheric sourced proto-kimberlites, all capable of carrying and preserving diamonds to Earth's surface. This criterion appears essential even for sub-lithospheric diamonds, that still require a diamond transport mechanism capable of preserving the high-pressure carbon polymorph via facilitating rapid transport of volatile-charged magma to the surface, without dilution from additional melting that takes place beneath thinner (<120 km) lithospheric "lids". Seismology can help to define the lateral extent of today's cratons, but a detailed understanding of the regional geological history, kimberlite eruption ages and geothermal conditions is required to evaluate periods of past diamond potential, no-longer evident today. This revised craton concept broadens the target terranes for diamond exploration away from only the Archean cores of cratons and an associated mentality that "the exception proves the rule". The revised definition is compatible with numerous occurrences of diamond in Proterozoic terranes or Archean terranes underpinned by Proterozoic mantle.
Abstract: A suite of peridotite xenoliths from the Chidliak kimberlite province provides an ideal opportunity to assess the age of the mantle lithosphere beneath the eastern Hall Peninsula Block (EHPB) in southern Baffin Island, Nunavut and to provide constraints on the lithospheric architecture of this region. The new dataset comprises highly siderophile element (HSE) abundances and Re-Os isotopic compositions for 32 peridotite xenoliths sampled from four Late Jurassic-Early Cretaceous kimberlite pipes (CH-1, -6, -7, and -44). These peridotites represent strongly depleted mantle residues, with bulk-rock and olivine chemistry denoting melt extraction extents of up to 40%. The vast majority of samples show PPGE (Pt and Pd) depletion relative to IPGE (Os, Ir, and Ru) ((Pt/Ir)N: 0.10-0.96, median = 0.57; (Pd/Ir)N: 0.03-0.79, median = 0.24), coupled with mostly unradiogenic Os isotopic compositions (187Os/188Os = 0.1084-0.1170). These peridotites display strong correlations between 187Os/188Os and melt depletion indicators (such as olivine Mg number and bulk-rock Al2O3, (Pd/Ir)N), suggesting that an ancient (~2.8 Ga) melt depletion event governed the formation of the Chidliak lithosphere. The prominent mode of TRDerupt model ages at ca. 2.8 Ga matches the main crust-building ages of the EHPB, demonstrating temporal crust-mantle coupled in the Meso-Neoarchean. These ancient melt-depletion ages are present throughout the depth of the ~ 200 km thick lithospheric mantle column beneath Chidliak. The Meso-Neoarchean formation age of the EHPB mantle broadly coincides with the timing of stabilization of the lithospheric mantle beneath the Greenlandic portion of the North Atlantic Craton (NAC). This, along with the similarity in modal mineralogy, chemical composition and evolutionary history, indicates that the EHPB, southern Baffin Island was once -contiguous with the Greenlandic NAC. The mantle lithosphere beneath both the EHPB and the NAC show a similar metasomatic history, modified by multiple pulses of metasomatism. These multiple metasomatic events combined to weaken and thin the lithospheric mantle, culminating in the formation of the Labrador Sea and Davis Strait separating the EHPB from the Greenlandic NAC in the Paleocene.
Abstract: Regolith-hosted rare earth element (REE) deposits predominate global resources of heavy REEs. Regoliths are underlain by various types of igneous rocks and do not always host economically valuable deposits. Thus a feasible and convenient method is desired to identify REE mineralization in a particular regolith. This study presents a detailed visible short-wave infrared reflectance (VSWIR) spectroscopic study of the Renju regolith-hosted REE deposit, South China, to provide diagnostic parameters for targeting REE orebodies in regoliths. The results show that the spectral parameters, M794_2nd and M800_2nd, derived from the VSWIR absorption of Nd3+ at approximately 800 nm, can be effectively used to estimate the total REE concentrations in regolith profiles. M1396_2nd/M1910_2nd ratios can serve as proxies to evaluate weathering intensities in a regolith. Abrupt changes of specific spectral features related to mineral abundances, chemical compositions, and weathering intensities can be correlated with variations of protolith that formed a regolith. These VSWIR proxies are robust and can be used for exploration of regolith-hosted REE deposits.
Geochimica et Cosmochimica Acta, Vol. 326, pp. 77-96.
China
allikites
Abstract: Despite being volumetrically minor components, carbonate-rich ultramafic magmas like aillikites represent good candidates to investigate the compositional variations in plume and/or lithospheric mantle sources because they represent low-degree melts which preferentially sample highly fusible components including recycled crustal material. To gain new insights into the composition of the plume-related magmas and, more broadly, the petrogenesis of ultramafic lamprophyres, we have undertaken the first comprehensive study of bulk rock and mineral (olivine and Ti-magnetite) highly siderophile element (HSE) abundances and Re-Os isotopes combined with in situ major-, trace-element and Sr-Nd isotope analyses of apatite and perovskite from the Permian Wajilitag aillikites of the Tarim large igneous province, China. The Wajilitag aillikites have high PPGE (Pt and Pd) contents relative to IPGE (Os, Ir and Ru), which can be ascribed to low-degree partial melting and/or fractionation of olivine and laurite. Measured 187Os/188Os ratios are moderately to highly radiogenic (0.186-0.313) with age-corrected ?Os values up to +113. In situ Sr and Nd isotope analyses of apatite phenocrysts (87Sr/86Sr(i) = 0.70349-0.70384; ?Nd(i) = +1.3 to +4.9) and fresh perovskite grains (87Sr/86Sr(i) = 0.70340-0.70390; ?Nd(i) = +1.3 to +3.8) exhibit limited variability both within and across samples from different aillikite dykes and the only volcanic pipe in the area. These Nd isotopic values resemble those from bulk-rock samples (?Nd(i) = +1.9 to +5.2), whereas Sr in apatite and perovskite extends to marginally less radiogenic values than the bulk-rock compositions (87Sr/86Sr(i) = 0.70362-0.70432). The moderately depleted Sr-Nd isotope compositions of magmatic apatite and perovskite, and the previously reported mantle-like C isotope values of these samples suggest that the aillikites and their carbon probably derived from a sub-lithospheric (plume) source with minimal contribution of deeply subducted material. Conversely, the radiogenic Os isotope compositions of the Tarim aillikites and separated minerals require some contribution from recycled crustal material in the plume source. Mass balance calculations suggest that the radiogenic Os isotopes and moderately depleted Sr-Nd isotopes can be reproduced by less than one third of eclogite component addition to a moderately depleted mantle source. We conclude that the combination of complementary isotopic systems can enlighten contributions from different components to mantle-derived magmas and, in this case, clarifies the occurrence of carbon-free subducted oceanic crust in the Tarim plume.
Late Archean to Early Proterozoic lithospheric mantle beneath the western North Chin a craton: Sr Nd Os isotopes of peridotite xenoliths from Yangyuan and Fansi
Abstract: Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab-mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo = 92.0-92.6, Ca = 42-115 ppm), porphyroblastic orthopyroxene (En = 91.8-92.8), Cr-spinel (Cr# = 50.4-73.0, TiO2 < 0.2 wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31-47.07 wt.%) and Mg# (91.5-91.9), and low Al2O3 (0.48-0.70 wt.%), CaO (0.25-0.44 wt.%) and TiO2 (< 0.03 wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re-Os isotopic data suggest that dunites formed in the early Proterozoic (~ 2.2 Ga). These data indicate that the Lijiatun dunites were the residues of ~ 30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo = 91.6-92.6, Ca = 233-311 ppm), clinopyroxene-bearing (Mg# = 95.9-96.7, Ti/Eu = 104-838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic peridotites derived from such a lithospheric mantle wedge may be heterogeneously modified prior to their incorporation into the subduction channel, which would set up a barrier for investigation of the mass transfer from the subducted crust to the mantle wedge through orogenic peridotites.
Abstract: Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab-mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo = 92.0-92.6, Ca = 42-115 ppm), porphyroblastic orthopyroxene (En = 91.8-92.8), Cr-spinel (Cr# = 50.4-73.0, TiO2 < 0.2 wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31-47.07 wt.%) and Mg# (91.5-91.9), and low Al2O3 (0.48-0.70 wt.%), CaO (0.25-0.44 wt.%) and TiO2 (< 0.03 wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re-Os isotopic data suggest that dunites formed in the early Proterozoic (~ 2.2 Ga). These data indicate that the Lijiatun dunites were the residues of ~ 30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo = 91.6-92.6, Ca = 233-311 ppm), clinopyroxene-bearing (Mg# = 95.9-96.7, Ti/Eu = 104-838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic peridotites derived from such a lithospheric mantle wedge may be heterogeneously modified prior to their incorporation into the subduction channel, which would set up a barrier for investigation of the mas
V(S) and density structure beneath the Colorado Plateau constrained by gravity anomalies and joint inversions of receiver function and phase velocity data.
Journal of Geophysical Research, Vol. 117, B2, B02313.
Mantle flow and lithosphere asthenosphere coupling beneath the southwestern edge of the North American craton: constraints from shear wave splitting measurements.
Earth and Planetary Science Letters, Vol. 402, pp. 209-220.
Geophysical Research Letters, Vol. 42, 20, pp. 8398-8405.
Africa, Botswana
Geophysics - gravity
Abstract: Rifting incorporates the fundamental processes concerning the breakup of continental lithosphere and plays a significant role in the formation and evolution of sedimentary basins. In order to decipher the characteristics of rifting at its earliest stage, we conduct the first teleseismic crustal study of one of the world's youngest continental rifts, the Okavango Rift Zone (ORZ), where the magma has not yet breached the surface. Results from receiver function stacking and gravity modeling indicate that the crust/mantle boundary beneath the ORZ is uplifted by 4-5 km, and the initiation of the ORZ is closely related to lithospheric stretching. Possible decompression melting of the subcrustal lithosphere occurs beneath the ORZ, as evidenced by a relatively low upper mantle density based on the gravity modeling.
Earth and Planetary Science Letters, Vol. 503, pp. 108-117.
Africa, South Africa, Zimbabwe
geophysics - seismic
Abstract: The depths of the 410 km (d410) and 660 km (d660) discontinuities beneath southern Africa, which is presumably underlain by the lower-mantle African Superswell, are imaged in 1? radius consecutive circular bins using over 6400 P-to-S receiver functions (RFs) recorded by 130 seismic stations over a 27 yr period. When the IASP91 standard Earth model is utilized for moveout correction and time-depth conversion, a normal mantle transition zone (MTZ) thickness of 246 ± 7 km is observed, suggesting that the Superswell has no discernible effect on mantle transition zone temperature. Based on the negligible disparity of the mean MTZ thicknesses between on (246 ± 6 km) and off (246 ± 8 km) cratonic regions, we conclude that the deep Archean cratonic keels possess limited influence on MTZ thermal structure. The apparently shallower-than-normal MTZ discontinuities and the parallelism between the d410 and d660 are mostly the results of upper mantle high wave speed anomalies probably corresponding to a thick lithosphere with a mean thickness of about 245 km beneath the Kaapvaal and 215 km beneath the Zimbabwe cratons. In contradiction to conclusions from some of the previous studies, the resulting spatial distribution of the stacking amplitudes of the P-to-S converted phases at the discontinuities is inconsistent with the presence of an excessive amount of water in the MTZ and atop the d410.
Geochemistry, Geophysics, Geosystems, Vol. 20, 7, pp. 3311-3327.
Africa, South Africa
geophysics
Abstract: We conduct a joint inversion of teleseismic receiver functions and Rayleigh wave phase velocity dispersion from both ambient noise and earthquakes using data from 79 seismic stations in southern Africa, which is home to some of the world's oldest cratons and orogenic belts. The area has experienced two of the largest igneous activities in the world (the Okavango dyke swarm and Bushveld mafic intrusion) and thus is an ideal locale for investigating continental formation and evolution. The resulting 3?D shear wave velocities for the depth range of 0-100 km and crustal thickness measurements show a clear spatial correspondence with known geological features observed on the surface. Higher than normal mantle velocities found beneath the southern part of the Kaapvaal craton are consistent with the basalt removal model for the formation of cratonic lithosphere. In contrast, the Bushveld complex situated within the northern part of the craton is characterized by a thicker crust and higher crustal Vp/Vs but lower mantle velocities, which are indicative of crustal underplating of mafic materials and lithospheric refertilization by the world's largest layered mafic igneous intrusion. The thickened crust and relatively low elevation observed in the Limpopo belt, which is a late Archean collisional zone between the Kaapvaal and Zimbabwe cratons, can be explained by eclogitization of the basaltic lower crust. The study also finds evidence for the presence of a stalled segment of oceanic lithosphere beneath the southern margin of the Proterozoic Namaqua?Natal mobile belt.
Abstract: The driving force for transient vertical motions of Earth's surface remains an outstanding question. A main difficulty lies in the uncertain role of the underlying mantle, especially during the geological past. Here I review previous studies on both observational constraints and physical mechanisms of North American topographic evolution since the Mesozoic. I first summarize the North American vertical motion history using proxies from structural geology, geochronology, sedimentary stratigraphy, and geomorphology, based on which I then discuss the published physical models. Overall, there is a progressive consensus on the contribution of mantle dynamic topography due to buoyancy structures associated with the past subduction. At the continental scale, a largely west-to-east migrating deformation pattern suggests an eastward translation of mantle dynamic effects, consistent with models involving an eastward subduction and sinking of former Farallon slabs since the Cretaceous. Among the existing models, the inverse model based on an adjoint algorithm and time-dependent data constraints provides the most extensive explanations for the temporal changes of North American topography since the Mesozoic. At regional scales, debates still exist on the predicted surface subsidence and uplift within both the western and eastern United States, where discrepancies are likely due to differences in model setup (e.g., mantle dynamic properties and boundary conditions) and the amount of time-dependent observational constraints. Toward the development of the next-generation predictive geodynamic models, new research directions may include (1) development of enhanced data assimilation capabilities, (2) exploration of multiscale and multiphysics processes, and (3) cross-disciplinary code coupling.
Geochemistry, Geophysics, Geosystems G3, Vol. 18, pp. 3854-3868.
Mantle
geotectonics
Abstract: Quantifying past mantle dynamic processes represents a major challenge in understanding the temporal evolution of the solid earth. Mantle convection modeling with data assimilation is one of the most powerful tools to investigate the dynamics of plate subduction and mantle convection. Although various data assimilation methods, both forward and inverse, have been created, these methods all have limitations in their capabilities to represent the real earth. Pure forward models tend to miss important mantle structures due to the incorrect initial condition and thus may lead to incorrect mantle evolution. In contrast, pure tomography-based models cannot effectively resolve the fine slab structure and would fail to predict important subduction-zone dynamic processes. Here we propose a hybrid data assimilation approach that combines the unique power of the sequential and adjoint algorithms, which can properly capture the detailed evolution of the downgoing slab and the tomographically constrained mantle structures, respectively. We apply this new method to reconstructing mantle dynamics below the western U.S. while considering large lateral viscosity variations. By comparing this result with those from several existing data assimilation methods, we demonstrate that the hybrid modeling approach recovers the realistic 4-D mantle dynamics the best.
Earth and Planetary Science Letters, Vol. 500, pp. 156-167.
United States
geodynamics
Abstract: The origin of the complex pattern of SKS splitting over the western United States (U.S.) remains a long-lasting debate, where a model that simultaneously matches the various SKS features is still lacking. Here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy. These tests reveal a configuration of mantle deformation more complex than ever envisioned before. In particular, we find that both lithospheric thickness variations and toroidal flows around the Juan de Fuca slab modulate flow locally, but their co-existence enhances large-scale mantle deformation below the western U.S. The ancient Farallon slab below the east coast pulls the western U.S. upper mantle eastward, spanning the regionally extensive circular pattern of SKS splitting. The prominent E-W oriented anisotropy pattern within the Pacific Northwest reflects the existence of sustaining eastward intrusion of the hot Pacific oceanic mantle to beneath the continental interior, from within slab tears below Oregon to under the Snake River Plain and the Yellowstone caldera. This work provides an independent support to the formation of intra-plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume.
Journal of Volcanology and Geothermal Research, Vol. 310, pp. 1-11.
China
LiDAR
Abstract: Dyke geometries are useful indicators of the palaeostress field during magma emplacement. In this paper, we present a multi-scale extraction method of dyke geometries by integrating WorldView-2 (WV2) imagery and terrestrial light detection and ranging (LiDAR) data. Color composite and fusion WV2 images with 0.5-m resolution were generated by using the Gramm-Schmidt Spectral Sharpening approach, which facilitates the discrimination of dyke swarms and provides the ability to measure the orientation, exposed length, and thickness of dykes in sub-horizontal topographic exposures. A terrestrial laser scanning survey was performed on a sub-vertical exposure of dykes to obtain LiDAR data with point spacing of ~ 0.02 m at 30 m. The LiDAR data were transformed to images for extracting dyke margins based on image segmentation, then the dyke attitudes, thicknesses, and irregularity of dyke margins were measured according to the points on dyke margins. This method was applied at Sijiao Island, Zhejiang, China where late Cretaceous mafic dyke swarms are widespread. The results show that integrating WV2 imagery and terrestrial LiDAR improves the accuracy, efficiency, and objectivity in determining dyke geometries in two and three dimensions. The ENE striking dykes are dominant, and intruded the host rock (mainly granite) with sub-vertical dips. Based on the aspect ratios of the dykes, the magmatic overpressure was estimated to be less than 11.5 MPa, corresponding to a magma chamber within 6.6 km in the lithosphere.
Abstract: Intergranular coesite is extremely rare in, and bears crucial information on the formation and preservation of, ultrahigh-pressure (UHP) rocks. Here, we report the first occurrence of intergranular coesite in a metasedimentary rock, which occurs in the Ganjialing area in the Dabie Shan, east-central China, and contains abundant coesite inclusions in both garnet and dolomite. We investigated the content of structural water in these minerals with Fourier transform infrared spectroscopy. Our new results undermine the ubiquity of the “pressure-vessel” model and highlight the role of reaction kinetics in preserving coesite due to the availability of water in UHP rocks.
Abstract: Three types of polyphase solid-inclusions (PSIs) with distinct mineral assemblages and micro-structures were found in garnet of an ultrahigh-pressure (UHP) eclogite-vein system from the Dabie Shan, east-central China. Type-1 PSI contains variable volumes of quartz, K-feldspar, plagioclase ± other phases, whereas Type-2 PSI contains variable volumes of quartz, calcite ± other phases. Both types display shapes that are compatible with those of euhedral coesite inclusions. Type-3 PSI always contains a rutile core that is surrounded by plagioclase ± quartz and generally displays the morphology of the rutile core. Variable amounts of K-feldspar are embedded within the plagioclase of Type-3 PSIs. The three PSI types developed fluid-mediated microstructures that include wedge-like offshoot and protrusion textures and inclusion-garnet interfaces controlled by the crystallographic structure of garnet. PSIs in peak minerals of UHP rocks have been previously thought to represent primary supercritical fluid or melt inclusions. Here we propose that the studied PSIs were formed under high-pressure (HP) eclogite-facies conditions during exhumation and represent reaction products between an enclosed mineral, such as coesite and rutile, and external fluids infiltrating the host garnet along fractures that have been healed later on. Two immiscible aqueous fluids (i.e., a siliceous and a carbonaceous) were involved in the formation of these PSIs. The siliceous fluid was rich in various large ion lithophile elements like Cs, Rb, Ba, K, Pb, Li, and Sr, whereas the carbonaceous fluid was rich in Pb and Sr. The new PSI formation mechanism proposed in this study brings significant implications for tracing fluid evolution and post-entrapment modifications of mineral inclusions in HP and UHP metamorphic rocks.
Deep subduction of continental crust in accretionary orogen: evidence from U-Pb dating on diamond-bearing zircons from the Qinling orogen, central China.
Contributions to Mineralogy and Petrology, Vol. 177, 2, 21p.pdf
Mantle
metasomatism
Abstract: Melting experiments on a carbonated pelite were performed at 5.5-15.5 GPa, 800-1875 °C using multi-anvil apparatuses to determine the melting phase relations and the P-T stability fields of various phases, which may shed some light on the source of silica-undersaturated magmas and the deep Earth carbon and water cycles. The subsolidus assemblages contain garnet, clinopyroxene, coesite/stishovite at all investigated pressures. Phengite, aragonite or magnesite, and topaz-OH occur below 9.5 GPa. Phase egg, K-hollandite, Ti-oxide, and CAS phase appear at 12-15.5 GPa. Phengite is stable up to 6 GPa and 800 °C, with the phengite-out boundary overlapping with the carbonate-out curve. Thus, the initial melt is carbonatitic and extremely potassium-rich, with K2O/Na2O weight-ratios larger than 40 at fluid-present conditions. The melting reaction phase egg?+?magnesite?+?aragonite?+?(clinopyroxene)?+?stishovite???melt?+?garnet?+?kyanite defines the solidus at 9.5 GPa, 1000-1100 °C. With increasing pressure, the composition of the near-solidus melts gradually evolves from potassium-rich to sodium-rich due to the formation of K-hollandite and the destabilization of clinopyroxene, and as a result of the clinopyroxene-out, the near-solidus melt has the lowest K2O/Na2O value and partitioning coefficient of sodium between clinopyroxene and melt (Dcpx/meltNa) at 15.5 GPa. In addition, phase egg remains stable up to 1400 °C at 15.5 GPa. Thus, phase egg is a good candidate as a deep-water carrier during subduction of pelitic sediments. This study concludes that low degree partial melting of carbonated pelite produces alkali-rich carbonatite melts evolving from potassium-rich (6-12 GPa) to sodium-rich (above 12 GPa) with increasing pressure, and if a slab stagnates at depth, and/or subduction slows down, the produced carbonatite melts will be more silicate-rich with increasing temperature. Moreover, the produced melts generally evolve from relatively silicate-rich to carbonatite-rich with increasing pressure. These alkali-rich carbonatite melts are compositionally similar to those in diamond inclusions, which provides strong evidence for the origin of deep-seated silica-undersaturated carbonatitic magma. Such magma is an ideal metasomatic agent that can give rise to mantle heterogeneity.
Contribution to Mineralogy and Petrology, Vol. 177, 19 , 18p. Pdf
Global
garnet
Abstract: Garnet is a common U-bearing mineral in various magmatic and metamorphic rocks with a high U-Pb closure temperature (>?850 °C), rendering it a potentially valuable U-Pb geochronometer. However, a high U (>?10 ppm) garnet reference material that suits both quadrupole and/or multi-collector inductively coupled plasma mass spectrometry (ICP-MS) is yet to be established. This study evaluates a potential reference material for in situ garnet U-Pb analysis with anomalously high U content from the Prairie Lake alkaline complex, Canada. The PL57 garnet, occurring in a calcite ijolite, has high TiO2 (6.5-15.0 wt%, average 12.7 wt%) and Fe2O3 (17.1-21.3 wt%) contents and is a member of the andradite (26-66 mol.%)-morimotoite (18-41 mol.%)-schorlomite (16-35 mol.%) solid solution series. Four samples were dated by U-Pb ID-TIMS to assess reproducibility. Twelve TIMS analyses produced concordant, equivalent results. Garnet PL57 yielded a concordant age of 1156.2?±?1.2 Ma (2?, n?=?10, MSWD?=?1.0), based on ten analyses with two results discarded due to possible mineral inclusions (if included, the concordia age is 1156.6?±?1.8 Ma; n?=?12, MSWD?=?2.0). PL57 had 27-76 ppm (average 41 ppm) U with Th/U of 0.51-0.68 (average 0.63). The total common Pb content ranged from 0.4 to 3.9 pg (average 1.1 pg). Laser ablation coupled with ICP-MS and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging provide direct evidence that U is incorporated and homogeneously distributed within the garnet lattice rather than as defects or pore spaces. Published garnet samples and standards were then tested by calibrating the Willsboro, Mali, Qicun, and Tonglvshan garnet against PL57, which gave accurate ages within the recommended values. Case studies of garnet from the Archean Musselwhite orogenic gold deposit in Canada and the Cenozoic Changanchong and Habo skarn deposits in China yield reliable ages. This suggests that PL57 is a robust U-Pb isotope reference material. The limited variations of U and Pb isotopic ratios, together with the high U concentration and extremely low initial common Pb, make PL57 an ideal calibration and monitor reference material for in situ measurements.
Diamond & Related Materials, Vol. 81, pp. 161-167.
Technology
microdiamonds
Abstract: Polylactic acid (PLA)-based composites filled with 20 or 50 ?m-diameter microdiamond are synthesized by hot pressing. Through improving the interface compatibility between the filler and the matrix enabled by octadecylamine (ODA) coating on the microdiamond particles, the maximum thermal conductivity of the composites is 2.22 Wm? 1 K? 1, which is a ~ 10-fold increase in comparison with that of pure PLA. According to the analysis on the glass transmission of the composites and the surface chemistry of the fillers using DSC, FI-IR, and Raman microscopy, it is found out that ODA is connected with the -OH group on the microdiamond surface through hydrogen bonding and an interfacial structure of PLA/ODA/microdiamond is formed. Thus, the interfacial thermal transport between PLA and microdiamond is significantly improved, leading to the enhancement of the thermal conductivity of the composites. Our work presents a simple method to modify the surface chemistry of microdiamond and to improve the interface compatibility between microdiamond and PLA. The microdiamond/PLA composites with large thermal conductivity are promising thermal management materials used for modern electronic products.
Abstract: Carbonatites and related alkaline rocks host most REE resources. Phosphate minerals, e.g., apatite and monazite, commonly occur as the main REE-host in carbonatites and have been used for tracing magmatic and mineralization processes. Many carbonatite intrusions undergo metamorphic and/or metasomatic modification after emplacement; however, the effects of such secondary events are controversial. In this study, the Miaoya and Shaxiongdong carbonatite-alkaline complexes, in the South Qinling Belt of Central China, are selected to unravel their magmatic and hydrothermal remobilization histories. Both the complexes are accompanied by Nb-REE mineralization and contain apatite and monazite-(Ce) as the major REE carriers. Apatite grains from the two complexes commonly show typical replacement textures related to fluid metasomatism, due to coupled dissolution-reprecipitation. The altered apatite domains, which contain abundant monazite-(Ce) inclusions or are locally surrounded by fine-grained monazite-(Ce), have average REE concentrations lower than primary apatite. These monazite-(Ce) inclusions and fine-grained monazite-(Ce) grains are proposed to have formed by the leaching REE from primary apatite grains during fluid metasomatism. A second type of monazite-(Ce), not spatially associated with apatite, shows porous textures and zoning under BSE imaging. Spot analyses of these monazite-(Ce) grains have variable U-Th-Pb ages of 210-410 Ma and show a peak age of 230 Ma, which is significantly younger than the emplacement age (440-430 Ma) but is roughly synchronous with a regionally metamorphic event related to the collision between the North China Craton and Yangtze Block along the Mianlue suture. However, in situ LA-MC-ICP-MS analyses of those grains show that they have initial Nd values same as those of magmatic apatite and whole rock. We suggest these monazite-(Ce) grains crystallized from the early Silurian carbonatites and have been partially or fully modified during a Triassic metamorphic event, partially resetting U-Pb ages over a wide range. Mass-balance calculations, based on mass proportions and the REE contents of monazite-(Ce) and apatite, demonstrate that the quantity of metasomatized early Silurian monazite-(Ce) is far higher than the proportion of monazite-(Ce) resulting from the metasomatic alteration of the apatite. Therefore, Triassic metamorphic events largely reset the U-Th-Pb isotopic system of the primary monazite-(Ce) and apatite but only had limited or local effects on REE remobilization in the carbonatite-alkaline complexes in the South Qinling Belt. Such scenarios may be widely applicable for other carbonatite and hydrothermal systems.
Abstract: The Paleo-Mesoproterozoic Zhongtiao aulacogen in the North China Craton and Cuddapah basin in the Indian Craton, have both been interpreted as intra-continental rift formed by a mantle plume that led to the breakup of Columbia supercontinent, but the mechanism has not been completely deciphered. In this paper, the mechanism of the Zhongtiao aulacogen and Cuddapah basin related to initial breakup of Columbia has been evaluated with 2D elastic finite element models of the North China Craton and the Indian Craton. The trajectories of the horizontal maximum principal compressive stress of the best-fit model fit well with the trends of dyke swarms in the North China Craton and the Indian Craton. When the other three models generated were compared with the best-fit model, it can be found that a mantle plume beneath the Zhongtiao and Cuddapah areas played the most vital role in developing the Zhongtiao aulacogen, Cuddapah basin and initial breakup of Columbia supercontinent. The boundary subduction forces, including the northern margin of the NCC, the northwest and southwest margins of the Indian Craton are indispensable factors for the rifting and breakup, whereas the mechanical properties have little influence on these modeling results. The initial breakup of Columbia supercontinent might have been resulted from the coupling between a mantle plume upwelling and some plate tectonic forces.
Abstract: Global deep?time plate motion models have traditionally followed a classical rigid plate approach, even though plate deformation is known to be significant. Here we present a global Mesozoic?Cenozoic deforming plate motion model that captures the progressive extension of all continental margins since the initiation of rifting within Pangea at ~240 Ma. The model also includes major failed continental rifts and compressional deformation along collision zones. The outlines and timing of regional deformation episodes are reconstructed from a wealth of published regional tectonic models and associated geological and geophysical data. We reconstruct absolute plate motions in a mantle reference frame with a joint global inversion using hotspot tracks for the last 80 million years and minimizing global trench migration velocities and net lithospheric rotation. In our optimized model net rotation is consistently below 0.2°/Myr, and trench migration scatter is substantially reduced. Distributed plate deformation reaches a Mesozoic peak of 30 million km2 in the Late Jurassic (~160?155 Ma), driven by a vast network of rift systems. After a mid?Cretaceous drop in deformation it reaches a high of 48 million km2 in the Late Eocene (~35 Ma), driven by the progressive growth of plate collisions and the formation of new rift systems. About a third of the continental crustal area has been deformed since 240 Ma, partitioned roughly into 65% extension and 35% compression. This community plate model provides a framework for building detailed regional deforming plate networks and form a constraint for models of basin evolution and the plate?mantle system.
Muller, D., Zahirovic, S., Williams, S.E., Cannon, J., Seton, M., Bower, D.J., Tetley, M., Heine, C., Le Breton, E., Liu, S., Russell, S.H.J., Yang, T., Leonard, J., Gurnis, M.
Abstract: Global deep?time plate motion models have traditionally followed a classical rigid plate approach, even though plate deformation is known to be significant. Here we present a global Mesozoic-Cenozoic deforming plate motion model that captures the progressive extension of all continental margins since the initiation of rifting within Pangea at ~240 Ma. The model also includes major failed continental rifts and compressional deformation along collision zones. The outlines and timing of regional deformation episodes are reconstructed from a wealth of published regional tectonic models and associated geological and geophysical data. We reconstruct absolute plate motions in a mantle reference frame with a joint global inversion using hot spot tracks for the last 80 million years and minimizing global trench migration velocities and net lithospheric rotation. In our optimized model, net rotation is consistently below 0.2°/Myr, and trench migration scatter is substantially reduced. Distributed plate deformation reaches a Mesozoic peak of 30 × 106 km2 in the Late Jurassic (~160-155 Ma), driven by a vast network of rift systems. After a mid?Cretaceous drop in deformation, it reaches a high of 48 x 106 km2 in the Late Eocene (~35 Ma), driven by the progressive growth of plate collisions and the formation of new rift systems. About a third of the continental crustal area has been deformed since 240 Ma, partitioned roughly into 65% extension and 35% compression. This community plate model provides a framework for building detailed regional deforming plate networks and form a constraint for models of basin evolution and the plate?mantle system.
Abstract: We studied a suite of mantle xenoliths carried by Cenozoic volcanism in the Borborema Province, NE Brazil. These xenoliths sample a subcontinental lithospheric mantle affected by multiple continental convergence and rifting events since the Archean. Equilibrium temperatures indicate a rather hot geotherm, implying a ca. 80?km thick lithosphere. Most xenoliths have coarse-granular and coarse-porphyroclastic microstructures, recording variable degrees of annealing following deformation. The high annealing degree and equilibrated pyroxene shapes in coarse-granular peridotites equilibrated at ~900?°C indicate that the last deformation event that affected these peridotites is several hundreds of Ma old. Coarse-porphyroclastic peridotites equilibrated at 950-1100?°C probably record younger (Cretaceous?) deformation in the deep lithospheric mantle. In addition, a few xenoliths show fine-porphyroclastic microstructures and equilibrium temperatures ?1200?°C, which imply recent deformation, probably related to the dykes that fed the Cenozoic volcanism. Chemical and microstructural evidence for reactive percolation of melts is widespread. Variation in textural and chemical equilibrium among samples implies multiple melt percolation events well spaced in time (from Neoproterozoic or older to Cenozoic). Crystal preferred orientations of olivine and pyroxenes point to deformation controlled by dislocation creep with dominant activation of the [100](010) and [001]{0kl} slip systems in olivine and pyroxenes, respectively, for all microstructures. Comparison of xenoliths' seismic properties to SKS splitting data in the nearby RCBR station together with the equilibrated microstructures in the low-temperature xenoliths point to coupled crust-mantle deformation in the Neoproterozoic (Brasiliano) continental-scale shear zones, which is still preserved in the shallow lithospheric mantle. This implies limited reworking of the lithospheric mantle in response to extension during the opening of the Equatorial Atlantic in the Cretaceous, which in the present sampling is restricted to the base of the lithosphere.
Abstract: The origin of dolomite that hosts the Bayan Obo REE-Nb-Fe deposit (57.4 Mt.@6% REE2O3, 2.16 [email protected]% Nb2O5, and >1500 Mt.@35% iron oxides) has been controversial for decades, but it is integral to understanding of the genesis of this giant deposit. In this study, based on the textures and in situ major and trace element composition of its carbonates, the dolomite was proved to be initially generated from magnesio-ferro?carbonatite melts. It subsequently experienced magmatic-hydrothermal alteration and recrystallization in a low strain environment, caused by calcio?carbonatitic fluids, with formation of finer-grained dolomite, interstitial calcite and increasing amounts of associated fluorocarbonates. Available stable isotope analyses indicate that the recrystallized ore-hosting dolomite has higher ?13C and ?18O ratios compared to its igneous coarse-grained precursor. Rayleigh fractionation during the recrystallization process, rather than crustal contamination, played a major role in the highly-variable stable isotope composition of carbonates in the dolomite. Low-T alteration increased variability with apparently random increases in ?18O within carbonates. The REE, Ba and Sr were added simultaneously with the elevated (La/Yb)cn from magnesio-ferro?carbonatite melts to calcio?carbonatitic fluids, and to carbonatite-derived aqueous fluids, through which extensive fluorine metasomatism and alkali alteration overlapped the recrystallization of the ore-hosting dolomite. Therefore, the multi-stage REE mineralization at Bayan Obo is closely related to metasomatism by calcio?carbonatitic fluids of previously-emplaced intrusive magnesio-ferro?carbonatite bodies during late evolution of the Bayan Obo carbonatite complex. Then, the ore-hosting dolomitic carbonatite was subjected to compressive tectonics during a Paleozoic subduction event, and suffered intense, largely brittle, deformation, which partially obscured the earlier recrystallization process. The complex, multi-stage evolution of the ore-hosting dolomite is responsible for the uniqueness, high grade and giant size of the Bayan Obo deposit, the world's largest single REE resource with million tonnes of REE oxides.
Abstract: The Bayan Obo REE-Nb-Fe deposit, which reserves the current largest REE resources globally, also hosts over 70% of China’s Nb resources. Unlike many world-class carbonatite-related Nb deposits (e.g. Morro dos Seis Lagos and Araxá, Brazil) with igneous or secondary origin, Nb was mainly stored in Nb-bearing minerals (aeschynite, ilmenorutile, baotite, fergusonite etc.) of hydrothermal origin at Bayan Obo, supported by evidence from petrography, element and isotopic geochemistry. Although igneous fersmite and columbite were occasionally discovered in local carbonatite dykes, the Mesoproterozoic and Paleozoic hydrothermal metasomatism occurred in the ore-hosting dolomite, related to carbonatite intrusion and the closure of Paleo-Asian Ocean respectively, has played a more significant role during the ultimate Nb enrichment. REE, however, was significantly enriched during both the carbonatite-related magmatic and hydrothermal processes. Consequently, there was differentiated mineralization between REE and Nb in the carbonatite dykes and the ores. Niobium mineralization at Bayan Obo is rather limited in Mesoproterozoic carbonatite, whereas more extensive in the metasomatized ore-hosting dolomite, and generally postdating the REE mineralization at the same stage. According to mineral geochemistry, Bayan Obo aeschynite was classified into 3 groups: aeschynite-(Nd) with convex REE patterns (Group 1); aeschynite-(Ce) (Group 2) and nioboaeschynite (Group 3) with nearly flat REE patterns. Aeschynite (Group 1), ilmenorutile and fergusonite precipitated from Paleozoic hydrothermal fluids with advanced fractionation of Ce-rich REE minerals. The Mesoproterozoic hydrothermal Nb mineralization, represented by aeschynite (Group 3) and baotite, occurred postdating REE mineralization at same stage. Besides, fersmite and aeschynite (Group 2) precipitated from the Mesoproterozoic REE-unfractionated melt and hydrothermal fluids, respectively. All above Nb-bearing minerals exhibit extreme Nb-Ta fractionation as a primary geochemical characteristic of mantle-derived carbonatite. The forming age of the aeschynite megacrysts (Group 1) has not been accurately determined. However, the potential age was constrained to ~430 Ma or alternatively ~270-280 Ma subjected to subduction and granite activity, respectively. These aeschynite crystals inherited REEs from multiphase former REE mineralization, with an intermediate apparent Sm-Nd isochron age between the Mesoproterozoic and the Paleozoic REE mineralization events.
Geochimica et Cosmochimica Acta, Vol. 295, pp. 207-223.
China
nephelinites, basanites
Abstract: Widespread Cenozoic intraplate basalts from eastern China offer the opportunity to investigate the consequences of interaction between the stagnant Pacific slab and overlying asthenosphere and chemical heterogeneity within this “big mantle wedge”. We present and compile a comprehensive study of highly siderophile elements and Mg-Zn isotopes of this magmatic suite (60 samples including nephelinites, basanites, alkali basalts and tholeiites). The large-scale Mg-Zn isotopic anomalies documented in these basalts have been ascribed to mantle hybridization by recycled Mg-carbonates from the stagnant western Pacific plate. Our results reveal that the nephelinites and basanites are characterized by unfractionated platinum-group element (PGE) patterns normalized to primitive upper mantle (PUM) (e.g., PdN/IrN normalized to PUM?=?1.1?±?0.8, 1?), relatively high total PGE contents (e.g., Ir?=?0.25?±?0.14?ppb) and modern mantle-like 187Os/188Os (0.142?±?0.020). These characteristics are coupled with lighter Mg isotope (?26Mg?=??0.48?±?0.07‰) and heavier Zn isotope (?66Zn = +0.46?±?0.06‰) compositions compared to the mantle values (?26Mg: ?0.25?±?0.07‰; ?66Zn: +0.18?±?0.05‰). Together, these data are interpreted to reflect the oxidative breakdown of low proportions of mantle sulfides in the sources of these small-degree melts, likely caused by recycled carbonates, which then release chalcophile-siderophile elements into carbonatitic melts. By contrast, the contemporaneous alkali basalts and tholeiites are characterized by highly fractionated PGE patterns (e.g., PdN/IrN?=?4.4?±?3.3; Ir?=?0.037?±?0.027?ppb) and radiogenic 187Os/188Os (0.279?±?0.115) coupled with less fractionated Mg-Zn isotope compositions (?26Mg: ?0.39?±?0.05‰; ?66Zn: +0.35?±?0.03‰). In combination with other isotopic (e.g., Sr-Nd) and chemical (SiO2, Ce/Pb, Ba/Th, Fe/Mn) constraints, the alkali basalts and tholeiites were derived from higher degree melting of ancient pyroxenite-bearing mantle in addition to mixing with the aforementioned nephelinitic and basanitic melts. Collectively, we suggest that deep recycled carbonates promoted melting within the "big mantle wedge" leading to the generation of Cenozoic intraplate basalts across eastern China and the "redox freezing of carbonates" may cause the oxidation of Fe0 and S2-. This process may provide an important mechanism to oxidize mantle sulfides and transfer precious metals from deep mantle to crust.
Journal of Asian Earth Sciences, Vol. 214, 104772, 8p. Pdf
Mantle
carbon
Abstract: The temperature of the upper mantle was a principal factor controlling the style of plate tectonics and influencing magmatism and metamorphism on Earth over geological history. Recent studies emphasized that Earth’s tectonic style has transited into the modern plate tectonics since the late Neoproterozoic, which is characterized by a global network of plate boundaries with deep and cold oceanic plate subduction. However, the consequence of the establishment of modern plate tectonics to Earth’s mantle temperature and deep carbon cycle has not been fully understood. Here we apply statistical analysis on the geochemical data of continental igneous rocks and identify an increased magnitude of nephelinitic volcanism at the end of the Ediacaran. Nephelinitic rocks, a silica-undersaturated high-alkaline rock group, are mostly formed by low-degree melting of carbonated mantle sources. We link their widespread emergence with an enhanced mantle cooling event and a dramatically increased flux of crustal carbonates transporting to the mantle. The rapid cooling of the mantle was ascribed to the onset of modern-style plate tectonics with global-scale cold oceanic and continental subduction since the late Neoproterozoic. The declined upper-mantle temperature could not only favor the low-degree melting but also allow the subduction of carbonates into the deep mantle without decarbonation at shallow depth. Considering the high oxygen fugacity feature of the nephelinitic rocks and some other high-alkaline volcanism, the establishment of modern plate tectonics and thereafter enhanced mantle cooling and deep carbon cycle might contribute to the high-level atmospheric oxygen content during the Phanerozoic.
Abstract: Rare metals including Lithium (Li), Beryllium (Be), Rubidium (Rb), Cesium (Cs), Zirconium (Zr), Hafnium (Hf), Niobium (Nb), Tantalum (Ta), Tungsten (W) and Tin (Sn) are important critical mineral resources. In China, rare metal mineral deposits are spatially distributed mainly in the Altay and Southern Great Xingán Range regions in the Central Asian orogenic belt; in the Middle Qilian, South Qinling and East Qinling mountains regions in the Qilian-Qinling-Dabie orogenic belt; in the Western Sichuan and Bailongshan-Dahongliutan regions in the Kunlun-Songpan-Garze orogenic belt, and in the Northeastern Jiangxi, Northwestern Jiangxi, and Southern Hunan regions in South China. Major ore?forming epochs include Indosinian (mostly 200-240 Ma, in particular in western China) and the Yanshanian (mostly 120-160 Ma, in particular in South China). In addition, Bayan Obo, Inner Mongolia, northeastern China, with a complex formation history, hosts the largest REE and Nb deposits in China. There are six major rare metal mineral deposit types in China: Highly fractionated granite; Pegmatite; Alkaline granite; Carbonatite and alkaline rock; Volcanic; and Hydrothermal types. Two further types, namely the Leptynite type and Breccia pipe type, have recently been discovered in China, and are represented by the Yushishan Nb-Ta- (Zr-Hf-REE) and the Weilasituo Li-Rb-Sn-W-Zn-Pb deposits. Several most important controlling factors for rare metal mineral deposits are discussed, including geochemical behaviors and sources of the rare metals, highly evolved magmatic fractionation, and structural controls such as the metamorphic core complex setting, with a revised conceptual model for the latter.
Methane rich fluid inclusions from ophiolitic dunite and post collisional mafic/ultramafic intrusion: the mantle dynamics underneath the Paleo-Asian Ocean floor
Earth and Planetary Science Letters, Vol. 242, 3-4, pp. 286-301.
Yellowknife Forum NWTgeoscience.ca, abstract volume p. 57.
Canada, Northwest Territories
deposit - Gahcho Kue
Abstract: Gahcho Kué Mine is owned as a joint venture between Mountain Province Diamonds Inc. and De Beers Canada Inc. Located about 280 km northeast of Yellowknife, it is Canada’s newest diamond mine and the world’s largest in the last 14 years. After two years of construction, commercial operations began in September 2016. As the mine enters into its fourth year of operation, De Beers is pleased to provide an update on the current mine operations, updated mine plan, safety, environment, and social performance.
Abstract: Calcium silicate perovskite, CaSiO3, is arguably the most geochemically important phase in the lower mantle, because it concentrates elements that are incompatible in the upper mantle, including the heat-generating elements thorium and uranium, which have half-lives longer than the geologic history of Earth. We report CaSiO3-perovskite as an approved mineral (IMA2020-012a) with the name davemaoite. The natural specimen of davemaoite proves the existence of compositional heterogeneity within the lower mantle. Our observations indicate that davemaoite also hosts potassium in addition to uranium and thorium in its structure. Hence, the regional and global abundances of davemaoite influence the heat budget of the deep mantle, where the mineral is thermodynamically stable.
High pressure/ultrahigh pressure eclogites from the Hongan Block, east central China: geochemical characterization, isotope disequilibrium, geochronology
Contributions to Mineralogy and Petrology, Vol. 149, 5, pp. 499-526.
High pressure ultrahigh pressure eclogites from the Hong an Block, east central China: geochemical characterization, isotope disequilibrium and geochronological controversy.
Contributions to Mineralogy and Petrology, On line
Melting of subducted continent: element and isotopic evidence for a genetic relationship between Neoproterozoic and Mesozoic granitoids in the Sulu orogen.
Chemical Geology, Vol. 229, 4, May 30, pp. 227-256.
Two stage collision related extrusion of the western Dabie HP-UHP metamorphic terranes, centra China: evidence from quartz c-axis fabrics and structures.
Abstract: Minerals recovered from the deep mantle provide a rare glimpse into deep Earth processes. We report the first discovery of ferric iron-rich majoritic garnet found as inclusions in a host garnet within an eclogite xenolith originating in the deep mantle. The composition of the host garnet indicates an ultrahigh-pressure metamorphic origin, probably at a depth of ~200 km. More importantly, the ferric iron-rich majoritic garnet inclusions show a much deeper origin, at least at a depth of 380 km. The majoritic nature of the inclusions is confirmed by mineral chemistry, x-ray diffraction, and Raman spectroscopy, and their depth of origin is constrained by a new experimental calibration. The unique relationship between the majoritic inclusions and their host garnet has important implications for mantle dynamics within the deep asthenosphere. The high ferric iron content of the inclusions provides insights into the oxidation state of the deep upper mantle.
Abstract: The oxygen fugacity of the upper mantle is 3-4 orders of magnitude higher than that of the lower mantle and this has been attributed to Fe2 + disproportionating into Fe3 + plus Fe0 at pressures > 24 GPa. The upper mantle might therefore have been expected to have evolved to more oxidizing compositions through geological time, but it appears that the oxygen fugacity of the upper mantle has remained constant for the last 3.5 billion years. Thus, it indicates that the mantle has been actively buffered from the accumulation of Fe3 +, and that this is linked to oxidation of diamond to carbonate coupled with reduction of Fe3 + to Fe2 +. When subducted plates penetrate into the lower mantle, compensational upwelling transports bridgmanite into the transition zone, where it breaks down to ringwoodite and majorite, releasing the ferric iron. The system returns to equilibrium through oxidation of diamond. Early in Earth history, diamond may have been enriched at the base of the transition zone in the Magma Ocean, because it is denser than peridotite melts at depths shallower than 660 km, and it is more buoyant below. Ongoing oxidation of diamond forms carbonate, leading to relatively high carbonate concentrations in the source of ocean island basalts.
Abstract: Mineral inclusions in diamonds reflect the compositional features of the diamondiferous sources so that they can provide important information of the upper mantle and even the transition zone. Since Moore and Gurney (1985) [1] found majoritic garnet inclusions in kimberlitic diamonds, the majoritic garnet inclusions have become significant samples to understand deep earth processes. According to Kiseeva et al. (2018) [2], the majoritic garnet inclusions have a ferric iron fraction (Fe3+/?Fe) increasing with depth. Recently, Xu et al. (2017) [3] discovered some majoritic garnets (from a depth of ~ 400 km) with ferric iron proportions as high as 0.81. In order to understand the origin of these Fe3+-rich garnet inclusions, the physical-chemical properties of the Fe3+-bearing garnets should be investigated. So far, no many studies have been carried out though. In this study, we performed experiments to synthesize garnets with their compositions along the grossular-andradite binary at 3 GPa and 1100 - 1200 °C, in order to investigate the effect on the physical-chemical properties of replacing Al3+ with Fe3+ on the octahedral sites of the garnet. The synthetic samples were investigated by BSE, EMPA, powder XRD and unpolarized Raman spectroscopy. The garnets had grain sizes between 20 ~ 100 um as indicated by BSI, and had generally homogenous compositions as suggested by the EMPA data. Charge balance calculations showed that most garnets had ferric iron fractions exceeding ~0.95. With an approximately linear dependence of the lattice parameters on the composition, the volume-mixing behavior should be generally ideal. We collected multiple Raman spectra from each sample, and evaluated the effects of crystal orientation and compositional heterogeneity on the Raman peak positions. Relations between the garnet compositions and Raman peak frequencies were also analyzed. Most Raman modes show linear behavior across the binary, whereas some of them do not. The reasons have been explored. According to the results of this study, we can infer that the elastic properties of the grossular-andradite binary might be linearly dependent to the composition. The Raman spectroscopic data are important for deriving some important thermodynamic data.
Abstract: The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2?Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66-3.20?Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2?Ga granulite-facies assemblage in a metagabbro of the AGC of central Swaziland and relate this to a major thermo-magmatic event that not only affected the AGC but also the neighbouring Barberton granitoid-greenstone terrane. Some previous models have related the 3.2?Ga event in the eastern Kaapvaal craton to subduction processes, but we see no evidence for long, narrow belts and metamorphic facies changes reflecting lithospheric suture zones, and there is no unidirectional asymmetry in the thermal structure across the entire region from Swaziland to the southern Barberton granite-greenstone terrane as is typical of Phanerozoic and Proterozoic belts. Instead, we consider an underplating event at ca. 3.2?Ga, giving rise to melting in the lower crust and mixing with mantle-derived under- and intraplated mafic magma to generate the voluminous granitoid assemblages now observed in the AGC and the southern Barberton terrane. This is compatible with large-scale crustal reworking during a major thermo-magmatic event and the apparent lack of a mafic lower crust in the Kaapvaal craton as shown by seismic data.
Journal of Geophysical Research , Vol. 124, 5, pp. 4617-4638.
Mantle
melting
Abstract: The oxidation state of the Earth`s mantle, often expressed as oxygen fugacity (fO2), could control the behavior of multivalent elements and thus exert a significant influence on the formation of magmatic ore deposits and the secular evolution of Earth`s atmosphere. Whether arc mantle is more oxidized than oceanic mantle remains a controversial topic. As a multivalent element, partitioning behavior of vanadium is fO2 sensitive and is capable of tracking mantle redox state. However, except fO2, other factors (temperature, pressure, and phase composition) that may affect vanadium partitioning behavior have not been clearly evaluated. Here we conducted high temperature and pressure experiments to determine partition coefficients of vanadium during mantle melting under various fO2 conditions. Combining our and published data, we evaluated the effects of fO2, T, P, and compositions of mineral and melt on the vanadium partitioning using multiple linear regressions. The results indicate that, in addition to fO2, temperature exerts a significant control on the vanadium partitioning. Additionally, we estimated fO2 of the arc mantle via numerical modelling using appropriate partition coefficients for vanadium. Our results clarify and reconcile the discrepancies between previous studies and reveal that arc mantle is generally ~10 times more oxidized than oceanic mantle.
Journal of African Sciences, Vol. 184, 104308, 12p. Pdf
Africa, Tanzania
deposit - Mbalizi
Abstract: The Mbalizi carbonatite is located in the middle of the Paleoproterozoic Ubendian Mobile Belt and the western branch of East Africa Rift, southwestern Tanzania. Calcite, dolomite, phlogopite, pyrochlore and apatite are found in the sample. Mineral chemistry studies have shown that the carbonatite phlogopite is linked to mantle-derived magmatism. The apatite is fluorapatite, means they are of magmatic origin. The analyses on two crystals of pyrochlore show high concentrations of Nb2O5, and therefore the Nb-oxide is classified as pyrochlore subspecies. Three types of zircon have been obtained from the Mbalizi carbonatite, including xenocrysts zircon, igneous zircon and metamorphic zircon. Zircon in-situ LA-ICP-MS U-Pb dating in this contribution indicates that the Mbalizi carbonatite was crystallized at ca. 116.0 ± 1.8 Ma. The ?Hf(t) values of igneous zircon ranging from ?13.9 to +5.7, indicates that the carbonatite parental magma was originated from the sub-continental lithospheric mantle, and evolves toward HIMU and EM. The whole-rock Sr-Nd isotopic data suggest more contribution of the HIMU and EM? material. We propose that the complex evolutionary history of the Ubendian Mobile Belt has stored the subduction oceanic crust which has the EM? and HIMU components, forming the compositional heterogeneity mantle beneath the Ubendian Mobile Belt. At 116.0 ± 1.8 Ma, with the extension stress field, deep faults cause the pressure reduction, resulting in reactive of the upwelling of the HIMU and EM? components. This provides the metamorphic conditions to induce the isotopic resetting and may result in large scatter of initial 176Hf/177Hf ratios of carbonatite melts.
Refertilization of ancient lithosphere mantle beneath the central North Chin a craton: evidence from petrology and geochemistry of peridotite xenoliths.
Deep subduction of continental crust in accretionary orogen: evidence from U-Pb dating on diamond-bearing zircons from the Qinling orogen, central China.
Lan, T-G., Fan, H-R., Santosh, M., Hu, F-F., Yang, Y-H, Liu, Y.
Geochemistry and Sr Nd Pb Hf isotopes of the Mesozoic Dadian alkaline intrusive complex in the Sulu orogenic belt, eastern China: implications for crust mantle interaction.
Precise age determin ation of the Paleozoic kimberlites in North Chin a craton and Hf isotopic constraint on the evolution of its subcontinental lithospheric mantle.
Geological Society of London Special Publication: Continent formation through time., No. 389, pp. 135-163.
India
Geotectonics
Abstract: Field and geochemical studies combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating set important constraints on the timing and petrogenesis of volcanic rocks of the Neoarchaean Kadiri greenstone belt and the mechanism of crust formation in the eastern Dharwar craton (EDC). The volcanic rocks are divided into three suites: tholeiitic basalts, calc-alkaline high-Mg# andesites and dominant dacites-rhyolites. The basalts (pillowed in places) show flat rare earth element (REE) and primordial mantle-normalized trace element patterns, but have minor negative Nb and Ta anomalies. They are interpreted as mantle plume-related oceanic plateau basalts whose source contained minor continental crustal input. The andesites are characterized by high Mg# (0.66-0.52), Cr and Ni, with depletion of high-field strength elements (HFSE) and enrichment of light REE (LREE) and large-ion lithophile elements (LILE). They were probably derived from a metasomatized mantle wedge overlying a subducted slab in a continental margin subduction zone. The dacites-rhyolites are silicic rocks (SiO2 = 61-72 wt%) with low Cr and Ni, K2O/Na2O mostly 0.5-1.1, highly fractionated REE patterns, enrichments of LILE and distinctly negative HFSE anomalies. One rhyolite sample yielded a zircon U-Pb age of 2353 ± 32 Ma. This suite is similar to potassic adakites and is explained as the product of deep melting of thickened crust in the arc with a significant older crustal component. Collision between a continental margin arc with an oceanic plateau followed by slab break-off, upwelling of hot asthenosphere and extensive crustal reworking in a sustained compressional regime is proposed for the geodynamic evolution of the area. This is in corroboration with the scenario of EDC as a Neoarchaean hot orogen as suggested recently by some workers.
Abstract: We have found >10 in situ microdiamonds in thin sections of eclogites from the Dabie and Su-Lu regions of central eastern China since the first occurrence of microdiamond in eclogites from the Dabie Mountains (DMT) reported in 1992. The microdiamonds are found not only in the central part but also in the northern part of the DMT. Several free crystals have been recovered from the crushed eclogites from the central DMT. Most in situ microdiamonds are inclusions in garnets but a few larger ones are intergranular. Most of the diamondiferous eclogites in the central part of the DMT are associated with coesite. Most importantly, the observation of microdiamonds in northern Dabie lead us to question the supposition that this is a low-P metamorphic terrane. All the diamondiferous eclogites from both the north and central DMT are of continental affinity as demonstrated by their negative ?Nd values. Therefore, both the north and central eclogite belts in the DMT are considered to be from the deep subducted terrane. Five in situ microdiamonds and two free crystals are first reported in this paper. The dimensions of the in situ microdiamonds are 30-80 ?m and the free crystals are up to 400–-00 ?m across. All the microdiamonds are confirmed as such by Raman spectroscopy. The results of an infrared spectroscopic investigation on two larger free crystals and two in situ microdiamonds show that all the microdiamonds from both the Dabie and Su-Lu regions are mixed types IaA and IaB diamonds and there is no indication of any synthetic microdiamonds in our samples because such synthetic microdiamonds are always rich in type Ib.
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Mantle
NAMs Nominally Anhydrous Minerals
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Low water contents in diamond mineral inclusions: proto-genetic origin in a dry cratonic lithosphere.
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Russia, Africa
Kaapvaal and Siberian SCLMs
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Geophysical Research Letters, Vol. 43, 1, pp. 144-153.
United States
Geophysics - gradiometry
Abstract: In order to better understand the tectonic evolution of the North American continent, we utilize data from the EarthScope Transportable Array network to calculate a three-dimensional shear velocity model for the continental United States. This model was produced through the inversion of Rayleigh wave phase velocities calculated using ambient noise tomography and wave gradiometry, which allows for sensitivity to a broad depth range. Shear velocities within this model highlight the influence of orogenic and postorogenic events on the evolution of the lithosphere. Most notable is the contrast in crustal and upper mantle structure between the relatively slow western and relatively fast eastern North America. These differences are unlikely to stem solely from thermal variations within the lithosphere and highlight both the complexities in lithospheric structure across the continental U.S. and the varying impacts that orogeny can have on the crust and upper mantle.
Journal of Asian Earth Sciences, in press available, 134p.
China, Tibet
Carbonatite
Abstract: The Cenozoic Mianning-Dechang (MD) rare earth element (REE) belt in eastern Tibet is an important source of light REE in southwest China. The belt is 270 km long and 15 km wide. The total REE resources are >3 Mt of light rare earth oxides (REO), including 3.17 Mt of REO at Maoniuping (average grade = 2.95 wt.%), 81,556 t at Dalucao (average grade = 5.21 wt.%), 0.1 Mt at Muluozhai (average grade = 3.97 wt.%), and 5764 t of REO at Lizhuang (average grade = 2.38 wt.%). Recent results from detailed geological surveys, and studies of petrographic features, ore-forming ages, ore forming conditions, and wallrock alteration are synthesized in this paper. REE mineralization within this belt is associated with carbonatite-syenite complexes, with syenites occurring as stocks intruded by carbonatitic sills or dikes. The mineralization is present as complex vein systems that contain veinlet, stringer, stockwork, and brecciated pipe type mineralization. Carbonatites in these carbonatite-related REE deposits (CARDs) are extremely rich in light REEs, Sr (>5000 ppm), and Ba (>1000 ppm), and have low Sr/Ba and high Ba/Th ratios, and radiogenic Sr-Nd isotopic compositions. These fertile magmas, which may lead to the formation of REE deposits, were generated by the partial melting of sub-continental lithospheric mantle (SCLM) that was metasomatized by REE- and CO2-rich fluids derived from subducted marine sediments. We suggest that this refertilization occurred along cratonic margins and, in particular, at a convergent margin where small-volume carbonatitic melts ascended along trans-lithospheric faults and transported REEs into the overlying crust, leading to the formation of the CARDs. The formation of fertile carbonatites requires a thick lithosphere and/or high pressures (>25 kbar), a metasomatized and enriched mantle source, and favorable pathways for magma to ascend into the overlying crust where REE-rich fluids exsolve from cooling magma. The optimal combination of these three factors only occurs along the margins of a craton with a continental root, rather than in modern subduction zones where the lithosphere is relatively thin. U-Pb zircon dating indicates that the Maoniuping, Lizhuang, and Muluozhai alkali igneous complexes in the northern part of the belt formed at 27-22 Ma, whereas the Dalucao complex in the southern part of the belt formed at 12-11 Ma. Biotite and arfvedsonite in Lizhuang and Maoniuping REE deposit have 40Ar/39Ar ages of 30.8 ± 0.4 Ma (MSWD = 0.98) and 27.6 ± 2.0 Ma (MSWD = 0.06), respectively. Biotitaion alteration in syenite and fenitization caused by the relatively amount of carbonatite on syenite and host rocks is the main alteration along the whole belt. Initial Sr (0.7059-0.7079), 143Nd/144Nd (0.5123-0.5127), and 207Pb/204Pb (15.601-15.628) and 208Pb/204Pb (38.422-38.604) isotopic compositions of fluorite, barite, celestite, and calcite in the MD belt are similar to those of the associated syenite and carbonatite. Given the relatively high contents of Cl, F, SO42-, and CO2 in the rocks of the complexes, it is likely that the REEs were transported by these ligands within hydrothermal fluids, and the presence of bastnäsite indicates that the REEs were precipitated as fluorocarbonates. Petrographic, fluid inclusion, and field studies of the ores indicate that bastnäsite and other REE minerals formed during the final stages (<300°C) of the evolution of magmatic-hydrothermal systems in the belt. The mineralization formed from magmatic and meteoric fluids containing CO2 derived from the decarbonation of carbonatite, as indicated by C-O isotopic values of hydrothermal calcite and bastnäsite (?13C= -4.8 to -8.7 and ?18O = 5.8 to 12.5 ‰) and O-H isotopic values of quartz (330°C) and arfvedsonite (260°C), which correspond to fluid isotope compositions of ?18O = 0.3 to 9.8‰ and ?D = -70.0 to -152.8‰ in the belt. This study indicates that formation the largest REE deposits are related to voluminous carbonatite-syenite complexes, compositionally similar ore-forming fluids, extensive alteration, multiple stages of REE mineralization, and tectonic setting.
Abstract: The Bayan Obo deposit in Inner Mongolia, North China Craton (NCC) is the largest rare-earth element (REE) resource in the world. Due to the complex element and mineral compositions and the activity of several geological events, the ore-forming mechanism is still controversial. Previous models are reviewed here to provide information for further investigation on the Bayan Obo deposit. In this study, we summarize all different types of Fe-REE-Nb mineralization using field observations and microscope work, in which we recognize 9 types of Fe-REE-Nb ores in the Bayan Obo ore district. By compiling and re-evaluating a large number of published geochemical data, this paper provides solid evidence that the Bayan Obo deposit formed through interaction between sedimentary rocks and carbonatite magmatism. From the results of our review, it can be conjectured that the formation of iron ores was originated from sedimentation (Pt1), whereas the formation of REE mineralized dolomite might be related to interaction and reaction between the carbonatite magmas and/or associated fluids with sedimentary carbonate rocks, with the REE-bearing carbonatite magmas having undergone intense fractionation enrichment process. The C-O-S-Fe-Mg isotopes indicate that the REE-Nb mineralization was derived from metasomatism (fenitic alteration) of sedimentary carbonate. A new model is proposed for this unique REE-Nb mineralization, which is related to the subduction of Siberian Craton beneath the North China Craton since Early Paleozoic period. We interpret that the Bayan Obo Fe-REE-Nb ore deposits and their massive barren host, H8 dolomite, were generated as a result of interaction of fluids expelled from a subcontinental lithospheric mantle (SCLM)-derived carbonatite magma with sedimentary carbonates.
Abstract: SiC and associated ultra-reduced minerals were reported in various geological settings, however, their genesis and preservation mechanism are poorly understood. Here, we reported a SiC-dominated ultra-reduced mineral assemblage, including SiC, TiC, native metals (Si, Fe, and Ni) and iron silicide, from carbonatitic xenoliths in Dalihu, Inner Mongolia. All minerals were identified in situ in polished/thin sections. SiC is 20-50 ?m in size, blue to colorless in color, and usually identified in the micro-cavities within the carbonatitic xenolith. Four types of SiC polytypes were identified, which are dominated by ?-SiC (3C polytype) and 4H polytype followed by 15R and 6H. These SiC are featured by 13C-depleted isotopic compositions (?13C = ?13.2 to ?22.8‰, average = ?17.7‰) with obvious spatial variation. We provided a numerical modeling method to prove that the C isotopic composition of the Dalihu SiC can be well-yielded by degassing. Our modeling results showed that degassing reaction between graphite and silicate can readily produce the low ?13C value of SiC, and the spatial variations in C isotopic composition could have been formed in the progressive growth process of SiC. The detailed in situ occurring information is beneficial for our understanding of the preservation mechanism of the Dalihu ultra-reduced phase. The predominant occurrence of SiC in micro-cavities implies that exsolution and filling of CO2 and/or CO in the micro-cavities during the diapir rising process of carbonatitic melt could have buffered the reducing environment and separated SiC from the surrounding oxidizing phases. The fast cooling of host rock, which would leave insufficient time for the complete elimination of SiC, could have also contributed to the preservation of SiC.
Abstract: The Bayan Obo in the northern North China Craton (NCC) is the world’s largest light rare earth element (LREE) deposit and is hosted in carbonatite sills emplaced into sedimentary rocks of the Bayan Obo Group. However, the timing and genesis of the Bayan Obo deposit has been highly controversial for many decades. Here we report a precise zircon 208Pb/232Th age of 1301 ± 12 Ma (N = 47, mean square of weighted deviates [MSWD] = 2.2) for a REE-Nb-rich carbonatite sill from the Bayan Obo deposit. Zircon morphology, trace element compositions and mineral inclusions demonstrate that these zircons were crystallized from REE-Nb-rich carbonatitic magmas and their ages represent the timing of carbonatites and REE-Nb mineralization. The newly obtained age of ca. 1.30 Ga is consistent with field observations of the Bayan Obo REE-Nb deposit and successfully explains why the carbonatites and REE-Nb mineralization in the Bayan Obo deposit occurred mainly in the Jianshan Formation and that no carbonatites and REE-Nb mineralization were identified from the rocks overlying the Jianshan Formation. The new results demonstrate that the Bayan Obo REE-Nb deposit is a product of mantle-derived carbonatite magmatism at ca. 1.30 Ga. Field relations show that emplacement of the Bayan Obo carbonatites was accompanied by pre-magmatic uplift that is considered to be related to rift-to-drift transition. The Bayan Obo carbonatites and REE-Nb deposit are spatially and temporally linked with the newly identified 1.33-1.30 Ga Yanliao large igneous province (LIP) in the northern NCC and were related to continental rifting that have led to breakup of the NCC from the Columbia (Nuna) supercontinent.
Journal of Asian Earth Sciences, Vol. 137, pp. 35-79.
China, Tibet
Carbonatite
Abstract: he Cenozoic Mianning-Dechang (MD) rare earth element (REE) belt in eastern Tibet is an important source of light REE in southwest China. The belt is 270 km long and 15 km wide. The total REE resources are >3 Mt of light rare earth oxides (REO), including 3.17 Mt of REO at Maoniuping (average grade = 2.95 wt.%), 81,556 t at Dalucao (average grade = 5.21 wt.%), 0.1 Mt at Muluozhai (average grade = 3.97 wt.%), and 5764 t of REO at Lizhuang (average grade = 2.38 wt.%). Recent results from detailed geological surveys, and studies of petrographic features, ore-forming ages, ore forming conditions, and wallrock alteration are synthesized in this paper. REE mineralization within this belt is associated with carbonatite-syenite complexes, with syenites occurring as stocks intruded by carbonatitic sills or dikes. The mineralization is present as complex vein systems that contain veinlet, stringer, stockwork, and brecciated pipe type mineralization. Carbonatites in these carbonatite-related REE deposits (CARDs) are extremely rich in light REEs, Sr (>5000 ppm), and Ba (>1000 ppm), and have low Sr/Ba and high Ba/Th ratios, and radiogenic Sr-Nd isotopic compositions. These fertile magmas, which may lead to the formation of REE deposits, were generated by the partial melting of sub-continental lithospheric mantle (SCLM) that was metasomatized by REE- and CO2-rich fluids derived from subducted marine sediments. We suggest that this refertilization occurred along cratonic margins and, in particular, at a convergent margin where small-volume carbonatitic melts ascended along trans-lithospheric faults and transported REEs into the overlying crust, leading to the formation of the CARDs. The formation of fertile carbonatites requires a thick lithosphere and/or high pressures (>25 kbar), a metasomatized and enriched mantle source, and favorable pathways for magma to ascend into the overlying crust where REE-rich fluids exsolve from cooling magma. The optimal combination of these three factors only occurs along the margins of a craton with a continental root, rather than in modern subduction zones where the lithosphere is relatively thin.
Abstract: Preliminary mineralogical and geochemical studies have been carried out on dolomite marble drill cores from the Bayan Obo REE deposit in China. Three types of apatites and four types of monazites have been identified based on textural features: Type 1 apatite occurs as grains with minor monazite (Type 1 monazite) on its border; Type 2 apatite veinlet shows clusters of assemblages with abundant bastnäsite and parisite at the rim; Type 3 apatite has a linear array associated with fluorite and bastnäsite veinlets. Type 2 monazite occurs as clusters intergrowing with parisite and fluorite. Type 3 and 4 monazites occur as polymineralic (fluorite and bastnäsite) and monomineralic veinlets, respectively. These four types of monazites have similar LREE composition but variable Y content (Y2O3 ranging from below determination limits to 0.7?wt%). The three types of apatites also show different REE content and distribution patterns, ranging from high REE abundance (?REE?+?Y: 27243-251789?ppm) and strong LREE enrichment [(La/Yb)CN ?101] in Type 1, less LREE enrichment [(La/Yb)CN ?8] in Type 2 to relatively low REE abundance (?REE?+?Y: 4323-11175?ppm) but high REE fractionation [(La/Yb)CN ?58] in Type 3. The primary apatite has high Sr (5461-6892?ppm) and REE content, implying a carbonatite origin. The late-stage apatites (Types 2 and 3) show different Sr and REE abundances. Significant differences in their Sr composition (6189?±?573, 6041?±?549 and 3492?±?802 for Types 1-3 samples, respectively) and Y/Ho ratio (20.9?±?0.11, 19.5?±?0.17 and 17.4?±?0.37, respectively) indicate that the three types of apatites may have crystallized from different metasomatic fluids. Multi-stage metasomatism resulted in remobilization and redeposition of primary REE minerals to form the Bayan Obo REE deposit.
Abstract: The Archean Yilgarn Craton in Western Australia is intruded by numerous mafic dykes of varying orientations, which are poorly exposed but discernible in aeromagnetic maps. Previous studies have identified two craton-wide dyke swarms, the 2408?Ma Widgiemooltha and the 1210?Ma Marnda Moorn Large Igneous Provinces (LIP), as well as limited occurrences of the 1075?Ma Warakurna LIP in the northern part of the craton. We report here a newly identified NW-trending mafic dyke swarm in southwestern Yilgarn Craton dated at 1888?±?9?Ma with ID-TIMS U-Pb method on baddeleyite from a single dyke and at 1858?±?54?Ma, 1881?±?37 and 1911?±?42?Ma with in situ SHRIMP U-Pb on baddeleyite from three dykes. Preliminary interpretation of aeromagnetic data indicates that the dykes form a linear swarm several hundred kilometers long, truncated by the Darling Fault in the west. This newly named Boonadgin dyke swarm is synchronous with post-orogenic extension and deposition of granular iron formations in the Earaheedy basin in the Capricorn Orogen and its emplacement may be associated with far field stresses. Emplacement of the dykes may also be related to initial stages of rifting and formation of the intracratonic Barren Basin in the Albany-Fraser Orogen, where the regional extensional setting prevailed for the following 300?million years. Recent studies and new paleomagnetic evidence raise the possibility that the dykes could be part of the coeval 1890?Ma Bastar-Cuddapah LIP in India. Globally, the Boonadgin dyke swarm is synchronous with a major orogenic episode and records of intracratonic mafic magmatism on many other Precambrian cratons.
Abstract: Rare-earth deposits associated with intrusive carbonatite complexes are the world’s most important source of these elements (REE). One of the largest deposits of this type is Maoniuping in the Mianning-Dechang metallogenic belt of eastern Tibet (Sichuan, China). In the currently mined central part of the deposit (Dagudao section), REE mineralization is hosted by a structurally and mineralogically complex Late Oligocene (26.4 ±?1.2 Ma, 40Ar/39Ar age of fluorphlogopite associated with bastnäsite) hydrothermal vein system developed in a coeval syenite intrusion. Low-grade stockworks of multiple veinlets and breccias in the lower part of the orebody grade upwards into progressively thicker veins (up to 12 m in width) that are typically zoned and comprise ferromagnesian micas (biotite to fluorphlogopite), sodium clinopyroxenes (aegirine to aegirine-augite), sodium amphiboles (magnesio-arfvedsonite to fluororichterite), K-feldspar, fluorite, barite, calcite, and bastnäsite. The latter four minerals are most common in the uppermost 80 m of the Dagudao section and represent the climax of hydrothermal activity. Systematic variations in the fluid inclusion data indicate a continuous hydrothermal evolution from about 230-400 °C (fluid inclusions in feldspar, clinopyroxene, and amphibole) to 140-240 °C (fluid inclusions in bastnäsite, fluorite, calcite). Hydrothermal REE transport was probably controlled by F?, (SO4)2?, Cl?, and (CO3)2? as complexing ligands. We propose that at Dagudao, silicate magmas produced orthomagmatic fluids that explored and expanded a fissure system generated by strike-slip faulting. Initially, the fluids had appreciable capacity to transport REE and, consequently, no major mineralization developed. The earliest minerals to precipitate were alkali- and Fe-rich silicates containing low levels of F, which caused progressive enrichment of the fluid in Ca, Mg, F, Cl, REE, (SO4)2?, and (CO3)2?, leading to the crystallization of aegirine-augite, fluororichterite, fluorphlogopite, fluorite, barite, calcite, and bastnäsite gradually. Barite, fluorite, calcite, and bastnäsite are the most common minerals in typical ores, and bastnäsite generally postdates these gangue minerals. Thus, it is very probable that fluid cooling and formation of large amount of fluorite, barite, and calcite triggered bastnäsite precipitation in the waning stage of hydrothermal activity.
Abstract: Multifractal features of element concentrations in the Earth’s crust have demonstrated to be closely associated with multiple probability distributions such as normal, lognormal and power law. However, traditional understanding of geochemical distribution satisfying normal, lognormal or power-law models still faces a serious problem in adjusting theoretical statistics with the empirical distribution. Given that the differences among different geochemical distribution populations may have considerable effects on the target estimation, a new perspective from the singularity of fractal density is adopted to investigate mixed geochemical distribution patterns within frequency and space domains. In the framework of fractal geometry, ordinary density such as volume density (e.g., g/cm3 and kg/m3) described in Euclidean space can be considered as a special case of the fractal density (e.g., g/cm? and kg/m?). According to the nature of fractal density, geochemical information obtained from Euclidean geometry may not sufficiently reflect inherent geochemical features, because some information might be hidden within fractal geometry that can be only revealed by means of a set of fractional dimensions. In the present study, stream sediment geochemical data collected from west Tianshan region, Xinjiang (China), were used to explore element distribution patterns in the Earth’s crust based on a fractal density model. Four elements Cu, Zn, K and Na were selected to study the differences between minor and major elements in terms of their geochemical distribution patterns. The results strongly suggest that element distribution patterns can be well revealed and interpreted by means of a fractal density model and related statistical and multifractal parameters.
Abstract: n recent years, nominal type IaAB and IIa diamonds with transient 2800 cm-1 FTIR absorption peaks arising from uncompensated boron produced under UV radiation have been reported (J. Li et al., A diamond with a transient 2804 cm-1 absorption peak, Journal of Gemmology, Vol. 35, 2016, pp. 248-252; Winter 2016 Lab Notes, pp. 412-413). The National Center of Supervision and Inspection on Quality of Gold and Silver Products recently examined a type IaB diamond that exhibited instantaneous 2803 cm-1 FTIR absorption shortly after exposure to an ultra-short-wave (< 230 nm) UV source.
Abstract: Bastnäsite is the main ore mineral in many carbonatite-related rare earth element (REE) deposits, which account for ?51% of rare-earth oxide reserves worldwide. However, the occurrence, geochemistry, and genetic significance of bastnäsite has not been methodically investigated. The Cenozoic Mianning-Dechang (MD) REE belt in Sichuan Province, SW China, contains the Maoniuping, Dalucao, Lizhuang, and Muluozhai deposits as well as numerous smaller REE occurrences. Individual deposits within the belt contain different types of bastnäsite-bearing ore, which provides a unique opportunity to explore in detail the common mechanisms controlling the formation of bastnäsite-rich REE deposits. Here, we present detailed results from field observations and petrographic, geochemical, and fluid inclusion studies of bastnäsite from the main MD deposits. Calcite, fluorite, and barite form stable mineral assemblages that are commonly overprinted by bastnäsite. Homogenization temperatures of fluid inclusions in bastnäsite of ?150-270?°C (Dalucao and Lizhuang deposits) and 155-210?°C (Maoniuping deposit) are systematically lower than those of fluid inclusions in gangue minerals. Therefore, the petrographic studies and homogenization temperatures both show that large-scale crystallization of bastnäsite took place during the later stage of the hydrothermal system. The bastnäsite, relatively geochemically homogeneous within all of the MD deposits, is enriched in Ba (293-8425?ppm), Th (16.4-2527?ppm), and U (4.19-92.7?ppm), and relatively depleted in high field strength elements such as Nb (0.15-17.4?ppm), Ta (0.06-6.48?ppm), Zr (0.71-31.1?ppm), Hf (0.62-5.65?ppm), and Ti (<60?ppm), the same to carbonatite, and ore veins. In comparison, the samples from the study area show an increase in average REE contents from syenites to carbonatites to ore veins (i.e., bastnäsite-bearing ores) and finally to bastnäsite. Lanthanum and Ce were commonly substituted by Th, U, Sc, Ba, and Sr supplied by more evolved hydrothermal fluids. Combining the present results with existing data, we present a three-stage model for the formation of carbonatite-related REE deposits. First, partial melting of metasomatized sub-continental lithospheric mantle, fluxed by REE- and CO2-rich fluids, forms the parental carbonatite-syenite magma. Second, Sr, Ba, and REEs are strongly partitioned into carbonatite melts during liquid immiscibility in the carbonatite-syenite magmatic system. Third, hydrothermal fluids exsolved from the crystalizing syenite and carbonatite magmas form ore veins with early gangue minerals and later bastnäsite overgrowths. Consequently, barite, calcite, and fluorite assemblages are a valuable guide in REE exploration.
Abstract: Carbonatite-related rare-earth element (REE) deposits are the most important source of the world’s REE resources. Hydrothermal fluids have been proposed to play a significant role in the transport and precipitation of REEs, but fluid inclusion data on the hydrothermal processes in carbonatitic settings are relatively sparse. The Dalucao deposit, located in the Mianning-Dechang (MD) REE belt, Sichuan, China, is a Cenozoic carbonatite-related REE deposit (c. 12?Ma) that offers an excellent opportunity to investigate the evolution of ore-forming fluids. Brecciated and weathered ores are common in this deposit. The former are characterized by mineral assemblages comprising fluorite?+?barite?+?celestite?+?calcite?+?quartz?+?bastnäsite (No. 1 orebody) or fluorite?+?celestite?+?pyrite?+?muscovite?+?calcite?+?quartz?+?bastnäsite (No. 3 orebody), whereas the latter contain REE minerals, clay minerals, and minor gangue minerals. We present a comprehensive study of fluid inclusions from the Dalucao deposit to constrain its hydrothermal evolution. Magmatic, pegmatitic, hydrothermal, and supergene stages have been recognized. During the pegmatitic stage, the main minerals that formed were coarse-grained fluorite, barite, celestite, calcite, and quartz, which host melt inclusions, melt-fluid inclusions, and minor high-salinity fluid inclusions. The presence of melt and melt-fluid inclusions suggests a magmatic origin for the ore-forming fluids. Hydrothermal processes included at least two stages, characterized by hydrothermal veins that are developed in fractures within the carbonatite-syenite complex: (1) Fluid inclusions during the formation of the fluorite-quartz-barite veins in the pre-REE stage were trapped under immiscible conditions, as evidenced by the presence of CO2-bearing inclusions coexisting with aqueous ones. These immiscible CO2-bearing inclusions recorded a range of pressures from 1050 to 1600?bar. All of fluid inclusions in this stage exhibited homogenization temperatures varying from 278 to 442?°C, with salinities ranging from 3.2 to 45.1?wt% NaCl equivalent (equiv.). (2) The REE-stage fluids were represented by abundant aqueous inclusions, characterized by homogenization temperatures ranging from 147 to 323?°C and salinities between 1.1 and 9.5?wt% NaCl equiv. These data suggest that the ore-forming fluids forming the Dalucao deposit evolved from high-temperature, high-pressure, high-salinity, CO2-rich to low-temperature, low-pressure, low-salinity, CO2-poor. Gas- and ion-chromatographic analyses combined with mineralogical features indicate that the initial fluids were rich in REEs, (SO4)2?, Cl?, F?, Na+, K+, Ca2+, and volatile components (e.g., H2O, CO2, N2, CH4, Ar, and C2H6). H-O isotope analyses of quartz suggest that the hydrothermal fluids had a dominantly magmatic signature and were gradually diluted by meteoric waters. Hydrothermal REE transport was probably controlled by F?, (SO4)2?, and Cl? as complexing ligands. We propose that fluid cooling and mixing rather than immiscibility led to the precipitation of bastnäsite during the waning stage of hydrothermal activity. Taken together, the inclusion data and observations of alteration, paragenesis and mineralization have provided insights into the development of REE mineralization and the further exploration of carbonatite-related REE resources.
Abstract: Carbonatite-related rare-earth element (REE) deposits (CARDs) are the major global source of REEs. The ore-forming fluids of CARDs usually comprise multiple components and record complicated evolutions. The Maoniuping REE deposit, located in the eastern Tibetan Plateau, is the second-largest CARD in China and contains total reserves of 3.17?Mt of light rare-earth oxides (REOs). Geochronological and geological data show that the deposit was formed at ?25?Ma and was only moderately affected by tectonic and hydrothermal activities, thereby allowing us to study the evolution of ore fluids as well as the mechanisms of REE mineralization. The Maoniuping REE deposit is spatially associated with a carbonatite-syenite complex and includes two sections: Guangtoushan and Dagudao. The Dagudao section is the main focus of exploration and hosts well-developed vein systems. In the uppermost vein system, minerals are zoned from the syenite wall-rock contact to the vein centers in the order of biotite, aegirine-augite, arfvedsonite, calcite, quartz, barite, fluorite, and bastnäsite-(Ce). Based on geological observations and the petrography of fluid inclusions, the mineralization processes are classified into magmatic, pegmatitic, hydrothermal I, hydrothermal II, and REE stages. The inclusions in these stages include melt (M), melt-fluid (M-L), pure CO2 (C), aqueous-CO2 (L-C), aqueous-CO2 with crystals (L???C?+?S), liquid-vapor aqueous with crystals (L???V?+?S), and liquid-vapor (L-V) type inclusions. The magmatic stage is marked by a carbonatite-syenite complex with minor bastnäsite-(Ce), whereas the pegmatitic stage consists of coarse-grained calcite, barite, fluorite, and quartz that contain M, M-L, and L-C type inclusions with a fluid system of NaCl-Na2SO4-H2O-CO2 at high temperature (>600?°C) and high salinity (>45?wt% NaCl equiv.). The hydrothermal I stage is characterized by fenitization and is marked by aegirine-augite and arfvedsonite containing abundant L-V and few L-C type inclusions. This stage is characterized by high temperatures (?480?°C) and moderate salinity (10.2-17.9?wt% NaCl equiv.), with a fluid system of NaCl-Na2SO4-H2O and minor CO2 and CH4?+?C2H6. The hydrothermal II stage is dominated by L-C, L???C?+?S, L???V?+?S, and L-V type inclusions that are hosted in barite, calcite, fluorite, and quartz, and formed at moderate to high temperatures (260-350?°C), with a wide range of salinity (9.4-47.8?wt% NaCl equiv.), a fluid system of NaCl-Na2SO4-CO2-H2O, and abundant CH4?+?C2H6. During the REE stage, pervasive bastnäsite-(Ce) containing abundant L-V type and few L-C type inclusions crystallized under low temperatures (160-240?°C) and low salinities (8.8-13.1?wt% NaCl equiv.) with a fluid system of NaCl-H2O and minor CO2 and CH4?+?C2H6. The results of ion-chromatographic analysis show that the ore fluids are rich in Na+, K+, Cl?, F?, and (SO4)2?, and have low Cl?/(SO4)2? ratios (0.78-2.00), showing a marked contrast with the fluids of granite-related REE deposits (Cl?/(SO4)2??>?50) and a similarity to subcontinental lithospheric mantle (SCLM). The ?D and ?18Ofluid values and the high N2/Ar ratios indicate that the ore fluids originated from carbonatitic magma and were dominated by magmatic water during the hydrothermal I stage, whereas magmatic and meteoric water co-existed during the hydrothermal II and REE stages. Moreover, the higher ratios of CO2/N2 (9-64) and CO2/CH4 (17-472) and the higher concentrations of CO2, CH4, C2H6, and N2 in the hydrothermal II stage compared with the hydrothermal I stage are attributed to intense immiscibility that resulted from decompression and is constrained to temperatures of 310-350?°C and pressures of 2.0-2.4?kbar. In contrast, microthermometric data and low CH4, C2H6, and N2 contents for the REE stage show that fluid cooling and mixing with meteoric water played an important role during the intensive mineralization of this stage, which occurred under shallow open-system conditions at temperatures of ?200?°C and pressures of <0.5?kbar. The mineral assemblages, together with experimental petrology results, suggest that the REE transport capability of the hydrothermal fluids was due to the high contents of (SO4)2?, Cl?, and F? complexes. In addition, CO2 that separates during immiscibility is known to act as a buffer that constrains the pH of ore fluids. Thus, immiscibility during the hydrothermal II stage could have provided favorable conditions for the migration of REEs. The subsequent cooling of fluids, the involvement of meteoric water, and increased fluid pH, favored the precipitation of REEs in the Maoniuping deposit.
Geostandards and Geoanalytical Research, Vol. 43, 2, pp. 291-300.
Global
carbonatites
Abstract: This study presents a high?precision method to measure barium (Ba) isotope compositions of international carbonate reference materials and natural carbonates. Barium was purified using chromatographic columns filled with cation exchange resin (AG50W?X12, 200-400 mesh). Barium isotopes were measured by MC?ICP?MS, using a 135Ba-136Ba double?spike to correct mass?dependent fractionation during purification and instrumental measurement. The precision and accuracy were monitored by measuring Ba isotope compositions of the reference material JCp?1 (coral) and a synthetic solution obtained by mixing NIST SRM 3104a with other matrix elements. The mean ?137/134Ba values of JCp?1 and the synthetic solution relative to NIST SRM 3104a were 0.21 ± 0.03‰ (2s, n = 16) and 0.02 ± 0.03‰ (2s, n = 6), respectively. Replicate measurements of NIST SRM 915b, COQ?1, natural coral and stalagmite samples gave average ?137/134Ba values of 0.10 ± 0.04‰ (2s, n = 18), 0.08 ± 0.04‰ (2s, n = 20), 0.27 ± 0.04‰ (2s, n = 16) and 0.04 ± 0.03‰ (2s, n = 20), respectively. Barium mass fractions and Ba isotopes of subsamples drilled from one stalagmite profile were also measured. Although Ba mass fractions varied significantly along the profile, Ba isotope signatures were homogeneous, indicating that Ba isotope compositions of stalagmites could be a potential tool (in addition to Ba mass fractions) to constrain the source of Ba in carbonate rocks and minerals.
Abstract: Most carbonatite-related REE (rare earth element) deposits record two stages of REE enrichment: magmatic and magmatic-hydrothermal. It is generally accepted that the first stage of enrichment, which occurs in magmas associated with carbonatite-syenite complexes, is a prerequisite to the formation of REE deposits. The magmatic-hydrothermal process is also important, as demonstrated by the fact that many fertile carbonatite-syenite complexes do not produce REE deposits. The Miaoya carbonatite-syenite complex is prospective for REE and is ideal for studies of the formation of REE deposits. The Miaoya REE prospect lies in the western member of the Wudan Terrane of the Qinling Belt, China, and is hosted by a carbonatite-syenite complex that was intruded along a fault zone between schist of the lower Silurian Meiziya Group and adjacent Proterozoic quartzite. Mineralization at the Miaoya REE prospect includes carbonatite-, syenite-, and mixed-type, all low grade (about 1%). Results of X-ray diffraction (XRD) and electron probe micro-analyzer (EMPA) analyses reveal that modes of REE minerals are low in all samples (<5%), which is consistent with the fact that less monazite, bastnäsite and other REE minerals have been found in the Miaoya REE prospect. REE mineralization is less likely to occur as an overprint on gangue minerals. Results of Photon Laser Ablation Inductively-Coupled-Plasma Mass-Spectrometer (LA-ICP-MS) analyses reveal that apatite and calcite in carbonatite have the highest REE concentrations which are responsible for the relatively high concentration in carbonatite rather than because of the presence of REE minerals. The consistence of Sr-Nd isotopes ratios between altered host rocks and fresh hosted rocks suggested REE mineralization originates directly from the unmineralized carbonatite-syenite complex rather than other host rocks. Carbon and oxygen isotope ratios of hydrothermal calcite are consistent with low-temperature alteration subsequent to ore. Trace element ratios for the Miaoya carbonatite-syenite complex lie in the barren carbonatite field (REEs vs. CaO/MgO, FeO/MgO, Ba and Sr/Ba) compared with those of other giant or large carbonatite-syenite complex related REE deposits, just below the boundary between fields for fertile and barren carbonatites. This suggests that the carbonatite-syenite complex at the Miaoya prospect did not have the potential to produce large or giant REE deposits. The low REE of the Miaoya prospect compared with other carbonatite-syenite hosted deposits may reflect: 1) as supported by petrography, minimal tectonic deformation in the area resulting in 2) restricted cycling of hydrothermal solutions that led to 3) minimal fluid scavenging from REE-rich apatite and calcite for local REE re-deposition and concentration.
Earth Science Reviews, in press available, 38p. Doi.org/1010.1016 /jearsciev.2019.103073
Global
carbon
Abstract: Although the deep recycling of carbon has been proposed to play a key role in producing intraplate magmatism, the question of how it controls or triggers mantle melting remains poorly understood. In addition, generation of incipient carbonated melts in the mantle and their subsequent reaction with the mantle are critical processes that can influence the geochemistry of intraplate basalts, but the details of such processes are also unclear. Here we present geochemical evidence for the existence of pervasive carbonate melt in the mantle source of Cenozoic continental intraplate highly alkali basalts (SiO2 < 45 wt%), which are volumetrically minor but widespread in eastern China. The primary magma compositions of these basalts cannot be explained by either partial melting of a single mantle source lithology or mixing of magmas derived from distinct mantle sources, but can be adequately explained by carbonate-fluxed melting of eclogite and subsequent reaction between silica-rich melts and peridotite that ultimately transformed the initial carbonated silica-rich melts into silica-undersaturated alkalic magmas. The source of the carbonate is in subducted eclogites associated with the Pacific plate, which stagnated in the mantle transition zone (MTZ). The spatial distribution of the alkali basalts is in accord with large-scale seismic low-velocity anomalies in the upper mantle above the MTZ. Similar scenarios in central-western Europe and eastern Australia lead us to propose that reaction between carbonated silica-rich melt and peridotite may be a pivotal mechanism for the generation of continental intraplate alkali basalts elsewhere in the world.
Earth Science Reviews, in press available 30p. Pdf
China
picrites
Abstract: Large igneous provinces on Earth result from anomalously enormous volcanic eruptions at high melt production rates. These eruptions are often linked to catastrophic events such as mass extinctions, global climate changes, or continental break-up. Decoding their petrogenesis is therefore of great importance for our comprehensive understanding of the evolution and geodynamics of our planet. The ~260 Ma Emeishan large igneous province is an important geological feature of SW China with world-class ore deposits and is also suggested to be linked with the Capitanian mass extinction. However, fundamental aspects of the genesis of Emeishan province's most primitive lavas (picrites), such as the source lithology (pyroxenite or peridotite), the origin of compositional variations of olivines and the melting temperature and pressure conditions, remain poorly constrained. Here, we compile information on melt inclusion and host olivine, and whole-rock compositions from the ELIP picrites and show that these data are consistent with decompression melting of a relatively homogeneous peridotitic mantle plume, with a potential temperature higher than 1560 °C. The compositional variability of the olivines and picrites can be explained by varying the equilibrium depth of primary magma segregation and does not require the contribution of a pyroxenite component as previously suggested. Our results favor a scenario for the origin of the Emeishan large igneous province in which the decompression melting during upwelling of a hot hydrous and oxidized mantle plume is accompanied by catastrophic lithospheric thinning. In combination with the now extensive multi-element geochemical data, our findings provide a starting point for re-evaluation of the petrogenesis models for large igneous provinces.
Abstract: Carbonates in subducting oceanic slabs can survive beyond slab dehydration and be transferred into the deep mantle. Such deep carbon cycling plays a critical role in generating carbonatitic to alkaline melts. However, whether and how this process has influenced the lithospheric mantle still remains enigmatic. To address these issues, here we provide a detailed petrographic, in-situ chemical and Sr isotopic study on two mantle xenoliths (a wehrlite and a melt pocket-bearing peridotite) entrained by the Changle Miocene basalts from the eastern China. The Changle wehrlite contains carbonate melt inclusions and apatites and is merely enriched in clinopyroxene relative to the lherzolites. The clinopyroxenes are characterized by high (La/Yb)N (4.7-41) and low Ti/Eu (873-2292) ratios and equilibrated with carbonated silicate melt-like compositions. These petrographic and chemical features indicate that the wehrlite was formed by reaction between peridotite and carbonated silicate melts. On the other hand, the Changle melt pocket-bearing peridotite is suggested to have been produced by in-situ melting/breakdown of amphiboles of an amphibole-rich dunite. Low olivine Fo (~89), presence of amphiboles with high (La/Yb)N (~50) and low Ti/Eu (~1070) ratios suggest that such amphibole-rich dunite would have been formed by reaction of peridotite with hydrous alkaline basaltic melts from a carbonated mantle. Our data, combined with previously reported data of the Changle lherzolite xenoliths, unravel a series of mantle metasomatisms by carbonatitic to alkaline melts from carbonated mantle sources. The consistently high 87Sr/86Sr ratios (up to 0.7036) of the clinopyroxenes in both the wehrlites and lherzolites indicate the carbonate components in the mantle sources were derived from the stagnant Pacific slab within the Mantle Transition Zone. This study provides a fresh perspective on the role of deep carbon cycling from subducted oceanic slabs in chemical modification of intracontinental lithospheric mantle through reaction with different types of melts.
Abstract: This paper presents some data of the Jiaopingdu gabbro and Caiyuanzi granite at the southwestern margin of the Yangtze Block, on the geochemical compositions, zircon LA-ICP-MS U-Pb ages and Hf isotopic data. The Jiaopingdu gabbro gives the age of 1721 ± 5 Ma, the Caiyuanzi granite 1732 ± 6 Ma and 1735 ± 4 Ma, and the Wenjiacun porphyry granite 1713 ± 4 Ma, suggesting nearly contemporaneous formation time of the gabbro and granite. The bimodal feature is demonstrated by the gabbro SiO2 content of 44.64-46.87 wt% and granite 73.81-77.03 wt%. In addition, the granite has high content of SiO2 and Na2O + K2O, low content of Al2O3 and CaO, enriched in REEs (except Eu) and Zr, Nb, Ga and Y, depleted in Sr, implying it belongs to A?type granite geochemistry and origin of within?plate environment. The zircon ?Hf(t) of the granite and gabbro is at the range of 2-6, which is near the 2.0 Ga evolution line of the crust, implying the parent magma of the gabbro being derived from the depleted mantle and a small amount of crustal material, and the parent magma of the granite from partial melting of the juvenile crust and some ancient crustal material at the same time. Compared with 1.8-1.7 Ga magmatism during breakup of other cratons in the world, we can deduce that the Columbia has initially broken since ca. 1.8 Ga, and some continental marginal or intra?continental rifts occurred at ca. 1.73 Ga.
Abstract: Carbonatites host the world's most important rare earth element (REE) resources. The origins of REE mineralization in carbonatite-related deposits, particularly the role of hydrothermal fluids in REE mobilization and mineralization, remain enigmatic. The Cenozoic Mianning-Dechang REE belt in eastern Tibet is one of the largest REE production regions worldwide, and is an ideal area for investigating REE mineralization. Geological investigations and fluid inclusion studies suggest that ore fluids in this belt evolved from hydrothermal stage I (fenitization at high temperatures of ~480 °C) to hydrothermal stage II (calcite, quartz, barite, and fluorite crystallization at temperatures of 300-350 °C and salinities of ~20 wt% NaCl equiv.), and then to the REE mineralization stage (temperatures of ~200 °C and low salinities of ~9 wt% NaCl equiv.). The bulk fluid compositions demonstrate that the ore fluids contained significant amounts of alkalis (up to 5 wt% Na + K), halogens (up to 12 wt% Cl; up to 7 wt% F), sulfate (>2 wt% SO42?), Ba (>1123 ppm), Sr (>1120 ppm), and REEs (>5 wt%). Chondrite-normalized REE patterns of these fluids are light REE-enriched and exhibit moderate depletion in Eu ([Eu/Eu?]CN = 0.85 ± 0.08), similar to the carbonatites and nordmarkites. These fluid characteristics and plots of Rb/Na vs. K/Na and Mn vs. Na suggest that the ore fluids in the Mianning-Dechang REE belt were derived from a late-stage alkaline-carbonatitic magma. High concentrations of Cl?, F?, SO42?, and REEs, and the absence of REE fluoride (REEF3) and fluorite (CaF2), suggest that the ore fluids in hydrothermal stage I were a high-temperature, SO42?-rich (>2 wt%), and acidic fluid system (pH < 3.5). In this system, chloride REE complexes were predominant over fluoride and sulfate REE complexes, which resulted in efficient transport of REEs. Sulfate species were predominant in hydrothermal stage II at temperatures of 260-350 °C and a pH between 3.5 and 5.2. The higher pH and fluid cooling from hydrothermal stage I to hydrothermal stage II caused an increase in F?, which in turn lowered fluid REE concentrations, owing to the formation of REE-rich fluorite. This suggests that F? was a depositional ligand in hydrothermal stage II. Continued fluid cooling (~200 °C) and increasing pH (~6), combined with the precipitation of barite and fluorite in the REE mineralization stage, destabilized the REE complexes because of the decreasing concentrations of SO42?, Cl?, and F?, which thus led to widespread REE deposition. A review of different-sized deposits in the Mianning-Dechang REE belt indicates that appreciable amounts of SO42?, Cl?, REEs, CO2, and particularly F? and alkalis in fluids, along with a high fluid exsolution temperature, represent the ideal conditions for potential REE mineralization in a carbonatite-related setting.
Abstract: Many Archean cratons exhibit Paleoproterozoic rifted margins, implying they were pieces of some ancestral landmass(es). The idea that such an ancient continental assembly represents an Archean supercontinent has been proposed but remains to be justified. Starkly contrasting geological records between different clans of cratons have inspired an alternative hypothesis where cratons were clustered in multiple, separate "supercratons". A new ca. 2.62 Ga paleomagnetic pole from the Yilgarn craton of Australia is compatible with either two successive but ephemeral supercontinents or two long-lived supercratons across the Archean-Proterozoic transition. Neither interpretation supports the existence of a single, long-lived supercontinent, suggesting that Archean geodynamics were fundamentally different from subsequent times (Proterozoic to present), which were influenced largely by supercontinent cycles.
Abstract: This manuscript presents results of the newest petrographic, mineralogical and bulk chemical, as well as H, C and O stable isotope study of carbonatites and associated silicate rocks from the Tajno Massif (NE Poland). The Tajno Intrusion is a Tournaisian-Visean ultramafic-alkaline-carbonatite body emplaced within the Paleoproterozoic rocks of the East European Craton (EEC). Carbonatites of the Tajno Massif can be subdivided into the calciocarbonatite (calcite), ferrocarbonatite (ankerite), and breccias with an ankerite-fluorite matrix. Due to location at the cratonic margin and abundance in the REE, Tajno classifies (Hou et al., 2015) as the carbonatite-associated REE deposit (CARD), and more precisely as the Dalucao-Style orebody (the breccia-hosted orebody). High Fe2O3 (13.8 wt%), MnO (2.1 wt%), total REE (6582 ppm), Sr (43895 ppm), Ba (6426 ppm), F (greater than10000 ppm) and CO2 contents points for the involvement of the slab - including pelagic metalliferous sediments - in the carbonatites formation. Spatial relations and Sr isotope composition ((87Sr/86Sr)i = 0.7043-0.7048; Wiszniewska et al., 2020) of alkali clinopyroxenite and syenite suggest that these are products of differentiation of the magma, generated by the initial melting of the SCLM due to influx of F-rich fluids from subducted marine sediments. Carbonatites Sr isotope composition ((87Sr/86Sr)i = 0.7037-0.7038), and Ba/Th (16-20620) and Nb/Y (0.01-6.25) ratios, link their origin with a more advanced melting of the SCLM, triggered by CO2-rich fluids from the subducted AOC and melts from sediments. The Tajno Massif - and coeval mafic-alkaline intrusions - age, high potassic composition, and location along the craton margin nearly parallel the Variscan deformation front, are suggesting Variscan subduction beneath the EEC. The oxygen isotope compositions of clinopyroxene (?18O value = 5.2‰) and alkali feldspar (?18O value = 5.7‰), from alkali clinopyroxenite and foid syenite, respectively, are consistent with mantle-derived magmas. Isotopic compositions of carbonatites and breccias (carbonate ?18O = 8.7‰ to 10.7‰; ?13C = -4.8‰ to ?0.4‰) span from values of primary carbonatites to carbonatites affected by a fractionation or sedimentary contamination. The highest values (?18O = 10.7‰; ?13C = -0.4‰) were reported for breccia cut by numerous veins confirming post-magmatic hydrothermal alteration. The lowest carbonate ?18O (9.3‰ to 10.7‰) and ?13C (?5.0‰ to ?3.8‰) values are reported for veins in alkali clinopyroxenites, whereas the highest ?18O (11.2‰) and ?13C (?1.2‰ to ?1.1‰) values are for veins in syenites and trachytes. Isotopic composition of veins suggests hydrothermal origin, and interaction with host mantle-derived rocks, as well as country rocks. In silicate rocks of the Tajno Massif, fluid influx leads to the development of Pb, Zn, Cu, Ag, Au sulfide mineralization-bearing stockwork vein system, with carbonate, silicate and fluorite infilling the veins. Bulk-rock contents of molybdenum (925 ppm), rhenium (905 ppb) and palladium (29 ppb) are notable. The Re-rich molybdenite association with galena, pyrite and Th-rich bastnäsite in carbonate veins is similar as in Mo deposits associated with carbonatites, implying the mantle source of Mo and Re.
Geochimica et Cosmochimica Acta, in press available, 53p.
China
eclogite
Abstract: Alkaline basalts occur widely in intraplate settings and carbonate-bearing mantle sources such as carbonated peridotites are increasingly regarded to play a key role in their formation. Carbonated eclogites, most likely the products of subducted carbonate-bearing altered oceanic crust, are probable alternative ingredients in the mantle sources of many intraplate alkaline basalts, highlighting the importance of the subduction-driven deep carbon cycle. However, this widely proposed hypothesis remains enigmatic because the recognition of low-MgO primitive alkaline basalts predicted by experiments is scarce. Here we show that Cenozoic continental intraplate alkaline basalts occurring above the stagnant oceanic slab in the mantle transition zone beneath the Hannuoba region, eastern China, display geochemical features consistent with their origin as low-degree partial melts of carbonate-bearing eclogites. Their MgO contents correlate positively with CaO, Ba/Th and Ti/Eu, but negatively with Dy/Yb and ?Nd. Remarkably, the most primitive alkaline basalts are characterized by low MgO (<5.25 wt.%), low heavy rare earth elements and Sc contents, low CaO/Al2O3 (<0.41), low Ti/Eu (<3380), but Dy/Yb (>7.1) higher than those of ocean island basalts (OIBs). These features cannot be ascribed to differentiation from high-MgO alkaline basalts because significant amounts of crystallization of clinopyroxene and garnet did not occur during ascent. Differentiation also cannot account for the correlations of time-integrated Sr-Nd isotopes with MgO, Dy/Yb and Ba/Th. Instead, the linear correlations mainly reflect strong interaction between ascending primitive alkaline melts and the lithospheric mantle. The compositions of primitive alkaline basalts reflect the key control of garnet and clinopyroxene in the mantle residue (eclogites), and the Ti, Zr and Hf anomalies further indicate the critical effect of carbonates in the eclogite source. Partial melting of the carbonate-bearing eclogites likely occurred in the uppermost asthenosphere. The production of alkaline basalts with low MgO contents by partial melting of carbonate-bearing eclogite below the peridotite solidus in an intraplate setting has been overlooked and the magmas were instead often considered to be highly evolved. Recycled altered oceanic crust thus may not only cause metasomatism of the deep mantle but may also serve as a direct source of mafic melts. These results on natural rocks support the experiment-based model for subducted altered oceanic crustal material and also indicate its diverse fate in the mantle.
Abstract: A total of 48 natural alluvial diamonds from the Yangtze Craton, China, also called Hunan diamonds, were studied using morphology and IR spectroscopy. These diamond samples, collected downstream of the Yuan River, Hunan Province, with unknown host-rock source(s), were observed by scanning electron microscope (SEM) and Fourier-transform infrared spectroscopy (FTIR). Most Hunan diamonds are monocrystal forms of octahedra, tetrahexahedra (THH) and dodecahedra; octahedral-rhom-dodecahedral transitional behaviors and irregular forms are also visible. Trigons and tetragons, terraces and shield-shaped laminae are surface features that frequently indicate dissolution and reabsorption; green and brown spots, network patterns, and other mechanical abrasion marks are typical evidence of long-time deposition and transportation of Hunan diamonds. The main types of Hunan diamonds are type IaAB and type ?a. Diamond samples have a wide range of total nitrogen content (Ntot) from 196-1094 ppm. Two populations are distinguished by two-peak distribution models of NA (A-center concentrations) and %B (proportion of aggregated nitrogen). Hunan diamonds are low in structure hydrogen (0.03-4.67 cm?1, mostly below 1 cm?1) and platelets (0.23-17 cm?1, mostly below 2 cm?1). Moreover, there is a significant positive correlation between the hydrogen correlation peak and Ntot, which is similar to Argyle diamonds. The temperature conditions of the diamond formation have been estimated at 1075-1180 °C, mainly conforming to the kimberlite diamond range. Besides, some samples with slightly higher temperatures are close to the ultramafic-related Juina diamonds. Therefore, the FTIR characteristics analysis and comparison indicate the multiple sources of Hunan diamonds.
Ultra high pressure metamorphism and multistage exhumation of eclogite of the Luotian dome, North Dabie Complex Zone: evidence from mineral inclusions -texture
Journal of Asian Earth Sciences, Vol. 42, 4, pp. 607-617.
Liu, Y-C., Deng, L-P., Gu, X-F., Groppo, C., Rolfo, F.
Application of Ti in zircon and Zr in rutile thermometers to constrain high temperature metamorphism in eclogites from the Dabie Orogen, central China.
European Journal of Mineralogy, Vol. 31, pp. 889-803. pdf
China
eclogite
Abstract: A combined study of detailed petrographic observation, mineral chemistry analysis and phase equilibrium modeling indicates that the high-temperature eclogites from the Dabie orogen, central China, experienced two episodes of anatexis: the first is phengite dehydration melting during the exhumation of deeply subducted slices, and the second is heating melting related to the post-orogenic collapse. Petrographic evidence and clues of the anatectic events include biotite + plagioclase + garnet ± amphibole intergrowth in matrix and biotite + plagioclase intergrowth within amphibole porphyroblast. Pressure-temperature (P-T) pseudosection and modal variation diagram indicate that the biotite + plagioclase + garnet ± amphibole in matrix was formed by the reactions phengite + clinopyroxene + quartz = melt + sanidine + garnet + plagioclase and later melt + sanidine + garnet = biotite + plagioclase, while the biotite + plagioclase intergrowths within poikiloblastic amphibole were formed by the reaction amphibole + muscovite + epidote = biotite + plagioclase + melt. In addition, the combination of petrological observations and P-T estimates suggests that the first melting event occurred at the late Triassic, while the second is related to the early Cretaceous mountain-root removal and subsequent asthenospheric upwelling and heat input. As the P-T paths of high-temperature/ultrahigh-pressure rocks have high probabilities to cross-cut phengite-melting curves, phengite melting during decompression may be a common process in these rocks. Moreover, the coexistence of multiple episodes of anatexis in a single tectonic slice suggests caution when identifying and dating partial melting in high-temperature/(ultra)high-pressure rocks.
Abstract: The Bayan Obo ore deposit in Inner Mongolia, North China, the largest-known rare earth element (REE) deposit in the world, is closely associated with carbonatite dykes. Scarce zircon grains, with a wide range of ages and diverse origins, have been extracted from the Wu dyke, a REE-enriched calcitic carbonatite dyke 2?km from the East Ore Body of the Bayan Obo deposit. Three zircon populations were identified based on ages and trace element compositions: 1) Captured zircons with Paleoproterozoic and Archean ages. These zircons have REE patterns and moderate Th/U ratios similar to zircon with silicate inclusions from basement igneous rocks, which have been recognized as contaminants from wall rocks. 2) Carbonatite magmatic zircons with Mesoproterozoic ages. These zircons have high to extremely high Th/U ratios (13-1600), a characteristic signature of the Bayan Obo deposit. Two zircon grains yielded concordant 206Pb/238U ages (1.27?±?0.11?Ga???1.42?±?0.18?Ga) and 208Pb/232Th age (1.26?±?0.20?Ga) with calcite inclusions, indicating that the Wu dyke was emplaced at ca. 1.34?Ga, which coincides with a worldwide generation of Mesoproterozoic kimberlites, lamprophyres, carbonatites, and anorogenic magmatism. 3) Hydrothermal zircons with Caledonian and Triassic ages. The Caledonian zircon has 206Pb/238U age of 381?±?4?Ma and 208Pb/232Th age of 367?±?14?Ma with dolomite inclusion. These evidences are consistent with multiple stages of mineralization, Mesoproterozoic calcite carbonatite magmatism interacted by protracted fluxing of subduction-released Caledonian fluids during the closure of the Palaeo-Asian Ocean, coupled with interaction with the mantle wedge and metasomatism of overlying sedimentary carbonate.
Abstract: The Bayan Obo ore deposit in Inner Mongolia, North China, the largest-known rare earth element (REE) deposit in the world, is closely associated with carbonatite dykes. Scarce zircon grains, with a wide range of ages and diverse origins, have been extracted from the Wu dyke, a REE-enriched calcitic carbonatite dyke 2?km from the East Ore Body of the Bayan Obo deposit. Three zircon populations were identified based on ages and trace element compositions: 1) Captured zircons with Paleoproterozoic and Archean ages. These zircons have REE patterns and moderate Th/U ratios similar to zircon with silicate inclusions from basement igneous rocks, which have been recognized as contaminants from wall rocks. 2) Carbonatite magmatic zircons with Mesoproterozoic ages. These zircons have high to extremely high Th/U ratios (13-1600), a characteristic signature of the Bayan Obo deposit. Two zircon grains yielded concordant 206Pb/238U ages (1.27?±?0.11?Ga???1.42?±?0.18?Ga) and 208Pb/232Th age (1.26?±?0.20?Ga) with calcite inclusions, indicating that the Wu dyke was emplaced at ca. 1.34?Ga, which coincides with a worldwide generation of Mesoproterozoic kimberlites, lamprophyres, carbonatites, and anorogenic magmatism. 3) Hydrothermal zircons with Caledonian and Triassic ages. The Caledonian zircon has 206Pb/238U age of 381?±?4?Ma and 208Pb/232Th age of 367?±?14?Ma with dolomite inclusion. These evidences are consistent with multiple stages of mineralization, Mesoproterozoic calcite carbonatite magmatism interacted by protracted fluxing of subduction-released Caledonian fluids during the closure of the Palaeo-Asian Ocean, coupled with interaction with the mantle wedge and metasomatism of overlying sedimentary carbonate.
Abstract: Carbonatitic magmatism plays a significant role in Earth's carbon cycle, which is also a lithoprobe of crust-mantle interaction, mantle metasomatism and partial melting. Due to different mineral assemblages and geochemical compositions, and diverse tectonic settings, the origin of carbonatite has long been debated. At subduction zones, sediments (including carbonates) are subducted into the mantle with the downgoing oceanic slab. However, the detailed mechanism of how subducted carbonates contribute to carbonatitic magmatism remains unclear. Here we present geochronological, geochemical and isotopic study on the Taohuala Mountain carbonatite at the southern margin of the Alxa Block, North China Craton. The classification of carbonatite from the Taohuala Mountain relies strongly on the observations of obvious intrusion contact relationships and flow structures in field outcrop. The Taohuala Mountain carbonatite has SiO2 ranging from 2.37 wt.% to 11.45 wt%, high CaO (45.93-53.86 wt%) and low MgO (0.51-4.39 wt%), and is characterized by enrichment of LILE (Ba, Sr), depletion of HFSE (Nb, Ta, Zr, Hf), and slightly negative Ce and Eu anomalies. Carbonates in the samples have high 87Sr/86Sr (0.70686-0.70694) and low 143Nd/144Nd (0.511635-0.511924). Remarkably, the highly fractionated ?18OVSMOW (11.83-25.92‰) indicates components of both sedimentary and mantle origin. Detailed zircon in situ U-Pb dating and oxygen isotope analysis exhibit contrast ages and ?18OVSMOW from core to rim, i.e., old ages (mainly > 800 Ma), high Th/U (mainly > 0.5) and low ?18OVSMOW (6.37-11.44‰) in cores (inherited), whereas young ages (~ 400 Ma), low Th/U (mainly < 0.01) and high ?18OVSMOW (20.04-24.54‰) in rims, suggesting that the Taohuala Mountain carbonatite may have been generated from melting of subducted sedimentary carbonates. Considering all these evidences, and that the collision along Qilian Mountains was older than the carbonatite, we propose that a large volume of sedimentary carbonates subducted and remained in the lithospheric mantle under the Alxa block during the closure of the Paleo-Qilian Ocean. Subsequently, the carbonatite was formed by melting of carbonates with minor contributions from the mantle during the breakoff or rollback of the Paleo-Asian oceanic slab.
Abstract:
Carbonatitic magmatism plays a significant role in outgassing carbon from mantle and the formation of rare earth element (REE), rare metal (e.g., Nb and Th) and other types of deposits. The mechanism of REE mineralization associated with carbonatite have been widely studied. However, it is hard to understand U-Nb mineralization without Th enrichment associated with carbonatite. Here we report a carbonatite-hosted U-Nb deposit in Huayangchuan, located in the north Qinling Orogenic Belt. Field observation, mineralogy and geochemical analysis on a suite of drillhole samples were conducted to decipher the mineralization mechanism and its relationship with carbonatite. Huayangchuan carbonatite samples mainly consist of calcite and augite with small volume of accessory minerals (e.g., allanite, fluorapatite, barite and celestite). Betafite [(Ca,U)2(Ti,Nb,Ta)2O6(OH)] is the major ore-bearing mineral in Huayangchuan deposit. The carbonatite shows high CaO, low MgO and alkali contents, which should be products to be differentiated from primary carbonatite (high MgO and alkali contents). The immiscibility and crystallization processes could explain the high CaO/(CaO + MgO + FeO) ratios and the enrichment of LILE. Numerical modeling also indicates positive ?18OSMOW (7.29 to 15.53‰) and negative ?13CPDB (?5.26 to ?10.08‰) shifts are induced by reduced sediments assimilation from source consistent with there being enriched Sr-Nd and low Mg isotopic compositions. LA-ICP-MS zircon U-Pb dating of Huayangchuan carbonatite yielded Triassic ages of 229 ± 3 Ma, which corresponds to the post-collision stage of Qinling Orogen during the middle-late Triassic. We then proposed that the recycling of subducted sediments and later re-melting of those materials in shallow mantle generated the Huayangchuan carbonatite and subsequently formed the Huayangchuan deposit. Fluorine concentration decrease, caused by fluorapatite crystallization, ultimately resulted in betafite mineralization.
Geochimica et Cosmochimica Acta, in press available, 53p.
China
eclogite
Abstract: Alkaline basalts occur widely in intraplate settings and carbonate-bearing mantle sources such as carbonated peridotites are increasingly regarded to play a key role in their formation. Carbonated eclogites, most likely the products of subducted carbonate-bearing altered oceanic crust, are probable alternative ingredients in the mantle sources of many intraplate alkaline basalts, highlighting the importance of the subduction-driven deep carbon cycle. However, this widely proposed hypothesis remains enigmatic because the recognition of low-MgO primitive alkaline basalts predicted by experiments is scarce. Here we show that Cenozoic continental intraplate alkaline basalts occurring above the stagnant oceanic slab in the mantle transition zone beneath the Hannuoba region, eastern China, display geochemical features consistent with their origin as low-degree partial melts of carbonate-bearing eclogites. Their MgO contents correlate positively with CaO, Ba/Th and Ti/Eu, but negatively with Dy/Yb and ?Nd. Remarkably, the most primitive alkaline basalts are characterized by low MgO (<5.25 wt.%), low heavy rare earth elements and Sc contents, low CaO/Al2O3 (<0.41), low Ti/Eu (<3380), but Dy/Yb (>7.1) higher than those of ocean island basalts (OIBs). These features cannot be ascribed to differentiation from high-MgO alkaline basalts because significant amounts of crystallization of clinopyroxene and garnet did not occur during ascent. Differentiation also cannot account for the correlations of time-integrated Sr-Nd isotopes with MgO, Dy/Yb and Ba/Th. Instead, the linear correlations mainly reflect strong interaction between ascending primitive alkaline melts and the lithospheric mantle. The compositions of primitive alkaline basalts reflect the key control of garnet and clinopyroxene in the mantle residue (eclogites), and the Ti, Zr and Hf anomalies further indicate the critical effect of carbonates in the eclogite source. Partial melting of the carbonate-bearing eclogites likely occurred in the uppermost asthenosphere. The production of alkaline basalts with low MgO contents by partial melting of carbonate-bearing eclogite below the peridotite solidus in an intraplate setting has been overlooked and the magmas were instead often considered to be highly evolved. Recycled altered oceanic crust thus may not only cause metasomatism of the deep mantle but may also serve as a direct source of mafic melts. These results on natural rocks support the experiment-based model for subducted altered oceanic crustal material and also indicate its diverse fate in the mantle.
Fluids in deeply subducted continental crust: petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu Orogen, eastern China.
Geochimica et Cosmochimica Acta, Vol. 72, 13, July 1, pp. 3200-3228.
Geochimica et Cosmochimica Acta, Vol. 197, pp. 356-377.
China
Picrite
Abstract: Iron-rich silicate melts in the Earth’s deep mantle have been seismologically and geochemically inferred in recent years. The origin of local enrichments in iron and low-velocity seismic anomalies that have been detected in dense mantle domains are critical to understanding the mantle’s evolution, which has been canonically explained by long-term chemical reactions between the Earth’s silicate mantle and its liquid iron outer core. However, the Pleistocene alkaline ferropicrites (?0.73 Ma) from Wudi, North China, show chemical and Sr-Nd-Os isotopic features that suggest derivation from the preferential melting of silica-deficient eclogite, a lithology of delaminated mafic lower continental crust that had stagnated at mid-upper mantle depths during the Mesozoic decratonization of the North China block. These rocks are characterized by substantial enrichment in iron (14.9-15.2 wt% Fe2O3), relative depletion in silica (40-41 wt% SiO2) and decoupled Y and heavy rare earth element (HREE) compositions. These ferropicrites have particularly higher Y/Yb ratios than the other Cenozoic basalts from North China. The pressure-dependent compatibility of Fe, Y and Yb in eclogitic garnet can adequately explain the Fe-enrichment and Y-HREE decoupling of the Wudi ferropicrites and indicates that the eclogites were melted at pressures of 5-8 GPa, as also constrained by previous high-P-T experiments. This melting depth ties together a seismically imaged high-velocity anomaly that extends from 150 km to 350 km in depth under the study area, which has been commonly interpreted as evidence for the stagnation of the missing, delaminated continental lithosphere. Our findings provide an alternative mechanism to produce an extremely iron-rich mantle reservoir in addition to core-mantle interaction. Iron-rich silicate melts that form by this process are likely to be denser than the ambient mantle peridotite (and therefore drive flow downward) and may play a more significant role in the deep-mantle geophysical and geochemical diversities than previously considered.
Abstract: Sedimentary carbonate rocks, which exist extensively in the oceanic realm, are subducted to differing degrees during the closure of oceanic basins. However, very few observational data exist to provide details on the mechanisms of transport of carbonate materials from the surface to mantle depths and back to the Earth’s surface. Here we presented a series of diamond-bearing carbonatite xenoliths, carbonatite intrusions and carbonatite veins along the northern margin of the North China Craton (NCC). These carbonatites show geochemical features of recycled limestone (similar trace element patterns and high 87Sr/86Sr ratios of 0.705-0.709), indicating that they had a sedimentary limestone precursor. However, the presence of diamond, reduced minerals (e.g., moissanite), mantle-derived silicate minerals (eg., Cpx and Opx), and high Ni content and 143Nd/144Nd ratio indicate their staying for a time in the mantle. Combining with the zircon age spectrums of the carbonatite xenoliths and intrusions and the extensive high-87Sr/86Sr (up to 0.708) carbonatite metasomatism in the lithospheric mantle along the northern margin of NCC, we suggest that the limestone precursor could have been derived from the Paleo-Asian Ocean, and these carbonatites mark the subduction of a carbonate platform of the Paleo-Asian Oceanic slab to mantle depths beneath the NCC. Extensive mantle recycling of sedimentary carbonate could have contributed to the modification of the lithospheric mantle along the northern margin of the North China Craton.
Abstract: Diamond, moissanite and a variety of other minerals, similar to those reported from ophiolites in Tibet and northern Russia, have recently been discovered in chromitites of the Hegenshan ophiolite of the Central Asian Orogenic Belt, north China. The chromitites are small, podiform and vein-like bodies hosted in dunite, clinopyroxene-bearing peridotite, troctolite and gabbro. All of the analysed chromite grains are relatively Al-rich, with Cr# [100Cr/(Cr+Al)] of about 47-53. Preliminary studies of mainly disseminated chromitite from ore body No. 3756 have identified more than 30 mineral species in addition to diamond and moissanite. These include oxides (mostly hematite, magnetite, rutile, anatase, cassiterite, and quartz), sulfides (pyrite, marcasite and others), silicates (magnesian olivine, enstatite, augite, diopside, uvarovite, pyrope, orthoclase, zircon, sphene, vesuvianite, chlorite and serpentine) and others (e.g., calcite, monazite, glauberite, iowaite and a range of metallic alloys). This study demonstrates that diamond, moissanite and other exotic minerals can occur in high-Al, as well as high-Cr chromites, and significantly extends the geographic and age range of known diamond-bearing ophiolites.
Physics and Chemistry of Minerals, in press available 9p.
Technology
Garnet morphology
Abstract: We present a systematic experimental study on the phase transition, lattice microstrain, and order-disorder of cations for garnets in the majorite-pyrope system. Polycrystalline gem-quality garnets were synthesized at high pressure and high temperature using a Kawai-type multi-anvil apparatus. A phase transition from a cubic to tetragonal structure is clearly observed for garnets with the majorite content of more than 74 mol % through X-ray diffraction (XRD) and Raman scattering studies. Microstrain of garnets, evaluated with the Williamson-Hall plot on XRD profiles, shows a nonlinear dependence of the garnet compositions. The variation of the XRD peak broadening suggests the lattice microstrain of these garnets may be associated with the local structural heterogeneities due to the substitution of different cations via the coupled substitution (Mg2+ + Si4+ = 2Al3+) in the garnet structure. The width variation of Raman scattering peaks indicates that cation disorder occurs in the garnet structure for intermediate compositions. It is found that intermediate garnets and end-members have a minimum of microstrain, while those between end-members and intermediate compositions possess a larger microstrain.
Journal of Asian Earth Sciences, Vol. 159, pp. 109-129.
China, Tibet
chromitites
Abstract: The Purang harzburgite massif in SW Tibet (China) hosts abundant chrome ore deposits. Ores consist of 20 to >95% modal chromian spinel (Cr-spinel) with mylonitic fabric in imbricate shaped pods. The composition of Cr-spinel in these ores ranges from Al-rich [Cr#Sp or Cr/(Cr?+?Al)?×?100?=?47.60-57.56] to Cr-rich (Cr#Sp: 62.55-79.57). Bulk platinum-group element (PGE) contents of chromitites are also highly variable ranging from 17.5?ppb to ?2.5?ppm. Both metallurgical and refractory chromitites show a general enrichment in the IPGE (Os, Ir and Ru) with respect to the PPGE (Rh, Pt and Pd), resulting mostly in right-sloping primitive mantle (PM)-normalized PGE profiles. The platinum-group mineral (PGM) assemblages of both chromitite types are dominated by heterogeneously distributed, euhedral Os-bearing laurite inclusions in Cr-spinel. The Purang chromitites have quite inhomogeneous 187Os/188Os ratios (0.12289-0.13194) that are within the range of those reported for mantle-hosted chromitites from other peridotite massifs. Geochemical calculations demonstrate that the parental melts of high-Cr chromitites were boninitic, whereas those of high-Al chromitites had an arc-type tholeiitic affinity. Chromite crystallization was most likely stimulated by changes in magma compositions due to melt-peridotite interaction, leading to the establishment of a heterogeneous physicochemical environment during the early crystallization of the PGM. The highly variable PGE contents, inhomogeneous Os-isotopic compositions and varying Cr#Sp ratios of these chromitites imply a polygenetic origin for them from spatially distinct melt inputs. The generally low ?Os values (<1) of chromitites indicate that their parental melts originated within different sections of a heterogeneously depleted mantle source region. These melts were most likely produced in the mantle wedge above a downgoing lithospheric slab.
Abstract: While mineral phases stable in the mantle transition zone (such as wadsleyite and ringwoodite) can store up to 3 wt% H2O, those in the lower mantle such as bridgmanite and ferropericlase can contain a very small amount (<50 ppm). While such dramatic differences can lead to dehydration/hydration and hydrous melting at 660-km depth in the mantle [1,2] it is uncertain how much water can be transported and stored at these depths. In order to answer this question, we have conducted a series of high pressure experiments in laser-heated diamondanvil cell and multi-anvil press combined with X-ray diffraction, infrared spectroscopy, laser Raman spectroscopy, and secondary ion mass spectrometry. Initially we examined the water storage capacity of dense (Al free) silica polymorphs at high pressure and temperature. We found that water can dramatically reduce the rutile-type to CaCl2-type phase transition from 55 GPa to 25 GPa and stabilize a new "disordered inverse" inverse NiAs-type phase at pressures above 50 GPa, which is not stable in dry SiO2 system. The CaCl2-type and NiAs-type silica polymorphs contain up to 8 wt% of H2O at 1400-2100 K up to at least 110 GPa. We next explored the effects of water on the mineralogy of the lower mantle and found that hydrous Mg2SiO4 ringwoodite (1 wt% H2O) breaks down to silica + bridgmanite + ferropericlase at pressures up to 60 GPa and 2100 K. The recovered silica samples contain 0.3-1.1 wt% H2O, suggesting that water stabilizes silica even under Si-undersaturated systems because of their large water storage capacity. Therefore, our observations support the stability of silica in hydrous regions in the pyrolitic lower mantle. In the subducting oceanic crust (basalt and sediment), silica represents 20-80% of the mineralogy. Because its stability range spans the mantle transition zone to the deep mantle, hydrous silica is expected to play a major role in the transport and storage of water in the deep mantle.
Abstract: The effect of aluminum (Al) on the elasticity of majorite-pyrope garnets was investigated by means of ultrasonic interferometry measurements on well-fabricated polycrystalline specimens. Both velocities and elastic moduli increase almost linearly with increasing Al content within analytical uncertainty. No significant variation of the velocities and elastic moduli is observed across the tetragonal-to-cubic phase transition at majorite with the pyrope content up to 26 mol% along the majorite-pyrope system. The elasticity variation of majorite-pyrope garnets is largely dominated by the Al content, while the phase transition as a result of cation ordering/disordering of Mg and Si via substitution of Al on octahedral sites cannot significantly affect elastic properties. Seismic velocity variations of a garnet-bearing mantle transition zone are therefore dominated by garnet composition (e.g., Al, Fe, Ca, and Na) rather than the tetragonal-to-cubic phase transition because of cation ordering/disordering.
Geophysical Research Letters, Vol. 46, 15, pp. 8731-8740.
Mantle
bridgmanite
Abstract: The lower mantle encompasses the largest region of the Earth's interior and is mainly composed of the perovskite?structured mineral (Mg,Fe,Al)(Al,Si)O3 bridgmanite. Its properties, therefore, control both the diffusive transport of elements and solid state flow in the lower mantle, which will be strongly influenced by point defects. We have identified and quantified defects in bridgmanite that arise from the replacement of silicon by aluminum and result in the creation of a vacant oxygen site. These oxygen defects are also found to form clusters in the structure, which in other perovskite structured minerals have been shown to strongly affect physical properties. As defect formation and ordering is dependent on composition and pressure, strong variations in physical properties may be expected within the upper 300 km of the lower mantle.
Abstract: The Earth’s mantle is characterized by a sharp seismic discontinuity at a depth of 660?km that can provide insights into deep mantle processes. The discontinuity occurs over only 2?km—or a pressure difference of 0.1?GPa—and is thought to result from the post-spinel transition, that is, the decomposition of the mineral ringwoodite to bridgmanite plus ferropericlase. Existing high-pressure, high-temperature experiments have lacked the pressure control required to test whether such sharpness is the result of isochemical phase relations or chemically distinct upper and lower mantle domains. Here, we obtain the isothermal pressure interval of the Mg-Fe binary post-spinel transition by applying advanced multi-anvil techniques with in situ X-ray diffraction with the help of Mg-Fe partition experiments. It is demonstrated that the interval at mantle compositions and temperatures is only 0.01?GPa, corresponding to 250?m. This interval is indistinguishable from zero at seismic frequencies. These results can explain the discontinuity sharpness and provide new support for whole-mantle convection in a chemically homogeneous mantle. The present work suggests that distribution of adiabatic vertical flows between the upper and lower mantles can be mapped on the basis of discontinuity sharpness.
Geochimica et Cosmochimica Acta, Vol. 281, pp. 67-90. pdf
Russia, Siberia
deposit - Udachnaya
Abstract: Cratonic lithospheric mantle is believed to have been formed in the Archean, but kimberlite-hosted coarse peridotites from Udachnaya in the central Siberian craton typically yield Paleoproterozoic Re-depletion Os isotope ages (TRD). By comparison, olivine megacrysts from Udachnaya, sometimes called “megacrystalline peridotites”, often yield Archean TRD ages, but the nature of these rare materials remains enigmatic. We provide whole-rock (WR) Re-Os isotope and PGE analyses for 24 olivine-rich xenoliths from Udachnaya as well as modal and petrographic data, WR and mineral major and trace element compositions. The samples were selected based on (a) high olivine abundances in hand specimens and (b) sufficient freshness and size to yield representative WR powders. They comprise medium- to coarse-grained (olivine??1?cm) dunite, olivine megacrysts and low-orthopyroxene (11-21% opx) harzburgites equilibrated at 783-1154?°C and 3.9-6.5 GPa; coarse dunites have not been previously reported from Udachnaya; two xenoliths contain ilmenite. The harzburgites and dunites have similar WR variation ranges of Ca, Al, Fe, Cr and Mg# (0.917-0.934) typical of refractory cratonic peridotites, but the dunites tend to have higher MgO, NiO and Mg/Si. Mineral abundances and those of Ca and Al are not correlated with Mg#WR; they are not due to differences in melting degrees but are linked to metasomatism. Several samples with high 187Re/188Os show a positive linear correlation with 187Os/188Os with an apparent age of 0.37?Ga, same as eruption age of host kimberlite. Robust TRD ages were obtained for 16 xenoliths with low 187Re/188Os (0.02-0.13). TRD ages for low-opx harzburgites (1.9-2.1?Ga; average 2.0?±?0.1?Ga, 1 ?) are manifestly lower than for dunites and megacrysts (2.4-3.1?Ga); the latter define two subsets with average TRD of 2.6?±?0.1?Ga and 3.0?±?0.1?Ga, and TMA of 3.0?±?0.2?Ga and 3.3?±?0.1?Ga, respectively. Differences in olivine grain size (coarse vs. megacrystalline) are not related to age. The age relations suggest that the dunites and megacrysts could not be produced by re-melting of harzburgites, e.g. in arc settings, nor be melt channel materials in harzburgites. Instead, they are relict fragments of lithospheric mantle formed in the Archean (likely in two events at or after 2.6?Ga and 3.0?Ga) that were incorporated into cratonic lithosphere during the final assembly of the Siberian craton in the Paleoproterozoic. A multi-stage formation of the Siberian lithospheric mantle is consistent with crustal basement ages from U-Pb dating of zircons from crustal xenoliths at Udachnaya and detrital zircons from the northern Siberian craton (1.8-2.0, 2.4-2.8 and 3.0-3.4?Ga). The new data from the Siberian and other cratons suggest that the formation of strongly melt-depleted cratonic lithosphere (e.g. Mg# ?0.92) did not stop at the Archean-Proterozoic boundary as is commonly thought, but continued in the Paleoproterozoic. The same may be valid for the transition from the ‘Archean’ (4-2.5?Ga) to modern tectonic regimes.
American Mineralogist, Vol. 105, pp. 1342-1348. pdf
Mantle
bridgmanite
Abstract: To understand the effects of H2O on the mineral phases forming under the pressure-temperature conditions of the lower mantle, we have conducted laser-heated diamond-anvil cell experiments on hydrous ringwoodite (Mg2SiO4 with 1.1 wt% H2O) at pressures between 29 and 59 GPa and temperatures between 1200 and 2400 K. Our results show that hydrous ringwoodite (hRw) converts to crystalline dense hydrous silica, stishovite (Stv) or CaCl2-type SiO2 (mStv), containing 1 wt% H2O together with Brd and MgO at the pressure-temperature conditions expected for shallow lower-mantle depths between approximately 660 to 1600 km. Considering the lack of sign for melting in our experiments, our preferred interpretation of the observation is that Brd partially breaks down to dense hydrous silica and periclase (Pc), forming the phase assembly Brd + Pc + Stv. The results may provide an explanation for the enigmatic coexistence of Stv and Fp inclusions in lower-mantle diamonds.
Minerals, Vol. 10, 9, 740 10.3390/ min10090740 24p. Pdf
Russia
deposit - Muna
Abstract: The oxidation state of the mantle plays an important role in many chemical and physical processes, including magma genesis, the speciation of volatiles, metasomatism and the evolution of the Earth’s atmosphere. We report the first data on the redox state of the subcontinental lithospheric mantle (SCLM) beneath the Komsomolskaya-Magnitnaya kimberlite pipe (KM), Upper Muna field, central Siberian craton. The oxygen fugacity of the KM peridotites ranges from ?2.6 to 0.3 logarithmic units relative to the fayalite-magnetite-quartz buffer (?logfO2 (FMQ)) at depths of 120-220 km. The enriched KM peridotites are more oxidized (?1.0-0.3 ?logfO2 (FMQ)) than the depleted ones (from ?1.4 to ?2.6 ?logfO2 (FMQ)). The oxygen fugacity of some enriched samples may reflect equilibrium with carbonate or carbonate-bearing melts at depths >170 km. A comparison of well-studied coeval Udachnaya and KM peridotites revealed similar redox conditions in the SCLM of the Siberian craton beneath these pipes. Nevertheless, Udachnaya peridotites show wider variations in oxygen fugacity (?4.95-0.23 ?logfO2 (FMQ)). This indicates the presence of more reduced mantle domains in the Udachnaya SCLM. In turn, the established difference in the redox conditions is a good explanation for the lower amounts of resorbed diamonds in the Udachnaya pipe (12%) in comparison with the KM kimberlites (33%). The obtained results advocate a lateral heterogeneity in the oxidation state of the Siberian SCLM.
Abstract: Many Cenozoic basaltic rocks in Eastern Australia exhibit an age-progressive trend from north to south, leading to the suggestion that one or more mantle plumes passed beneath the Australian plate. Trace element patterns indicate that the source regions have been metasomatised by infiltrating melts, but the source rock assemblages have never been closely identified. Here, trace element analyses of olivine and whole rock geochemistry for several occurrences in New South Wales (Bingara-Inverell, Dubbo, Barrington and Ebor) are combined to characterize the mineralogy of the source and identify the nature of the melts that caused the metasomatic enrichment. According to Ni/Mg against Mn/Fe and Zn/Fe ratios in olivines, Zn/Fe and FC3MS (FeOT/CaO-3*MgO/SiO2) parameters in whole rocks, tholeiite, alkali basalt, and basanite rich in olivine xenocrysts from Dubbo were derived from pyroxenite-dominated mixed source, mixed pyroxenite+peridotite source, and peridotite-dominated source, respectively. Similarly, basalts from Ebor and Bingara/Inverell are suggested to originate from a mixed pyroxenite+peridotite source based on their high FC3MS values. In contrast, the source of basanite and picrobasalt from Barrington was peridotite with little pyroxenite. High Li and Zn in olivines, high P2O5/TiO2 and Zr/Hf at low Ti/Eu in whole rocks illustrate that the pyroxenite sources of basanites from Bingara/Inverell, Barrington and Dubbo resulted from variable degrees of carbonatitic metasomatism. Partial melting of peridotite metasomatised by carbonatite melts at around the spinel-garnet peridotite transition depth produced basalts and basanites from Dubbo, Barrington, Ebor, Bingara/Inverell and Buckland (Queensland). Carbonatitic metasomatism is widespread in the eastern Australian mantle lithosphere, occurring seaboard of a ledge between thick lithosphere beneath the Australian continent that stretches from Queensland, through New South Wales to Victoria.
Earth and Planetary Science Letters, Vol. 570, 117088
Mantle
bridgemanite
Abstract: Water solubility in the dominant lower-mantle bridgmanite phase remains controversial. Discrepancies between previous results highlight the importance of the growth high-quality single crystals of bridgmanite under high-pressure and high-temperature conditions corresponding to the top of the lower mantle. Here we synthesized high-quality single crystals of aluminous bridgmanite up to 300 ?m in size that were saturated with hydrous melt at 24-26 GPa and 1700-1900 K using both stoichiometric and MgO-rich non-stoichiometric hydrous starting materials in a multi-anvil press. Fourier-transform infrared spectroscopy measurements on clear and pure spots of the single-crystal bridgmanites did not detect any pronounced OH-stretching bands, which were prominent in some earlier studies. The present results support that the lower-mantle dominated bridgmanite is nearly dry, at least at the top of the lower mantle, and that Al3+ and Fe3+ cannot enhance water incorporation into the crystal structure even in the presence of oxygen vacancies. Large partition coefficients of water between transition-zone minerals and dry lower-mantle dominated bridgmanite further support dehydration melting at the top of the lower mantle. We suggest that the majority of the top of a pyrolitic lower mantle is nearly dry based on the dry principal minerals and stability of hydrous phases in this region.
Studies on the defect structures and metasomatism of olivine and pryoxenein lherzolite xenoliths from basalts in Fujian and Zhejiang Provinces, southeastern China
International Mineralogical Association Meeting Held June, 1990 Beijing China, Vol. 1, extended abstract p. 363-364
Abstract: Carbonatite and related alkaline silicate rocks contain one of the most significant rare earth element (REE) reserves in the world. It is well-known that these REE deposits are characterized by a strong light REE enrichment with a steep fractionation from La to Lu in the chondrite-normalized diagram. However, the origin of their REE enrichment remains debatable. The Shaxiongdong (SXD) carbonatite in the South Qinling orogenic belt hosts one of the most important REE deposits in central China. In this study, in situ chemical and isotopic data have been obtained for carbonate minerals from the complex. Our results show that calcite has variable trace element abundances, especially REEs. In situ Pb isotope data for calcite reveal extreme variations of 206Pb/204Pb (18.05-31.71) and 207Pb/204Pb (15.49-16.36) ratios. Interestingly, Pb isotope variations display positive correlations with REE enrichments [i.e., (La/Yb)N and (La/Nd)N]. Calcite with extreme radiogenic Pb isotopic compositions displays upper mantle C and O isotopic compositions (?13Cavg?=??5.74‰, ?18Oavg?=?7.13‰) and depleted 87Sr/86Sr isotopic ratios (~0.7030). The observed various REE enrichments accompanying the variable Pb isotopic composition within SXD calcite possibly result from a closed-system metasomatic event. The U-bearing mineral (i.e., pyrochlore) accumulating abundant uranogenic lead since their Silurian formation serves as the radiogenic Pb and LREE source for the metasomatism. Alternatively, the chemical and isotopic composition observed might suggest involvement of two mantle sources (PREMA and the distinct radiogenic Pb mantle reservoir).
Journal of Geophysical Research: Solid Earth, Vol. 174, 7, pp. 6625-6652.
United States
geophysics - seismic
Abstract: The Southeastern United States is an ideal location to understand the interactions between mountain building, rifting, and magmatism. Line 2 of the Suwannee suture and Georgia Rift basin refraction seismic experiment in eastern Georgia extends 420 km from the Inner Piedmont to the Georgia coast. We model crustal and upper mantle VP and upper crustal VS. The most dramatic model transition occurs at the Higgins?Zietz magnetic boundary, north of which we observe higher upper crustal VP and VS and lower VP/VS. These observations support the interpretation of the Higgins?Zietz boundary as the Alleghanian suture. North of this boundary, we observe a low?velocity zone less than 2 km thick at ~5?km depth, consistent with a layer of sheared metasedimentary rocks that forms the Appalachian detachment. To the southeast, we interpret synrift sediments and decreasing crustal thickness to represent crustal thinning associated with the South Georgia Rift Basin and subsequent continental breakup. The correspondence of the northern limit of thinning with the interpreted suture location suggests that the orogenic suture zone and/or the Gondwanan crust to the south of the suture helped localize subsequent extension. Lower crustal VP and VP/VS preclude volumetrically significant mafic magmatic addition during rifting or associated with the Central Atlantic Magmatic Province. Structures formed during orogenesis and/or extension appear to influence seismicity in Georgia today; earthquakes localize along a steeply dipping zone that coincides with the northern edge of the South Georgia Basin and the change in upper crustal velocities at the Higgins?Zietz boundary.
Kovach, V.,Salnikova, E., Wang, K-L., Jahn, B-M., Chiu, H-Y., Reznitskiy, L., Kotov, A., Lizuka, Y., Chung, S-L.
Zircon ages and Hf isotopic constraints on sources of clastic metasediments of the Slyudyansky high grade complex, southeastern Siberia: implication for continental growth and evolution of the Central Asian orogenic belt.
Journal of Asian Earth Sciences, Vol. 62, pp. 18-36.
Journal of Geochemical Exploration, Vol. 204, pp. 270-288.
Europe, Spain
REE
Abstract: Gran Canaria is a hotspot-derived, intraplate, oceanic island, comprising a variety of alkaline felsic magmatic rocks (i.e. phonolites, trachytes, rhyolites and syenites). These rocks are enriched in rare-earth elements (REE) in relation to the mean concentration in the Earth's crust and they are subsequently mobilised and redistributed in the soil profile. From a set of 57 samples of felsic rocks and 12 samples from three paleosol profiles, we assess the concentration and mobility of REE. In the saprolite that developed over the rhyolites, we identified REE-bearing minerals such as primary monazite-(Ce), as well as secondary phases associated with the edaphic weathering, such as rhabdophane-(Ce) and LREE oxides. The averaged concentration of REE in the alkaline bedrock varies from trachytes (449?mg?kg?1), to rhyolites (588?mg?kg?1) and to phonolites (1036?mg?kg?1). REE are slightly enriched in saprolites developed on trachyte (498?mg?kg?1), rhyolite (601?mg?kg?1) and phonolite (1171?mg?kg?1) bedrocks. However, B-horizons of paleosols from trachytes and phonolites showed REE depletion (436 and 994?mg?kg?1, respectively), whereas a marked enrichment was found in soils developed on rhyolites (1584?mg?kg?1). According to our results, REE resources on Gran Canaria are significant, especially in Miocene alkaline felsic magmatic rocks (declining stage) and their associated paleosols. We estimate a total material volume of approximately 1000?km3 with REE concentrations of 672?±?296?mg?kg?1, yttrium contents of 57?±?30?mg?kg?1, and light and heavy REE ratios (LREE/HREE) of 17?±?6. This mineralisation can be considered as bulk tonnage and low-grade ore REE deposits but it remains necessary to develop detailed mineral exploration on selected insular zones in the future, without undermining environmental and socioeconomic interests.
Distribution of fluorine between minerals and glass in lamproites, lamprophyres and kamafugites: implications for the role of F in deep mantle derived magmas
Proceedings of Fifth International Kimberlite Conference held Araxa June 1991, Servico Geologico do Brasil (CPRM) Special, pp. 79-81
light rare earth element (LREE) distribution in accessory minerals from southwest Ugand an xenoliths and their kamafugite hosts: an electron microprobe study
Rare earth Minerals: chemistry, origin and ore deposits, International Geological Correlation Programme (IGCP) Project, pp. 73-75. abstract
Journal of Metamorphic Geology, doi.org/10/111/jmg.12566
Mantle
coesite
Abstract: Ultrahigh?pressure (UHP) metamorphism observed in continental terranes implies that continental crust can subduct to ~40 kbar before exhuming to the surface. This process is one of the least understood and widely debated parts of the orogenic cycle. The dominantly felsic composition of UHP continental terranes means that many petrology?based techniques for determining peak pressures and temperatures are often not possible. In such cases, the detection of UHP conditions depends on the preservation of coesite, a rarely preserved mineral in exhumed UHP terranes as it rapidly transforms to quartz on decompression. Consequently, the qualitative identification of palisade quartz microstructures that form during the retrograde transformation of coesite to quartz is often used to identify UHP terranes. In this study, we conduct EBSD and misorientation analysis of palisade quartz inclusions in the coesite?bearing pyrope quartzite from the Dora Maira massif in the Alps, and matrix?scale palisade quartz in the Polokongka La granite from Tso Morari in the Ladakh Himalaya, in order to quantitatively define crystallographic characteristics of quartz after coesite. The repeatability of our observations in two unrelated occurrences of UHP rocks supports our interpretation that the following features provide a systematic and predictable set of criteria to identify the coesite to quartz transition: (1) Quartz crystallographic orientations define spatially and texturally distinct subdomains of palisade quartz grains with ‘single crystal’ orientations defined by distinct c?axis point?maxima. (2) Adjacent subdomains are misorientated with respect to each other by a misorientation angle/axis of 90°/. (3) Within each subdomain, palisade quartz grain boundaries commonly have intra? and inter?granular misorientations of 60°/[0001], consistent with the dauphiné twin law. Our observations imply that the coesite?to?quartz transformation is crystallographically controlled by the epitaxial nucleation of palisade quartz on the former coesite grain, specifically on potential coesite twin planes such as (101) and (021).
Journal of African Earth Sciences, Vol. 179, 104206, 21p. pdf
Africa, Cameroon, Central African Republic
geophysics
Abstract: The Bangui Magnetic Anomaly (BMA) is one of the largest magnetic anomalies in the world whose origin is still not known. This research investigated the crustal thickness, Curie depths and thermal structures in the Central African sub-regions - Cameroon, Central African Republic and adjacent countries - which are largely characterized by the Bangui Magnetic Anomaly. To achieve a better understanding and clearer idea of the location of the possible sources of the BMA, analyses of geothermal structures were conducted. Two potential methods were used: gravity to evaluate the crustal thickness and magnetics for geothermal analysis. Spectral analysis of gravity data shows that crustal thickness range between 14 and 55 km. The highest depths were found in Central African Republic. The lower values of crustal thickness were obtained in South-Chad basin with a minimum of roughly 14 km. Geothermal analysis is carried out using the Curie point depth, thermal gradient and heat-flow evaluations. The results show that the BMA is related to a thick crust of roughly 40 km. Depth to the bottom of possible sources does not exceed the lower crust. The mean Curie point depth estimated is 38 km with an error of ±2 km. Geothermal results also show the difference in the thermal behaviour between the crust in the Pan African and Precambrian domain. The mobile zone which constitutes the Pan African domain is associated with a thin crust of high heat-flow values of 65 mW/m2. However, the Precambrian domain beneath the BMA is associated with a thick crust with lower heat-flow values (roughly 45 mW/m2). The difference between crustal thickness and Curie point depths shows that all the sources of the BMA are crustal. The present results are in favour of a geological origin for the Bangui Magnetic anomaly.
Arabian Journal of Geosciences, Vol. 13, , 209 orchid.org/ 0000-002-3287-9537
Africa, Mauritania
craton
Abstract: We used remote sensing, geographical information systems, Google Earth™ images, and regional geology in order to (i) improve the mapping of linear structures and understand the chronology of different mafic dyke swarms in the Ahmeyim area that belongs to the Archean Tasiast-Tijirit Terrane of the Reguibat Shield, West African craton, NW Mauritania. The spatial and temporal distributions with the trends of the dyke swarms provide important information about geodynamics. The analysis of the mafic dyke swarms map and statistical data allow us to distinguish four mafic dyke swarm sets: a major swarm trending NE-SW to NNE-SSW (80%) and three minor swarms trending EW to ENE-WSW (9.33%), NW-SE to WNW-ESE (9.06%), and NS (1.3%). The major swarms extend over 35 km while the minor swarms do not exceed 13 km. The Google Earth™ images reveal relative ages through crossover relationships. The major NE-SW to NNE-SSW and the minor NS swarms are the oldest generations emplaced in the Ahemyim area. The NW-SE-oriented swarm dykes which are cutting the two former swarms are emplaced later. The minor E-W to WSW-ENE swarms are probably the youngest. A precise U-Pb baddeleyite age of 2733?±?2 Ma has been obtained for the NNE-SSW Ahmeyim Great Dyke. This dyke is approximately 1500 m wide in some zone and extends for more than 150 km. The distinct mafic dyke swarms being identified in this study can potentially be linked with coeval magmatic events on other cratons around the globe to identify reconstructed LIPs and constrain continental reconstructions.
Physics of the Earth and Planetary Interiors, Vol. 247, pp. 11-16.
Mantle
Experimental Petrology
Abstract: Lattice thermal conductivity of ferropericlase and radiative thermal conductivity of iron bearing magnesium silicate perovskite (bridgmanite) - the major mineral of Earth’s lower mantle- have been measured at room temperature up to 30 and 46 GPa, respectively, using time-domain thermoreflectance and optical spectroscopy techniques in diamond anvil cells. The results provide new constraints for the pressure dependencies of the thermal conductivities of Fe bearing minerals. The lattice thermal conductivity of ferropericlase Mg0.9Fe0.1O is 5.7(6) W/(m * K) at ambient conditions, which is almost 10 times smaller than that of pure MgO; however, it increases with pressure much faster (6.1(7)%/GPa vs 3.6(1)%/GPa). The radiative conductivity of a Mg0.94Fe0.06SiO3 bridgmanite single crystal agrees with previously determined values for powder samples at ambient pressure; it is almost pressure-independent in the investigated pressure range. Our results confirm the reduced radiative conductivity scenario for the Earth’s lower mantle, while the assessment of the heat flow through the core-mantle boundary still requires in situ measurements at the relevant pressure-temperature conditions.
Earth and Planetary Science Letters, Vol. 479, pp. 43-49.
Mantle
perovskite
Abstract: Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal conductivity at relevant pressure-temperature conditions of Earth's core-mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core-mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg0.9Fe0.1SiO3 post-perovskite (?1.1 W/m/K) is almost two times smaller than that of bridgmanite (?2.0 W/m/K) at the base of the mantle. By combining this result with the present-day core-mantle heat flow and available estimations on the lattice thermal conductivity we conclude that post-perovskite is at least as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth.
Abstract: The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO3-Fe0 system at 70-150 GPa and 800-2600 K in a laser-heated diamond anvil cell. Using in situ synchrotron X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by: 6MgCO3 + 19Fe = 8FeO +10(Mg0.6Fe0.4)O + Fe7C3 + 3C. The presented results suggest that the interaction of carbonates with Fe0 or Fe0-bearing rocks can produce Fe-carbide and diamond, which can accumulate in the D’’ region, depending on its carbon to Fe ratio. Due to the sluggish kinetics of the transformation, diamond can remain metastable at the core-mantle boundary (CMB) unless it is in a direct contact with Fe-metal. In addition, it can be remobilized by redox melting accompanying the generation of mantle plumes.
Abstract: The heat flux across the core-mantle boundary (QCMB) is the key parameter to understand the Earth/s thermal history and evolution. Mineralogical constraints of the QCMB require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (krad) of a realistic lower mantle (pyrolite) in situ using an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to ~3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from ~0.8 W/m/K at 1000 km to ~0.35 W/m/K at the CMB, the latter being ~30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of ~8.5 TW, at odds with present estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo.
Abstract: Mesozoic (125-135 Ma) cratonic low-Ti lamproites from the northern part of the Aldan Shield do not conform to typical classification schemes of ultrapotassic anorogenic rocks. Here we investigate their origins by analyzing olivine and olivine-hosted inclusions from the Ryabinoviy pipe, a well preserved lamproite intrusion within the Aldan Shield. Four types of olivine are identified: (1) zoned phenocrysts, (2) high-Mg, high-Ni homogeneous macrocrysts, (3) high-Ca and low-Ni olivine and (4) mantle xenocrysts. Olivine compositions are comparable to those from the Mediterranean Belt lamproites (Olivine-1 and -2), kamafugites (Olivine-3) and leucitites. Homogenized melt inclusions (MIs) within olivine-1 phenocrysts have lamproitic compositions and are similar to the host rocks, whereas kamafugite-like compositions are obtained for melt inclusions within olivine-3. Estimates of redox conditions indicate that “lamproitic” olivine crystallized from anomalously oxidized magma (?NNO +3 to +4 log units.). Crystallization of "kamafugitic" olivine occurred under even more oxidized conditions, supported by low V/Sc ratios. We consider high-Ca olivine (3) to be a fingerprint of kamafugite-like magmatism, which also occurred during the Mesozoic and slightly preceded lamproitic magmatism. Our preliminary genetic model suggests that low-temperature, extension-triggered melting of mica- and carbonate-rich veined subcontitental lithospheric mantle (SCLM) generated the kamafugite-like melts. This process exhausted carbonate and affected the silicate assemblage of the veins. Subsequent and more extensive melting of the modified SCLM produced volumetrically larger lamproitic magmas. This newly recognized kamafugitic "fingerprint" further highlights similarities between the Aldan Shield potassic province and the Mediterranean Belt, and provides evidence of an overlap between "orogenic" and "anorogenic" varieties of low-Ti potassic magmatism. Moreover, our study also demonstrates that recycled subduction components are not an essential factor in the petrogenesis of low-Ti lamproites, kamafugites and leucitites.
Botswana Journal of Earth Sciences, Vol. 7, pp. 29-34. pdf
Africa, Botswana
deposit - KX36
Abstract: The KX36 kimberlite pipe is situated in the southeastern part of Central Kalahari Game Reserve (CKGR), Botswana, approximately 60 km from the known Gope and Kikao kimberlite fields (see figure 1).The kimberlite is covered by 75m of Kalahari sand, has a surface area of 3.6 Ha at the base of the sand cover and was discovered by Petra Diamonds Botswana (Pty) Ltd in 2008. Application of modern geophysical techniques (Ultra hi-resolution low level flying Xcalibur magnetics) and improved sampling method led to the discovery of KX36.The kimberlite was emplaced into the Karoo Supergroup, which comprised the older sedimentary rocks (300 - 185 Ma) overlain by the flood basalts (185Ma). The Karoo Supergroup rocks are overlain by approximately 80m of Kalahari Group sediments.
Klein, E.L., Luzardo, R., Moura, Lobato, Brito, Armstrong
Geochronology, Nd isotopes and reconnaissance geochemistry of volcanic and metavolcanic rocks of Sao Luis Craton, northern Brazil: tectonics and crustal
Journal of South American Earth Sciences, Vol. 27, 2-3, pp. 129-145.
The Journal of the Southern African Insitute of Mining and Metallurgy, Vol. 119, Feb. 10p. Pdf
Africa, Tanzania
financing
Abstract: Government's equity role in the minerals sector is one of the nationalist measures implemented in order to ensure greater control and management of a country's mineral resources. This paper evaluates the Tanzanian government's equity participation in the minerals sector from 1996 to 2015. The research methodology included determination of the number of mineral rights, minimum allowable exploration expenditures in prospecting licences (PLs), and forms of equity role of the government. Data was collected and analysed for PLs, mining licences (MLs), and special mining licences (SMLs). The study revealed a number of challenges faced by the Tanzanian government as regards its equity strategy in the mineral sector. One of the major challenges was the secrecy surrounding agreements and contracts entered into between the government and private sector investors, which were concluded via various business ownership and mineral development projects. This secrecy resulted in non-transparency and lack of accountability in the mining industry. The financial benefits accruing to the government were inadequately realized, evident through inconsistent payments of corporate income tax and mining royalties by the mining companies. Furthermore, the government does not have solid mechanisms and frameworks for assessing non-financial benefits, thus it is difficult to measure the impact of these factors. It is recommended that the Tanzanian government review the Mining Act and Regulations of 2010 to include the provision of solid mechanisms and frameworks for all forms of government equity role.
Earth Science Reviews , Vol. 219, 103616 231p. Pdf
Africa, Namibia
Craton - Congo
Abstract: Otavi Group is a 1.5-3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran?Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S?N crustal stretching (770-655?Ma), followed by post-rift thermal subsidence (655-600?Ma). Rifting under southern Swakop Group continued until 650-635?Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S?N plate divergence. Rift-related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746?Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770-717?Ma; (2) glacial early Cryogenian/Sturtian, 717-661?Ma; (3) non-glacial middle Cryogenian, 661-646?±?5?Ma; (4) glacial late Cryogenian/Marinoan, 646?±?5-635?Ma; and (5) non-glacial early Ediacaran, 635-600?±?5?Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585?measured sections (totaling >190?km of strata) and?>?8764 pairs of ?13C/?18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5-9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3-8.8?m Myr?1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ?1.6?km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9?m regional average thickness, n?=?140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ?90?m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ?0.6?km of paleobathymetric relief. Late Tonian shallow-neritic ?13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770-760?Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737-717?Ma) in Swakop Group. Devede Fm is isotopically heavy, +4-8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746?Ma volcanics and shows two negative excursions bridged by heavy ?13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic ?13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling-stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0-3.5‰. Early Ediacaran neritic ?13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (??+?11‰) with extreme point-to-point variability (?10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform ?13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9?±?1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60-0.58?Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ?1.85?km of megakarstic relief, and km-thick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0628-1 14p.
Africa, Angola
kimberlites
Abstract: Based on a comprehensive analysis of kimberlite pipes of Angola, including the near surface structural setting, deep lithospheric structure, pipe morphology and emplacement, mineralogical and petrographic features, diamond characteristics and locations of secondary deposits four geographical regions have been outlined within Angola representing four types of diamond bearing potential. These areas include high diamond bearing potential pipes, possible potential, no potential, and unclear potential areas. It was found that the depth of magmatism and diamond potential of kimberlites increases from the Atlantic coast in southwestern Angola into the continent in the north-easterly direction. Areas prospective for the discovery of new primary diamond deposits have been identified.
Geochemistry International, Vol. 55, 4, pp. 360-366.
Russia, Siberia
diamondoid
Abstract: A broad suite of geological materials were studied a using a handheld laser-induced breakdown spectroscopy (LIBS) instrument. Because LIBS is simultaneously sensitive to all elements, the full broadband emission spectrum recorded from a single laser shot provides a ‘chemical fingerprint’ of any material - solid, liquid or gas. The distinguishing chemical characteristics of the samples analysed were identified through principal component analysis (PCA), which demonstrates how this technique for statistical analysis can be used to identify spectral differences between similar sample types based on minor and trace constituents. Partial least squares discriminant analysis (PLSDA) was used to distinguish and classify the materials, with excellent discrimination achieved for all sample types. This study illustrates through four selected examples involving carbonate minerals and rocks, the oxide mineral pair columbite-tantalite, the silicate mineral garnet and native gold how portable, handheld LIBS analysers can be used as a tool for real-time chemical analysis under simulated field conditions for element or mineral identification plus such applications as stratigraphic correlation, provenance determination and natural resources exploration.
The Southern African Institute of Mining and Metallurgy, Vol. 119, pp. 155-164.
Africa, Botswana
deposit - Jwaneng
Abstract: Despite the pre-eminence of the Jwaneng Diamond Mine as the world's richest diamond mine, the discovery story has long been clouded in mystery. This is the 45-year old untold story of the Jwaneng discovery and contemporaneous Bechuanaland/Botswana political and socioeconomic history.
Botswana Journal of Earth Sciences, Vol. 7, pp. 3-12. pdf
Africa, Botswana
deposit - Jwaneng
Abstract: When Leon Daniels first contacted us, and others, about the Botswana Diamond Exploration Conference 2017, he wrote: ‘The future of diamond exploration lies in us "old-timers" who have retired or are about to retire to inspire the next generation to go out there and make the next discoveries so vitally important for Botswana.’ Sadly, in presenting on behalf of both Stuart and I, I had to own to being one of those old-timers!
Abstract: Projects with long histories must be documented in current disclosures with transparency and materiality, using historical data and historical estimates. Historical data may be of great value if it is from a reliable source, and the raw data can be validated and/or duplicated. Historical estimates can and should be reported, but with qualification of the ever-changing economic parameters of ‘Reasonable Prospects for Eventual Economic Extraction’ (RPEEE). The SAMREC Code requires current sampling results and diamond valuations, without which RPEEE cannot be assessed; consequently, historical estimates cannot ever be declared as current Diamond Resources or Reserves. The SAMREC Code defines historical estimates and provides guidance on the use of historical data. Examples from real projects and reports in the public domain are reviewed in this paper. Opinions on use and misuse are those of the writer; judgment on good or bad practice is not the intention and is left to the opinion of the reader. Comparison, with both the JORC Code (Australasia) and CIM Definition Standards and National Instrument 43-101 (Canada), is provided. The SAMREC Code appears to be more closely aligned with the Canadian standards.
Contrasting garnet lherzolite xenoliths suites from the Letseng kimberlite pipes: inferences for the northern Lesotho geotherm.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 29-44.
Nature Communications, doi.org/10.1038/s41467-019-11065-4 pdf 10p.
Mantle
subduction
Abstract: Ore deposits are loci on Earth where energy and mass flux are greatly enhanced and focussed, acting as magnifying lenses into metal transport, fractionation and concentration mechanisms through the lithosphere. Here we show that the metallogenic architecture of the lithosphere is illuminated by the geochemical signatures of metasomatised mantle rocks and post-subduction magmatic-hydrothermal mineral systems. Our data reveal that anomalously gold and tellurium rich magmatic sulfides in mantle-derived magmas emplaced in the lower crust share a common metallogenic signature with upper crustal porphyry-epithermal ore systems. We propose that a trans-lithospheric continuum exists whereby post-subduction magmas transporting metal-rich sulfide cargoes play a fundamental role in fluxing metals into the crust from metasomatised lithospheric mantle. Therefore, ore deposits are not merely associated with isolated zones where serendipitous happenstance has produced mineralisation. Rather, they are depositional points along the mantle-to-upper crust pathway of magmas and hydrothermal fluids, synthesising the concentrated metallogenic budget available.
Abstract: Perovskite mineral oxides commonly exhibit extensive solid-solution, and are therefore classified on the basis of the proportions of their ideal end-members. A uniform sequence of calculation of the end-members is required if comparisons are to be made between different sets of analytical data. A Microsoft Excel spreadsheet has been programmed to assist with the classification and depiction of the minerals of the perovskite- and vapnikite-subgroups following the 2017 nomenclature of the perovskite supergroup recommended by the International Mineralogical Association (IMA). Compositional data for up to 36 elements are input into the spreadsheet as oxides in weight percent. For each analysis, the output includes the formula, the normalized proportions of 15 end-members, and the percentage of cations which cannot be assigned to those end-members. The data are automatically plotted onto the ternary and quaternary diagrams recommended by the IMA for depiction of perovskite compositions. Up to 200 analyses can be entered into the spreadsheet, which is accompanied by data calculated for 140 perovskite compositions compiled from the literature.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0632-5 20p.
Canada, Nunavut, Melville Peninsula
deposit - Pelly Bay, Darby, Aviat, Qilalugaq
Abstract: Detailed geochronology along with petrographic, mineralogical and geochemical studies have been conducted on recently found diamond-bearing kimberlitic and related rocks in the Rae Craton at Aviat and Qilalugaq, Melville Peninsula, north-east Canada. Magmatic rocks from the Aviat pipes have geochemical (both bulk rock and isotopic) and mineralogical signatures (e.g., core to rim Al and Ba enrichment in phlogopite) similar to Group I kimberlite. In contrast, Aviat intrusive sheets are similar to ‘micaceous’ Group II kimberlite (orangeite) in their geochemical and mineralogical characteristics (e.g., phlogopite and spinel compositions, highly enriched Sr isotopic signature). Qilalugaq rocks with the least crustal contamination have geochemical and mineralogical signatures [e.g., high SiO2, Al2O3 and H2O; low TiO2 and CO2; less fractionated REE (rare earth elements), presence of primary clinopyroxene, phlogopite and spinel compositions] that are similar to features displayed by olivine lamproites from Argyle, Ellendale and West Greenland. The Naujaat dykes, in the vicinity of Qilalugaq, are highly altered due to extensive silicification and carbonation. However, their bulk rock geochemical signature and phlogopite chemistry are similar to Group I kimberlite. U-Pb perovskite geochronology reveals that Aviat pipes and all rocks from Qilalugaq have an early Cambrian emplacement age (540-530 Ma), with the Aviat sheets being ~30 Ma younger. This volatile-rich potassic ultramafic magmatism probably formed by varying degrees of involvement of asthenospheric and lithospherically derived melts. The spectrum of ages and compositions are similar to equivalent magmatic rocks observed from the nearby north-eastern North America and Western Greenland. The ultimate trigger for this magmatism could be linked to Neoproterozoic continental rifting during the opening of the Iapetus Ocean and breakup of the Rodinia supercontinent.
Abstract: Nixonite (IMA 2018-133), ideally Na2Ti6O13, is a new mineral found within a heavily-metasomatized pyroxenite xenolith from the Darby kimberlite field, beneath the west central Rae Craton, Canada. It occurs as microcrystalline aggregates, 15 to 40 ?m in length. Nixonite is isostructural with jeppeite, K2Ti6O13, with a structure consisting of edge- and corner-shared titanium-centered octahedra that enclose alkali-metal ions. The Mohs hardness is estimated to be between 5 and 6 by comparison to jeppeite and the calculated density is 3.51(1) g/cm3. Electron microprobe wavelength-dispersive spectroscopic analysis (average of 6 points) yielded: Na2O 6.87, K2O 5.67 CaO 0.57, TiO2 84.99, V2O3 0.31, Cr2O3 0.04, MnO 0.01, Fe2O3 0.26, SrO 0.07, total 98.79 wt%. The empirical formula, based on 13 O atoms, is: (Na1.24K0.67Ca0.06)?1.97(Ti5.96V0.023Fe0.018)?6.00O13 with minor amounts of Cr and Mn. Nixonite is monoclinic, space group C2/m, with unit-cell parameters a = 15.3632(26) Å, b = 3.7782(7) Å, c = 9.1266(15) Å, ? = 99.35(15)º and V = 522.72(1) Å3, Z = 2. Based on the average of seven integrated multi-grain diffraction images, the strongest diffraction lines are [dobs in Å (I in %) (h k l)]: 3.02 (100) (3 1 0) , 3.66 (75) (1 1 0), 7.57 (73) (2 0 0), 6.31 (68) (2 0 -1), 2.96 (63) (3 1 -1), 2.96 (63) (2 0 -3) and 2.71 (62) (4 0 2). The five main Raman peaks of nixonite, in order of decreasing intensity, are at: 863, 280, 664, 135 and 113 cm-1. Nixonite is named after Peter H. Nixon, a renowned scientist in the field of kimberlites and mantle xenoliths. Nixonite occurs within a pyroxenite xenolith in a kimberlite, in association with rutile, priderite, perovskite, freudenbergite and ilmenite. This complex Na-K-Ti rich metasomatic mineral assemblage may have been produced by a fractionated Na-rich kimberlitic melt that infiltrated a mantle-derived garnet pyroxenite and reacted with rutile during kimberlite crystallization.
Abstract: Goldschmidtite is a new perovskite-group mineral (IMA No. 2018-034) with the ideal formula (K,REE,Sr)(Nb,Cr)O3. A single grain of goldschmidtite with a maximum dimension of ?100 ?m was found as an inclusion in a diamond from the Koffiefontein pipe in South Africa. In addition to the dark green and opaque goldschmidtite, the diamond contained a Cr-rich augite (websteritic paragenesis) and an intergrowth of chromite, Mg-silicate, and unidentified K-Sr-REE-Nb-oxide. Geothermobarometry of the augite indicates that the depth of formation was ?170 km. The chemical composition of gold-schmidtite determined by electron microprobe analysis (n = 11, WDS, wt%) is: Nb2O5 44.82, TiO2 0.44, ThO2 0.10, Al2O3 0.35, Cr2O3 7.07, La2O3 11.85, Ce2O3 6.18, Fe2O3 1.96, MgO 0.70, CaO 0.04, SrO 6.67, BaO 6.82, K2O 11.53, total 98.53. The empirical formula (expressed to two decimal places) is (K0.50La0.15Sr0.13Ba0.09Ce0.08)?0.95(Nb0.70Cr0.19Fe0.05Al0.01Mg0.04Ti0.01)?1.00O3. Goldschmidtite is cubic, space group Pm3m, with unit-cell parameters: a = 3.9876(1) Å, V = 63.404(6) Å3, Z = 1, resulting in a calculated density of 5.32(3) g/cm3. Goldschmidtite is the K-analog of isolueshite, (Na,La)NbO3. Raman spectra of goldschmidtite exhibit many second-order broad bands at 100 to 700 cm-1 as well as a pronounced peak at 815 cm-1, which is possibly a result of local ordering of Nb and Cr at the B site. The name goldschmidtite is in honor of the eminent geochemist Victor Moritz Goldschmidt (1888-1947), who formalized perovskite crystal chemistry and identified KNbO3 as a perovskite-structured compound.
Geotectonic aspects of West Africa to the east of the Gulf of Guinea with reference to the connected structures and lithologies between Brasil andAfrica
Rev. Bra. Geoscience, (in Portugese)., Vol. 14, No. 1, March pp. 47-53
Geochemistry, Vol. 80, doi.org/10.1016/ j.chemer. 2019.125594 40p. Pdf
Mantle
geochemistry
Abstract: We review some issues relevant to volatile element chemistry during accretion of the Earth with an emphasis on historical development of ideas during the past century and on issues we think are important. These ideas and issues include the following: (1) whether or not the Earth accreted hot and the geochemical evidence for high temperatures during its formation, (2) some chemical consequences of the Earth’s formation before dissipation of solar nebular gas, (3) the building blocks of the Earth, (4) the composition of the Earth and its lithophile volatility trend, (5) chemistry of silicate vapor and steam atmospheres during Earth’s formation, (6) vapor - melt partitioning and possible loss of volatile elements, (7) insights from hot rocky extrasolar planets. We include tabulated chemical kinetic data for high-temperature elementary reactions in silicate vapor and steam atmospheres. We finish with a summary of the known and unknown issues along with suggestions for future work.
Short hold time parameters. Diavik mine water treatment plant.
2018 Yellowknife Geoscience Forum , p. 74-75. abstract
Canada, Northwest Territories
deposit - Diavik
Abstract: It is well known that it is very difficult to transport samples from remote locations to the laboratory and allow sufficient time to commence analysis within the prescribed short hold times for certain parameters. Also, the majority of published hold times are based on legacy as opposed to hard science. In an attempt to determine the validity of specific short hold times, a joint study between Diavik and Maxxam was undertaken. The purpose was to determine the stability of short hold time parameters over time using real samples from Diavik sites. Data from two sites will be presented. The first from the Diavik mine water treatment plant influent, which had relatively high levels of the target analytes. The second from a lake water sample with lower native levels of the target analytes. The parameters studied were ammonia (preserved and unpreserved), total nitrogen, nitrite, nitrate, phosphate, total phosphorus and turbidity. All target parameters have a prescribed 3-day hold time.1 pH was also monitored. Samples were collected by Diavik personnel in one-litre containers and extraordinary logistical measures were taken to get them to Maxxam's Burnaby laboratory as soon as possible. On receipt, they were immediately subsampled into appropriate containers. Each parameter (except pH and turbidity) was split into three containers: 1) as received; 2) low level spike added and 3) medium level spike added. All samples were analyzed within 3 at approximately 3-day intervals thereafter for a period of two weeks.
Recovery Plant Practice at de Beers Consolidated Mines Limited, Kimberley with Particular Reference to Improvements Made for the Sorting of the Final Concentrates.
South African Institute of Mining and Metallurgy. Journal, Vol. 80, No. 9, PP. 317-328.
Russian Geology and Geophysics, doi:10.15372 /RG2020144 21p. Pdf
Russia
deposit - Mir, Udachnaya, Aikal, Yubileinya
Abstract: The orientation of 76 mineral inclusions represented by olivine (25 inclusions), pyrope (13 inclusions), and magnesiochromite (38 inclusions) was measured in 16 diamond samples from the major primary diamond deposits of Yakutia: Mir, Udachnaya, Internatsionalnaya, Aikhal, and Yubileynaya kimberlite pipes. The novelty of the study is that it provides a special purposeful approach to selection of samples containing not only olivine inclusions that have been extensively studied in the most recent years after the publication of the book Carbon in Earth (2013). The present collection accounts for more than 25% of all samples studied across the world and includes the most typical mineral inclusions of the predominant peridotitic paragenesis in almost all known kimberlites. Both this experiment and similar studies conducted by foreign colleagues in 2014-2019 have found no inclusions whose orientation meets the epitaxial criterion. Only single magnesiochromite inclusions in three diamonds demonstrate an orientation close to the regular one. A significant correlation between the carbon isotope composition and the mineral composition of inclusions of peridotitic and eclogitic paragenesis diamonds as well as the lack of a correlation with other properties may be considered one of the geochemical features. However, given the numerous published and proprietary data demonstrating the complex diamond growth history and, in some cases, wide variations in the composition of mineral inclusions in different zones, along with the difference in their morphology, the authors a believe that syngenetic and protogenetic inclusions can coexist in the same diamond. This is also confirmed by the discoveries of diamondiferous peridotite and eclogite xenoliths in kimberlites where diamonds are completely enclosed in garnet or olivine. Of particular note is the constant presence of heavy hydrocarbons (rel.%), from pentane (C5H12) to hexadecane (C16H34), that are predominant in fluid inclusions in kimberlite and placer diamonds as well as in pyrope and olivine of diamondiferous peridotite xenoliths.
Pyrope inclusions in chrome spinels from kimberlites and lamproites and their significance for estimation of the paragenetic assemblage and formation depth.
Doklady Earth Sciences, Vol. 399, Oct-Nov. pp. 1074-8.
Formation and evolution of hypabyssal kimberlites from the Siberian craton: part 1 - new insights from cathodluminescence of the carbonates. Anabar and Olenek area
Journal of Asian Earth Sciences, Vol. 145, pt. B, pp. 670-678.
Solid Earth Discussions, Vol. 5, pp. 1-75. pdf * note date
Russia, Yakutia
picroilmenites
Abstract: Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modeling, shows long compositional trends for the ilmenites. These are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite minerals from stage to stage and splitting of the magmatic system in the final stages.
High-pressure (5.5-7.0 GPa) Cr-bearing Mg-rich ilmenites (group 1) reflect the conditions of high-temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1-10/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb > 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) trace the high-temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10-100)/PM with La / Ybn ~ 10-25, similar to those derived from kimberlites, with high-field-strength elements (HFSE) peaks (typical megacrysts). A series of similar patterns results from polybaric Assimilation + fractional crystallization (AFC) crystallization of protokimberlite melts which also precipitated sulfides (Pb < 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high-Cr content (group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr-rich ilmenites are typical of veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. Ilmenites which have the highest trace element contents (1000/PM) have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet-spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn-Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.
Geology of Ore Deposits, Vol. 62, 6, pp. 497-507. pdf
Russia, Yakutia
deposit Yubileinaya
Abstract: An experimental study was carried out on the dissolution of natural octahedral diamonds from the Internatsionalnaya and Yubileinaya kimberlite pipes (Yakutia) in an Fe-S melt at 4 GPa and 1450-1500°C with different sulfur contents (10-25 wt %). It was found that with an increase in sulfur content in the iron melt, the degree of diamond dissolution sharply decreases. The stationary (final) shape of diamond crystal dissolution under the achieved conditions corresponds to an octahedroid with trigonal etching layers, which is confirmed by photogoniometry. Diamonds with similar morphology are common in kimberlite pipes, especially in mantle xenoliths from kimberlites. It was concluded that diamonds with this shape did not undergo natural dissolution in a kimberlite magma, but, similar to flat-faced octahedra, were probably isolated from it in xenoliths. Therefore, the higher the content of octahedroid-shaped diamonds with trigonal layers in a deposit, the smaller the direct influence of an aggressive kimberlite magma on the diamond content.
Lithos, Vol. 406-407. doi: 10.1016/j.lithos.2021.106499 77p. Pdf
Russia
kimberlite genesis
Abstract: Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modeling, shows long compositional trends for the ilmenites. These are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite minerals from stage to stage and splitting of the magmatic system in the final stages. High-pressure (5.5–7.0 GPa) Cr-bearing Mg-rich ilmenites (group 1) reflect the conditions of high-temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1–10/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb > 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) trace the high-temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10–100)/PM with La / Ybn ~ 10–25, similar to those derived from kimberlites, with high-field-strength elements (HFSE) peaks (typical megacrysts). A series of similar patterns results from polybaric Assimilation + fractional crystallization (AFC) crystallization of protokimberlite melts which also precipitated sulfides (Pb < 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high-Cr content (group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr-rich ilmenites are typical of veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. Ilmenites which have the highest trace element contents (1000/PM) have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet–spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn–Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.
Doklady Earth Sciences, Vol. 501, pt. 1, pp. 919-924. pdf
Russia, Yakutia
cathodluminescence
Abstract: The zoning of diamonds was studied using cathodoluminescence (CL) and the chemical composition of mineral inclusions in six typical diamonds from kimberlites of Yakutia. The diamonds were ground on special equipment until inclusions with dimensions of 10-200 ?m were brought to the surface. The inclusions are characterized by a morphology reflecting the influence of the host diamonds. Multiple inclusions and intergrowths of magnesiochromite, olivine, pyrope, and phlogopite are located in both the central and peripheral zones of diamonds. In three diamonds, significant differences in the composition of magnesiochromites in different growth zones were observed, while in the other three such differences were not found. The overwhelming majority (five out of the six diamonds studied), according to the compositional features of magnesiochromite, olivine, and phlogopite, belong to the dunite-harzburgite paragenesis prevailing in diamonds from various diamond-bearing provinces of the Earth. In one of the diamonds, a lherzolite paragenesis, identified by the composition of the pyrope inclusion in magnesiochromite, was observed for the first time. The complex history of diamond growth and the variations in the chemical composition of the included minerals indicate the possibility of coexistence of syngenetic and protogenetic inclusions in the same diamond crystal.
Variations of the oxygen conditions in mantle column beneath Siberian kimberlite pipes and it's application to lithospheric structure of feeding systems.
Vladykin: VI International Workshop, held Mirny, Deep seated magmatism, its sources and plumes, pp. 125-144.
Logvinova, A., Wirth, R., Taylor, L.A., Sobolev, N.V.
Aragonite, magnesite and dolomite inclusions in Yakutian diamonds: TEM observations.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 1p. Abstract
Ashchepkov, I., Ntaflos, T., Logvinova, A., Vladykin, N., Ivanov, A., Spetsius, Z., Stegnitsky, Y., Kostrovitsky, S., Salikhov, R., Makovchuk, I., Shmarov, G., Karpenko, M., Downes, H., Madvedev, N.
Abstract: The PTX diagrams for the separate phases in Sytykanskaya (Ashchepkov et al., 2016) Dalnyaya (Ashchepkov et al., 2017), pipes shows that the PK show the relatively simple P-X trends and geotherms and shows more contrast and simple layering. The PK contain most abundant material from the root of the magma generation they are dunitic veins as the magma feeders represented by the megacrysts. New results for the Aykhal, Zarya and Komsomolskaya pipes in Alake field and Zarnitsa and Udachnaya pipes in Daldyn field show that evolution is accompanied by the developing of metasomatites and branching and veining of the wall rock peridotites . In Aykhal pipe in PK the Gar- dunites prevail, the xenoliths from the dark ABK "Rebus" contain Cr-Ti - rich garnets and ilmenites, more abundant compared with the grey carbonited breccia Nearly the same features were found for Yubileinaya pipe. The example of Komsomolskya pipes show that the ABK contain more eclogitic xenolith than PK. The developing of the magma channel shown in satellite Chukukskaya and Structurnaya pipe was followed by the separation of some parts of the magmatic feeders and crystallization of abundant Gar megacrysts near o the walls blocking the peridotites from the magma feeder. This drastically decrease diamond grade of pipes. Such blocking seems to be the common features for the latest breccias. In Zarnitsa pipe, the dark PK and ABK also contain fresh xenoliths but not only dunites but also sheared and metasomatic varieties and eclogites. Most of dark ABK in Yakutia contain the intergrowth of ilmenites with brown Ti- Cpx showing joint evolution trends. The late breccia contains completely altered peridotite xenoliths mainly of dunite- harzburgite type. The comparison of the trace elements of the coexisting minerals in megacryst show that they were derived from the protokimberlites but are not in complete equilibrium as well as other megacrystalline phases. Ilmenites show inflections of the trace element patterns of most Ilmenites but more regular for the Cpx and Garnets revealing the sub parallel patterns elevating LREE with the rising TRE. But commonly these are not continuous sequances because they developed in the pulsing moving systems like beneath Zarnitsa. The minerals from the feeders like dunites also show the inflected or S-type REE patterns. From the earlier to later phases the TRE compositions became more evolved reflecting the evolution of protokimberlites. The wall rocks also often show the interaction with the more evolved melts and sometimes "cut" spectrums due to the dissolution some phases and repeated melting events So we could suggest the joint evolution of the mantle column protokimberlites and megacrysts composition and type of kimberlites with the diamond grade. The mantle lithospheric base captured by the PK. The developing and rising protokimbelrites was followed by the crystallization of the diamonds in the gradient in FO2 zone in wall rocks due to reductions of C -bearing fluids and carbonatites (> 1 QMF) on peridotites ((< -2 -5 QMF). The most intensive reactions are near the graphite - diamond boundary where protokimberlites are breaking and where most framesites are forming.
Abstract: The microinclusions in cuboid diamonds from Ebelyakh River deposits (northeastern Siberian craton) have been investigated by FIB/TEM techniques. It was found that these microinclusions have multiphase associations, containing silicates, oxides, carbonates, halides, sulfides, graphite, and fluid phases. The bulk chemical composition of the microinclusions indicates two contrasting growth media: Mg-rich carbonatitic and Al-rich silicic. Each media has their own specific set of daughter phases. Carbonatitic microinclusions are characterized by the presence of dolomite, phlogopite, apatite, Mg, Fe-oxide, KCl, rutile, magnetite, Fe-sulfides, and hydrous fluid phases. Silicic microinclusions are composed mainly of free SiO2 phase (quartz), high-Si mica (phengite), Al-silicate (paragonite), F-apatite, Ca-carbonates enriched with Sr and Ba, Fe-sulfides, and hydrous fluid phases. These associations resulted from the cooling of diamond-forming carbonatitic and silicic fluids/melts preserved in microinclusions in cuboid diamonds during their ascent to the surface. The observed compositional variations indicate different origins and evolutions of these fluids/melts.
Abstract: Diamonds of eclogitic paragenesis are dominant in the placer deposits in the northeastern part of the Siberian Craton. Multiple inclusions and host diamonds carbon isotopes composition are consistent with a mixing model in which they result from the interaction of slab-derived melt/fluid with surrounding mantle [1,2]. A significant portion of diamonds contains black inclusions usually interpreted as graphite or sulphides. Twenty six dark inclusions from the 22 diamonds were exposed by polishing for chemical microanalysis. Inclusions were studied with SEM, TEM and EMP. Fe-C-O melt inclusions in association with with Kfsp, Ol and silicate melt inclusions were identified. Most of the inclusions are heterogeneous in composition and consist of iron carbides, iron in various oxidation states and carbon. Carbides contain impurities of Ni (0-0.6%), Sr (up to 3.4%), Cr (up to 0.8%) Si (up to 1%). Inclusions of wustite and Fe-Ti-O melt were identified in one diamond along with inclusions of Fe-C-O melt. In two cases diamond inclusions found within host diamond crystal. Diamond inclusions are surronded by a border consisting of wustite and siderite. Inclusions of Fe-C-O melt in allivial diamonds are best explained by carbonate melt-iron reaction [3].
Sobolev, N., Wirth, R., Logvinova, A.M., Pokhilenko, N.P., Kuzmin, D.V.
Retrograde phase transitions of majorite garnets included in diamonds: a case study of subcalcic Cr rich majorite pyrope from a Snap Lake diamond, Canada.
American Geological Union, Fall meeting Dec. 15-19, Eos Trans. Vol. 89, no. 53, meeting supplement, 1p. abstract
Oxygen isotope variations of garnets and clinopyroxenes in a layered Diamondiferous calcsilicate rock from Kokchetav Massif, Kazakhstan: a window into geochemical
Contributions to Mineralogy and Petrology, Vol. 162, 5, pp.1079-1092.
Shertl, H.P., Neuser, R.D., Logvinova, A.M., Wirth, R., Sobolev, N.V.
Cathodluminescence microscopy of the Kokchetav ultra high pressure calcsilicate rocks: what can we learn from silicates, carbon hosting minerals and diamond?
Russian Geology and Geophysics, Vol. 56, 1-2, pp. 100-112.
Abstract: Mantle xenoliths (>150) and concentrates from late autolithic breccia and porphyritic kimberlite from the Sytykanskaya pipe of the Alakit field (Yakutia) were analyzed by EPMA and LAM ICP methods. In P-T-X-f(O2) diagrams minerals from xenoliths show widest variations, the trends P-Fe#-CaO, f(O2) for minerals from porphyric kimberlites are more stepped than for xenocrysts from breccia. Ilmenite PTX points mark moving for protokimberlites from the lithosphere base (7.5 GPa) to pyroxenite lens (5-3.5 GPa) accompanied by Cr increase by AFC and creation of two trends P-Fe#Ol ?10-12% and 13-15%. The Opx-Gar-based mantle geotherm in Alakit field is close to 35 mW/m2 at 65 GPa and 600 °C near Moho was determined. The oxidation state for the megacrystalline ilmenites is lower for the metasomatic associations due to reduction of protokimberlites on peridotites than for uncontaminated varieties at the lithosphere base. Highly inclined linear REE patterns with deep HFSE troughs for the parental melts of clinopyroxene and garnet xenocrysts from breccia were influenced by differentiated protokimberlite. Melts for metasomatic xenoliths reveal less inclined slopes without deep troughs in spider diagrams. Garnets reveal S-shaped REE patterns. The clinopyroxenes from graphite bearing Cr-websterites show inclined and inflected in Gd spectrums with LREE variations due to AFC differentiation. Melts for garnets display less inclined patterns and Ba-Sr troughs but enrichment in Nb-Ta-U. The 40Ar/39Ar ages for micas from the Alakit mantle xenoliths for disseminated phlogopites reveal Proterozoic (1154 Ma) age of metasomatism in early Rodinia mantle. Veined glimmerites with richterite - like amphiboles mark ?1015 Ma plume event in Rodinia mantle. The ?600-550 Ma stage manifests final Rodinia break-up. The last 385 Ma metasomatism is protokimberlite-related.
Deep-seated magmatism, its sources and plumes, Proceedings of XIII International Workshop held 2014., Vol. 2014, pp. 203-232.
Russia, Yakutia
Deposit - Sytykanskaya
Abstract: The concentrate from two phases of the kimberlite (breccia and porphyritic kimberlite) and about 130 xenoliths from the Sytykanskaya pipe of the Alakit field (Yakutia) were studied by EPMA and LAM ICP methods. Reconstructions of the PTXfO2 mantle sections were made separately for the two phases. The porphyritic kimberlites and breccia show differences in the minerals although the layering and pressure interval remains the same. For the porphyritic kimberlite the trends P- Fe# - CaO in garnet, fO2 are sub-vertical while the xenocrysts from the breccia show stepped and curved trends possibly due to interaction with fluids. Minerals within xenoliths show the widest variation in all pressure intervals. PT points for the ilmenites which trace the magmatic system show splitting of the magmatic source into two levels at the pyroxenite lens (4GPa) accompanied by peridotite contamination and an increase in Cr in ilmenites. Two groups of metasomatites with Fe#Ol ~ 10-12% and 13-15% were created by the melts derived from protokimberlites and trace the mantle columns from the lithosphere base (Ilm - Gar - Cr diopside) to Moho becoming essentially pyroxenitic (Cr-diopside with Phl). The first Opx-Gar-based mantle geotherm from the Alakit field has been constructed from15 associations and is close to 35 mw/m2 in the lower part of mantle section but deviates to high temperatures in the upper part of the mantle section. The oxidation state for the protokimberlite melts determined from ilmenites is higher than for the other pipes in the Yakutian kimberlite province which probably accounts for the decrease in the diamond grade of this pipe. The geochemistry of the minerals (garnets and clinopyroxenes) from breccias, metasomatic peridotite xenoliths and pyroxenites systematically differ. Xenocrysts from the breccia were produced by the most differentiated melts and enriched protokimberlite or carbonatite; they show highly inclined nearly linear REE patterns and deep troughs of HFSE. Minerals of the metasomatic xenoliths are less inclined with lower La/Cen ratios and without troughs in spider diagrams. The garnets often show S-shaped patterns. Garnets from the Cr websterites show round REE patterns and deep troughs in Ba-Sr but enrichment in Nb-Ta-U. The clinopyroxenes reveal the inclined and inflected on Gd spectrums with variations in LREE due to AFC differentiation. The 40Ar-39Ar ages for micas from the Alakit field reveal three intervals for the metasomatism. The first (1154 Ma) relates to dispersed phlogopites found throughout the mantle column, and probably corresponds to the continental arc stage in the early stage of Rodinia. Veined highly alkaline and Ti-rich veins with richterite ~1015 Ma corresponds to the plume event within the Rodinia mantle. The ~600-550 Ma stage marks the final Rodinia break-up. The last one near 385 Ma is protokimberlite related.
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Mantle
NAMs Nominally Anhydrous Minerals
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Low water contents in diamond mineral inclusions: proto-genetic origin in a dry cratonic lithosphere.
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Russia, Africa
Kaapvaal and Siberian SCLMs
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Abstract: Homogeneity of a peridotitic garnet inclusion in diamond demonstrating excess in Si concentration (i.e. presence of majorite component) was investigated by TEM using FIB prepared foils. The host diamond is a low-nitrogen brown stone, which can be related to type IIa with features of strong plastic deformation. The studied sample is represented by Ca-poor Cr-pyrope of harzburgitic (H) paragenesis from Snap Lake dyke, Canada The garnet had been previously reported to contain Si = 3.16 apfu. The revised examination of the sample, resulted in detection of extremely fine-grained symplectite consisting of low Ca-orthopyroxene, clinopyroxene, Cr-spinel and coesite completely located and isolated in the inner part of the garnet crystal, which forms a sharp interface with the surrounding homogeneous garnet. XRD study confirmed the presence of the minerals constituting the symplectite. EPMA showed an identical bulk chemistry of the nanometer-sized symplectite and garnet. Further polishing of the garnet inclusion on the same surface with diamond removed the symplectite, which possibly was present as a thin lens within garnet. The remaining garnet is completely homogeneous as checked by two profiles, and contains unusually high Ni (118.2 ppm) and depleted REE patterns. Estimated PT formation conditions of this garnet are 10.8 GPa and 1450 °C within asthenosphere. Symplectite testifies partial retrograde isochemical phase transformation of the examined garnet which is suggested to be caused by decompression along with plastic deformation of diamond within the coesite stability field at T > 1000 °C and depth no less than 100 km. Because previously published studies of rare majoritic garnets composition were performed by EPMA only, it is possible that the traces of partial phase transformation (symplectite formation) could have been overlooked without additional XRD and/or TEM/AEM studies.
Abstract: Mineral data from Yakutian kimberlites allow reconstruction of the history of lithospheric mantle. Differences occur in compositions of mantle pyropes and clinopyroxenes from large kimberlite pipes in the Alakit and Daldyn fields. In the Alakit field, Cr-diopsides are alkaline, and Stykanskaya and some other pipes contain more sub-calcic pyropes and dunitic-type diamond inclusions, while in the Daldyn field harzburgitic pyropes are frequent. The eclogitic diamond inclusions in the Alakit field are sharply divided in types and conditions, while in the Daldyn field they show varying compositions and often continuous Pressure-Temperature (P-T) ranges with increasing Fe# with decreasing pressures. In Alakit, Cr-pargasites to richterites were found in all pipes, while in Daldyn, pargasites are rare Dalnyaya and Zarnitsa pipes. Cr-diopsides from the Alakit region show higher levels of light Rare Earth Elements (LREE) and stronger REE-slopes, and enrichment in light Rare Earth Elements (LREE), sometimes Th-U, and small troughs in Nb-Ta-Zr. In the Daldyn field, the High Field Strength Elements HFSE troughs are more common in clinopyroxenes with low REE abundances, while those from sheared and refertilized peridotites have smooth patterns. Garnets from Alakit show HREE minima, but those from Daldyn often have a trough at Y and high U and Pb. PTXfO2 diagrams from both regions show similarities, suggesting similar layering and structures. The degree of metasomatism is often higher for pipes which show dispersion in P-Fe# trends for garnets. In the mantle beneath Udachnaya and Aykhal, pipes show 6-7 linear arrays of P-Fe# in the lower part of the mantle section at 7.5-3.0 GPa, probably reflecting primary subduction horizons. Beneath the Sytykanskaya pipe, there are several horizons with opposite inclinations which reflect metasomatic processes. The high dispersion of the P-Fe# trend indicating widespread metasomatism is associated with decreased diamond grades. Possible explanation of the differences in mineralogy and geochemistry of the mantle sections may relate to their tectonic positions during growth of the lithospheric keel. Enrichment in volatiles and alkalis possibly corresponds to interaction with subduction-related fluids and melts in the craton margins. Incorporation of island arc peridotites from an eroded arc is a possible scenario.
Abstract: New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites, pyroxenites and eclogites, and also garnet barometer for eclogites and peridotites were developed. They were checked using large experimental data sets for eclogitic (?530) and peridotitic systems (>650). The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermakite method produced by Nimis and Taylor (2000). Cpx barometer was transformed by the substitution of major multiplier for KD by the equations dependent from Al-Na-Fe. Obtained equation in combination with the thermometer of Nimis and Taylor (2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites. These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks. They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals. They are useful for PT estimates for diamond inclusions. The positions of eclogite groups in mantle sections are similar to those determined with new Gar-Cpx barometer produced by C. Beyer et al. (2015). The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle (SCLM) at 3-4 GPa marking pyroxenite eclogites layer. Ca-rich eclogites and especially grospydites in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts. The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5-6 GPa. Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch. The Mg- and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums.
Abstract: Mineralogical studies and U-Pb dating have been carried out on rutile included in peridotitic and eclogitic garnets from the Internatsionalnaya pipe, Mirny field, Siberian craton. We also describe a unique peridotitic paragenesis (rutile + forsterite + enstatite + Cr-diopside + Cr-pyrope) preserved in diamond from the Mir pipe, Mirny field. Compositions of rutile from the heavy mineral concentrates of the Internatsionalnaya pipe and rutile inclusions in crustal almandine-rich garnets from the Mayskaya pipe (Nakyn field), as well as from a range of different lithologies, are presented for comparison. Rutile from cratonic mantle peridotites shows characteristic enrichment in Cr, in contrast to lower-Cr rutile from crustal rocks and off-craton mantle. Rutile with Cr2O3 > 1.7 wt% is commonly derived from cratonic mantle, while rutiles with lower Cr2O3 may be both of cratonic and off-cratonic origin. New analytical developments and availability of standards have made rutile accessible to in situ U-Pb dating by laser ablation ICP-MS. A U-Pb age of 369 ± 10 Ma for 9 rutile grains in 6 garnets from the Internatsionalnaya pipe is consistent with the accepted eruption age of the pipe (360 Ma). The equilibrium temperatures of pyropes with rutile inclusions calculated using Ni-in-Gar thermometer range between ~ 725 and 1030 °C, corresponding to a depth range of ca ~ 100-165 km. At the time of entrainment in the kimberlite, garnets with Cr-rich rutile inclusions resided at temperatures well above the closure temperature for Pb in rutile, and thus U-Pb ages on mantle-derived rutile most likely record the emplacement age of the kimberlites. The synthesis of distinctive rutile compositions and U-Pb dating opens new perspectives for using rutile in diamond exploration in cratonic areas.
Abstract: The results of integrated studies of inclusion-containing diamonds from kimberlites of the Snap Lake dike complex (Canada) are presented. Features of the morphology, defect–impurity composition, and internal structure of the diamonds were determined by optic and scanning microscopy. The chemical composition of crystalline inclusions (olivine, garnet, and pyroxene) in diamonds was studied using a microanalyzer with an electronic probe. The inclusions of ultramafic paragenesis in the diamond (87%) are predominant. Carbonates, sulfide and hydrated silicate phases were found only in multiphase microinclusions. The large phlogopite inclusion studied was similar in composition to earlier studied nanosize inclusions of high-silica mica in diamonds from Snap Lake kimberlites. Revealed features of studied diamonds and presence of high-silica mica suggest that diamonds from Snap Lake have formed as the result of interaction between enriched in volatile and titanium high-potassium carbonate–silicate melts and peridotitic substrate at the base of thick lithospheric mantle.
Abstract: The nitrogen concentration and aggregation form reflect the conditions of diamond formation and diamond evolution in primary source [1]. FTIR measurements were made on 40 colorless or slightly greenish octahedral diamonds from Snap Lake kimberlite dyke system. Studied diamonds differ in nitrogen content, distribution and aggregation degree. The total nitrogen content in different diamond zones is up to 1600 ppm. Diamonds have been classified into two groups on the basis of nitrogen aggregation degree in them. Group 1 includes poorly-aggregated-nitrogen diamonds. We suggest that such diamonds belong to the same generation such as cubic diamonds from Snap Lake [2]. The low degree of nitrogen aggregation in diamonds is indicative of short mantle residence and suggests that they crystallized shortly before kimberlite eruption. Diamonds of Group 2 are characterized by high nitrogen aggregation degree (up to 98.6%). Group 2 includes diamonds either with uniform nitrogen distribution throughout the crystal volume or with a predominance of Bdefect in the center. Inhomogeneity in nitrogen distribution from the center to the edge of the octahedral crystals indicates, at least, about the two, or even more growth stages of a part of the studied diamonds. High nitrogen aggregation degree according to “annealing” model is evidence of diamond staying in the high temperature region or of their residence in the mantle conditions. Results obtained support that significant part of octahedral diamonds from Snap Lake may have formed at the base of a thick lithospheric mantle at depth below 300 km [3].
Abstract: Is there the existence of a water-rich zone in the mantle, currently one of the most discussed problem in mantle petrology? There are recent studies of low-water content in nominally anhydrous minerals in diamonds [1] and the chemistry of exceptionally rare phlogopite inclusions coexisting with peridotitic and eclogitic minerals in kimberlite-hosted diamonds [2]. Previous studies have shown that some rapidly formed diamonds reflect the composition of the environment in which they formed [3]. The minerals trapped during nucleation stage remain shielded from any changing conditions during further diamond growth or later mantle metasomatism. Thus, the analysis of diamond microinclusions is a major tool for the direct study of mantle high-density fluids (HDFs) from which the diamonds have precipitated [4]. Using transmission electron microscopy (TEM) techniques, we have investigated hydrous silicates inside nanometerscale, polyphased unclusions, especially in dark cloudy alluvial and kimberlite diamonds. Clinohumite, phlogopite, and phengite were detected. Hydrous silicate phases are accompanied by Ba-Sr-Ca -Fe-Mg carbonates, in addition to sulfides, oxides (magnetite, rutile, ilmenite), F-apatite, KCl, graphite, and fluid bubbles. A contrast occurs between clinohumite associated with phlogopite, F-apatite and highMg carbonates, but phengite, accompanied by a Al, Kbearing, unidentified silicate. These inclusions reflect the composition of fluid from which the host diamond crystallized. The mica composition, in most cases, has excess Si, similar to the high-silica mica identified within diamond microinclusions from Diavik [5]. The fluid-bearing carbonatitic-silicic diamonds grew in water-rich environments with extremely high K-activity, compared to most diamonds, which grew only within limited zones in the Earth’s mantle.
Doklady Earth Sciences, Vol. 476, 2, pp. 1155-1158.
Russia, Yakutia
deposit - Udachnaya
Abstract: The crystallographic orientation of three diamonds and 19 olivine inclusions from Udachnaya kimberlite pipe was studied using monocrystal X-ray diffractometry. No epitaxial olivine inclusions were found.
Abstract: New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites, pyroxenites and eclogites, and also garnet barometer for eclogites and peridotites were developed. They were checked using large experimental data sets for eclogitic (?530) and peridotitic systems (>650). The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermakite method produced by Nimis and Taylor (2000). Cpx barometer was transformed by the substitution of major multiplier for KD by the equations dependent from Al-Na-Fe. Obtained equation in combination with the thermometer of Nimis and Taylor (2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites. These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks. They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals. They are useful for PT estimates for diamond inclusions. The positions of eclogite groups in mantle sections are similar to those determined with new Gar–Cpx barometer produced by C. Beyer et al. (2015). The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle (SCLM) at 3–4 GPa marking pyroxenite eclogites layer. Ca-rich eclogites and especially grospydites in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts. The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5–6 GPa. Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch. The Mg- and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums.
Abstract: Pyrope-almandine garnets (Mg# = 28.3-44.9, Ca# = 15.5-21.3) from a heavy mineral concentrate of diamondiferous kimberlites of the largest diamond deposit, the Yubileinaya pipe, along with kimberlite- like rocks and diamondiferous volcano-sediments of the Laptev Sea coast, have been found to contain polymineral, predominantly acicular inclusions, composed of aggregates of shrilankite (Ti2ZrO6), rutile, ilmenite, clinopyroxene, and apatite. The presence of shrilankite as an inclusion in garnets from assumed garnet-pyroxene rocks of the lower crust, lifted up by diamond-bearing kimberlite, allows it to be considered as an indicator mineral of kimberlite, which expands the possibilities when searching for kimberlite in the Arctic.
Abstract: Pyrope-almandine garnets (Mg# = 28.3-44.9, Ca# = 15.5-21.3) from a heavy mineral concentrate of diamondiferous kimberlites of the largest diamond deposit, the Yubileinaya pipe, along with kimberlite- like rocks and diamondiferous volcano-sediments of the Laptev Sea coast, have been found to contain polymineral, predominantly acicular inclusions, composed of aggregates of shrilankite (Ti2ZrO6), rutile, ilmenite, clinopyroxene, and apatite. The presence of shrilankite as an inclusion in garnets from assumed garnet-pyroxene rocks of the lower crust, lifted up by diamond-bearing kimberlite, allows it to be considered as an indicator mineral of kimberlite, which expands the possibilities when searching for kimberlite in the Arctic.
Abstract: The healed internal conjugated cleavages at the core of Aykhal octahedral diamond sample AH2 were decorated with {111}dia-facetted ultrapotassic fluid/melt inclusion pockets containing nanosized graphite, phlogopite and olivine (Fo92) inclusions. These olivines are either rounded in pockets with ample fluid, or facetted by the {111}dia mold in the pockets with a fluid film. Transmission electron microscopy revealed two distinct crystallographic characteristics of olivine inclusions: (1) pronounced crystallographic texture of olivines grouped in specific diamond domain, and (2) frequent parallelism or sub-parallelism of specific low-energy faces of the two phases, mainly (010)ol, {120}ol, (001)ol and {111}dia, {110}dia, {100}dia in the order of decreasing preference, forming prominent (010)ol/{111}dia, (010)ol/{110}dia, (001)ol/{110}dia, {120}ol/{111}dia, and {120}ol/{110}dia low-energy phase boundaries with thin liquid film of 1-2?nm in between. These findings not only testify to the extremely low adhesion energies of olivine-diamond boundary pairs, but also imply that, in the presence of a fluid phase, the interfacial energetics and the energetically favored crystallographic orientations of olivine inclusions in diamond can be controlled simply by the settlement/attachment of low-energy facets of olivine crystals precipitating from the parental fluid upon the low-energy {111}dia or {110}dia surfaces of diamond. Such interfacial energetics control and the resultant low-energy boundary pairs are characteristically distinct from the common topotaxy or epitaxy between oxide/silicate mineral pairs, but are in a sense like the Van der Waals heteroepitaxy in artificial systems.
Abstract: Monocrystalline type Ia diamonds with octahedral growth morphology prevail among lithospheric diamonds, including precious stones. Unlike less common ‘fibrous’ diamonds that grew from alkali-rich carbonate-bearing melts and fluids, the growth medium of ‘monocrystalline’ type Ia diamonds remains debatable. Here we report the first finding of an optically visible (~30??m in size) carbonate inclusion in the center of a gem type Ia octahedral diamond from the Sytykanskaya kimberlite pipe, Yakutia. We found that the inclusion consists of submicron size carbonate phases represented by K2Ca(CO3)2 bütschliite (~15?vol%), Na2Mg(CO3)2 eitelite (~5?vol%), and dolomite (~80?vol%). Although neither bütschliite nor eitelite can coexist with dolomite under mantle P-T conditions, these phases readily appear all together in the quenched products of carbonatite melt under mantle pressures. Thus, at the moment of capture, the inclusion material was a carbonatite melt with the following composition 10(K0.75Na0.25)2CO3?90(Ca0.57Mg0.43)CO3. The content of alkali carbonates at the level of 10?mol% indicates that the melt was formed at a temperature of ?1300?°C. The high K/Na and Ca/(Ca?+?Mg) ratios in this melt indicate its derivation by partial melting of recycled marine sediments (pelites). Considering an age of the last subduction event beneath the Siberian craton, our new finding implies that subducting slabs drag carbonated material of the continental crust beneath ancient cratons, where it experiences partial melting to form a potassic dolomitic melt responsible for the formation of most diamonds, since the Late Archean.
Nature Scientific Reports, doi.org/10.1038/ s41598-019-46556-3 8p. Pdf
Global
diamond morphology, impact craters
Abstract: Diamond is a material of immense technological importance and an ancient signifier for wealth and societal status. In geology, diamond forms as part of the deep carbon cycle and typically displays a highly ordered cubic crystal structure. Impact diamonds, however, often exhibit structural disorder in the form of complex combinations of cubic and hexagonal stacking motifs. The structural characterization of such diamonds remains a challenge. Here, impact diamonds from the Popigai crater were characterized with a range of techniques. Using the MCDIFFaX approach for analysing X-ray diffraction data, hexagonality indices up to 40% were found. The effects of increasing amounts of hexagonal stacking on the Raman spectra of diamond were investigated computationally and found to be in excellent agreement with trends in the experimental spectra. Electron microscopy revealed nanoscale twinning within the cubic diamond structure. Our analyses lead us to propose a systematic protocol for assigning specific hexagonality attributes to the mineral designated as lonsdaleite among natural and synthetic samples.
Abstract: Monocrystalline type Ia diamonds with octahedral growth morphology prevail among lithospheric diamonds, including precious stones. Unlike less common ‘fibrous’ diamonds that grew from alkali-rich carbonate-bearing melts and fluids, the growth medium of ‘monocrystalline’ type Ia diamonds remains debatable. Here we report the first finding of an optically visible (~30??m in size) carbonate inclusion in the center of a gem type Ia octahedral diamond from the Sytykanskaya kimberlite pipe, Yakutia. We found that the inclusion consists of submicron size carbonate phases represented by K2Ca(CO3)2 bütschliite (~15?vol%), Na2Mg(CO3)2 eitelite (~5?vol%), and dolomite (~80?vol%). Although neither bütschliite nor eitelite can coexist with dolomite under mantle P-T conditions, these phases readily appear all together in the quenched products of carbonatite melt under mantle pressures. Thus, at the moment of capture, the inclusion material was a carbonatite melt with the following composition 10(K0.75Na0.25)2CO3?90(Ca0.57Mg0.43)CO3. The content of alkali carbonates at the level of 10?mol% indicates that the melt was formed at a temperature of ?1300?°C. The high K/Na and Ca/(Ca?+?Mg) ratios in this melt indicate its derivation by partial melting of recycled marine sediments (pelites). Considering an age of the last subduction event beneath the Siberian craton, our new finding implies that subducting slabs drag carbonated material of the continental crust beneath ancient cratons, where it experiences partial melting to form a potassic dolomitic melt responsible for the formation of most diamonds, since the Late Archean.
Abstract: Diamonds and their inclusions are unique fragments of deep Earth, which provide rare samples from inaccessible portions of our planet. Inclusion-free diamonds cannot provide information on depth of formation, which could be crucial to understand how the carbon cycle operated in the past. Inclusions in diamonds, which remain uncorrupted over geological times, may instead provide direct records of deep Earth’s evolution. Here, we applied elastic geothermobarometry to a diamond-magnesiochromite (mchr) host-inclusion pair from the Udachnaya kimberlite (Siberia, Russia), one of the most important sources of natural diamonds. By combining X-ray diffraction and Fourier-transform infrared spectroscopy data with a new elastic model, we obtained entrapment conditions, Ptrap?=?6.5(2) GPa and Ttrap?=?1125(32)-1140(33) °C, for the mchr inclusion. These conditions fall on a ca. 35?mW/m2 geotherm and are colder than the great majority of mantle xenoliths from similar depth in the same kimberlite. Our results indicate that cold cratonic conditions persisted for billions of years to at least 200?km in the local lithosphere. The composition of the mchr also indicates that at this depth the lithosphere was, at least locally, ultra-depleted at the time of diamond formation, as opposed to the melt-metasomatized, enriched composition of most xenoliths.
Geochimica et Cosmochimica Acta, Vol. 266, pp. 197-212.
Russia, Urals
diamond inclusions
Abstract: The compositions of mineral inclusions from a representative collection (more than 140 samples) of diamonds from the placer deposits in the Ural Mountains were studied to examine their compositional diversity. The overwhelming majority of rounded octahedral and dodecahedral stones typical of placers contain eclogitic (E-type) mineral inclusions (up to 80%) represented by garnets with Mg# 40-75 and Ca# 10-56, including the unique high calcic “grospydite” composition, omphacitic pyroxenes containing up to 65% of jadeite, as well as kyanite, coesite, sulfides, and rutile. Peridotitic (P-type) inclusions are represented by olivine, subcalcic Cr-pyrope, chrome diopside, enstatite and magnesiochromite that are typical for diamonds worldwide. Comparing the chemical composition of olivine, pyrope and magnesiochromite in diamonds of the Urals, north-east of the Siberian platform placers and Arkhangelsk province kimberlites show striking similarity. There are significant differences only in the variations of carbon isotopic composition of the diamonds from the placers of the Urals and north-east of the Siberian platform. One typical rounded dodecahedral diamond was found to contain abundant primary oriented submicrometer-sized (<3.0?µm) octahedral fluid inclusions identified by transmission electron microscopy, which caused the milky color of the entire diamond crystal. The electron energy-loss spectrum of a singular inclusion has a peak at ?405?eV, indicating that nitrogen is present. The Raman spectra with peaks at 2346-2350?cm?1 confirmed that nitrogen exists in the solid state at room temperature. This means that fossilized pressure inside fluid inclusions may be over 6.0 GPa at room temperature, so the diamond may be considered sublithospheric in origin. However, identification of unique fluid inclusions in one typical placer diamond allows one to expand the pressure limit to at least more than 8.0 GPa. The volatile components of four diamonds from the Urals placers were analyzed by gas chromatography-mass spectrometry (GC-MS). They are represented (rel. %) by hydrocarbons and their derivatives (14.8-78.4), nitrogen and nitrogenated compounds (6.2-81.7), water (2.5-5.5), carbon dioxide (2.8-12.1), and sulfonated compounds (0.01-0.96). It is shown that high-molecular-weight hydrocarbons and their derivatives, including chlorinated, nitrogenated and sulfonated compounds, appear to be stable under upper mantle P-T conditions. A conclusion is drawn that Urals placer diamonds are of kimberlitic origin and are comparable in their high E-type/P-type inclusion ratios to those from the northeastern Siberian platform and in part to diamonds of the Arkhangelsk kimberlite province.
Abstract: We demonstrate for the first time the presence of iron carbides in placer diamonds from the northeastern region of the Siberian craton. It was found that the inclusions are polycrystalline aggregates, and iron carbides filling the fissures in the diamonds, thus providing clear evidence that the iron melts were captured first. Iron carbides were identified in diamonds containing mineral inclusions of eclogitic (Kfs, sulfide) and peridotitc (olivine) paragenesis. Iron carbides with minor amounts of admixed nickel were detected in a diamond sample containing an olivine inclusion (0.3 wt% Ni), indicating that the iron melt was not in equilibrium with the mantle peridotite.The low nickel contents of the iron carbides provide the best evidence that the subducted crust is a likely source of the iron melt. Diamonds containing carbide inclusions are characterised by a relatively low nitrogen aggregation state (5-35%), which is not consistent with the high temperature of the transition zone. Therefore, we have reason to assume that the studied diamonds are from the lower regions of the lithosphere. Considering all factors, the model for the interaction of the ascending asthenospheric mantle with the subducting slab seems to be more realistic.
Abstract: Diamond is a material of immense technological importance and an ancient signifier for wealth and societal status. In geology, diamond forms as part of the deep carbon cycle and typically displays a highly ordered cubic crystal structure. Impact diamonds, however, often exhibit structural disorder in the form of complex combinations of cubic and hexagonal stacking motifs. The structural characterization of such diamonds remains a challenge. Here, impact diamonds from the Popigai crater were characterized with a range of techniques. Using the MCDIFFaX approach for analysing X-ray diffraction data, hexagonality indices up to 40% were found. The effects of increasing amounts of hexagonal stacking on the Raman spectra of diamond were investigated computationally and found to be in excellent agreement with trends in the experimental spectra. Electron microscopy revealed nanoscale twinning within the cubic diamond structure. Our analyses lead us to propose a systematic protocol for assigning specific hexagonality attributes to the mineral designated as lonsdaleite among natural and synthetic samples.
Pyrope inclusions in chrome spinels from kimberlites and lamproites and their significance for estimation of the paragenetic assemblage and formation depth.
Abstract: here are three major reservoirs for carbon in the Earth at the present time, the core, the mantle, and the continental crust. The carbon in the continental crust is mainly in carbonates (limestones, marbles, etc.). In this paper we consider the origin of the carbonates. In 1952, Harold Urey proposed that calcium silicates produced by erosion reacted with atmospheric CO2 to produce carbonates, this is now known as the Urey reaction. In this paper we first address how the Urey reaction could have scavenged a significant mass of crustal carbon from the early atmosphere. At the present time the Urey reaction controls the CO2 concentration in the atmosphere. The CO2 enters the atmosphere by volcanism and is lost to the continental crust through the Urey reaction. We address this process in some detail. We then consider the decay of the Paleocene-Eocene thermal maximum (PETM). We quantify how the Urey reaction removes an injection of CO2 into the atmosphere. A typical decay time is 100 000 yr but depends on the variable rate of the Urey reaction.
Abstract: Comprehensive research of ore-bearing differentiated intrusions of the Keulik-Kenirim structural unit, which represents a fragment of the Paleoproterozoic Pechenga-Varzuga Belt, has been carried out for the first time. The intrusions are subvolcanic by type and lenticular in shape, nearly conformable and steeply dipping. They are made up of peridotite, olivine and plagioclase pyroxenites, and gabbro metamorphosed under amphibolite facies conditions along with host basic volcanics. All intrusive rocks are enriched in TiO2 and FeO. Sulfide Cu-Ni mineralization is represented by disseminated, pocket, and stringer-disseminated types, which are clustered in the peridotitic zone as hanging units and bottom lodes. The Ni content in disseminated ore is estimated at 0.45-0.55 wt % and 1.15-3.32 wt % in ore pockets; the Cu grades are 0.17-0.20 and 0.46-5.65 wt %, respectively. To determine the age of intrusions and metamorphism of intrusive and volcanic rocks, various isotopic systems have been used: Sm-Nd (TIMS) in rock and U-Pb (SIMS SHRIMP) and Lu-Hf (LA-ICP-MS) in zircon. Conclusions on the origin of zircons are based on concentrations of trace elements including REE therein and Hf-Nd correlation in zircons and rocks. The U-Pb system of zircons reflects episodes of igneous rock formation (1982 ± 12 Ma) and their postmagmatic transformation (1938 ± 20 Ma). The last disturbance of the U-Pb isotopic system occurred 700 and 425 Ma. Xenogenic zircons dated from 3.17 to 2.65 Ga have been revealed in the studied samples. These zircons were captured by magma from the Archean basement during its ascent. The intrusions were emplaced synchronously with economic ore formation in the Pechenga ore field (1985 ± 10 Ma). The peak metamorphism of intrusive rocks under amphibolite facies conditions is recorded at 40 Ma later. The differentiated intrusions of the Keulik-Kenirim structural unit are close in their internal structure, mineralogy, and geochemistry, as well as in age and features of related Cu-Ni mineralization to ore-bearing intrusions of the Pechenga ore field, which are derivatives of ferropicritic (ferriferous) magmatism.
U-Pb and LU-HF isotopic systems in zircons from some kimberlites of the Siberian platform and from Ebeliakh alluvial deposit: age and geochemical pecularities of the source rocks.
10th. International Kimberlite Conference Held Bangalore India Feb. 6-11, Poster abstract
Abstract: Comprehensive research of ore-bearing differentiated intrusions of the Keulik-Kenirim structural unit, which represents a fragment of the Paleoproterozoic Pechenga-Varzuga Belt, has been carried out for the first time. The intrusions are subvolcanic by type and lenticular in shape, nearly conformable and steeply dipping. They are made up of peridotite, olivine and plagioclase pyroxenites, and gabbro metamorphosed under amphibolite facies conditions along with host basic volcanics. All intrusive rocks are enriched in TiO2 and FeO. Sulfide Cu-Ni mineralization is represented by disseminated, pocket, and stringer-disseminated types, which are clustered in the peridotitic zone as hanging units and bottom lodes. The Ni content in disseminated ore is estimated at 0.45-0.55 wt % and 1.15-3.32 wt % in ore pockets; the Cu grades are 0.17-0.20 and 0.46-5.65 wt %, respectively. To determine the age of intrusions and metamorphism of intrusive and volcanic rocks, various isotopic systems have been used: Sm-Nd (TIMS) in rock and U-Pb (SIMS SHRIMP) and Lu-Hf (LA-ICP-MS) in zircon. Conclusions on the origin of zircons are based on concentrations of trace elements including REE therein and Hf-Nd correlation in zircons and rocks. The U-Pb system of zircons reflects episodes of igneous rock formation (1982 ± 12 Ma) and their postmagmatic transformation (1938 ± 20 Ma). The last disturbance of the U-Pb isotopic system occurred 700 and 425 Ma. Xenogenic zircons dated from 3.17 to 2.65 Ga have been revealed in the studied samples. These zircons were captured by magma from the Archean basement during its ascent. The intrusions were emplaced synchronously with economic ore formation in the Pechenga ore field (1985 ± 10 Ma). The peak metamorphism of intrusive rocks under amphibolite facies conditions is recorded at 40 Ma later. The differentiated intrusions of the Keulik-Kenirim structural unit are close in their internal structure, mineralogy, and geochemistry, as well as in age and features of related Cu-Ni mineralization to ore-bearing intrusions of the Pechenga ore field, which are derivatives of ferropicritic (ferriferous) magmatism.
Geochimica et Cosmochimica Acta, Vol. 294. pp. 295-314. pdf
Canada
carbon
Abstract: The recent expansion of studies at hydrothermal submarine vents from investigation of abiotic methane formation to include abiotic production of organics such acetate and formate, and rising interest in processes of abiotic organic synthesis on the ocean-world moons of Saturn and Jupiter, have raised interest in potential Earth analogs for investigation of prebiotic/abiotic processes to an unprecedented level. The deep continental subsurface provides an attractive target to identify analog environments where the influence of abiotic carbon cycling may be investigated, particularly in hydrogeological isolated fracture fluids where the products of chemical water-rock reactions have been less overprinted by the biogeochemical signatures of the planet’s surficial water and carbon cycles. Here we report, for the first time, a comprehensive set of concentration measurements and isotopic signatures for acetate and formate, as well as the dissolved inorganic and organic carbon pools, for saline fracture waters naturally flowing 2.4?km below surface in 2.7 billion year-old rocks on the Canadian Shield. These geologically ancient fluids at the Kidd Creek Observatory were the focus of previous investigations of fracture fluid geochemistry, microbiology and noble gas-derived residence times. Here we show the fracture waters of Kidd Creek contain high concentrations of both acetate and formate with concentrations from 1200 to 1900?µmol/L, and 480 to 1000?µmol/L, respectively. Acetate and formate alone account for more than 50-90% of the total DOC - providing a very simple "organic soup". The unusually elevated concentrations and profoundly 13C-enriched nature of the acetate and formate suggest an important role for abiotic organic synthesis in the deep carbon cycle at this hydrogeologically isolated site. A variety of potential abiotic production reactions are discussed, including a radiolytically driven H, S and C deep cycle that could provide a mechanism for sustaining deep subsurface habitability. Scientific discoveries are beginning to reveal that organic-producing reactions that would have prevailed on Earth before the rise of life, and that may persist today on planets and moons such as Enceladus, Europa and Titan, can be accessed in some specialized geologic settings on Earth that provide valuable natural analog environments for the investigation of abiotic organic chemistry outside the laboratory.
Abstract: This article, based on ethnographic and archival research in the northeastern parklands of the Central African Republic (CAR), explores the area’s history of armed conservation. Critical scholarly accounts of armed conservation practices and projects often starkly contrast the people involved in them: there are agents of the state, or state-like actors, who seek to dominate, territorialize, and discipline, often using violence to do so, and there are local populations who are dispossessed of their lands and resources without compensation and forced into new kinds of poverty, despite rhetoric and practices meant to inculcate "local participation". The case presented here forces us to re-think these accounts. Rather than pursuing authority in the sense of expanding control over other people, people in northeastern CAR (whether putatively in favor of or opposed to conservation) are working to create and maintain access to the status of an income. To do so they engage in practices of threatening and hiding. While the means to use physical force are not equally shared, capacities to threaten and hide are widely held, and organizational and other hierarchies are unstable, making it difficult to describe any of this as a matter of domination and resistance. Expanding on literature that examines processes of green militarization (Lunstrum, 2014), the article focuses on the interactional dynamics of armed conservation to show that threats are as important as acts of physical violence, and that hiding-whether in the bush or plain sight-is critical to understanding armed conservation in an area where the state is largely seen as absent.
International Journal of Earth Sciences, Vol. 107, 7, pp. 2627-2633.
Africa, Botswana
kimberlites
Abstract: Glassy beads were found in the sand cover associated with known and suspected kimberlites on the Kalahari plateau, Botswana. Morphology and chemistry were examined by ESEM and EDAX. The polymetallic and quartzose "beads", here described for the first time and termed fulguroids, formed at very high temperatures, well in excess of those reached by the kimberlites. They solidified in free fall. We propose that they were melted in the atmosphere by lightning strikes on Kalahari overburden entrained when the kimberlites erupted.
