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SDLRC - Scientific Articles all years by Author - Cl+
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
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Abstract: Rare earth mineralization in the Bear Lodge alkaline complex (BLAC) is mainly associated with an anastomosing network of carbonatite dikes and veins, and their oxidized equivalents. Bear Lodge carbonatites are LREE-dominant, with some peripheral zones enriched in HREEs. We describe the unique chemistry and mineralogy one such peripheral zone, the Cole HFSE(+HREE) Occurrence (CHO), located ~2 km from the main carbonatite intrusions. The CHO consists of anatase, xenotime-(Y), brockite, fluorite, zircon, and K-feldspar, and contains up to 44.88% TiO2, 3.12% Nb2O5, 6.52% Y2O3, 0.80% Dy2O3, 2.63% ThO2, 6.0% P2O5, and 3.73% F. Electron microprobe analyses of xenotime-(Y) overgrowths on zircon show that oscillatory zoning is a result of variable Th and Ca content. Cheralite-type substitution, whereby Th and Ca are incorporated at the expense of REEs, is predominant over the more commonly observed thorite-type substitution in xenotime-(Y). Th/Ca-rich domains are highly beam sensitive and accompanied by high-F concentrations and low-microprobe oxide totals, suggesting cheralite-type substitution is more easily accommodated in fluorinated and hydrated/hydroxylated xenotime-(Y). Analyses of xenotime-(Y) and brockite show evidence of Embedded Image substitution for Embedded Image with patches of an undefined Ca-Th-Y-Ln phosphovanadate solid-solution composition within brockite clusters. Fluorite from the CHO is HREE-enriched with an average Y/Ho ratio of 33.2, while other generations of fluorite throughout the BLAC are LREE-enriched with Y/Ho ratios of 58.6-102.5. HFSE(+HREE) mineralization occurs at the interface between alkaline silicate intrusions and the first outward occurrence of calcareous Paleozoic sedimentary rocks, which may be local sources of P, Ti, V, Zr, and Y. U-Pb zircon ages determined by LA-ICP-MS reveal two definitive 207Pb/206Pb populations at 2.60-2.75 and 1.83-1.88 Ga, consistent with derivation from adjacent sandstones and Archean granite. Therefore, Zr and Hf are concentrated by a physical process independent of the Ti/Nb-enriched fluid composition responsible for anatase crystallization. The CHO exemplifies the extreme fluid compositions possible after protracted LREE-rich crystal fractionation and subsequent fluid exsolution in carbonatite-fluid systems. We suggest that the anatase+xenotime-(Y)+brockite+fluorite assemblage precipitated from highly fractionated, low-temperature (<200 °C), F-rich fluids temporally related to carbonatite emplacement, but after significant fractionation of ancylite and Ca-REE fluorocarbonates. Low-temperature aqueous conditions are supported by the presence of fine-grained anatase as the sole Ti-oxide mineral, concentrically banded botryoidal fluorite textures, and presumed hydration of phosphate minerals. Fluid interaction with Ca-rich lithologies is known to initiate fluorite crystallization which may cause destabilization of (HREE,Ti,Nb)-fluoride complexes and precipitation of REE+Th phosphates and Nb-anatase, a model valuable to the exploration for economic concentrations of HREEs, Ti, and Nb.
Geostandards and Geoanalytical Research, doi:10.111/ GGR.12373. 51p. Pdf
Global
spectroscopy, mineralogy
Abstract: Photo?induced force microscopy (PiFM) is a new?frontier technique that combines the advantages of atomic force microscopy with infrared spectroscopy and allows for the simultaneous acquisition of 3D topographic data with molecular chemical information at high spatial (~ 5 nm) and spectral (~ 1 cm?1) resolution at the nanoscale. This non?destructive technique is time efficient as it requires only conventional mirror?polishing and has fast mapping rates on the order of a few minutes that allow the study of dynamic processes via time series. Here, we review the method’s historical development, working principle, data acquisition, evaluation, and provide a comparison with traditional geochemical methods. We review PiFM studies in the areas of materials science, chemistry, and biology. In addition, we provide the first applications for geochemical samples including the visualisation of faint growth zonation in zircons, the identification of fluid speciation in high?pressure experimental samples, and of nanoscale organic phases in biominerals. We demonstrate that PiFM analysis is a time? and cost?efficient technique combining high?resolution surface imaging with molecular chemical information at the nanoscale and, thus, complements and expands traditional geochemical methods.
Abstract: Pangea results from the progressive amalgamation of continental blocks achieved at 320?Ma. Assuming that the ancient concept of “active” versus “passive” rifting remains pertinent as end-members of more complex processes, we show that the progressive Pangea breakup occurred through a succession of rifting episodes characterized by different tectonic evolutions. A first episode of passive continental rifting during the Upper Carboniferous and Permian led to the formation of the Neo-Tethys Ocean. Then at the beginning of Triassic times, two short episodes of active rifting associated to the Siberian and Emeishan large igneous provinces (LIPs) failed. The true disintegration of Pangea resulted from (1) a Triassic passive rifting leading to the emplacement of the central Atlantic magmatic province (200?Ma) LIP and the subsequent opening of the central Atlantic Ocean during the lowermost Jurassic and from (2) a Lower Jurassic active rifting triggered by the Karoo-Ferrar LIP (183?Ma), which led to the opening of the West Indian Ocean. The same sequence of passive then active rifting is observed during the Lower Cretaceous with, in between, the Parana-Etendeka LIP at 135?Ma. We show that the relationships between the style of rifts and their breakdown or with the type of resulting margins (as magma poor or magma dominated) are not straightforward. Finally, we discuss the respective role of mantle global warming promoted by continental agglomeration and mantle plumes in the weakening of the continental lithosphere and their roles as rifting triggers.
Abstract: Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of “plate tectonic rules” such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continent-margin and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here still predicts less subduction (ca. 90%) than on the modern earth, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet's icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.
Abstract: Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of “plate tectonic rules” such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continent-margin and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here still predicts less subduction (ca. 90%) than on the modern earth, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet's icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.
Abstract: The extraction of geological lineaments from digital satellite data is a fundamental application in remote sensing. The location of geological lineaments such as faults and dykes are of interest for a range of applications, particularly because of their association with hydrothermal mineralization. Although a wide range of applications have utilized computer vision techniques, a standard workflow for application of these techniques to mineral exploration is lacking. We present a framework for extracting geological lineaments using computer vision techniques which is a combination of edge detection and line extraction algorithms for extracting geological lineaments using optical remote sensing data. It features ancillary computer vision techniques for reducing data dimensionality, removing noise and enhancing the expression of lineaments. We test the proposed framework on Landsat 8 data of a mineral-rich portion of the Gascoyne Province in Western Australia using different dimension reduction techniques and convolutional filters. To validate the results, the extracted lineaments are compared to our manual photointerpretation and geologically mapped structures by the Geological Survey of Western Australia (GSWA). The results show that the best correlation between our extracted geological lineaments and the GSWA geological lineament map is achieved by applying a minimum noise fraction transformation and a Laplacian filter. Application of a directional filter instead shows a stronger correlation with the output of our manual photointerpretation and known sites of hydrothermal mineralization. Hence, our framework using either filter can be used for mineral prospectivity mapping in other regions where faults are exposed and observable in optical remote sensing data.
Abstract: The timing of final assembly and initiation of subsequent rifting of Rodinia is disputed. New rutile ages (913?±?9?Ma, 900?±?8?Ma and 873?±?3?Ma) and published zircon, monazite, titanite, biotite, muscovite and xenotime geochronology from the Capricorn Orogen (West Australian Craton) reveal a significant early Neoproterozoic event characterized by very low to low metamorphic grade, abundant metasomatism, minor leucogranitic and pegmatitic magmatism and NW-SE fault reactivation episodes between ca. 955 and 830?Ma. Collectively, these are termed the ca. 955-830?Ma Kuparr Tectonic Event. An age range of ca. 955-830?Ma is concomitant with the final stages of Rodinia assembly and the initial stages of its attempted breakup. Very low- to low-grade metamorphic and structural geological evidence favor a distal north-south compressional regime as the driver for hydrothermal activity during ca. 955-830?Ma. Nearby continental collision or accretion from the west (e.g., South China and/or Tarim) are ruled out. The cessation of metasomatism and magmatism in the West Australian Craton after ca. 830?Ma is concomitant with the emplacement of the Gairdner-Amata dyke swarm and associated magmatic activity in South China and Laurentia, the inception of the Adelaide Rift Complex and the deposition of the Centralian Superbasin. We posit that the cessation of hydrothermal activity in the Capricorn Orogen was caused by a tectonic switch from compressional to extensional at ca. 830?Ma. Magmatic and hydrothermal fluids were transferred away from the Capricorn Orogen to the incipient Adelaide Rift Complex, terminating metasomatism in the West Australian Craton. Ultimately, the Kuparr Tectonic Event marked the final stages of Rodinia assembly and its cessation marks the initial stages of its attempted breakup.
Abstract: The Southern Irumide Belt (SIB) of Zambia consists of predominantly Mesoproterozoic terranes that record a pervasive tectono-metamorphic overprint from collision between the Congo and Kalahari cratons in the final stages of Gondwana amalgamation. This study applies multi-method thermochronology to samples throughout southern Zambia to constrain the post-collisional, Phanerozoic thermo-tectonic evolution of the region. U-Pb apatite and 40Ar/39Ar muscovite data are used to constrain the cooling history of the region following Congo-Kalahari collision, and reveal ages of c. 550-450?Ma. Variations in the recorded cooling ages are interpreted to relate to localised post-tectonic magmatism and the proximity of analysed samples to the Congo-Kalahari suture. Apatite fission track data are used to constrain the low-temperature thermo-tectonic evolution of the region and identify mean central ages of c. 320-300, 210-200 and 120-110?Ma. Thermal modelling of these samples identifies a number of thermal events occurring in the region throughout the Phanerozoic. Carboniferous to Permian-Triassic heating is suggested to relate to the development of Karoo rift basins found throughout central Africa and constrain the timing of sedimentation in the basin. Permian to Jurassic cooling is identified in a number of samples, reflecting exhumation as a result of the Mauritanian-Variscan and Gondwanide orogenies. Subsequent cooling of the majority of samples occurs from the Cretaceous and persists until present, reflecting exhumation in response to larger scale rifting associated with the break-up of Gondwana. Each model reveals a later phase of enhanced cooling beginning at c. 30?Ma that, if not an artefact of modelling, corresponds to the development of the East African Rift System. The obtained thermochronological data elucidate the previously unconstrained thermal evolution of the SIB, and provides a refined regional framework for constraining the tectonic history of central Africa throughout the Phanerozoic.
The Canadian Mineralogist, Vol. 59, pp. 1261-1276. doi:10.3749/canmin.2000048 pdf
Canada, Ontario
deposit - Kirkland Lake
Abstract: We report new isotopic data for H2 and CH4 gases and Sr for groundwater collected from Jurassic Kirkland Lake kimberlites in northern Ontario, Canada. Groundwaters interacting with kimberlites have elevated pH (up to 12.4), are reducing (Eh as low as the H2-H2O couple), are dominated by OH? alkalinity, and have non-radiogenic (mantle) 87Sr/86Sr values (?0.706-0.707). Most significantly, the highest pH groundwaters have low Mg, high K/Mg, and are associated with abundant reduced gases (H2 ± CH4). Open system conditions favor higher dissolved inorganic carbon and CH4 production, whereas under closed system conditions low DIC, elevated OH? alkalinity, and H2 production are enhanced. Hydrogen gas is isotopically depleted (?2HH2 = ?771 to ?801‰), which, combined with ?2HH2O, yields geothermometry temperatures of serpentinization of 5-25 °C. Deviation of H2-rich groundwaters (by up to 10‰) from the meteoric water line is consistent with Rayleigh fractionation during reduction of water to H2. Methane is characterized by ?13CCH4 = ?35.8 to ?68‰ and ?2HCH4 = ?434‰. The origin of CH4 is inconclusive and there is evidence to support both biogenic and abiogenic origins. The modeled groundwater-kimberlite reactions and production of elevated concentrations of H2 gas suggest uses for diamond-production tailings, as a source of H2 for fuel cells and as a carbon sink.
Abstract: The Eocene (ca. 55–38 Ma) Bear Lodge alkaline complex in the northern Black Hills region of northeastern Wyoming (USA) is host to stockwork-style carbonatite dikes and veins with high concentrations of rare earth elements (e.g., La: 4140–21000 ppm, Ce: 9220–35800 ppm, Nd: 4800–13900 ppm). The central carbonatite dike swarm is characterized by zones of variable REE content, with peripheral zones enriched in HREE including yttrium. The principle REE-bearing phases in unoxidized carbonatite are ancylite and carbocernaite, with subordinate monazite, fluorapatite, burbankite, and Ca-REE fluorocarbonates. In oxidized carbonatite, REE are hosted primarily by Ca-REE fluorocarbonates (bastnäsite, parisite, synchysite, and mixed varieties), with lesser REE phosphates (rhabdophane and monazite), fluorapatite, and cerianite. REE abundances were substantially upgraded (e.g., La: 54500–66800 ppm, Ce: 11500–92100 ppm, Nd: 4740–31200 ppm) in carbonatite that was altered by oxidizing hydrothermal and supergene processes. Vertical, near surface increases in REE concentrations correlate with replacement of REE(±Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing ?18O values. These vertical changes demonstrate the importance of oxidizing meteoric water during the most recent modifications to the carbonatite stockwork. Scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis were used to investigate variations in mineral chemistry controlling the lateral complex-wide geochemical heterogeneity. HREE-enrichment in some peripheral zones can be attributed to an increase in the abundance of secondary REE phosphates (rhabdophane group, monazite, and fluorapatite), while HREE-enrichment in other zones is a result of HREE substitution in the otherwise LREE-selective fluorocarbonate minerals. Microprobe analyses show that HREE substitution is most pronounced in Ca-rich fluorocarbonates (parisite, synchysite, and mixed syntaxial varieties). Peripheral, late-stage HREE-enrichment is attributed to: 1) fractionation during early crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates in the central Bull Hill dike swarm, 2) REE liberated during breakdown of primary calcite and apatite with higher HREE/LREE ratios, and 3) differential transport of REE in fluids with higher PO43?/CO32? and F?/CO32? ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Supergene weathering processes were important at the stratigraphically highest peripheral REE occurrence, which consists of fine, acicular monazite, jarosite, rutile/pseudorutile, barite, and plumbopyrochlore, an assemblage mineralogically similar to carbonatite laterites in tropical regions.
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.
Precambrian Research, in press available, 43p. Pdf
United States, Iowa
geophysics - seismics
Abstract: Large amplitude aeromagnetic and gravity anomalies over a ~9500 km2 area of northeast Iowa and southeast Minnesota have been interpreted to reflect the northeast Iowa intrusive complex (NEIIC), a buried intrusive igneous complex composed of mafic/ultramafic rocks in the Yavapai Province (1.8-1.7 Ga). Hundreds of meters of Paleozoic sedimentary cover and a paucity of basement drilling have prevented detailed studies of the NEIIC. Long considered, but not proven, to be related to the ~1.1 Ga Midcontinent Rift System (MRS), the NEIIC is comparable in areal extent to the richly mineralized Duluth Complex and is similarly located near the margin of the MRS. New geochronological and geophysical data together support an MRS affinity for the NEIIC. A dike swarm imaged in aeromagnetic data is cut by intrusions of the NEIIC, and a new apatite U-Pb date of ~1170 Ma on one of the dikes thus represents a maximum age for the NEIIC. A minimum age constraint is suggested by (1) large-volume magmatism associated with the MRS that was the last such event to affect the region; and (2) the presence of reversely magnetized dikes, similar in character to MRS-related dikes elsewhere, that cut several intrusions of the NEIIC. The NEIIC is largely characterized by the presence of multiple zoned intrusions, many of which contain large volumes of mafic-ultramafic rocks and have strong geophysical similarities to alkaline intrusive complexes elsewhere, including the MRS-related Coldwell Complex of Ontario. The largest of the zoned intrusions are ~40 km in diameter and are interpreted to have thicknesses of many kilometers. Suspected faults, alignments of intrusions, and intrusive margins tend to be aligned along northwest and northeast trends that match the trends of the Belle Plaine fault zone and Fayette structural zone, both previously interpreted as pre-MRS, possibly lithospheric-scale discontinuities that may have controlled NEIIC emplacement. These interpretations collectively imply notable potential for the NEIIC to host several different types of undiscovered base metal and critical mineral deposits.
Abstract: Large amplitude aeromagnetic and gravity anomalies over a ~9500 km2 area of northeast Iowa and southeast Minnesota have been interpreted to reflect the northeast Iowa intrusive complex (NEIIC), a buried intrusive igneous complex composed of mafic/ultramafic rocks in the Yavapai Province (1.8-1.7 Ga). Hundreds of meters of Paleozoic sedimentary cover and a paucity of basement drilling have prevented detailed studies of the NEIIC. Long considered, but not proven, to be related to the ~1.1 Ga Midcontinent Rift System (MRS), the NEIIC is comparable in areal extent to the richly mineralized Duluth Complex and is similarly located near the margin of the MRS. New geochronological and geophysical data together support an MRS affinity for the NEIIC. A dike swarm imaged in aeromagnetic data is cut by intrusions of the NEIIC, and a new apatite U-Pb date of ~1170 Ma on one of the dikes thus represents a maximum age for the NEIIC. A minimum age constraint is suggested by (1) large-volume magmatism associated with the MRS that was the last such event to affect the region; and (2) the presence of reversely magnetized dikes, similar in character to MRS-related dikes elsewhere, that cut several intrusions of the NEIIC. The NEIIC is largely characterized by the presence of multiple zoned intrusions, many of which contain large volumes of mafic-ultramafic rocks and have strong geophysical similarities to alkaline intrusive complexes elsewhere, including the MRS-related Coldwell Complex of Ontario. The largest of the zoned intrusions are ~40 km in diameter and are interpreted to have thicknesses of many kilometers. Suspected faults, alignments of intrusions, and intrusive margins tend to be aligned along northwest and northeast trends that match the trends of the Belle Plaine fault zone and Fayette structural zone, both previously interpreted as pre-MRS, possibly lithospheric-scale discontinuities that may have controlled NEIIC emplacement. These interpretations collectively imply notable potential for the NEIIC to host several different types of undiscovered base metal and critical mineral deposits.
Earth and Planetary Science Letters, Vol. 467, pp. 89-98.
Mantle
melting
Abstract: We present the results of thermo-mechanical modelling of extension and breakup of a heterogeneous continental lithosphere, subjected to plume impingement in presence of intraplate stress field. We incorporate partial melting of the extending lithosphere, underlying upper mantle and plume, caused by pressure-temperature variations during the thermo-mechanical evolution of the conjugate passive margin system. Effects of melting included in the model account for thermal effects, causing viscosity reduction due to host rock heating, and mechanical effects, due to cohesion loss. Our study provides better understanding on how presence of melts can influence the evolution of rifting. Here we focus particularly on the role of melting for the temporal and spatial evolution of passive margin geometry and rift migration. Depending on the lithospheric structure, melt presence may have a significant impact on the characteristics of areas affected by lithospheric extension. Pre-existing lithosphere heterogeneities determine the location of initial breakup, but in presence of plumes the subsequent evolution is more difficult to predict. For small distances between plume and area of initial rifting, the development of symmetric passive margins is favored, whereas increasing the distance promotes asymmetry. For a plume-rifting distance large enough to prevent interaction, the effect of plumes on the overlying lithosphere is negligible and the rift persists at the location of the initial lithospheric weakness. When the melt effect is included, the development of asymmetric passive continental margins is fostered. In this case, melt-induced lithospheric weakening may be strong enough to cause rift jumps toward the plume location.
Abstract: We constructed a thermomechanical model to examine the changes in rheology caused by the periodic intrusion of basaltic dykes in a two-layered continental crust. Dyke intrusion can locally change the mineralogical composition of the crust in space and time as a result of temperature-induced metamorphism. In our models we paid particular attention to determine how different mineral assemblages and reaction kinetics during metamorphism impact on the thermomechanical behavior of the crust, in terms of differential stress values. We investigated several lithologies characteristic for intracontinental crust: (1) a quartz-feldspathic crust (QF), (2) a crust with a mineralogical assemblage resembling the average chemical composition occurring in literature (CC), and (3) a micaschist crust (MS). Our model shows that temperature profiles are weakly influenced by metamorphism, with negligible variations in the T-t paths. The results indicate that intrusion-induced changes in the crustal rheology are strongly dependent on mineralogical assemblage variation. The strength of a dyke aureole in the upper crust increases during dyke emplacement, which may cause migration of later dykes and influence the dyke spacing. In contrast, in the lower crust the strength of a dyke aureole decreases during dyke emplacement. Fast kinetics results in a ductile lower crust in proximity of the dykes, whereas slower kinetics leads to the formation of partial melts and subsequent switch from ductile to brittle behavior. Lithology exerts a dominant role on the quantity of melt produced, with higher volume percentages occurring in the MS case study. Produced melts may migrate and support acidic volcanic activity.
Gemesis laboratory created diamonds. a study of the jewelry quality yellow synthetic diamonds being grown on a commercial scale by Genesis Corp. Sarasota Florida
The Geology of the Southern Part of Homa Mountain Carbonatite Complex, Western Kenya, with Particular Reference to the Petrology of Alkaline Silicate, Metasomatic and Melilite Bearing Suites.
Geothermal Studies in the San Juan Basin and the Four Corners Area of the Colorado Plateau. Ii Steady State Models of The Thermal Source San Juan Volcanic Field.
How can I speak up effectively on behalf of the mining industry. Focus on facts, statements ( not author). Stay on topic. Use Wikipedia (refs are checked in)
Metasomatism of the Pan-African lithospheric mantle beneath the Damara Belt, Namibia, by the Tristan mantle plume: geochemical evidence from mantle xenoliths.
Contributions to Mineralogy and Petrology, Vol. 168, pp. 1046-
Abstract: Xenoliths of mantle peridotite have been sampled from four kimberlite intrusions, Melton Wold, Hebron, Uintjiesberg and Markt, emplaced through the Mesoproterozoic Namaqua-Natal Belt, along the southern border of the Kaapvaal Craton. Although many of the xenoliths are heavily altered, constituent clinopyroxene, garnet and phlogopite are fresh and have been analysed by electron microprobe for major elements and by laser ablation ICP-MS for trace elements. Primitive mantle-normalised REE abundances in clinopyroxene are all strongly LREE enriched and show a range of patterns including uniformly MREE-HREE sloped (referred to here as ‘normal’), sinusoidal and humped sinusoidal patterns. HREE abundances are extremely low (Yb = 0.3-0.06 × PM). REEN patterns in coexisting garnets show a similar range of patterns. When normalised to primitive mantle values, trace element patterns in some clinopyroxenes show strong relative depletion in Rb-Ba, Ta-Nb and Ti, with some samples also being relatively depleted in Zr-Hf. These trace element characteristics are indistinguishable from those found in clinopyroxene and garnet from peridotites from the adjacent cratonic mantle. Numerical modelling of reactive porous flow of an enriched metasomatic melt through a geochemically depleted peridotite matrix can account for the full range in observed REEN patterns. The relative depletion in Rb-Ba, Ta-Nb and Ti can be accounted for by an early crystallisation of phlogopite from the percolating melt. The relative depletion in Zr-Hf in some clinopyroxenes requires either zircon to crystallise in the proximal metasomatic assemblage, or metasomatism by a carbonatitic melt. Modelling results, together with the absence of clinopyroxene with depleted or even partially enriched REEN patterns, suggest that all clinopyroxene has been modally introduced through metasomatism into an initially highly depleted harzburgitic protolith. The range in Sr and Pb isotopic composition of the clinopyroxenes indicates regional metasomatism by melts of various compositions. The strong HREEN depletion is interpreted to reflect the effect of initial melt depletion in the early Proterozoic, with melting extending into the spinel stability field requiring an oceanic realm, and again later in the Mesoproterozoic (Namaqua Orogeny). The superimposed incompatible element enrichment indicates subsequent multiple enrichment events by rising alkaline melts similar in composition to kimberlite or ultramafic alkaline lamprophyre, possibly related to Mesozoic plume upwelling beneath the region, that reintroduced clinopyroxene into the depleted Proterozoic harzburgite protolith.
Abstract: Mantle melting, which leads to the formation of oceanic and continental crust, together with crust recycling through plate tectonics, are the primary processes that drive the chemical differentiation of the silicate Earth. The present-day mantle, as sampled by oceanic basalts, shows large chemical and isotopic variability bounded by a few end-member compositions1. Among these, the HIMU end-member (having a high U/Pb ratio, ?) has been generally considered to represent subducted/recycled basaltic oceanic crust2, 3, 4, 5. However, this concept has been challenged by recent studies of the mantle source of HIMU magmas. For example, analyses of olivine phenocrysts in HIMU lavas indicate derivation from the partial melting of peridotite, rather than from the pyroxenitic remnants of recycled oceanic basalt6. Here we report data that elucidate the source of these lavas: high-precision trace-element analyses of olivine phenocrysts point to peridotite that has been metasomatized by carbonatite fluids. Moreover, similarities in the trace-element patterns of carbonatitic melt inclusions in diamonds7 and HIMU lavas indicate that the metasomatism occurred in the subcontinental lithospheric mantle, fused to the base of the continental crust and isolated from mantle convection. Taking into account evidence from sulfur isotope data8 for Archean to early Proterozoic surface material in the deep HIMU mantle source, a multi-stage evolution is revealed for the HIMU end-member, spanning more than half of Earth’s history. Before entrainment in the convecting mantle, storage in a boundary layer, upwelling as a mantle plume and partial melting to become ocean island basalt, the HIMU source formed as Archean-early Proterozoic subduction-related carbonatite-metasomatized subcontinental lithospheric mantle.
Abstract: The raw materials of some volcanic islands are shaped by some of the same processes that form diamonds deep under the continents, according to a new study. The study asserts that material from diamond-forming regions journeys nearly to Earth's core and back up to form such islands, a process that could take two and a half billion years or longer -- more than half of Earth's entire history.
Abstract: Chemical events involving deep carbon- and water-rich fluids impact the continental lithosphere over its history. Diamonds are a by-product of such episodic fluid infiltrations, and entrapment of these fluids as microinclusions in lithospheric diamonds provide unique opportunities to investigate their nature. However, until now, direct constraints on the timing of such events have not been available. Here we report three alteration events in the southwest Kaapvaal lithosphere using U-Th-He geochronology of fluid-bearing diamonds, and constrain the upper limit of He diffusivity (to D???1.8?×?10?19 cm2 s?1), thus providing a means to directly place both upper and lower age limits on these alteration episodes. The youngest, during the Cretaceous, involved highly saline fluids, indicating a relationship with late-Mesozoic kimberlite eruptions. Remnants of two preceding events, by a Paleozoic silicic fluid and a Proterozoic carbonatitic fluid, are also encapsulated in Kaapvaal diamonds and are likely coeval with major surface tectonic events (e.g. the Damara and Namaqua-Natal orogenies).
Brovarone, A.V., Butch, C.J., Ciappa, A., Cleaves, H.J., Elmaleh, A., Faccenda, M., Feineman, M., Hermann, J., Nestola, F., Cordone, A., Giovannelli., D.
American Mineralogist, Vol. 105, pp. 1152-1160. pdf
Mantle
carbon
Abstract: Water plays a key role in shaping our planet and making life possible. Given the abundance of water on Earth's surface and in its interior, chemical reactions involving water, namely hydration and dehydration reactions, feature prominently in nature and are critical to the complex set of geochemical and biochemical reactions that make our planet unique. This paper highlights some fundamental aspects of hydration and dehydration reactions in the solid Earth, biology, and man-made materials, as well as their connections to carbon cycling on our planet.
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.
Proceedings of the National Academy of Sciences PNAS, Vol. 118, 1 doi.org/10.1073 /pnas.2015370118 9p. Pdf
Global
mineral classification
Abstract: The advancement of science depends upon developing classification protocols that systematize natural objects and phenomena into “natural kinds”—categorizations that are conjectured to represent genuine divisions in nature by virtue of playing central roles in the articulation of successful scientific theories. In the physical sciences, theoretically powerful classification systems, such as the periodic table, are typically time independent. Similarly, the standard classification of mineral species by the International Mineralogical Association’s Commission on New Minerals, Nomenclature, and Classification relies on idealized chemical composition and crystal structure, which are time-independent attributes selected on the basis of theoretical considerations from chemical theory and solid-state physics. However, when considering mineral kinds in the historical context of planetary evolution, a different, time-dependent classification scheme is warranted. We propose an "evolutionary" system of mineral classification based on recognition of the role played by minerals in the origin and development of planetary systems. Lacking a comprehensive theory of chemical evolution capable of explaining the time-dependent pattern of chemical complexification exhibited by our universe, we recommend a bootstrapping approach to mineral classification based on observations of geological field studies, astronomical observations, laboratory experiments, and analyses of natural samples and their environments. This approach holds the potential to elucidate underlying universal principles of cosmic chemical complexification.
Sequencing Reelfoot extension based on relations from southeast Missouri and interpretations of the interplay between offset preexisting zones ofweakness
Abstract: The stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world's rifted margins leads to the unraveling of an unexpected variety of structures. It provides unprecedented access to the processes occurring in the middle and lower continental crust. We present a series of so far unreleased profiles that allows the identification of various rift-related geological processes such as crustal boudinage, ductile shear and low-angle detachment faulting, and a rifting history that differs from the classical models of oceanward-dipping normal faults. The lower crust in rifted margins appears much more intensely deformed than usually represented. At the foot of both magma-rich and magma-poor margins, we observe clear indications of ductile deformation of the deep continental crust along large-scale shallow dipping shear zones. These shear zones generally show a top-to-the-continent sense of shear consistent with the activity of Continentward Dipping Normal Faults (CDNF) observed in the upper crust. This pattern is responsible for a migration of the deformation and associated sedimentation and/or volcanic activity toward the ocean. We discuss the origin of these CDNF and investigate their implications and the effect of sediment thermal blanketing on crustal rheology. In some cases, low-angle shear zones define an anastomosed pattern that delineates boudin-like structures. The maximum deformation is localized in the inter-boudin areas. The upper crust is intensely boudinaged and the highly deformed lower crust fills the inter-boudins underneath. The boudinage pattern controls the position and dip of upper crustal normal faults. We present some of the most striking examples from the margins of Uruguay, West Africa, South China Sea and Barents Sea, and discuss their implications for the time-temperature history of the margins.
Geophysical Research Letters, doi.org/10.1029/2018GL078142
Mantle
melting
Abstract: Despite support from indirect observations, the existence of a layer of carbon?rich, partially molten rock (~60 km) below oceanic crust, made possible by the presence of CO2, remains uncertain. In particular, abrupt decreases in the velocity that seismic waves propagate at depths of 40-90 and 80-180 km beneath the ocean basins remain unexplained. In this study, we test whether these seismic discontinuities can be attributed to the presence of a layer of carbon?rich melt. Melt generation occurs only where the mantle is upwelling; thus, we predict the locations of carbonate?enhanced melting using a mantle convection model and compare the resulting melt distribution with the seismic observations. We find that the shallower seismic discontinuities (at 40? to 90?km depth) are not associated with regions of predicted melting but that the deeper discontinuities (80-180 km) occur preferentially in areas of greater mantle upwelling—suggesting that these deep observations may reflect the presence of localized melt accumulation at depth. Finally, we show that carbonate melting far from mid?ocean ridges produces an additional CO2 flux previously overlooked in deep carbon cycle estimates, roughly equivalent to the flux of CO2 due to seafloor volcanism.
Bol. Mus. Nac. Hist. Paraguay, , Vol. 20, 2, pp. 188-204. pdf
South America, Paraguay
geochemistry
Abstract: After some works of Jaime Baez-Presse that quoted the presence of diamonds in Eastern Paraguay, we have perfprmed a whole sampling a study relative to the indicator mineral for diamonds. Indicator minerals are mineral species that, when appearing as transported grains in clastic sediments, indicate the presence in bedrock of a specific type of mineralization, hydrothermal alteration or lithology. Their physical and chemical characteristics, including a relatively high density (heavy minerals), facilitate their preservation and identification. The heavy minerals represent an important exploration method for detecting a variety of ore deposit types including diamond, gold, Ni-Cu, PGE, and so on.. One of the most significant events in the application of indicator mineral methods in the past was the diamond exploration. This paper provides an overview of indicator mineral methods, i.e. presence of Cr-diopside, Pyrope-rich garnet and Picroilmenite, for diamond exploration along the Eastern Paraguay river. Unfortunately the above heavy mineraks, generally associated to the diamonds, do not appear in Eastern Paraguay, excluding this Country as a potential source for the diamond as economic potential source.
Abstract: Accretion from primordial material and its subsequent differentiation into a planet with core and mantle are fundamental problems in terrestrial and solar system. Many of the questions about the processes, although well developed as model scenarios over the last few decades, are still open and much debated. In the early Earth, during its formation and differentiation into rocky mantle and iron-rich core, it is likely that silicate melts played an important part in shaping the Earth's main reservoirs as we know them today. Here, we review several recent results in a deep magma ocean scenario that give tight constraints on the early evolution of our planet. These results include the behaviour of some siderophile elements (Ni and Fe), lithophile elements (Nb and Ta) and one volatile element (Helium) during Earth's core formation. We will also discuss the melting and crystallization of an early magma ocean, and the implications on the general feature of core-mantle separation and the depth of the magma ocean. The incorporation of Fe2 + and Fe3 + in bridgmanite during magma ocean crystallization is also discussed. All the examples presented here highlight the importance of the prevailing conditions during the earliest time of Earth's history in determining the composition and dynamic history of our planet.
Abstract: Carbon isotope compositions and the distribution of nitrogen and hydrogen in diamonds from 18 eclogites from Nurbinskaya kimberlites were studied in situ in polished plates. Cathodoluminescence images show that most of the diamonds have complex growth structures with distinctive core, intermediate and rim zones. In some diamonds the cores display dissolution features, and intermediate growth zones are separated from the cores by narrow rounded oscillatory zones. At least three crystals show interrupted multistage diamond growth; variations in ?13C of 2–3‰ occur across the contacts between distinct zones. Generally, ?13C within the diamond cores varies only by 1–2‰, in rare cases up to 3.3‰. ?13C values are usually lower in the intermediate zones and drop further towards the rims by up to 3‰. High-resolution SIMS profiles show that variations in ?13C across the diamond growth zones are sharp with no evidence of diffusive relaxation. Diamonds with predominantly tangential octahedral growth have a wide range in ?13C from ? 15.2‰ up to 9.0‰ (± 0.4‰), and their nitrogen (N) contents vary between 30 and 1500 at. ppm. Six diamonds show little internal variation along the isotopic profiles with changes in ?13C of only 0.3–0.9‰ around mean values ranging from ? 6‰ to ? 3‰. Five crystals are isotopically heavy, with relatively homogeneous ?13C up to 9‰. FTIR data show markedly different N concentrations and nitrogen aggregation states between major growth zones. This implies that the diamonds in eclogitic xenoliths from Nyurbinskaya pipe grew in multiple and interrupted growth events, probably from fluids enriched in K and H. The wide variations of ?13C in the studied eclogitic diamonds and identification of their anomalously positive ?13C values, combined with the wide range of high ?18O in garnets from the diamondiferous xenoliths of the Nyurbinskaya pipe, which are mostly outside of the mantle range, suggest a crustal contribution to the parental mantle-related fluids forming diamonds in these xenoliths and indicate the complex metasomatic evolution of the lithospheric mantle beneath the Nakynsky kimberlite field.
Abstract: Lateral variations of seismic wave speeds and attenuation (dissipation of strain energy) in the Earth’s upper mantle have the potential to map key characteristics such as temperature, major-element composition, melt fraction and water content1,2,3. The inversion of these data into meaningful representations of physical properties requires a robust understanding of the micromechanical processes that affect the propagation of seismic waves2,3. Structurally bound water (hydroxyl) is believed to affect seismic properties2,3 but this has yet to be experimentally quantified. Here we present a comprehensive low-frequency forced-oscillation assessment of the seismic properties of olivine as a function of water content within the under-saturated regime that is relevant to the Earth’s interior. Our results demonstrate that wave speeds and attenuation are in fact strikingly insensitive to water content. Rather, the redox conditions imposed by the choice of metal sleeving, and the associated defect chemistry, appear to have a substantial influence on the seismic properties. These findings suggest that elevated water contents are not responsible for low-velocity or high-attenuation structures in the upper mantle. Instead, the high attenuation observed in hydrous and oxidized regions of the upper mantle (such as above subduction zones) may reflect the prevailing oxygen fugacity. In addition, these data provide no support for the hypothesis whereby a sharp lithosphere-asthenosphere boundary is explained by enhanced grain boundary sliding in the presence of water.
Cogenetic silica rich and carbonate rich melts trapped in mantle minerals in Kerguelen ultramafic xenoliths -implications for metasomatism in the oceanic upper mantl
Earth Planet. Sci. Letters, Vol. 123, No. 1-2, May pp. 167-178.
Journal of South African Institute of Mining and Metallurgy, Vol. 116, Aug. pp. 745-753.
Africa, South Africa
Deposit - Cullinan
Abstract: In 2012, Cullinan Diamond Mine began an expansion programme with the shaft deepening and development of access to the C-Cut 1 block at approximately 839 m below surface. The expansion programme is funded by a combination of bank loans and retained operating profit generated by the mine. Continuous production during deepening of the No. 1 Shaft, which is the rock hoisting shaft, was therefore critical for sustainability and efficiency as well as overall funding of the project. The deepening method, support design and verification, as well as learning outcomes pertaining to the extension of the No. 1 Shaft underneath the existing operating shaft are summarized, with emphasis on the importance of gaining some understanding of the shaft's host rock mass.
South African Institute of Mining and Metallurgy, Vol. 116, 8, pp. 745-752.
Africa, South Africa
deposit - Cullinan
Abstract: In 2012, Cullinan Diamond Mine began an expansion programme with the shaft deepening and development of access to the C-Cut 1 block at approximately 839 m below surface. The expansion programme is funded by a combination of bank loans and retained operating profit generated by the mine. Continuous production during deepening of the No. 1 Shaft, which is the rock hoisting shaft, was therefore critical for sustainability and efficiency as well as overall funding of the project. The deepening method, support design and verification, as well as learning outcomes pertaining to the extension of the No. 1 Shaft underneath the existing operating shaft are summarized, with emphasis on the importance of gaining some understanding of the shaft's host rock mass.
Earth and Planetary Science Letters, Vol. 467, pp. 89-98.
Mantle
melting
Abstract: We present the results of thermo-mechanical modelling of extension and breakup of a heterogeneous continental lithosphere, subjected to plume impingement in presence of intraplate stress field. We incorporate partial melting of the extending lithosphere, underlying upper mantle and plume, caused by pressure-temperature variations during the thermo-mechanical evolution of the conjugate passive margin system. Effects of melting included in the model account for thermal effects, causing viscosity reduction due to host rock heating, and mechanical effects, due to cohesion loss. Our study provides better understanding on how presence of melts can influence the evolution of rifting. Here we focus particularly on the role of melting for the temporal and spatial evolution of passive margin geometry and rift migration. Depending on the lithospheric structure, melt presence may have a significant impact on the characteristics of areas affected by lithospheric extension. Pre-existing lithosphere heterogeneities determine the location of initial breakup, but in presence of plumes the subsequent evolution is more difficult to predict. For small distances between plume and area of initial rifting, the development of symmetric passive margins is favored, whereas increasing the distance promotes asymmetry. For a plume-rifting distance large enough to prevent interaction, the effect of plumes on the overlying lithosphere is negligible and the rift persists at the location of the initial lithospheric weakness. When the melt effect is included, the development of asymmetric passive continental margins is fostered. In this case, melt-induced lithospheric weakening may be strong enough to cause rift jumps toward the plume location.
Abstract: Using numerical thermo?mechanical experiments we analyse the role of an active mantle plume and pre?existing lithospheric thickness differences in the structural development of the central and southern East African Rift system. The plume?lithosphere interaction model setup captures the essential features of the studied area: two cratonic bodies embedded into surrounding lithosphere of normal thickness. The results of the numerical experiments suggest that localization of rift branches in the crust is mainly defined by the initial position of the mantle plume relative to the cratons. We demonstrate that development of the Eastern branch, the Western branch and the Malawi rift can be the result of non?uniform splitting of the Kenyan plume, which has been rising underneath the southern part of the Tanzanian craton. Major features associated with Cenozoic rifting can thus be reproduced in a relatively simple model of the interaction between a single mantle plume and pre?stressed continental lithosphere with double cratonic roots.
Abstract: In the Arabian-Northern African region, interaction of the Nubian, Arabian and Eurasian plates and many small tectonic units is conspicuous. In order to better understand this interaction, we use satellite derived gravity data (retracked to the Earth’s surface) recognized now as a powerful tool for tectono-geodynamic zonation. We applied the polynomial approximation to the gravity data which indicated the presence of a large, deep ring structure in the eastern Mediterranean centered below the Island of Cyprus. Quantitative analysis of residual gravity anomaly provides an estimate of the deep anomalous body’s upper edge at a depth of about 1700 km. Computations of the residual gravity anomalies for the lower mantle also indicate presence of anomalous sources. The GPS vector pattern coinciding with the gravity trend implies counter clockwise rotation of this structure. Independent analyses of the geoid isolines map and seismic tomography data support the existence of a deep anomaly. Paleomagnetic data analysis from the surrounding regions confirms a counter clockwise rotation. Numerous petrological, mineralogical, geodynamical and tectonic data suggest a relation between this deep structure and near-surface processes. This anomaly sheds light on a number of phenomena including the Cyprus gravity anomaly, counter clockwise rotation of the Mesozoic terrane belt and asymmetry of basins along continental transform faults.
Density, temperature and composition of the North American lithosphere - new insights from a joint analysis of seismic, gravity and mineral physics data: 1. density structure of the crust and upper mantle.
Geochemistry, Geophysics, Geosystems: G3, Vol. 15, 12, pp. 4781-4807.
Abstract: We constructed a thermomechanical model to examine the changes in rheology caused by the periodic intrusion of basaltic dykes in a two-layered continental crust. Dyke intrusion can locally change the mineralogical composition of the crust in space and time as a result of temperature-induced metamorphism. In our models we paid particular attention to determine how different mineral assemblages and reaction kinetics during metamorphism impact on the thermomechanical behavior of the crust, in terms of differential stress values. We investigated several lithologies characteristic for intracontinental crust: (1) a quartz-feldspathic crust (QF), (2) a crust with a mineralogical assemblage resembling the average chemical composition occurring in literature (CC), and (3) a micaschist crust (MS). Our model shows that temperature profiles are weakly influenced by metamorphism, with negligible variations in the T-t paths. The results indicate that intrusion-induced changes in the crustal rheology are strongly dependent on mineralogical assemblage variation. The strength of a dyke aureole in the upper crust increases during dyke emplacement, which may cause migration of later dykes and influence the dyke spacing. In contrast, in the lower crust the strength of a dyke aureole decreases during dyke emplacement. Fast kinetics results in a ductile lower crust in proximity of the dykes, whereas slower kinetics leads to the formation of partial melts and subsequent switch from ductile to brittle behavior. Lithology exerts a dominant role on the quantity of melt produced, with higher volume percentages occurring in the MS case study. Produced melts may migrate and support acidic volcanic activity.
Abstract: The tectonic-geodynamic characteristics of the North African-Arabian region are complicated by interaction of numerous factors. To study this interaction, we primarily used the satellite gravity data (retracked to the Earth's surface), recognized as a powerful tool for tectonic-geodynamic zonation. The applied polynomial averaging of gravity data indicated the presence of a giant, deep quasi-ring structure in the Eastern Mediterranean, the center of which is located under the Island of Cyprus. Simultaneously, the geometrical center of the revealed structure coincides with the Earth's critical latitude of 35?. A quantitative analysis of the obtained gravitational anomaly made it possible to estimate the depth of the upper edge of the anomalous body as 1650?1700 km. The GPS vector map coinciding with the gravitational trend indicates counterclockwise rotation of this structure. Review of paleomagnetic data on the projection of the discovered structure into the earth's surface also confirms its counterclockwise rotation. The analysis of the geoid anomalies map and seismic tomography data commonly approve presence of this deep anomaly. The structural and geodynamic characteristics of the region and paleobiogeographic data are consistent with the proposed physical-geological model. Comprehensive analysis of petrological, mineralogical, and tectonic data suggests a relationship between the discovered deep structure and near-surface processes. The revealed geological deep structure sheds light on specific anomalous effects in the upper crustal layer, including the high-intensity Cyprus gravity anomaly, counterclockwise rotation of the Mesozoic terrane belt, configuration of the Sinai plate, and the asymmetry of sedimentary basins along the continental faults.
The Frimingen garnet peridotite ( central Swedish Caledonides). A good example of the characteristic PTt path of a cold mantle wedge garnet peridotite.
Philosphical Transactions of the Royal Society, doi.org/10.1098 / rsta.2018.0103 21p.
Mantle
plate tectonics
Abstract: The secular evolution of the Earth's crust is marked by a profound change in average crustal chemistry between 3.2 and 2.5?Ga. A key marker for this change is the transition from Archaean sodic granitoid intrusions of the tonalite-trondhjemite-granodiorite (TTG) series to potassic (K) granitic suites, akin (but not identical) to I-type granites that today are associated with subduction zones. It remains poorly constrained as to how and why this change was initiated and if it holds clues about the geodynamic transition from a pre-plate tectonic mode, often referred to as stagnant lid, to mobile plate tectonics. Here, we combine a series of proposed mechanisms for Archaean crustal geodynamics in a single model to explain the observed change in granitoid chemistry. Numeric modelling indicates that upper mantle convection drives crustal flow and subsidence, leading to profound diversity in lithospheric thickness with thin versus thick proto-plates. When convecting asthenospheric mantle interacts with lower lithosphere, scattered crustal drips are created. Under increasing P-T conditions, partial melting of hydrated meta-basalt within these drips produces felsic melts that intrude the overlying crust to form TTG. Dome structures, in which these melts can be preserved, are a positive diapiric expression of these negative drips. Transitional TTG with elevated K mark a second evolutionary stage, and are blends of subsided and remelted older TTG forming K-rich melts and new TTG melts. Ascending TTG-derived melts from asymmetric drips interact with the asthenospheric mantle to form hot, high-Mg sanukitoid. These melts are small in volume, predominantly underplated, and their heat triggered melting of lower crustal successions to form higher-K granites. Importantly, this evolution operates as a disseminated process in space and time over hundreds of millions of years (greater than 200?Ma) in all cratons. This focused ageing of the crust implies that compiled geochemical data can only broadly reflect geodynamic changes on a global or even craton-wide scale. The observed change in crustal chemistry does mark the lead up to but not the initiation of modern-style subduction.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.
Journal of Metamorphic Geology, doi.org/10.1111/jmg.12465
Mantle
melting
Abstract: Partial melting of continental crust and evolution of granitic magmas are inseparably linked to the availability of H2O. In the absence of a free aqueous fluid, melting takes place at relatively high temperatures by dehydration of hydrous minerals, whereas in its presence, melting temperatures are lowered, and melting need not involve hydrous minerals. With the exception of anatexis in water?saturated environments where anhydrous peritectic minerals are absent, there is no reliable indicator that clearly identifies the presence of a free aqueous fluid during anatexis. Production of Ab?rich magmas or changes in LILE ratios, such as an increase in Sr and decrease in Rb indicating increased involvement of plagioclase, are rough guidelines to the presence of aqueous fluids. Nevertheless, all of them have caveats and cannot be unequivocally applied, allowing for the persistence of a bias in the literature towards dehydration melting. Investigation of mineral equilibria modelling of three metasedimentary protoliths of the Kangaroo Island migmatites in South Australia, shows that the main indicator for the presence of small volumes of excess water under upper amphibolite to lower granulite facies conditions (660?750°C) is the melt volume produced. Melt composition, modal content or chemical composition of peritectic minerals such as cordierite, sillimanite or garnet are relatively insensitive to the presence of free water. However, the mobility of melt during open system behaviour makes it difficult to determine the melt volume produced. We therefore argue that the presence of small volumes of excess water might be much more common than so far inferred, with large impact on the buffering of crustal temperatures and fertility, and therefore rheology of the continental crust.
Journal of Metamorphic Geology, Vol. 37, 3, pp. 339-358.
Mantle
water
Abstract: Partial melting of continental crust and evolution of granitic magmas are inseparably linked to the availability of H2O. In the absence of a free aqueous fluid, melting takes place at relatively high temperatures by dehydration of hydrous minerals, whereas in its presence, melting temperatures are lowered, and melting need not involve hydrous minerals. With the exception of anatexis in water?saturated environments where anhydrous peritectic minerals are absent, there is no reliable indicator that clearly identifies the presence of a free aqueous fluid during anatexis. Production of Ab?rich magmas or changes in LILE ratios, such as an increase in Sr and decrease in Rb indicating increased involvement of plagioclase, are rough guidelines to the presence of aqueous fluids. Nevertheless, all indicators have caveats and cannot be unequivocally applied, allowing for the persistence of a bias in the literature towards dehydration melting. Investigation of mineral equilibria modelling of three metasedimentary protoliths of the Kangaroo Island migmatites in South Australia, shows that the main indicator for the presence of small volumes of excess water under upper amphibolite to lower granulite facies conditions (660-750°C) is the melt volume produced. Melt composition, modal content or chemical composition of peritectic minerals such as cordierite, sillimanite or garnet are relatively insensitive to the presence of free water. However, the mobility of melt during open system behaviour makes it difficult to determine the melt volume produced. We therefore argue that the presence of small volumes of excess water might be much more common than so far inferred, with large impact on the buffering of crustal temperatures and fertility, and therefore rheology of the continental crust.
Earth and Planetary Science letters, Vol. 525, 115755, 12p.
Mantle
plate tectonics
Abstract: The processes that operated on the early Earth and the tectonic regimes in which it was shaped are poorly constrained, reflecting the highly fragmentary rock record and uncertainty in geodynamic conditions. Most models of early Earth geodynamics invoke a poorly mobile lid regime, involving little or episodic movement of the lithosphere, above a convecting mantle. However, such a regime does not reconcile with the record of Archean tectonics, which displays contrasting environments associated with either non-plate tectonics or plate tectonics. Here, we propose a regime for the early Earth in which progressive melt extraction at sites of divergence led to the formation of large portions of stiffer lithospheric lid, called proto-plates. These proto-plates enabled stress propagation to be focussed at their margins, which were then the locus for extension as oppose to shortening, under-thrusting and thickening to form adjoining proto-cratons. We test this hypothesis embedding lithospheric stiffening during melt extraction in thermo-mechanical models of mantle convection, under conditions that prevailed in the Archean. We demonstrate the emergence of migrating, rigid proto-plates in which regions of prolonged focused compression coexist with remnants of the stagnant lid, thereby reproducing the widespread dichotomy proposed for the Archean tectonic record. These diverse tectonic modes coexist in a single regime that is viable since the Hadean and lasted until the transition to modern plate tectonics.
Abstract: Alkaline igneous rocks host many global high-field-strength element (HFSE) and rare-earth element (REE) deposits. While HFSEs are commonly assumed to be immobile in hydrothermal systems, transport by late-stage hydrothermal fluids associated with alkaline magmas is reported. However, the magnitude of the flux and the conditions are poorly constrained and yet essential to understanding the formation of REE-HFSE ores. We examined the alteration of country rocks (“fenitization”) accompanying the emplacement of a syenite magma at Illerfissalik in Greenland, through analysis of changes in rock chemistry, mineralogy, and texture. Our novel geochemical maps show a 400-m-wide intrusion aureole, within which we observed typically tenfold increases in the concentrations of many elements, including HFSEs. Textures suggest both pervasive and structurally hosted fluid flow, with initial reaction occurring with the protolith's quartz cement, leading to increased permeability and enhancing chemical interaction with a mixed Ca-K-Na fenitizing fluid. We estimated the HFSE masses transferred from the syenite to the fenite by this fluid and found ~43 Mt of REEs were mobilized (~12% of the syenite-fenite system total rare-earth-oxide [TREO] budget), a mass comparable to the tonnages of some of the world's largest HFSE resources. We argue that fenite can yield crucial information about the tipping points in magma evolution because retention and/or loss of volatile-bonded alkali and HFSEs are key factors in the development of magmatic zirconosilicate-hosted HFSE ores (e.g., Kringlerne, at Ilímaussaq), or the formation of the syenite-hosted Nb-Ta-REE (Motzfeldt-type) roof-zone deposits.
An investigation of upper mantle heterogeneity beneath the Archean and Proterozoic crust of western Canada from lithoprobe controlled source seismic experiments.
Tectonophysics, Vol. 416, 1-4, April 5, pp. 187-207.
Late Pliocene lamproites from Bucak, I sparta ( southwestern Turkey): implications for mantle wedge evolution during Africa-Anatoloan plate convergence.
Journal of Asian Earth Sciencs, Vol. 29, 1, pp.160-176.
Mineral composition in cognate inclusions in Late Miocene-Early Pliocene potassic lamprophyres with affinities to lamproites from the Denizli region, Western Anatolia, Turkey: implications for uppermost mantle processes in a back arc setting.
Cobden, L., Goes, S., Ravenna, M., Styles, E., Cammarano, F., Gallagher, K., Connolly, J.
Thermochemical interpretation of 1-D seismic dat a for the lower mantle: the significance of nonadiabiatic thermal gradients and compositional heterogeneity.
Journal of Geophysical Research, Vol. 114, B 11, B11309
Physics of the Earth and Planetary Interiors, Vol. 250, pp. 46-63.
Mantle
Water
Abstract: Although water has a major influence on tectonic and other geodynamic processes, little is known about its quantity and distribution within the deep Earth. In the last few decades, laboratory experiments on nominally anhydrous minerals (NAMs) of the transition zone have shown that these minerals can contain significant amounts of water, up to 3.3 wt%. In this study, we investigate if it is possible to use seismic observations to distinguish between a hydrous and anhydrous transition zone. We perform an extensive literature search of mineral experimental data, to generate a compilation of the water storage capacities, elastic parameters and phase boundary data for potentially hydrous minerals in the transition zone, and use thermodynamic modelling to compute synthetic seismic profiles of density, VP and VS at transition zone temperatures and pressures. We find that large uncertainties on the mineral phase equilibria (ca. 2 GPa) and elastic properties produce a wide range of seismic profiles. In particular, there is a lack of data at temperatures corresponding to those along a 1300 °C adiabat or hotter, which may be expected at transition zone pressures. Comparing our hydrous transition zone models with equivalent profiles at anhydrous conditions, we see that the depths of the 410 and 660 discontinuities cannot at present be used to map the water content of the transition zone due to these uncertainties. Further, while average velocities and densities inside the transition zone clearly decrease with increasing water content, there is a near-perfect trade-off with increases in temperature. It is therefore difficult to distinguish thermal from water effects, and the conventional view of a slow and thick transition zone for water and slow and thin transition zone for high temperature should be regarded with caution. A better diagnostic for water may be given by the average velocity gradients of the transition zone, which increase with increasing water content (but decrease for increasing temperature). However the significance of this effect depends on the degree of water saturation and partitioning between the NAMs. Since seismology is better able to constrain the thickness of the transition zone than velocity gradients, our study indicates that the most useful input from future mineral physics experiments would be to better constrain the phase relations between hydrous olivine and its high-pressure polymorphs, especially at high temperatures. Additionally, the uncertainties on the mineral seismic properties could be reduced significantly if the experimentally-observable correlations between bulk and shear moduli and their corresponding pressure derivatives would be published.
Geological Society of America Special Paper, No. 514, pp. SPE514-13.
Mantle
Geophysics - seismics
Abstract: We use the fact that geoid anomalies are directly related to the local dipole moment of the density-depth distribution to help constrain density variations within the lithosphere and the associated tectonic stresses. The main challenge with this approach is isolating the upper mantle geoid contribution from the full geoid (which is dominated by sources in the lower mantle). We address this issue by using a high-pass spherical harmonic filtering of the EGM2008-WGS84 geoid to produce an "upper mantle" geoid. The tectonic implications of the upper mantle are discussed in terms of plate tectonics and intraplate stresses. We find that globally there is about a 9 meter geoid step associated with the cooling oceanic lithosphere that imparts a net force of ~2.5x1012 N/m in the form of "ridge push" - a magnitude that is consistent with 1-d models based on first-order density profiles. Furthermore, we ind a consistent 6 meter geoid step across passive a continental margin which has the net effect of educing the compressive stresses in the continents due to the ridge force. Furthermore, we use the pper mantle geoid to reevaluate the tectonic reference state which previously studies estimated using n assumption of Airy-based isostasy. Our evaluation of the upper mantle geoid confirms the near quivalence of the gravitational potential energy of continental lithosphere with an elevation of about 750 meters and the mid-ocean ridges. This result substantiates early conclusions about the tectonic reference state and further supports the prediction that continental regions are expected to be in a slightly extensional state of stress.
Abstract: Constructing palaeogeographical maps is best achieved through the integration of data from hotspotting (since the Cretaceous), palaeomagnetism (including ocean-floor magnetic anomalies since the Jurassic), and the analysis of fossils and identification of their faunal and floral provinces; as well as a host of other geological information, not least the characters of the rocks themselves. Recently developed techniques now also allow us to determine more objectively the palaeolongitude of continents from the time of Pangaea onwards, which palaeomagnetism alone does not reveal. This together with new methods to estimate true polar wander have led to hybrid mantle plate motion frames that demonstrate that TUZO and JASON, two antipodal thermochemical piles in the deep mantle, have been stable for at least 300 Ma, and where deep plumes sourcing large igneous provinces and kimberlites are mostly derived from their margins. This remarkable observation has led to the plume generation zone reconstruction method which exploits the fundamental link between surface and deep mantle processes to allow determination of palaeolongitudes, unlocking a way forward in modelling absolute plate motions prior to the assembly of Pangaea. The plume generation zone method is a novel way to derive ‘absolute’ plate motions in a mantle reference frame before Pangaea, but the technique assumes that the margins of TUZO and JASON did not move much and that Earth was a degree-2 planet, as today.
Abstract: Constructing palaeogeographical maps is best achieved through the integration of data from hotspotting (since the Cretaceous), palaeomagnetism (including ocean-floor magnetic anomalies since the Jurassic), and the analysis of fossils and identification of their faunal and floral provinces; as well as a host of other geological information, not least the characters of the rocks themselves. Recently developed techniques now also allow us to determine more objectively the palaeolongitude of continents from the time of Pangaea onwards, which palaeomagnetism alone does not reveal. This together with new methods to estimate true polar wander have led to hybrid mantle plate motion frames that demonstrate that TUZO and JASON, two antipodal thermochemical piles in the deep mantle, have been stable for at least 300 Ma, and where deep plumes sourcing large igneous provinces and kimberlites are mostly derived from their margins. This remarkable observation has led to the plume generation zone reconstruction method which exploits the fundamental link between surface and deep mantle processes to allow determination of palaeolongitudes, unlocking a way forward in modelling absolute plate motions prior to the assembly of Pangaea. The plume generation zone method is a novel way to derive ‘absolute’ plate motions in a mantle reference frame before Pangaea, but the technique assumes that the margins of TUZO and JASON did not move much and that Earth was a degree-2 planet, as today.
Mysen, B.O., Kumamoto, K., Cody, G.D., Fogel, M.L.
Solubility and solution mechanisms of C-O-H volatiles in silicate melt with variable redox conditions and melt composition at upper mantle temperatures and pressures.
Geochimica et Cosmochimica Acta, Vol. 75, 9, pp. 6183-6199.
Progress in Earth and Planetary Science, Vol. 2, 22p.
Mantle
Magmatism
Abstract: Degassing of water during the ascent of hydrous magma in a volcanic edifice produces dramatic changes in the magma density and viscosity. This can profoundly affect the dynamics of volcanic eruptions. The water exsolution history, in turn, is driven by the water solubility and solution mechanisms in the silicate melt. Previous studies pointed to dissolved water in silicate glasses and melts existing as molecules (H 2 O mol species) and hydroxyl groups, OH. These latter OH groups commonly are considered bonded to Si 4+ but may form other bonds, such as with alkali or alkaline-earth cations, for instance. Those forms of bonding influence the structure of hydrous melts in different ways and, therefore, their properties. As a result, exsolution of water from magmas may have different eruptive consequences depending on the initial bonding mechanisms of the dissolved water. However, despite their importance, the solution mechanisms of water in silicate melts are not clear. In particular, how chemical composition of melts affects water solubility and solution mechanism is not well understood. In the present experimental study, components of such information are reported via determination of how water interacts with the cationic network of alkali (Li, Na, and K) silicate quenched melts. Results from 29 Si single-pulse magic-angle spinning nuclear magnetic resonance ( 29 Si SP MAS NMR), infrared, and Raman spectroscopies show that decreasing the ionic radius of alkali metal cation in silicate melts results in decreasing fraction of water dissolved as OH groups. The nature of OH bonding also changes as the alkali ionic radius changes. Therefore, as the speciation and bonding of water controls the degree of polymerization of melts, water will have different effects on the transport properties of silicate melts depending on their chemical composition. This conclusion, in turn, may affect volcanic phenomena related to the viscous relaxation of hydrous magmas, such as for instance the fragmentation process that occurs during explosive eruptions.
Abstract: All chondrites accreted ?3.5 wt.% C in their matrices, the bulk of which was in a macromolecular solvent and acid insoluble organic material (IOM). Similar material to IOM is found in interplanetary dust particles (IDPs) and comets. The IOM accounts for almost all of the C and N in chondrites, and a significant fraction of the H. Chondrites and, to a lesser extent, comets were probably the major sources of volatiles for the Earth and the other terrestrial planets. Hence, IOM was both the major source of Earth’s volatiles and a potential source of complex prebiotic molecules. Large enrichments in D and 15N, relative to the bulk solar isotopic compositions, suggest that IOM or its precursors formed in very cold, radiation-rich environments. Whether these environments were in the interstellar medium (ISM) or the outer Solar System is unresolved. Nevertheless, the elemental and isotopic compositions and functional group chemistry of IOM provide important clues to the origin(s) of organic matter in protoplanetary disks. IOM is modified relatively easily by thermal and aqueous processes, so that it can also be used to constrain the conditions in the solar nebula prior to chondrite accretion and the conditions in the chondrite parent bodies after accretion. Here we review what is known about the abundances, compositions and physical nature of IOM in the most primitive chondrites. We also discuss how the IOM has been modified by thermal metamorphism and aqueous alteration in the chondrite parent bodies, and how these changes may be used both as petrologic indicators of the intensity of parent body processing and as tools for classification. Finally, we critically assess the various proposed mechanisms for the formation of IOM in the ISM or Solar System.
Journal of African Earth Sciences, Vol. 158, available 14p. pdf
Africa, South Africa
magmatism
Abstract: Field and sub-surface data from the Victoria West sill complex in the Karoo Large Igneous Province (ca. 180 Ma) of South Africa are used to constrain the emplacement controls of the regional-scale sill complexes in the central Karoo basin. Cross-cutting relationships point to the presence of five distinct and successively emplaced saucer-shaped sills. Growth of the sill complex was achieved through magmatic underaccretion of magma batches below earlier sills and associated uplift of the overlying strata. The magmatic underaccretion suggests that earlier sills were fully crystallized during the emplacement of later magma pulses and that the rigid (high E) dolerites, in particular, acted as stress barriers that impeded further upward propagation of steep feeder sheets. The resulting nested structure of sills-in-sills within a confined area of less than 2000 km2 also suggests the reutilization of the same or similar feeder system even after full crystallization thereof. The emplacement controls of sills in the central Karoo through stress barriers implies that sill emplacement occurred under very low deviatoric stresses or in a mildly compressional stress regime prior to the break-up of Gondwana. The swap from earlier (184-180?Ma), mainly sill complexes to later (182-174?Ma) dykes and dyke swarms is indicative of a switch in the stress field during the early stages of Gondwana break-up. We speculate that loading, thermal subsidence and lithospheric flexure associated with the emplacement of the earlier, stacked and voluminous sill complexes in the Karoo basins may have determined the formation of the large Karoo dyke swarms, particularly when coinciding with deeper crustal structures. The original and inherited basin geometry and lithospheric structure is pivotal in the development of later Karoo magmatism.
Andreoli, M.A.G., Hart, R.J., Ashwal, L.D., Coetzee, H.
Correlations between U, Th content and metamorphic grade in the Western Namaqualand Belt, South Africa: with implications for radioactive heating of the crust.
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.
Lithos, doi.org/10.1016/ j.lithos.2020.105918 67p. Pdf
Africa, South Africa
deposit - Roberts Victor
Abstract: Platinum-group elements (PGE) display a chalcophile behaviour and are largely hosted by base metal sulphide (BMS) minerals in the mantle. During partial melting of the mantle, BMS release their metal budget into the magma generated. The fertility of magma sources is a key component of the mineralisation potential of large igneous provinces (LIP) and the origin of orthomagmatic sulphide deposits hosted in cratonic mafic magmatic systems. Fertility of mantle-derived magma is therefore predicated on our understanding of the abundance of metals, such as the PGE, in the asthenospheric and lithospheric mantle. Estimations of the abundance of chalcophile elements in the upper mantle are based on observations from mantle xenoliths and BMS inclusions in diamonds. Whilst previous assessments exist for the BMS composition and chalcophile element budget of peridotitic mantle, relatively few analyses have been published for eclogitic mantle. Here, we present sulphide petrography and an extensive in situ dataset of BMS trace element compositions from Roberts Victor eclogite xenoliths (Kaapvaal Craton, South Africa). The BMS are dominated by pyrite-chalcopyrite-pentlandite (± pyrrhotite) assemblages with S/Se ratios ranging 1200 to 36,840 (with 87% of analyses having S/Se this editing is incorrect. This should read "(with 87% of analyses having S/Se < 10,000)" Please note the 100 ppm) and are characteristically enriched in Os, Ir, Ru and Rh. Nano- and micron-scale Pd-Pt antimonide, telluride and arsenide platinum-group minerals (PGM) are observed spatially associated with BMS. We suggest that the predominance of pyrite in the xenoliths reflects the process of eclogitisation and that the trace element composition of the eclogite BMS was inherited from oceanic crustal protoliths of the eclogites, introduced into the SCLM via ancient subduction during formation of the Colesberg Magnetic Lineament c. 2.9 Ga and the cratonisation of the Kaapvaal Craton. Crucially, we demonstrate that the PGE budget of eclogitic SCLM may be substantially higher than previously reported, akin to peridotitic compositions, with significant implications for the PGE fertility of cratonic mafic magmatism and metallogenesis. We quantitatively assess these implications by modelling the chalcophile geochemistry of an eclogitic melt component in parental magmas of the mafic Rustenburg Layered Suite of the Bushveld Complex.
Abstract: The Good Hope carbonatite is located adjacent to the Prairie Lake ijolite-malignite-calcite carbonatite complex in northwestern Ontario. The carbonatite is a breccia consisting of diverse calcite and dolomite carbonatites, with lesser REE-rich ferrocarbonatites, containing xenoliths of amphibole syenite, potassium feldspar+phlogopite and pyrochlore-apatite cumulates. The occurrence outcrops over an area of 500 m x 500 m and has been proven by diamond drilling to extend to a minimum depth of 650 m. Pyrochlore-apatite cumulates occur as elongated and/or irregular clasts up to 5 cm in maximum dimension. In these, pyrochlore has crystallized before apatite and occurs as euhedral crystals (0.1-1 cm; up to 5 cm) and can comprise up to ca. 25 vol % of a clast. Prismatic apatite is commonly flow-aligned and in some instances forms isoclinal folds. The apatite does not exhibit optical- or BSE-compositional zonation. However, cathodoluminescence imagery shows blue-green cores with thin (< 500 ?m) blue margins. The cores are enriched in light REE (833-941 ppm La; 1790-2200 ppm Ce; 8.2-13.6 Yb ppm; (La/Yb)CN 62-42. The pyrochlores are Na-Ca-F-pyrochlore of relatively-uniform composition with fully-occupied A-sites, and minor SrO (l-1.5 wt %) and low Ta2O5 (< 0.5 wt %). Some pyrochlores have irregular cores of resorbed Sr-bearing (6-11 wt % SrO) pyrochlore with overgrowths of Na-Ca-F-pyrochlore. Others contain inclusions of fersmite and/or columbite-(Fe). Pyrochlore also occurs as discrete crystals in calcite and dolomite hosts and represents disaggregated clasts. In accord with experimental data on the liquidus phase relationships of apatite and pyrochlore in haplocarbonatite melts the formation of apatite-pyrochlore cumulates in the initial stages of crystallization of such melts is to be expected. These cumulates were subsequently disrupted, disaggregated, and transported by pulses of later batches of carbonatite of diverse composition.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 276.
Global
diamond color
Abstract: Diamond characterization is carried out via a wide variety of gemological and chemical analyses. An important analytical tool for this purpose is spectroscopic characterization utilizing both absorption and emission measurements. The main techniques are UV-visible and infrared spectroscopy, though Raman as well as cathodoluminescence spectroscopy are also used. We have used electron paramagnetic resonance (EPR) spectroscopy to compare the properties of treated colored diamonds to the pretreated stones. The colors studied were blue, orange, yellow, green, and pink. The EPR technique determines radicals (atoms with unpaired electrons) and is very sensitive, capable of measuring concentrations as low as ~1 × 10–17 radicals/cm3. The results, shown in table 1, indicate that all the carbon radicals determined are affected by adjacent nitrogen atoms, with the spectra showing a hyperfine structure attributed to the presence of nitrogen. The highest concentration of radicals and hyperfine structures is observed in pink and orange treated diamonds. The results concerning nitrogen concentration were correlated with the infrared spectra, which determine the absorption peaks of the diamonds as well as those of the nitrogen contamination in their crystal structure.
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%.
Effect of chemistry on the stability and elasticity of the perovskite and post-perovskite phase in the MgSiO3 FeSi03 Al203 system and implications for the lowermost mantle.
Geophysical Research Letters, Vol. 32, 16, Aug. 28, L16310
Abstract: Mg2GeO4 is important as an analog for the ultrahigh-pressure behavior of Mg2SiO4, a major component of planetary interiors. In this study, we have investigated magnesium germanate to 275 GPa and over 2,000 K using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction and density functional theory (DFT) computations. The experimental results are consistent with the formation of a phase with disordered Mg and Ge, in which germanium adopts eightfold coordination with oxygen: the cubic, Th3P4-type structure. DFT computations suggest partial Mg-Ge order, resulting in a tetragonal I4¯2d structure indistinguishable from I4¯3d Th3P4 in our experiments. If applicable to silicates, the formation of this highly coordinated and intrinsically disordered phase may have important implications for the interior mineralogy of large, rocky extrasolar planets.
Abstract: The Fen Complex is a 2 km-wide subcircular intrusion composed mainly of sovite, Fe-dolomite carbonatite, damtjernite (lamprophyre) and minor alkaline rocks such as nepheline syenite and ijolite, emplaced at 580 Ma through Mesoproterozoic orthogneisses forming the Fennoscandian Shield. Previous bulk-rock isotopic study indicates that the carbonatite magma originated in the upper mantle [(87Sr/86Sr)i = 0.7029] and underwent contamination during its ascent throughout the crust. This study focuses on two deep cores (1000 m and 700 m), drilled to assess the distribution of REE mineralizations in the Fe-dolomite carbonatite. Hyperspectral data, allowing investigators to log cores objectively and quantify lithologies, were acquired using a SisuRock Gen 2 system composed of three cameras gathering data in the following wavelengths: RGB, Near-Visible Short-Wave Infrared (VN-SWIR) and Long-Wave Infrared (LWIR). In addition, every meter of the first core and 500 m of the second one were analyzed for bulk-rock geochemistry to characterize the distribution of elements. In this study, we compare the results obtained by the imaging technique with the bulk-rock data and present preliminary results of the textural variations observed in rare-earth mineralizations. Preliminary results indicate that neither of the deep bore holes reached the fenitized host-rock and that the Fe-dolomite carbonatite continues at depth. In both cores, the dominant carbonate is Fe-rich dolomite, although calcite and Fe-Mg carbonate have been observed locally. REE-minerals, composed mainly of bastnäsite, parisite/synchisite and monazite, display variable textural relationships and often occur together in clusters associated with barite and minor Fe-oxides, sulfides (pyrite ± sphalerite) and locally thorite.
Diamonds and Related Materials, in press available 36p. Pdf
Africa, Ghana
deposit - Akwatia
Abstract: Magnetic mineral inclusions, as iron oxides or sulfides, occur quite rarely in natural diamonds. Nonetheless, they represent a key tool not only to unveil the conditions of formation of host diamonds, but also to get hints about the paleointensity of the geomagnetic field present at times of the Earth's history otherwise not accessible. This possibility is related to their capability to carry a remanent magnetization dependent on their magnetic history. However, comprehensive experimental studies on magnetic inclusions in diamonds have been rarely reported so far. Here we exploit X-ray diffraction, Synchrotron-based X-ray Tomographic Microscopy and Alternating Field Magnetometry to determine the crystallographic, morphological and magnetic properties of ferrimagnetic Fe-oxides entrapped in diamonds coming from Akwatia (Ghana). We exploit the methodology to estimate the natural remanence of the inclusions, associated to the Earth's magnetic field they experienced, and to get insights on the relative time of formation between host and inclusion systems. Furthermore, from the hysteresis loops and First Order Reversal Curves we determine qualitatively the anisotropy, size and domain state configuration of the magnetic grains constituting the inclusions.
Abstract: Magnetic mineral inclusions, as iron oxides or sulfides, occur quite rarely in natural diamonds. Nonetheless, they represent a key tool not only to unveil the conditions of formation of host diamonds, but also to get hints about the paleointensity of the geomagnetic field present at times of the Earth's history otherwise not accessible. This possibility is related to their capability to carry a remanent magnetization dependent on their magnetic history. However, comprehensive experimental studies on magnetic inclusions in diamonds have been rarely reported so far. Here we exploit X-ray diffraction, Synchrotron-based X-ray Tomographic Microscopy and Alternating Field Magnetometry to determine the crystallographic, morphological and magnetic properties of ferrimagnetic Fe-oxides entrapped in diamonds coming from Akwatia (Ghana). We exploit the methodology to estimate the natural remanence of the inclusions, associated to the Earth's magnetic field they experienced, and to get insights on the relative time of formation between host and inclusion systems. Furthermore, from the hysteresis loops and First Order Reversal Curves we determine qualitatively the anisotropy, size and domain state configuration of the magnetic grains constituting the inclusions.
Abstract: Late Cretaceous global plate reorganization associated with the inception of counterclockwise rotation of Africa relative to Europe initiated in the Balkan region small-volume magmatism of diverse geochemical signature along the enigmatic Sava-Vardar Zone. We study a Late Cretaceous lamprophyric sill in Ripanj village near Belgrade to constrain this magmatic episode. The lamprophyre is characterized by high contents of Na, P, Fe and Al, and low contents of K, Ca and Mg. Its original nature (Na, K, Ca and Mg) is concealed by intense alteration (albitization of feldspar and partial chloritization of phlogopite) that erased the ultrapotassic affinity of the rocks and resulted in extremely low K/Na ratios. The recalculated chemical composition demonstrates that the rocks are ultrapotassic, with K2O and MgO > 3 wt % and K2O/Na2O > 2, and belong to the durbachite-vaugnerite series, i. e., the plutonic equivalents of minettes and kersantites. Two phlogopite concentrates gave Ar-Ar ages of 86.80 ± 0.5 Ma and 86.90 ± 0.5 Ma. Our combined elemental and Sr-Nd-Pb isotope data (87Sr/86Sr 0.70667-0.70677, 143Nd/144Nd 0.512426-0.512429, 206Pb/204Pb 18.82-19.13, 207Pb/204Pb 15.67-15.68, 208Pb/204Pb 38.92-39.19) for representative lamprophyric samples suggests magma derivation from a light rare earth elements (LREE) and K enriched, metasomatized mantle source. The content of LREE of the rocks is enriched, whereas heavy rare earth elements (HREE) is depleted. Rare earth elements (REE) of the whole rock and REE of diopside all indicate that garnet was present in their source. There are two viable and mutually-excluding geodynamic scenarios for the Late Cretaceous magmatism in the Balkans: (i) If the Sava-Vardar ocean still existed in the Late Cretaceous and was subducted under the European plate with arc volcanism along the Apuseni-Banat-Timok-Panagyurishte-Srednjogorje belt, coeval magmatism in the Sava-Vardar Zone occurred in a fore-arc setting, and may be related to ridge subduction; (ii) If the Mesozoic ocean closed already during the Upper Jurassic or Lower Cretaceous, the Late Cretaceous volcanism within the Sava-Vardar Zone represents intracontinental volcanism associated with transtensional tectonics.
Abstract: Podiform chromitites occur in mantle peridotites of the Late Triassic Puerto Nuevo Ophiolite, Baja California Sur State, Mexico. These are high-Cr chromitites [Cr# (Cr/Cr + Al atomic ratio = 0.61-0.69)] that contain a range of minor- and trace-elements and show whole-rock enrichment in IPGE (Os, Ir, Ru). That are similar to those of high-Cr ophiolitic chromitites crystallised from melts similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction-zone mantle wedges. Crystallisation of these chromitites from S-undersaturated melts is consistent with the presence of abundant inclusions of platinum-group minerals (PGM) such as laurite (RuS2)-erlichmanite (OsS2), osmium and irarsite (IrAsS) in chromite, that yield TMA ? TRD model ages peaking at ~ 325 Ma. Thirty-three xenocrystic zircons recovered from mineral concentrates of these chromitites yield ages (2263 ± 44 Ma to 278 ± 4 Ma) and Hf-O compositions [?Hf(t) = ? 18.7 to + 9.1 and 18O values < 12.4‰] that broadly match those of zircons reported in nearby exposed crustal blocks of southwestern North America. We interpret these chromitite zircons as remnants of partly digested continental crust or continent-derived sediments on oceanic crust delivered into the mantle via subduction. They were captured by the parental melts of the chromitites when the latter formed in a supra-subduction zone mantle wedge polluted with crustal material. In addition, the Puerto Nuevo chromites have clinopyroxene lamellae with preferred crystallographic orientation, which we interpret as evidence that chromitites have experienced high-temperature and ultra high-pressure conditions (< 12 GPa and ~ 1600 °C). We propose a tectonic scenario that involves the formation of chromitite in the supra-subduction zone mantle wedge underlying the Vizcaino intra-oceanic arc ca. 250 Ma ago, deep-mantle recycling, and subsequent diapiric exhumation in the intra-oceanic basin (the San Hipólito marginal sea) generated during an extensional stage of the Vizcaino intra-oceanic arc ca. 221 Ma ago. The TRD ages at ~ 325 Ma record a partial melting event in the mantle prior to the construction of the Vizcaino intra-oceanic arc, which is probably related to the Permian continental subduction, dated at ~ 311 Ma.
Abstract: Recent findings of diamonds in ophiolitic peridotites and chromitites challenge our traditional notion of Earth mantle dynamics. Models attempting to explain these findings involve incorporation of diamonds into chromite near the mantle transition zone. However, the occurrence of metastable diamonds in this context has not been considered. Here, we report for the first time in situ microdiamonds in chromite from ophiolitic chromitite pods hosted in the Tehuitzingo serpentinite (southern Mexico). Here, diamonds occur as fracture-filling inclusions along with quartz, clinochlore, serpentine, and amorphous carbon, thus indicating a secondary origin during the shallow hydration of chromitite. Chromite chemical variations across the diamond-bearing healed fractures indicate formation during the retrograde evolution of chromitite at temperatures between 670 °C and 515 °C. During this stage, diamond precipitated metastably at low pressure from reduced C-O-H fluids that infiltrated from the host peridotite at the onset of serpentinization processes. Diamond was preserved as a result of fracture healing at the same temperature interval in which the chromite alteration began. These mechanisms of diamond formation challenge the idea that the occurrence of diamond in ophiolitic rocks constitutes an unequivocal indicator of ultrahigh-pressure conditions.
Abstract: Recent findings of diamonds in ophiolitic peridotites and chromitites challenge our traditional notion of Earth mantle dynamics. Models attempting to explain these findings involve incorporation of diamonds into chromite near the mantle transition zone. However, the occurrence of metastable diamonds in this context has not been considered. Here, we report for the first time in situ microdiamonds in chromite from ophiolitic chromitite pods hosted in the Tehuitzingo serpentinite (southern Mexico). Here, diamonds occur as fracture-filling inclusions along with quartz, clinochlore, serpentine, and amorphous carbon, thus indicating a secondary origin during the shallow hydration of chromitite. Chromite chemical variations across the diamond-bearing healed fractures indicate formation during the retrograde evolution of chromitite at temperatures between 670 °C and 515 °C. During this stage, diamond precipitated metastably at low pressure from reduced C-O-H fluids that infiltrated from the host peridotite at the onset of serpentinization processes. Diamond was preserved as a result of fracture healing at the same temperature interval in which the chromite alteration began. These mechanisms of diamond formation challenge the idea that the occurrence of diamond in ophiolitic rocks constitutes an unequivocal indicator of ultrahigh-pressure conditions.
Abstract: The highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au) exist in solid solution in accessory base-metal sulfides (BMS) as well as nano-to-micron scale minerals in rocks of the subcontinental lithospheric mantle (SCLM). The latter include platinum-group minerals (PGM) and gold minerals, which may vary widely in morphology, composition and distribution. The PGM form isolated grains often associated with larger BMS hosted in residual olivine, located at interstices in between peridotite-forming minerals or more commonly in association with metasomatic minerals (pyroxenes, carbonates, phosphates) and silicate glasses in some peridotite xenoliths. The PGM found inside residual olivine are mainly Os-, Ir- and Ru-rich sulfides and alloys. In contrast, those associated with metasomatic minerals or silicate glasses of peridotite xenoliths consist of Pt, Pd, and Rh bonded with semimetals like As, Te, Bi, and Sn. Nanoscale observations on natural samples along with the results of recent experiments indicate that nucleation of PGM is mainly related with the uptake of HSE by nanoparticles, nanominerals or nanomelts at high temperature (> 900?°C) in both silicate and/or sulfide melts, regardless of the residual or metasomatic origin of their host minerals. A similar interpretation can be assumed for gold minerals. Our observations highlight that nanoscale processes play an important role on the ore-forming potential of primitive mantle-derived magmas parental to magmatic-hydrothermal deposits enriched in noble metals. The metal inventory in these magmas could be related with the physical incorporation of HSE-bearing nanoparticles or nanomelts during processes of partial melting of mantle peridotite and melt migration from the mantle to overlying continental crust.
Abstract: Diamond is commonly regarded as an indicator of ultra-high pressure conditions in Earth System Science. This canonical view is challenged by recent data and interpretations that suggest metastable growth of diamond in low pressure environments. One such environment is serpentinisation of oceanic lithosphere, which produces highly reduced CH4-bearing fluids after olivine alteration by reaction with infiltrating fluids. Here we report the first ever observed in situ diamond within olivine-hosted, CH4-rich fluid inclusions from low pressure oceanic gabbro and chromitite samples from the Moa-Baracoa ophiolitic massif, eastern Cuba. Diamond is encapsulated in voids below the polished mineral surface forming a typical serpentinisation array, with methane, serpentine and magnetite, providing definitive evidence for its metastable growth upon low temperature and low pressure alteration of oceanic lithosphere and super-reduction of infiltrated fluids. Thermodynamic modelling of the observed solid and fluid assemblage at a reference P-T point appropriate for serpentinisation (350 °C and 100 MPa) is consistent with extreme reduction of the fluid to logfO2 (MPa) = ?45.3 (?logfO2[Iron-Magnetite] = ?6.5). These findings imply that the formation of metastable diamond at low pressure in serpentinised olivine is a widespread process in modern and ancient oceanic lithosphere, questioning a generalised ultra-high pressure origin for ophiolitic diamond.
Abstract: The Neoproterozoic Bou Azzer ophiolite in the Moroccan Anti-Atlas Panafrican belt hosts numerous chromitite orebodies within the peridotite section of the oceanic mantle. The chromitites are strongly affected by serpentinization and metamorphism, although they still preserve igneous relicts amenable for petrogenetic interpretation. The major, minor and trace element composition of unaltered chromite cores reveal two compositional groups: intermediate-Cr (Cr# = 0.60 - 0.74) and high-Cr (Cr# = 0.79 - 0.84) and estimates of parental melt compositions suggest crystallization from pulses of fore-arc basalts (FAB) and boninitic melts, respectively, that infiltrated the oceanic supra-subduction zone (SSZ) mantle. A platinum group elements (PGE) mineralization dominated by Ir-Ru-Os is recognized in the chromitites, which has its mineralogical expression in abundant inclusions of Os-Ir alloys and coexisting magmatic laurite (RuS2) and their products of metamorphic alteration. Unusual mineral phases in chromite, not previously reported in this ophiolite, include super-reduced and/or nominally ultra-high pressure minerals moissanite (SiC), native Cu and silicates (oriented clinopyroxene lamellae), but “exotic” zircon and diaspore have also been identified. We interpret that clinopyroxene lamellae have a magmatic origin, whereas super-reduced phases originated during serpentinization processes and diaspore is linked to late circulation of low-silica fluids related to rodingitization. Zircon grains, on the other hand, with apatite and serpentine inclusions, could either have formed after the interaction of chromitite with mantle-derived melts or could represent subducted detrital sediments later incorporated into the chromitites. We offer a comparison of the Bou Azzer chromitites with other Precambrian ophiolitic chromitites worldwide, which are rather scarce in the geological record. The studied chromitites are very similar to the Neoproterozoic chromitites reported in the Arabian-Nubian shield, which are also related to the Panafrican orogeny. Thus, we conclude that the Bou Azzer chromitites formed in a subduction-initiation geodynamic setting with two-stages of evolution, with formation of FAB-derived intermediate-Cr chromitites in the early stage and formation of boninite-derived high-Cr chromitites in the late stage.
Abstract: Temperature (T), Pressure (P) and Oxygen fugacity (fO2) conditions were established for the Três Ranchos IV (diamond-bearing) and Limeira I (sterile) kimberlites of the Coromandel-Três Ranchos kimberlite field (Minas Gerais and Goiás, Brazil), Alto Paranaíba Alkaline Province (APAP), in order to draw a possible correlation between these intensive crystallization parameters and diamond instability. Both Três Ranchos IV and Limeira I are classified as coherent macrocrystic kimberlites, with an inequigranular texture formed by partially-to-fully altered olivine, phlogopite megacrysts up to 1 cm wide, macrocrysts (0.5-10 mm-sized), and crustal xenoliths set in a very fine groundmass composed mainly by perovskite, olivine, phlogopite, spinel, serpentine and carbonates identified in both intrusions. Apatite, ilmenite and monticellite are also present, but only in LM-I. Garnet macrocrysts and centimetric pyroxene xenocrysts phases are also present in Três Ranchos IV and Limeira I, respectively. The samples, strongly enriched in incompatible elements, are all MgO-rich, with high Mg# content. In order to apply different geotherm-and-oxybarometers in the calculation of P-T-fO2 conditions and to characterize the compositional variation of TR-IV and LM-I kimberlites, major, minor and trace-element concentrations of the main mineral phases were obtained by electron microprobe and LA-ICP-MS. Olivine cores of Limeira I present higher NiO, CaO and lower Cr2O3 contents than those from Três Ranchos IV. Mg# [(Mg/Mg+FeT), mol.%) ranges from 87 to 92 mol.% in TR-IV and from 83 to 92 mol.% in LM-I. The trace-element contents of olivine are similar in both kimberlites, the concentrations of Li, Zn and Mn appearing to be higher at olivine rims. In olivines from both intrusions, a pattern of enrichment in Zr, Ga, Nb, Sc, V, P, Al, Ti, Cr, Ca, and Mn in rims regions, is observed in the "melt trend" whereas enrichment in Zn, Co, Ni and possibly Na in cores regions, is found in the "mantle trend." In monticellite specimens from Limeira I, Mg# ranges from 72 to 93.8, while Ca/(Ca+Mg) ratios range from 35 to 58 mol.%. The perovskite composition in both LM-I and TR-IV remains close to the ideal CaTiO3, perovskite, but a variation from core endmembers (average Lop16 and Prv78) towards the rims (average Lop13 and Prv81) can be noticed in TR-IV samples. The highest concentrations of light rare earth elements (LREE), Nb, and Fe3+ are also observed in perovskites from the TR-IV kimberlite. Macrocrystic spinels of TR-IV kimberlite are Al-rich, whereas the groundmass crystals range from magnesiochromite to chromite. Ilmenites from LM-I are characterized by high MgO values at a given TiO2, with a large variation in Cr2O3. Pyrope garnets (62 to 73 mol.%) are present only in TR-IV, with Mg# ranging from 72 to 79 mol.%, being classified as lherzolitic (G9) and pyroxenitic (G4, G5). Diopside occurs as xenocrysts in LM-I and as microphenocrysts in TR-IV, with Mg# ranging from 85 to 91 and from 87 to 92, respectively. Xenocrystic diopsides from LM-I present higher MgO and FeO concentrations with monticellite grains along crystal rims and fractures. Temperature estimates for the LM-I kimberlite, obtained from the composition of diopside xenocrysts and Al-in olivine concentrations, ranging from 718 to 985 °C. Pressure ranges from 34 to 47 Kbar, as calculated using an empirical curve from a 37-mW/m2 geotherm proposed in the literature for Alto Paranaíba magmas. For TR-IV, temperature values ranging from 975 to 1270°C were obtained from Al-in olivine and Ni-in garnet concentrations. Pressures in the range from 18 to 34 Kbar were obtained from major element composition of garnet samples from TR-IV kimberlite. The fO2 of the TR-IV constrained by perovskite (kimberlite cognate phase) oxygen barometry ranges from NNO-7 to NNO+4, while for LM-I values range from NNO+6 to NNO-4. For the LM-I intrusion, monticellite, another cognate phase used as an oxybarometer, yielded a value range of NNO-4 to NNO+2. A change in the oxygen fugacity from cores towards rim recorded in the perovskites and the monticellite crystals is also noticed. The oxygen fugacity estimates of this work are the first ever calculated for magmas of the Alto Paranaíba Alkaline Province. All P-T-fO2 values obtained are consistent with literature data on the APAP. Clinopyroxene xenocrysts from LM-I were classified as garnet-facies clinopyroxene, according to the compositions obtained in this work. Such results, along with pressure, and temperature data from and the presence of Mg-ilmenite in LM-I (known to be sterile), indicate that the kimberlite magma might have at least crossed the diamond stability field. The variation in oxygen fugacity observed in both kimberlites possibly reflects the instability of diamonds in these magmas since LM-I presents slightly higher oxidation conditions.
Abstract: Oxygen fugacity (ƒO2) conditions were established for Três Ranchos IV (TR-IV, diamond-bearing) and Limeira I (LM-I, barren) kimberlite intrusions, in Alto Paranaíba Alkaline Province, to constrain a possible correlation between fO2 and diamond instability. Temperature and pressure estimates obtained from the xenocryst assemblage composition are compatible up to garnet lherzolite levels. It suggests that both intrusions could cross the diamond-stability field. The ƒO2 of the TR-IV constrained by perovskite oxygen barometry presents an average value of -2.4 for ?NNO, with standard deviation of 1.30 (n = 120), whereas those calculations for LM-I have an average value of -1.31 for ?NNO, with standard deviation of 1.38 (n = 81). Considering these uncertainties, there is an important superposition of fO2 values for both intrusions, in which there is higher tendency of more reduced conditions for TR-IV. For the LM-I, an oxybarometer based on the composition of monticellite yielded a similar ?NNO range: -4.2 and +2.5. Some crystals and samples present trends towards more reduced conditions, while others display more oxidized conditions for each intrusion. Due to the superposition of ranges and absence of a preferential trend, the influence of fO2 for the possible instability of diamonds in the study area remains uncertain.
Lithospheric structure, evolution and diamond prospectivity of the Rehoboth Terrane and western Kaapvaal Craton, southern Africa: constraints from broadband
Abstract: The late Jurassic Masontown dyke in Fayette County, SW Pennsylvania, preserves abundant rounded, mm to cm-diameter masses of olivine and serpentine cemented together in serpentine-rich kimberlite groundmass. Each mass is interpreted to be a partially serpentinized olivine xenocryst or peridotite xenocryst. Each rounded clast is jacketed by a distinct rim of serpentine; probably originally olivine. The (1) ubiquitous roundness of clasts and (2) the presence of distinct serpentine jackets around each clast, supports emplacement of the dyke by a 'kimberlite factory' (Brett et al., 2015). Due to the paucity of available samples, we have used non-destructive imaging by computed tomography (CT) at the National Energy Technology Lab in Morgantown, WV, to construct 3D models of the internal structure of hand samples loaned from the Smithsonian Institute's Museum of Natural History. MicroCT (1-3 micron resolution) and industrial CT (~15 microns resolution) serial scans processed in ImageJ and Blob3D allow for 3D characterizations of individual clasts, including their shape factors (sphericity, roughness, etc.) and sizes (i.e. crystal size distributions).
Abstract: Although traditionally considered the realm of igneous petrologists and geochemists, kimberlites have received attention from physical volcanologists interested in how they are emplaced in the crust and how they can erupt. This presentation will review the evidence for the volcaniclastic (i.e. fragmental) nature of kimberlites from examples in Canada's Northwest Territories and in Pennsylvania. A growing body of evidence indicates that kimberlite magmas are gas-dominated (overwhelmingly CO2) suspensions of molten kimberlite liquid and crystals, usually olivines. The olivines, like other mineral phases and xenoliths, are entrained from the surrounding mantle peridotite wall-rock, rather than crystallized from the meager kimberlite liquid, and are, therefore, overwhelmingly xenocrystic. This crystal and rock fragment load is sampled and mechanically processed by a turbulent gas-jet before being immersed in a bath of kimberlite liquid: this is the kimberlite factory. As the gas-charged crack-tip propagates and ascends, new mantle is processed into the kimberlite factory. Each emplacement event records the passage of a kimberlite factory through the mantle and lithosphere. The Masontown kimberlite in Pennsylvania is a solitary hypabyssal kimberlite dyke but it preserves evidence of the passage of a single kimberlite factory. Although many kimberlites stall in the crust, many erupt explosively to produce indisputably volcaniclastic kimberlite lithofacies associated with diatremes. Open-pit mining of several diatremes in Canada reveals the complex temporal-spatial nature of different emplacement events within the same volcanic field, and the ubiquitous presence of hypabyssal kimberlite dykes that fed or attempted to feed explosive eruptions. Such explosive eruptions sustained tephra plumes that produced kimberlite fall deposits and pyroclastic density currents that produced kimberlite ignimbrites; both of which exited their source diatremes and inundated the surrounding landscape.
Garnet Bearing Lherzolites and Discrete Nodule Suites from The Malaita Alnoite, Solomon Islands and Their Bearing on The Nature and Origin of the Ontong Java Plateau.
Aust. Society of Exploration Geophysics Bulletin., Vol. 9, No. 3, AUGUST PP. 103-107.
Age and geochemical characteristics of a mafic dyke swarm in the Archean vestfold block Antarctica- inferences about Proterozoic dyke emplacement inGondwana
Journal of Petrology, Vol. 27, No. 4, August pp. 853-886
The early Proterozoic Mt. Weld carbonatite: implications for mantle Strontium, neodymium, and lead isotopic evolution of subcontinental lithosphere beneath the Yilgarnblock, Western
Structural evolution and tectonic significance of the Early Proterozoic Virgin River shear zone, northwest Saskatchewan:implications for relations between Rae and Hearne cratons
Eos, Vol. 70, No. 43, October 24, p. 1310. Abstract
samarium-neodymium (Sm-Nd) isotopic restraints on the age of buried Precambrian basement In central and southernSaskatchewan: implications for diamond exploration
Saskatchewan Geological Survey Summary of Investigations for 1989, Report No. 89-4, pp. 168-171
The evolution of Wyoming craton lower crust: uranium-lead (U-Pb) (U-Pb) shrimp and neodymium-Sr isotopic evidence for middle Archean and Early Proterozoic events.
The Xenolith window into the lower crust, abstract volume and workshop, p. 4.
Origin of the Archean Sask Craton and its extent within the Trans-Hudson orogen: evidence Pb Nd isotopic compositions basement rocks, post-orogenic intrusions.
Canadian Journal of Earth Sciences, Vol. 42, 4, April pp. 659-684.
Majoritic garnet: a new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamonds.
Geochimica et Cosmochimica Acta, Vol. 74, 20, pp. 5939-5937.
Abstract: Interpretations of magnetic anomolies, fracture-zone geometry, and continental-oceanic crust transitions frame the debate surrounding the tectonic opening of the Equatorial and South Atlantic Ocean. Misinterpretations of movement in this region lead to misalignments and misunderstanding of the depositional environments at the margins. This article presents further evidence of the Guiana Shield's role during the initial separation between the American and African plates. Research suggests that the structure of the larger Guiana basin was controlled by the reactivation of Paleozoic and early Mesozoic faults, inherited from older orogenic belts. The same river paths fed the basin with clastic deposits for several tens to hundreds of millions of years. A network of NNW-SSE and NNE-SSW lineaments along the Atlantic margin coast and their onshore continuity at the edge of the Guiana Shield denote relics of deep faulting associated with the early rifting of the central Atlantic Ocean during early Jurassic, between 190 and 170 million years (Ma) (Fig. 1). These older faults were reactivated during Cretaceous E-W drift, a fact that created a favorable Tertiary-to-present structural and depositional environment for southward-shoreward hydrocarbon migration in the South American-Central Equatorial Atlantic margin (OGJ, Jan. 4, 2016, p. 42).
Gemmological Research Industries Inc. Vancouver B.C., isbn 978-1777369231 184p.
Global
Book - notice
Abstract: For those who have some portable and advanced instruments, this book will serve as a handbook with many useful spectra, cross polarised filters and fluorescence reactions to compare, plus an Appendix with results of tests conducted using 11 portable instruments on 64 samples, and suggestions as to what instruments to use depending on budget and needs. Even if you are not a diamond specialist but are merely interested in the science of diamond, or you trade in diamonds, the information in this book will make you more knowledgeable and confident to talk about this beautiful gem with friends and clients.
Santosj, M., Tanaka, K., Yokoyama, K., Collins, A.S.
Late Neoproterozoic Cambrian felsic magmatism along transcrustral shear zones in southern India: U Pb electron microprobe ages implications for amalagamtion
McGee, B., Collins, A.S., Trindade, R.I.F., Jourdan, F.
Investigating mid-Edicaran glaciation and final Gondwana amalgamation using coupled sedimentology and 40 Ar/39Ar detrital muscovite provenance from the Paraguay Belt, Brazil.
Abstract: Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of “plate tectonic rules” such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continent-margin and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here still predicts less subduction (ca. 90%) than on the modern earth, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet's icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.
Journal of Asian Earth Sciences, in press available, 47p.
India, Africa, Madagascar
tectonis
Abstract: It has long been recognised that Madagascar was contiguous with India until the Late Cretaceous. However, the timing and nature of the amalgamation of these two regions remain highly contentious as is the location of Madagascar against India in Gondwana. Here we address these issues with new U-Pb and Lu-Hf zircon data from five metasedimentary samples from the Karwar Block of India and new Lu-Hf data from eight previously dated igneous rocks from central Madagascar and the Antongil-Masora domains of eastern Madagascar. New U-Pb data from Karwar-region detrital zircon grains yield two dominant age peaks at c. 3100 Ma and c. 2500 Ma. The c. 3100 Ma population has relatively juvenile ?Hf(t) values that trend toward an evolved signature at c. 2500 Ma. The c. 2500 Ma population shows a wide range of ?Hf(t) values reflecting mixing of an evolved source with a juvenile source at that time. These data, and the new Lu-Hf data from Madagascar, are compared with our new compilation of over 7000 U-Pb and 1000 Lu-Hf analyses from Madagascar and India. We have used multidimensional scaling to assess similarities in these data in a statistically robust way. We propose that the Karwar Block of western peninsular India is an extension of the western Dharwar Craton and not part of the Antananarivo Domain of Madagascar as has been suggested in some models. Based on ?Hf(t) signatures we also suggest that India (and the Antongil-Masora domains of Madagascar) were palaeogeographically isolated from central Madagascar (the Antananarivo Domain) during the Palaeoproterozoic. This supports a model where central Madagascar and India amalgamated during the Neoproterozoic along the Betsimisaraka Suture.
Abstract: Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of “plate tectonic rules” such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continent-margin and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here still predicts less subduction (ca. 90%) than on the modern earth, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet's icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.
Journal of the Geological Society of London, in press available 25p.
Africa, Madagascar
thermochronology
Abstract: Madagascar occupied an important place in the amalgamation of Gondwana, and preserves a record of several Neoproterozoic events that can be linked to orogenesis of the East African Orogen. We integrate remote sensing and field data to unravel complex deformation in the Ikalamavony and Itremo domains of central Madagascar. The deformation sequence comprises a gneissic foliation (S1), followed by south to south-west directed, tight to isoclinal, recumbent folding (D2). These are overprinted by north-trending upright folds that formed during a ~E-W shortening event. Together these produced type 1 and type 2 fold interference patterns throughout the Itremo and Ikalamavony domains. Apatite U-Pb and muscovite and biotite Rb-Sr thermochronometers indicate that much of central Madagascar was thermally reset to at least ~500oC at c. 500 Ma. Deformation in west-central Madagascar occurred between c. 750 Ma and c. 550 Ma, and we suggest this deformation formed in response to the c. 650 Ma collision of Azania with Africa along the Vohibory Suture in southwestern Madagascar. In eastern Madagascar, deformation is syn- to post-550 Ma, which formed in response to the final closure of the Mozambique Ocean along the Betsimisaraka Suture that amalgamated Madagascar with the Dharwar Craton of India.
Abstract: The extent of continental rifts and subduction zones through deep geological time provides insights into the mechanisms behind supercontinent cycles and the long term evolution of the mantle. However, previous compilations have stopped short of mapping the locations of rifts and subduction zones continuously since the Neoproterozoic and within a self-consistent plate kinematic framework. Using recently published plate models with continuously closing boundaries for the Neoproterozoic and Phanerozoic, we estimate how rift and peri-continental subduction length vary from 1 Ga to present and test hypotheses pertaining to the supercontinent cycle and supercontinent breakup. We extract measures of continental perimeter-to-area ratio as a proxy for the existence of a supercontinent, where during times of supercontinent existence the perimeter-to-area ratio should be low, and during assembly and dispersal it should be high. The amalgamation of Gondwana is clearly represented by changes in the length of peri-continental subduction and the breakup of Rodinia and Pangea by changes in rift lengths. The assembly of Pangea is not clearly defined using plate boundary lengths, likely because its formation resulted from the collision of only two large continents. Instead the assembly of Gondwana (ca. 520 Ma) marks the most prominent change in arc length and perimeter-to-area ratio during the last billion years suggesting that Gondwana during the Early Palaeozoic could explicitly be considered part of a Phanerozoic supercontinent. Consequently, the traditional understanding of the supercontinent cycle, in terms of supercontinent existence for short periods of time before dispersal and re-accretion, may be inadequate to fully describe the cycle. Instead, either a two-stage supercontinent cycle could be a more appropriate concept, or alternatively the time period of 1 to 0 Ga has to be considered as being dominated by supercontinent existence, with brief periods of dispersal and amalgamation.
Abstract: The Southern Irumide Belt (SIB) of Zambia consists of predominantly Mesoproterozoic terranes that record a pervasive tectono-metamorphic overprint from collision between the Congo and Kalahari cratons in the final stages of Gondwana amalgamation. This study applies multi-method thermochronology to samples throughout southern Zambia to constrain the post-collisional, Phanerozoic thermo-tectonic evolution of the region. U-Pb apatite and 40Ar/39Ar muscovite data are used to constrain the cooling history of the region following Congo-Kalahari collision, and reveal ages of c. 550-450?Ma. Variations in the recorded cooling ages are interpreted to relate to localised post-tectonic magmatism and the proximity of analysed samples to the Congo-Kalahari suture. Apatite fission track data are used to constrain the low-temperature thermo-tectonic evolution of the region and identify mean central ages of c. 320-300, 210-200 and 120-110?Ma. Thermal modelling of these samples identifies a number of thermal events occurring in the region throughout the Phanerozoic. Carboniferous to Permian-Triassic heating is suggested to relate to the development of Karoo rift basins found throughout central Africa and constrain the timing of sedimentation in the basin. Permian to Jurassic cooling is identified in a number of samples, reflecting exhumation as a result of the Mauritanian-Variscan and Gondwanide orogenies. Subsequent cooling of the majority of samples occurs from the Cretaceous and persists until present, reflecting exhumation in response to larger scale rifting associated with the break-up of Gondwana. Each model reveals a later phase of enhanced cooling beginning at c. 30?Ma that, if not an artefact of modelling, corresponds to the development of the East African Rift System. The obtained thermochronological data elucidate the previously unconstrained thermal evolution of the SIB, and provides a refined regional framework for constraining the tectonic history of central Africa throughout the Phanerozoic.
Journal of the Geological Society, Vol. 177, pp. 784-798.
Africa, Madagascar
geothermometry
Abstract: Madagascar occupied an important place in the amalgamation of Gondwana and preserves a record of several Neoproterozoic events that are linked to orogenesis of the East African Orogen. In this study, we integrate remote sensing, field data and thermochronology to unravel complex deformation in the Ikalamavony and Itremo domains of central Madagascar. The deformation sequence comprises a gneissic foliation (S1), followed by south- to SW-directed, tight to isoclinal, recumbent folding (D2). These are overprinted by north-trending upright folds that formed during an approximately east-west shortening event (D3). Together these produced type 1 and type 2 fold interference patterns throughout the Itremo and Ikalamavony domains. We show that the Itremo and Ikalamavony domains were deformed together in the same orogenic system, which we interpret as the c. 630 Ma collision of Azania with Africa along the Vohibory Suture in southwestern Madagascar. In eastern Madagascar, deformation is syn- to post-550 Ma, and probably formed in response to final closure of the Mozambique Ocean along the Betsimisaraka Suture that amalgamated Madagascar with the Dharwar Craton of India. Apatite U-Pb and novel laser ablation triple quadrupole inductively coupled plasma mass spectrometry (LA-QQQ-ICP-MS) muscovite and biotite Rb-Sr thermochronology indicates that much of central Madagascar cooled through c. 500°C at c. 500 Ma.
Abstract: The Ongole Domain in the southern Eastern Ghats Belt of India formed during the final stages of Columbia amalgamation at ca. 1600 Ma. Yet very little is known about the protolith ages, tectonic evolution or geographic affinity of the region. We present new detrital and igneous U-Pb-Hf zircon data and in-situ monazite data to further understand the tectonic evolution of this Columbia-forming orogen. Detrital zircon patterns from the metasedimentary rocks are dominated by major populations of Palaeoproterozoic grains (ca. 2460, 2320, 2260, 2200-2100, 2080-2010, 1980-1920, 1850 and 1750 Ma), and minor Archaean grains (ca. 2850, 2740, 2600 and 2550 Ma). Combined U-Pb ages and Lu-Hf zircon isotopic data suggest that the sedimentary protoliths were not sourced from the adjacent Dharwar Craton. Instead they were likely derived from East Antarctica, possibly the same source as parts of Proterozoic Australia. Magmatism occurred episodically between 1.64 and 1.57 Ga in the Ongole Domain, forming felsic orthopyroxene-bearing granitoids. Isotopically, the granitoids are evolved, producing ?Hf values between ? 2 and ? 12. The magmatism is interpreted to have been derived from the reworking of Archaean crust with only a minor juvenile input. Metamorphism between 1.68 and 1.60 Ga resulted in the partial to complete resetting of detrital zircon grains, as well as the growth of new metamorphic zircon at 1.67 and 1.63 Ga. In-situ monazite geochronology indicates metamorphism occurred between 1.68 and 1.59 Ga. The Ongole Domain is interpreted to represent part of an exotic terrane, which was transferred to proto-India in the late Palaeoproterozoic as part of a linear accretionary orogenic belt that may also have included south-west Baltica and south-eastern Laurentia. Given the isotopic, geological and geochemical similarities, the proposed exotic terrane is interpreted to be an extension of the Napier Complex, Antarctica, and may also have been connected to Proterozoic Australia (North Australian Craton and Gawler Craton).
Abstract: Madagascar hosts several Paleoproterozoic sedimentary sequences that are key to unravelling the geodynamic evolution of past supercontinents on Earth. New detrital zircon U-Pb and Hf data, and a substantial new database of ?15,000 analyses are used here to compare and contrast sedimentary sequences in Madagascar, Africa and India. The Itremo Group in central Madagascar, the Sahantaha Group in northern Madagascar, the Maha Group in eastern Madagascar, and the Ambatolampy Group in central Madagascar have indistinguishable age and isotopic characteristics. These samples have maximum depositional ages > 1700 Ma, with major zircon age peaks at c. 2500 Ma, c. 2000 Ma and c. 1850 Ma. We name this the Greater Itremo Basin, which covered a vast area of Madagascar in the late Paleoproterozoic. These samples are also compared with those from the Tanzania and the Congo cratons of Africa, and the Dharwar Craton and Southern Granulite Terrane of India. We show that the Greater Itremo Basin and sedimentary sequences in the Tanzania Craton of Africa are correlatives. These also tentatively correlate with sedimentary protoliths in the Southern Granulite Terrane of India, which together formed a major intra?Nuna/Columbia sedimentary basin that we name the Itremo?Muva?Pandyan Basin. A new Paleoproterozoic plate tectonic configuration is proposed where central Madagascar is contiguous with the Tanzania Craton to the west and the Southern Granulite Terrane to the east. This model strongly supports an ancient Proterozoic origin for central Madagascar and a position adjacent to the Tanzania Craton of East Africa.
Abstract: Recent progress in plate tectonic reconstructions has seen models move beyond the classical idea of continental drift by attempting to reconstruct the full evolving configuration of tectonic plates and plate boundaries. A particular problem for the Neoproterozoic and Cambrian is that many existing interpretations of geological and palaeomagnetic data have remained disconnected from younger, better-constrained periods in Earth history. An important test of deep time reconstructions is therefore to demonstrate the continuous kinematic viability of tectonic motions across multiple supercontinent cycles. We present, for the first time, a continuous full-plate model spanning 1 Ga to the present-day, that includes a revised and improved model for the Neoproterozoic-Cambrian (1000-520 Ma) that connects with models of the Phanerozoic, thereby opening up pre-Gondwana times for quantitative analysis and further regional refinements. In this contribution, we first summarise methodological approaches to full-plate modelling and review the existing full-plate models in order to select appropriate models that produce a single continuous model. Our model is presented in a palaeomagnetic reference frame, with a newly-derived apparent polar wander path for Gondwana from 540 to 320 Ma, and a global apparent polar wander path from 320 to 0 Ma. We stress, though while we have used palaeomagnetic data when available, the model is also geologically constrained, based on preserved data from past-plate boundaries. This study is intended as a first step in the direction of a detailed and self-consistent tectonic reconstruction for the last billion years of Earth history, and our model files are released to facilitate community development.
Earth Science Reviews , Vol. 214, 103477, 44p. Pdf
Mantle
plate tectonics, Rodinia, Gondwana
Abstract: Recent progress in plate tectonic reconstructions has seen models move beyond the classical idea of continental drift by attempting to reconstruct the full evolving configuration of tectonic plates and plate boundaries. A particular problem for the Neoproterozoic and Cambrian is that many existing interpretations of geological and palaeomagnetic data have remained disconnected from younger, better-constrained periods in Earth history. An important test of deep time reconstructions is therefore to demonstrate the continuous kinematic viability of tectonic motions across multiple supercontinent cycles. We present, for the first time, a continuous full-plate model spanning 1 Ga to the present-day, that includes a revised and improved model for the Neoproterozoic-Cambrian (1000-520 Ma) that connects with models of the Phanerozoic, thereby opening up pre-Gondwana times for quantitative analysis and further regional refinements. In this contribution, we first summarise methodological approaches to full-plate modelling and review the existing full-plate models in order to select appropriate models that produce a single continuous model. Our model is presented in a palaeomagnetic reference frame, with a newly-derived apparent polar wander path for Gondwana from 540 to 320 Ma, and a global apparent polar wander path from 320 to 0 Ma. We stress, though while we have used palaeomagnetic data when available, the model is also geologically constrained, based on preserved data from past-plate boundaries. This study is intended as a first step in the direction of a detailed and self-consistent tectonic reconstruction for the last billion years of Earth history, and our model files are released to facilitate community development.
Diamond and Related Materials, in press available 33p.
Global
DiaMap
Abstract: Type IIb diamonds are those that contain more boron than nitrogen. The presence of this uncompensated boron gives rise to absorption in the infrared part of the electromagnetic spectrum, extending into the visible region and often resulting in blue colouration. Here we report on the expansion of the DiaMap freeware (for the automated spectral deconvolution of Type I [nitrogen containing] diamonds) to work on Type IIb diamonds, returning concentrations from three boron-related absorption bands, and determining which band provides the most reliable value. The program uses the calibration coefficients of Collins (2010), which show good relative agreement between the three bands, but might require some further study to confirm their absolute accuracy to the uncompensated boron concentration. The methodology of DiaMap_IIb is applicable to all Type IIb diamonds, both natural and synthetic. Analysis of high-resolution Fourier-transform infrared (FTIR) maps of two high-pressure high-temperature (HPHT) synthetic diamonds using DiaMap_IIb, confirm the growth sector dependence of the boron incorporation. Partitioning of boron strongly favours the octahedral {111} sectors.
Diamonds & Related Materials, In press available, 30p. Pdf
Global
synthetics
Abstract: Type IIb diamonds are those that contain more boron than nitrogen. The presence of this uncompensated boron gives rise to absorption in the infrared part of the electromagnetic spectrum, extending into the visible region and often resulting in blue colouration. Here we report on the expansion of the DiaMap freeware (for the automated spectral deconvolution of Type I [nitrogen containing] diamonds) to work on Type IIb diamonds, returning concentrations from three boron-related absorption bands, and determining which band provides the most reliable value. The program uses the calibration coefficients of Collins (2010), which show good relative agreement between the three bands, but might require some further study to confirm their absolute accuracy to the uncompensated boron concentration. The methodology of DiaMap_IIb is applicable to all Type IIb diamonds, both natural and synthetic. Analysis of high-resolution Fourier-transform infrared (FTIR) maps of two high-pressure high-temperature (HPHT) synthetic diamonds using DiaMap_IIb, confirm the growth sector dependence of the boron incorporation. Partitioning of boron strongly favours the octahedral {111} sectors.
Geostandards and Geoanalytical Research, in press available
Technology
spectroscopy
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.
Granite-greenstone terranes in the Pilbara Block,Australia, as coeval volcano plutonic complexes; evidence from uranium-lead (U-Pb) (U-Pb) zircon dating of the Mt. EdgarBatholith
Earth and Planetary Science Letters, Vol. 97, No. 1-2, February pp. 41-53
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 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.
Earth-Science Reviews, Vol. 211, doi.org/10.1016 /j.earscirev.2020 .103413 17p. Pdf
Global
cratons
Abstract: Long-lived (800?Ma) Paleo- to Mesoproterozoic accretionary orogens on the margins of Laurentia, Baltica, Amazonia, and Kalahari collided to form the core of the supercontinent, Rodinia. Accretionary orogens in Laurentia and Baltica record predominately radiogenic zircon ?Hf(t) and whole-rock Pb isotopic compositions, short crustal residence times (ca. 0.5?Ga), and the development of arc-backarc complexes. The accretionary orogenic record of Laurentia and Baltica is consistent with a retreating accretionary orogen and analogous to the Phanerozoic western Pacific orogenic system. In contrast, the Mesoproterozoic orogens of Amazon and Kalahari cratons record unradiogenic zircon ?Hf(t) values, ca. 0.8?Ga crustal residence times, and more ancient whole-rock Pb isotopic signatures. The accretionary orogenic record of Amazonia and Kalahari indicates the preferential incorporation of cratonic material in continental arcs of advancing accretionary orogens comparable to the Phanerozoic eastern Pacific orogenic system. Based on similarities in the geodynamic evolution of the Phanerozoic circum-Pacific orogens peripheral to Gondwana/Pangea, we suggest that the Mesoproterozoic accretionary orogens formed as peripheral subduction zones along the margin of the supercontinent Nuna (ca. 1.8-1.6?Ga). The eventual collapse of this peripheral subduction zone onto itself and closure of the external ocean around Nuna to form Rodinia is equivalent to the projected future collapse of the circum-Pacific subduction system and juxtaposition of Australia-Asia with South America. The juxtaposition of advancing and retreating accretionary orogens at the core of the supercontinent Rodinia demonstrates that supercontinent assembly can occur by the closure of external oceans and indicates that future closure of the Pacific Ocean is plausible.
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: Continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle. When two continents collide, the mass balance for the continental crust is therefore assumed to be maintained. Here we use estimates of pre-collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the India-Asia collisional system. Using the current best estimates for the timing of the diachronous onset of collision between India and Eurasia, we find that about 50% of the pre-collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at Earth’s surface today—represented by the mass preserved in the thickened crust that makes up the Himalaya, Tibet and much of adjacent Asia, as well as southeast Asian tectonic escape and exported eroded sediments. This implies large-scale subduction of continental crust during the collision, with a mass equivalent to about 15% of the total oceanic crustal subduction flux since 56 million years ago. We suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent-continent collisions is reflected in some ocean crust and ocean island basalt geochemistry. The subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry.
Journal of Volcanology and Geothermal Research, in press available 15p.
Africa, Algeria
Lamproite
Abstract: The late Miocene (11-9 Ma) volcanic rocks of Kef Hahouner, ~ 40 km NE of Constantine (NE Algeria), are commonly classified as lamproites in literature. However, these rocks are characterized by an anhydrous paragenesis with plagioclase and Mg-rich olivine phenocrysts, set in a groundmass made up of feldspars, pyroxenes and opaque minerals. Thus, we classify the Kef Hahouner rocks as ultrapotassic shoshonites and latites, having K2O > 3 wt.%, K2O/Na2O > 2.5, MgO > 3-4 wt.%, SiO2 < 55-57 wt.% and SiO2/K2O < 15. All the investigated samples show primitive mantle-normalized multi-element patterns typical of orogenic (arc-type) magmas, i.e. enriched in LILE (e.g. Cs, Rb and Ba) and LREE (e.g. La/Yb = 37-59) with respect to the HFSE, peaks at Pb and troughs at Nb and Ta. Initial isotopic ratios are in the range of 87Sr/86Sr = 0.70874-0.70961, 143Nd/144Nd = 0.51222-0.51223, 206Pb/204Pb = 18.54-18.60, 207Pb/204Pb = 15.62-15.70 and 208Pb/204Pb = 38.88-39.16. The Kef Hahouner volcanic rocks show multi-element patterns similar to the other circum-Mediterranean lamproites and extreme Sr, Nd and Pb isotopic compositions. Nevertheless, the abundant plagioclase, the presence of Al-rich augite coupled with high Al2O3 whole rock compositions (9.6-21.4 wt.%), and the absence of phlogopite are all at inconsistent with the definition of lamproite. We reviewed the rocks classified as lamproites worldwide, and found that many of these rocks, as for the Kef Hahouner samples, should be actually defined as "normal" potassic to ultrapotassic volcanic rocks. Even the grouping of lamproites into "orogenic" and "anorogenic" types appears questionable.
Abstract: The Rio Apa cratonic fragment crops out in Mato Grosso do Sul State of Brazil and in northeastern Paraguay. It comprises Paleo-Mesoproterozoic medium grade metamorphic rocks, intruded by granitic rocks, and is covered by the Neoprotero-zoic deposits of the Corumbá and Itapocumi Groups. Eastward it is bound by the southern portion of the Paraguay belt. In this work, more than 100 isotopic determina-tions, including U-Pb SHRIMP zircon ages, Rb-Sr and Sm-Nd whole-rock determina-tions, as well as K-Ar and Ar-Ar mineral ages, were reassessed in order to obtain a complete picture of its regional geological history. The tectonic evolution of the Rio Apa Craton starts with the formation of a series of magmatic arc complexes. The oldest U-Pb SHRIMP zircon age comes from a banded gneiss collected in the northern part of the region, with an age of 1950 23 Ma. The large granitic intrusion of the Alumiador Batholith yielded a U-Pb zircon age of 1839 33 Ma, and from the southeastern part of the area two orthogneisses gave zircon U-Pb ages of 1774 26 Ma and 1721 25 Ma. These may be coeval with the Alto Tererê metamorphic rocks of the northeastern corner, intruded in their turn by the Baía das Garças granitic rocks, one of them yielding a zircon U-Pb age of 1754 49 Ma. The original magmatic protoliths of these rocks involved some crustal component, as indicated by the Sm-Nd T DM model ages, between 1.9 and 2.5 Ga. Regional Sr isotopic homogenization, associated with tectonic deformation and medium-grade metamorphism occurred at approximately 1670 Ma, as suggested by Rb-Sr whole rock reference isochrons. Finally, at 1300 Ma ago, the Ar work indicates that the Rio Apa Craton was affected by widespread regional heating, when the temperature probably exceeded 350°C. Geographic distribution, age and isotopic signature of the lithotectonic units suggest the existence of a major suture separating two different tectonic domains, juxtaposed at about 1670 Ma. From that time on, the unified Rio Apa continental block behaved as one coherent and stable tectonic unit. It correlates well with the SW corner of the Amazonian Craton, where the medium-grade rocks of the Juruena-Rio Negro tectonic province, with ages between 1600 and 1780 Ma, were reworked at about 1300 Ma. Looking at the largest scale, the Rio Apa Craton is probably attached to the larger Amazonian Craton, and the actual configuration of southwestern South America is possibly due to a complex arrangement of allochthonous blocks such as the Arequipa, Antofalla and Pampia, with different sizes, that may have originated as disrupted parts of either Laurentia or Amazonia, and were trapped during later collisions of these continental masses.
Abstract: The Rosário-6 is a non-diamondiferous hypabyssal kimberlite located above the Rio de la Plata craton and near the south-eastern edge of the Paraná Basin, in southern Brazil. It is petrographically an inequigranular texture, macrocrystal kimberlite, fresh and the groundmass exhibits a microporphyritic texture and round megacrysts of olivine, which are derived from disaggregated mantle xenoliths. Olivine is also present as macrocrysts, microphenocrysts and in the groundmass together with phlogopite and apatite. These microphenocrysts are immersed in a groundmass of olivine, monticellite, phlogopite, CaTiO3-perovskite, apatite, Mg-chromite and Mg-ulvöspinel and melilite. A mesostasis assemblage of phlogopite, melilite, soda melilite, akermanite and calcium carbonate is segregated from the groundmass. Its geochemical signature is similar to those of transitional kimberlites of Kaapvaal Craton, South Africa, and the U-Pb ages of ~ 128 Ma on perovskite reveal that Rosário-6 kimberlite post-dates the main pulse of volcanism in the Paraná-Etendeka Large Igneous Province (LIP). The high Ti content of some minerals, such as Mg-chromite, Mg-ulvöspinel, phlogopite and melilite, and the presence of perovskite suggest a Ti-rich source. The petrographic, geochemical and isotopic data indicate that the Rosário-6 kimberlite source is a depleted mantle metasomatized by H2O-rich fluids, CO2-rich and silicate melts derived from the recycling of an ancient subducted oceanic plate (eclogite) before the South Atlantic opening. Although several authors indicate the influence of Tristan da Cunha plume for the generation of alkaline magmatism associated to the Paraná-Etendeka flood basalts, our data demonstrates that Tristan da Cunha plume has no chemical contribution to the generation of Rosário-6 kimberlite, except by its thermal influence.
Abstract: The theory of plate tectonics describes how the surface of Earth is split into an organized jigsaw of seven large plates1 of similar sizes and a population of smaller plates whose areas follow a fractal distribution2, 3. The reconstruction of global tectonics during the past 200 million years4 suggests that this layout is probably a long-term feature of Earth, but the forces governing it are unknown. Previous studies3, 5, 6, primarily based on the statistical properties of plate distributions, were unable to resolve how the size of the plates is determined by the properties of the lithosphere and the underlying mantle convection. Here we demonstrate that the plate layout of Earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere. Using three-dimensional spherical models of mantle convection that self-consistently produce the plate size -frequency distribution observed for Earth, we show that subduction geometry drives the tectonic fragmentation that generates plates. The spacing between the slabs controls the layout of large plates, and the stresses caused by the bending of trenches break plates into smaller fragments. Our results explain why the fast evolution in small back-arc plates7, 8 reflects the marked changes in plate motions during times of major reorganizations. Our study opens the way to using convection simulations with plate-like behaviour to unravel how global tectonics and mantle convection are dynamically connected.
Abstract: The concept of interplay between mantle convection and tectonics goes back to about a century ago, with the proposal that convection currents in the Earth’s mantle drive continental drift and deformation (Holmes, 1931). Since this time, plate tectonics theory has established itself as the fundamental framework to study surface deformation, with the remarkable ability to encompass geological and geophysical observations. Mantle convection modeling has progressed to the point that connections with plate tectonics can be made, pushing the idea that tectonics is a surface expression of the global dynamics of one single system: the mantle-lithosphere system. Here, we present our perspective, as modelers, on the dynamics behind global tectonics with a focus on the importance of self-organisation. We first present an overview of the links between mantle convection and tectonics at the present-day, examining observations such as kinematics, stress and deformation. Despite the numerous achievements of geodynamic studies, this section sheds light on the lack of self-organisation of the models used, which precludes investigations on feedbacks and evolution of the mantle-lithosphere system. Therefore, we review the modeling strategies, often focused on rheology, that aim at taking into account self-organisation. The fundamental objective is that plate-like behaviour emerges self-consistently in convection models. We then proceed with the presentation of studies of continental drift, seafloor spreading and plate tectonics in convection models allowing for feedbacks between surface tectonics and mantle dynamics. We discuss the approximation of the rheology of the lithosphere used in these models (pseudo-plastic rheology), for which empirical parameters differ from those obtained in experiments. In this section, we analyse in detail a state-of-the-art 3D spherical convection calculation, which exhibits fundamental tectonic features (continental drift, one-sided subduction, trench and ridge evolution, transform shear zones, small-scale convection, and plume tectonics). This example leads to a discussion where we try to answer the question: can mantle convection models transcend the limitations of plate tectonics theory?
Geochemistry, Geophysics, Geosystems, Vol. 18, 8, pp. 3197-3208.
Mantle
tectonics
Abstract: Mantle convection models with plate-like behavior produce surface structures comparable to Earth's plate boundaries. However, analyzing those structures is a difficult task, since convection models produce, as on Earth, diffuse deformation and elusive plate boundaries. Therefore we present here and share a quantitative tool to identify plate boundaries and produce plate polygon layouts from results of numerical models of convection: Automatic Detection Of Plate Tectonics (ADOPT). This digital tool operates within the free open-source visualization software Paraview. It is based on image segmentation techniques to detect objects. The fundamental algorithm used in ADOPT is the watershed transform. We transform the output of convection models into a topographic map, the crest lines being the regions of deformation (plate boundaries) and the catchment basins being the plate interiors. We propose two generic protocols (the field and the distance methods) that we test against an independent visual detection of plate polygons. We show that ADOPT is effective to identify the smaller plates and to close plate polygons in areas where boundaries are diffuse or elusive. ADOPT allows the export of plate polygons in the standard OGR-GMT format for visualization, modification, and analysis under generic softwares like GMT or GPlates.
Abstract: The concept of interplay between mantle convection and tectonics goes back to about a century ago, with the proposal that convection currents in the Earth's mantle drive continental drift and deformation (Holmes, 1931). Since this time, plate tectonic theory has established itself as the fundamental framework to study surface deformation, with the remarkable ability to encompass geological and geophysical observations. Mantle convection modeling has progressed to the point where connections with plate tectonics can be made, pushing the idea that tectonics is a surface expression of the global dynamics of one single system: the mantle-lithosphere system. Here, we present our perspective, as modelers, on the dynamics behind global tectonics with a focus on the importance of self-organisation. We first present an overview of the links between mantle convection and tectonics at the present-day, examining observations such as kinematics, stress and deformation. Despite the numerous achievements of geodynamic studies, this section sheds light on the lack of self-organisation of the models used, which precludes investigations of the feedbacks and evolution of the mantle-lithosphere system. Therefore, we review the modeling strategies, often focused on rheology, that aim at taking into account self-organisation. The fundamental objective is that plate-like behaviour emerges self-consistently in convection models. We then proceed with the presentation of studies of continental drift, seafloor spreading and plate tectonics in convection models allowing for feedbacks between surface tectonics and mantle dynamics. We discuss the approximation of the rheology of the lithosphere used in these models (pseudo-plastic rheology), for which empirical parameters differ from those obtained in experiments. In this section, we analyse in detail a state-of-the-art 3-D spherical convection calculation, which exhibits fundamental tectonic features (continental drift, one-sided subduction, trench and ridge evolution, transform shear zones, small-scale convection, and plume tectonics). This example leads to a discussion where we try to answer the following question: can mantle convection models transcend the limitations of plate tectonic theory?
Abstract: The existence of undulations of the geoid, gravity and bathymetry in ocean basins, as well as anomalies in heat flow, point to the existence of small scale convection beneath tectonic plates. The instabilities that could develop at the base of the lithosphere are sufficiently small scale (< 500 km) that they remain mostly elusive from seismic detection. We take advantage of 3D spherical numerical geodynamic models displaying plate-like behavior to study the interaction between large-scale flow and small-scale convection. We find that finger-shaped instabilities develop at seafloor ages > 60 Ma. They form networks that are shaped by the plate evolution, slabs, plumes and the geometry of continental boundaries. Plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere. They create a wake in which new instabilities form downstream. These wakes form channels that are about 1000 km wide, and thus are possibly detectable by seismic tomography. Beneath fast plates, cold sinking instabilities are tilted in the direction opposite to plate motion, while they sink vertically for slow plates. These instabilities are too small to be detected by usual seismic methods, since they are about 200 km in lateral scale. However, this preferred orientation of instabilities below fast plates could produce a pattern of large-scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models.
Nonlinear Processes Geophysics, Vol. 25, pp. 99-123. pdf
Mantle
convection
Abstract: Recent advances in mantle convection modeling led to the release of a new generation of convection codes, able to self-consistently generate plate-like tectonics at their surface. Those models physically link mantle dynamics to surface tectonics. Combined with plate tectonic reconstructions, they have the potential to produce a new generation of mantle circulation models that use data assimilation methods and where uncertainties in plate tectonic reconstructions are taken into account. We provided a proof of this concept by applying a suboptimal Kalman filter to the reconstruction of mantle circulation (Bocher et al., 2016). Here, we propose to go one step further and apply the ensemble Kalman filter (EnKF) to this problem. The EnKF is a sequential Monte Carlo method particularly adapted to solve high-dimensional data assimilation problems with nonlinear dynamics. We tested the EnKF using synthetic observations consisting of surface velocity and heat flow measurements on a 2-D-spherical annulus model and compared it with the method developed previously. The EnKF performs on average better and is more stable than the former method. Less than 300 ensemble members are sufficient to reconstruct an evolution. We use covariance adaptive inflation and localization to correct for sampling errors. We show that the EnKF results are robust over a wide range of covariance localization parameters. The reconstruction is associated with an estimation of the error, and provides valuable information on where the reconstruction is to be trusted or not.
Geochemistry, Geophysics, Geosystems, Vol. 19, 9, pp. 3140-3163.
United States, California
subduction
Abstract: Mantle convection shapes Earth's surface by generating dynamic topography. Observational constraints and regional convection models suggest that surface topography could be sensitive to mantle flow for wavelengths as short as 1,000 and 250 km, respectively. At these spatial scales, surface processes including sedimentation and relative sea?level change occur on million?year timescales. However, time?dependent global mantle flow models do not predict small?scale dynamic topography yet. Here we present 2?D spherical annulus numerical models of mantle convection with large radial and lateral viscosity contrasts. We first identify the range of Rayleigh number, internal heat production rate and yield stress for which models generate plate?like behavior, surface heat flow, surface velocities, and topography distribution comparable to Earth's. These models produce both whole?mantle convection and small?scale convection in the upper mantle, which results in small?scale (<500 km) to large?scale (>104 km) dynamic topography, with a spectral power for intermediate scales (500 to 104 km) comparable to estimates of present?day residual topography. Timescales of convection and the associated dynamic topography vary from five to several hundreds of millions of years. For a Rayleigh number of 107, we investigate how lithosphere yield stress variations (1050 MPa) and the presence of deep thermochemical heterogeneities favor small?scale (200500 km) and intermediate?scale (500104 km) dynamic topography by controlling the formation of small?scale convection and the number and distribution of subduction zones, respectively. The interplay between mantle convection and lithosphere dynamics generates a complex spatial and temporal pattern of dynamic topography consistent with constraints for Earth.
Abstract: The existence of undulations of the geoid, gravity and bathymetry in ocean basins, as well as anomalies in heat flow, point to the existence of small scale convection beneath tectonic plates. The instabilities that could develop at the base of the lithosphere are sufficiently small scale (< 500 km) that they remain mostly elusive from seismic detection. We take advantage of 3D spherical numerical geodynamic models displaying plate-like behavior to study the interaction between large-scale flow and small-scale convection. We find that finger-shaped instabilities develop at seafloor ages > 60 Ma. They form networks that are shaped by the plate evolution, slabs, plumes and the geometry of continental boundaries. Plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere. They create a wake in which new instabilities form downstream. These wakes form channels that are about 1000 km wide, and thus are possibly detectable by seismic tomography. Beneath fast plates, cold sinking instabilities are tilted in the direction opposite to plate motion, while they sink vertically for slow plates. These instabilities are too small to be detected by usual seismic methods, since they are about 200 km in lateral scale. However, this preferred orientation of instabilities below fast plates could produce a pattern of large-scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models.
Abstract: Although plate tectonics has pushed the frontiers of geosciences in the past 50 years, it has legitimate limitations and among them we focus on both the absence of dynamics in the theory, and the difficulty of reconstructing tectonics when data is sparse. In this manuscript, we propose an anticipation experiment, proposing a singular outlook on plate tectonics in the digital era. We hypothesize that mantle convection models producing self?consistently plate?like behavior will capture the essence of the self?organisation of plate boundaries. Such models exist today in a preliminary fashion and we use them here to build a database of mid?ocean ridge and trench configurations. To extract knowledge from it we develop a machine learning framework based on Generative Adversarial Networks (GANs) that learns the regularities of the self?organisation in order to fill gaps of observations when working on reconstructing a plate configuration. The user provides the distribution of known ridges and trenches, the location of the region where observations lack, and our digital architecture proposes a horizontal divergence map from which missing plate boundaries are extracted. Our framework is able to prolongate and interpolate plate boundaries within an unresolved region, but fails to retrieve a plate boundary that would be completely contained inside of it. The attempt we make is certainly too early because geodynamic models need improvement and a larger amount of geodynamic model outputs, as independent as possible, is required. However, this work suggests applying such an approach to expand the capabilities of plate tectonics is within reach.
Earth and Planetary Science Letters, Vol. 547, 15p. Pdf
Mantle
geodynamics
Abstract: Mantle plumes provide valuable information about whole-mantle convection: they originate at the core-mantle boundary, cross Earth's mantle and interact with the lithosphere. For instance, it has been proposed that the mobility/stability of plumes depends on plume intrinsic properties, on how slabs interact with the basal boundary layer, on mantle flow, or on their proximity to mid-ocean ridges. Here, we use 3D-spherical models of mantle convection generating self-consistent plate-like behaviour to investigate the mechanisms linking tectonics and mantle convection to plume dynamics. Our models produce fully-dynamic mantle plumes that rise vertically with deflection and present excess temperatures, rising speeds, buoyancy and heat fluxes comparable to observations. In the absence of plate tectonics, plumes are stable and their lifetime exceeds hundreds of million years. With plate tectonics, plumes are more mobile, and we identify four physical mechanisms controlling their stability. 1/ Fixed plumes are located at saddle points of basal mantle flow. 2/ Plumes moving at speeds between 0.5-1 cm yr?1 are slowly entrained by passive mantle flow. 3/ Fast plume motions between 2-5 cm yr?1 lasting several tens of million years are caused by slab push. 4/ Plumes occasionally drift at speeds >5 cm yr?1 over <10 Myr through plume merging. We do not observe systematic anchoring of plumes to mid-oceanic ridges. Independent of the presence of a dense basal layer, plate-like regimes decrease the lifetime of plumes compared to stagnant-lid models. Plume age, temperature excess or buoyancy flux are not diagnostic of plume lateral speed. The fraction of plumes moving by less than 0.5 cm yr?1 is >25%, which suggests that fixed hotspot reference frames can be defined from carefully selected hotspot tracks.
Bonadiman, C., Beccaluva, L., Coltort, M., Siena, F.
Kimberlite like metasomatism and garnet signature in spinel peridotite xenoliths from Sal, Cape Verde Archipelago: relics of subcontinental mantle domain.
Abstract: Unravelling the physical state and properties of mantle rocks is crucial for understanding both plate tectonics, seismic activity, and volcanism. In this context, the knowledge of accurate elastic parameters of constituent mineral phases, and their variations with pressure (P) and temperature (T), is an essential requirement, that coupled with the thermal state of the lithosphere can provide a better understanding of its petrophysics and thermochemical structure. In this paper, we present an assessment of the thermoelastic parameters [in the form of P-V-T-K Equations of State (EoS)] of orthopyroxene, clinopyroxene, spinel and garnet based on X-Ray diffraction data and direct elastic measurements available in literature. The newly developed EoS are appropriate to describe the elastic behaviour of these phases under the most relevant P-T conditions and bulk compositions of the Earth's mantle. In combination with the published EoS for mantle olivine and magnesiochromite, these EoS are suitable to calculate the physical properties of mantle peridotites and their variation with P and T. Thanks to these EoS, we can evaluate how the variations in bulk composition and thermal regimes affect the density structure of the lithospheric mantle. Accordingly, the density structure of fertile and depleted peridotitic systems was calculated along the 35, 45 and 60 mWm?2 geothermal gradients at P comprised between 1 and 8 GPa. Under very cold geothermal gradients, the density of both fertile and depleted peridotitic systems progressively increases with depth, whereas under relatively hot conditions a first downwards decrease from 1 to ca 3 GPa is observed, followed by an increase downward. In mantle sections characterized by intermediate geotherms (45 mWm?2), the behaviour of the two systems differs up to ca 4 GPa, as the density of the depleted system remains nearly constant down to this depth whereas it moderately increases in the fertile system. The results of our simplified parameterisation, in agreement with classical thermodynamic modelling, indicate that the density structure of the lithospheric mantle is predominantly controlled by the P - T gradient variations, with some compositional control mostly arising at cold-intermediate thermal conditions. Integrated by geophysical and thermodynamic modelling, the newly developed and selected EoS could provide an alternative strategy to infer the elastic properties of mineral phases and peridotite rocks, under the most relevant P-T conditions and compositions of the Earth's mantle, without requiring sets of end-member properties and solution models.
Bol. Mus. Nac. Hist. Paraguay, , Vol. 20, 2, pp. 188-204. pdf
South America, Paraguay
geochemistry
Abstract: After some works of Jaime Baez-Presse that quoted the presence of diamonds in Eastern Paraguay, we have perfprmed a whole sampling a study relative to the indicator mineral for diamonds. Indicator minerals are mineral species that, when appearing as transported grains in clastic sediments, indicate the presence in bedrock of a specific type of mineralization, hydrothermal alteration or lithology. Their physical and chemical characteristics, including a relatively high density (heavy minerals), facilitate their preservation and identification. The heavy minerals represent an important exploration method for detecting a variety of ore deposit types including diamond, gold, Ni-Cu, PGE, and so on.. One of the most significant events in the application of indicator mineral methods in the past was the diamond exploration. This paper provides an overview of indicator mineral methods, i.e. presence of Cr-diopside, Pyrope-rich garnet and Picroilmenite, for diamond exploration along the Eastern Paraguay river. Unfortunately the above heavy mineraks, generally associated to the diamonds, do not appear in Eastern Paraguay, excluding this Country as a potential source for the diamond as economic potential source.
Journal of South American Earth Sciences, Vol. 58, March pp. 72-81.
South America, Paraguay, Brazil
Geochronology
Abstract: The magmatic rocks from Alto Paraguay (High Paraguay River extensional lineament), western Apa craton, mainly consist of several major circular alkaline complexes and some rhyolitic domes and ignimbrites. The former are characterized by intrusive Na-alkaline rock-types (nepheline syenites and syenites and effusive equivalents) topped by lava flows and ignimbrites. Two main evolved suites were defined using petrochemical and Sr- isotope data: an agpaitic suite in the north and a miaskitic suite in the south. The domes of subalkaline rhyolitic lavas and ignimbrites occur to the north of the alkaline complexes, along the Paraguay River, near the town of Fuerte Olimpo. The emplacement ages of the alkaline complexes were constrained using the K-Ar, Ar-Ar, Rb-Sr and Sm-Nd dating methods on whole rocks and/or mineral separates (amphibole, alkali feldspar and biotite). Ages are quite variable (Upper Permian to Middle Triassic), with average K-Ar and Ar-Ar ages of 248.8 ± 4.8 and 241.8 ± 1.1 Ma, respectively, and Rb-Sr and Sm-Nd age data giving best values from 248 ± 4 to 244 ± 27 Ma and from 256 ± 3 to 257 ± 3 Ma, respectively. In contrast, the Fuerte Olimpo volcanics show a Mesoproterozoic age (1.3 Ga, K-Ar and Ar-Ar radiometric methods; and 1.42 ± 0.24 to 1.30 ± 0.03 Ga, Rb-Sr and Sm-Nd methods, respectively). Rb-Sr systematics (87Sr/86Sr initial ratios ? 0.7038) highlight a relatively "primitive" character of the Na-alkaline magmatic source(s), in contrast with the "crustal" values (87Sr/86Sr initial ratio ? 0.7105) of the Fuerte Olimpo rhyolites. Thus, magmatism in the Alto Paraguay area is related to two extensional events: a younger event corresponding to the Permian-Triassic alkaline rocks, and an older event connected to the Precambrian volcanic acidic rocks.
Journal of South American Earth Sciences, Vol. 71, pp. 248-261.
South America, Paraguay
Carbonatite
Abstract: This work describes rare accessory minerals in volcanic and subvolcanic silica-undersaturated peralkaline and agpaitic rocks from the Permo-Triassic Cerro Boggiani complex (Eastern Paraguay) in the Alto Paraguay Alkaline Province. These accessory phases consist of various minerals including Th-U oxides/silicates, Nb-oxide, REE-Sr-Ba bearing carbonates-fluorcarbonates-phosphates-silicates and Zr-Na rich silicates. They form a late-stage magmatic to deuteric/metasomatic assemblage in agpaitic nepheline syenites and phonolite dykes/lava flows made of sodalite, analcime, albite, fluorite, calcite, ilmenite-pyrophanite, titanite and zircon. It is inferred that carbonatitic fluids rich in F, Na and REE percolated into the subvolcanic system and metasomatically interacted with the Cerro Boggiani peralkaline and agpaitic silicate melts at the thermal boundary layers of the magma chamber, during and shortly after their late-stage magmatic crystallization and hydrothermal deuteric alteration.
Omarini, R.H., Gasparon, M., De Min, A.M., Comin-Chiaramonti, P.
An overview of the Mesozoic-Cenozoic magmatism and tectonics of Eastern Paraguay and central Andes ( western Gondwana): implications for the composition of mantle sources.
Journal of South American Earth Sciences, In press available, 19p.
Journal of South American Earth Sciences, Vol. 72, pp. 302-314.
South America, Paraguay
Magmatism
Abstract: The amalgamation of the Western Gondwana (including the Greater Gondwana supercraton) occurred at 600 Ma during the Brazilian - Pan African orogeny. A plate junction related to this event is marked by the Transbrazilian lineament which separates the South American continent into two sectors: the Eastern Paraguay-Brazilian and Central Andean domains. An overview of the geodynamic data from these two sectors indicates that the two domains were subjected to distinct evolutions from the Proterozoic to the present. The Andean domain is characterized by long-lived subduction processes linked to the convergence and consequent collision of microplates since the Middle Proterozoic (western Amazonian Craton) with a peak at about 600-580 Ma. The Paraguay-Brazilian domain remained relatively stable but was affected by extension episodes that reactivated ancient (Early and Middle Proterozoic) suture zones. These different geodynamic evolutions seem to reflect broadly distinct mantle compositions. In the subduction zones of the Andean domain the mantle was deeply modified by metasomatic processes following the subduction of oceanic plates. Consequently, the Andean type magma sources show a clear HIMU imprint inherited from the MORB, whereas the Paraguay-Brazilian sector shows a prevalent EMI and subordinate EMII character. The petrological data mainly from Mesozoic and Cenozoic magmatic events in the two sectors are reviewed to investigate the current mantle plume and mantle dome models for the uprising of the asthenospheric (or sub-lithospheric) material.
Boletin del Museo Nacional de Historia Narural del Paraguay, Vol. 20, 2, pp. 188-204. pdf available in * Port
South America, Paraguay
Geochemistry - indicator minerals
Abstract: Many diamondiferous kimberlites in the Lac de Gras region of the Northwest Territories are concealed by glacial drift, rendering them challenging to detect by traditional exploration techniques that exploit residual surface chemistry. Much research has been aimed at the development of deep penetrating geochemical exploration technologies to increase the rate of discovery whilst reducing risk and exploration cost. However, results from a detailed study of soil geochemistry above the DO-18 kimberlite (Peregrine Diamonds) demonstrate the potential to apply conventional surface geochemical techniques coupled with surface material mapping and landscape evolution models to the evaluation of discrete targets. 50 soil samples from the oxidized upper B-horizon in a detailed grid crossing the concealed kimberlite were collected. Samples, screened to -180 microns, were analysed by multi-element ICP-MS following 4-acid, aqua-regia and deionized water extractions. Fp-XRF was utilised as an equivalent total method to evaluate its applicability. Sequential leach on selected samples was undertaken to understand the deportment of the elements of interest within the soils. Surficial mapping included soil type, topographic variation, landforms, environment and vegetation. This allows an assessment of surface controls on the geochemistry, in particular the generation of false anomalies from chemical traps such as swamps; and allows the generation of a landscape development model. Hydrocarbons, analysed using the SGH and Gore-sorber techniques, were evaluated to characterize the type and abundance of complex hydrocarbons above the kimberlite relative to above the host granitic gneiss. Geochemical data is subject to landform generation processes. The northern half of the grid comprises till with numerous frost boils. The southern half, at lower topography below a distinct break, is dominated by sand-rich material and fine clay. Results from the 4-acid and aqua regia extraction show a dispersion of Nb, Ni, Mg, Ce, Cr and Cs from directly above the northern part of the kimberlite to the edge of the sampling grid, approximately 500 metres to the northwest, following glacial dispersion. SGH-hydrocarbon results exhibit a similar pattern in light-alkyl benzenes. Fp-XRF data repeats the pattern in all elements except Mg, where the concentrations are too low for reliable detection. In the southern half of the grid, at a lower topographic level, geochemical responses are considerably more subtle. It is hypothesised that anomaly formation in the till followed standard glacial dispersion in the down ice trend. Material was entrained to the surface from deeper in the till, locally above the kimberlite, by frost boil action. The southern part of the area is considered to have been inundated with water, the remains of which comprise the current lake over the DO-27 kimberlite approximately 400m to the south. Sediments in this area are clay rich - comprising material deposited by the lake, or re-worked sandy material along the palaeo-lake margin and subsequent erosional channels. These later processes acted to further disperse, conceal and dilute the signal of the underlying body.
Bol. Mus. Nac. Hist. Parag. *** ENG, Vol. 20, 2, pp. 188-204. pdf
South America, Paraguay
geochemistry
Abstract: After some works of Jaime Baez-Presse that quoted the presence of diamonds in Eastern Paraguay, we have perfprmed a whole sampling a study relative to the indicator mineral for diamonds. Indicator minerals are mineral species that, when appearing as transported grains in clastic sediments, indicate the presence in bedrock of a specific type of mineralization, hydrothermal alteration or lithology. Their physical and chemical characteristics, including a relatively high density (heavy minerals), facilitate their preservation and identification. The heavy minerals represent an important exploration method for detecting a variety of ore deposit types including diamond, gold, Ni-Cu, PGE, and so on.. One of the most significant events in the application of indicator mineral methods in the past was the diamond exploration. This paper provides an overview of indicator mineral methods, i.e. presence of Cr-diopside, Pyrope-rich garnet and Picroilmenite, for diamond exploration along the Eastern Paraguay river. Unfortunately the above heavy mineraks, generally associated to the diamonds, do not appear in Eastern Paraguay, excluding this Country as a potential source for the diamond as economic potential source.
Ferrando, S., Frzzotti, M.L., Dallai, L., Compagnoni, R.
Multiphase solid inclusions in UHP rocks ( Su-Lu, China): remnants of supercritical silicate rich aqueous fluids released during continental subduction.
Chemical Geology, Vol. 223, 1-3, Nov. 22, pp. 68-81.
Malaspina, N., Hermann, J., Scambelluri, M., Compagnoni, R.
Polyphase inclusions in garnet orthopyroxenite (Dabie Shan, China) as monitors for metasomatism and fluid related trace element transfer in subduction zone.
Geochimica et Cosmochimica Acta, In press available
Campbell, L.S., Compston, W., Sircombe, K.N., Wilkinson, C.C.
Zircon from the East orebody of the Bayan Obo Fe Nb REE deposit, China, and SHRIMP ages for carbonatite related magmatism and REE mineralization events.
Contributions to Mineralogy and Petrology, Vol. 168, pp. 1041-
Lithos, doi.org/10.1016/ j.lithos.2020.105918 67p. Pdf
Africa, South Africa
deposit - Roberts Victor
Abstract: Platinum-group elements (PGE) display a chalcophile behaviour and are largely hosted by base metal sulphide (BMS) minerals in the mantle. During partial melting of the mantle, BMS release their metal budget into the magma generated. The fertility of magma sources is a key component of the mineralisation potential of large igneous provinces (LIP) and the origin of orthomagmatic sulphide deposits hosted in cratonic mafic magmatic systems. Fertility of mantle-derived magma is therefore predicated on our understanding of the abundance of metals, such as the PGE, in the asthenospheric and lithospheric mantle. Estimations of the abundance of chalcophile elements in the upper mantle are based on observations from mantle xenoliths and BMS inclusions in diamonds. Whilst previous assessments exist for the BMS composition and chalcophile element budget of peridotitic mantle, relatively few analyses have been published for eclogitic mantle. Here, we present sulphide petrography and an extensive in situ dataset of BMS trace element compositions from Roberts Victor eclogite xenoliths (Kaapvaal Craton, South Africa). The BMS are dominated by pyrite-chalcopyrite-pentlandite (± pyrrhotite) assemblages with S/Se ratios ranging 1200 to 36,840 (with 87% of analyses having S/Se this editing is incorrect. This should read "(with 87% of analyses having S/Se < 10,000)" Please note the 100 ppm) and are characteristically enriched in Os, Ir, Ru and Rh. Nano- and micron-scale Pd-Pt antimonide, telluride and arsenide platinum-group minerals (PGM) are observed spatially associated with BMS. We suggest that the predominance of pyrite in the xenoliths reflects the process of eclogitisation and that the trace element composition of the eclogite BMS was inherited from oceanic crustal protoliths of the eclogites, introduced into the SCLM via ancient subduction during formation of the Colesberg Magnetic Lineament c. 2.9 Ga and the cratonisation of the Kaapvaal Craton. Crucially, we demonstrate that the PGE budget of eclogitic SCLM may be substantially higher than previously reported, akin to peridotitic compositions, with significant implications for the PGE fertility of cratonic mafic magmatism and metallogenesis. We quantitatively assess these implications by modelling the chalcophile geochemistry of an eclogitic melt component in parental magmas of the mafic Rustenburg Layered Suite of the Bushveld Complex.
Journal of South American Earth Sciences, Vol. 97, 102416, 7p. Pdf
South America, Brazil, Minas Gerais
carbonatite
Abstract: The Alto Parnaíba Igneous Province (APIP) is a voluminous magmatic province composed of various alkaline-carbonatite complexes emplaced in the Brasilia Mobile Belt during the Cretaceous. Relative timing of emplacement of silicate and carbonate magmas in most of these complexes remains mostly unresolved due to conflicting geochronological results. To determine the duration of magmatism and to test the possible existence of multiple magmatic events, we employ 40Ar/39Ar phlogopite single crystal dating to determine the history of magma emplacement at the Tapira alkaline-carbonatite complex, Minas Gerais, Brazil. The new single crystal data indicate at least two magmatic events during the emplacement of this complex, the first at > 96.2 ± 0.8 Ma and the second at 79.15 ± 0.6 Ma. The first igneous event was responsible for emplacement of the silicate plutonic series, while the second event corresponds to the emplacement of primarily carbonatitic magmas, generating metasomatic phlogopite alteration in bebedourites. The ages of intrusion and cooling of the alkaline-carbonatite complexes in the APIP must be investigated in other complexes to determine if intrusion intervals of ~17 Ma or more are common regionally. Protracted intrusive events, if related to magma generation by passage of South America over a stationary Trindade plume, requires complex ponding and lateral magma flow below a slow-moving continent.
Geochemistry and radiogenic isotope characteristics of xenoliths in Archean Diamondiferous lamprophyres: implications for the Superior Province cratonic keel.
Abstract: The presence and speciation of volatile C-H-O elements in the silicate systems play an important role in the genesis of magmas on the Earth’s mantle, due to the fact that these elements, mainly in the form of H2O, CO2, CH4 and CxHy, decrease the solidi temperatures of source rocks, making magmatism possible in Earth’s present day thermal conditions [1]. Among those elements, carbon is the only element that changes its valence according to the oxygen fugacity (fO2) conditions of the environment, resulting in different speciation, as: CO3 -2, CO2, Cgraphite/diamond, CH4 or heavier hydrocarbons. In the present work, we are determining phase stability of minerals, water, CO2 and CH4 in the system Nefeline-Kalsilite-Diopside. Our experiments are conducted under 4.0 GPa and temperatures up to 1300°C, using a 1000 tonf hydraulic press coupled with toroidal chambers. Preliminary experiments performed at 1300°C and 4.0GPa (initial composition in the Olivine-Quartz- Kalsalite/Nepheline system: 40mol% Ol90, 40mol% Nph50Kls50 and 20mol% Qz, PH2O,CO2=Ptotal) resulted in the formation of forsterite (Fo90) in equilibrium with phlogopite (Phl), melt and volatile phases (CO2 and CH4). Closer to the Diopside vertice, the addition of CO3 to the sample resulted in a imisibility of a carbonatitic and a silicatic melt, in which the carbonititic melt is enriched in sodium, while the silcate melt is enriched in potassium. Appart from that, experiments in different parts of the diagram suggest compositions from nephelinite-kalsilitite to lamproites composition for the silicate melt in equilibrium with diopside (solid solution with omphacite) and phlogopite. This work is a continuation of previous work in the anhydrous diagram and future works will provide the addition of CH4 as the volatile phase
Abstract: The Rosário-6 alnöite is an alkaline occurrence that belongs to the Rosário do Sul kimberlitic field, situated in the south-eastern edge of the Paraná Basin, in the South of Brazil, and erupted concomitant or just after the volcanism of the Paraná-Etendeka Large Igneous Province (LIP). Following recent published nomenclature, Rosário-6 was classified as a kimberlite from a deep mantle source with a distinctive inequigranular texture resulting from the presence of olivine macrocrysts set in a finer-grained matrix. Trace element compositions of olivine, monticellite, spinel, phlogopite, perovskite and apatite show an enrichment of Nb, Ce, Ta and U, which implies that the Rosário-6 mantle source was enriched by recycled oceanic crust. The positive anomalies of Rb, Ba and Sr, the enrichment in LREE, and the negative anomalies of HREE in the Rosário-6 minerals, are indicative of a metasomatic process in the mantle source that could be caused by fluids from recycled oceanic crust. Temperature, pressure and redox conditions (fO2) of Rosário-6 crystallization are estimated from olivine, spinel, perovskite and monticellite compositions: Rosário-6 crystallization temperatures using olivine-spinel geothermobarometry were around 1390(±56)°C at a pressure of 2 GPa, and 1405(±56)°C at 3 GPa with ?NNO = 2.8, at pressures constrained by the silica activity limited by the crystallization of monticellite. Using a perovskite oxybarometer, we obtained a larger range of ?NNO (from -2.8 to 3.4), whereas the monticellite oxybarometer results in fO2 of -2.6 to -0.8 ?NNO units. The fO2 indicate that the mantle source of Rosário-6 at the time of crystallization was possibly oxidized by materials from ancient subduction, which may be the cause for Rosário-6's low potential to carry and preserve diamonds. Horizontal tomographic images derived from P-wave velocity data constrain the thickness of the lithosphere in this region and the overall information indicates that mantle cooling at depths below 200 km may have resulted of an accumulation of oceanic plate slabs from old subduction. The geochemical data in conjunction with the geophysical characterizes the conditions of Rosário-6 mineral crystallization and also the mantle of this part of South America during Gondwana breakup.
Abstract: To test if hydrogen incorporation by ionic diffusion can occur between a volatile-rich kimberlitic liquid and forsterite, results of high-pressure and high-temperature experiments using a piston-cylinder apparatus at 1200–1300 °C and 1 GPa for durations of 1 min, 5 h, and 23 h, are reported here. Kim-berlitic liquid in the system CaO-MgO-Al 2 O 3-SiO 2-CO 2-H 2 O and synthetic forsterite single crystals were chosen as a first simplification of the complex natural kimberlite composition. Unpolarized Fourier transform infrared spectroscopy was used to quantify the concentrations of OH in the crystallographically oriented forsterite. Scanning electron microscopy, electron backscattered diffraction, electron microprobe analyses, and transmission electron microscopy were performed to identify the run products. After 5 and 23 h, a forsterite overgrowth crystallized with the same orientation as the initial forsterite single crystal. The kimberlitic liquid has crystallized as micrometer-scale euhedral forsterite neocrystals with random crystallographic orientations, as well as a nanoscale aluminous phase and a calcic phase. Despite theoretical water-saturation of the system and long duration, none of the initial forsterite single crystals display signs of hydration such as hydrogen diffusion profile from the border toward the center of the crystal. Most likely, the presence of CO 2 in the system has lowered the H 2 O fugacity to such an extent that there is no significant hydration of the starting forsterite single crystal or its overgrowth. Also, the presence of CO 2 enhances rapid forsterite crystal growth. Forsterite growth rate is around 2 × 10 8 mm 3 /h at 1250 °C. These experimental results suggest a deep mantle origin of the high OH content found in natural mantle-derived xenoliths transported in kimberlites, as reported from the Kaapvaal craton. In agreement with previous studies, it also points out to the fact that significant hydration must take place in a CO 2-poor environment.
Contributions to Mineralogy and Petrology, Vol. 171, pp. 95-121.
Technology
Melting - peridotite
Abstract: Melting experiments have been performed at 3 GPa, between 1150 and 1450 °C, on a phlogopite-peridotite source in the garnet stability field. We succeeded to extract and determine the melt compositions of both phlogopite-bearing lherzolite and harzburgite from low to high degrees of melting (? = 0.008-0.256). Accounting for the presence of small amounts of F in the mantle, we determined that phlogopite coexists with melt >150 °C above the solidus position (1150-1200 °C). Fluorine content of phlogopite continuously increases during partial melting from 0.2 to 0.9 wt% between 1000 and 1150 °C and 0.5 to 0.6 wt% between 1150 and 1300 °C at 1 and 3 GPa, respectively. The phlogopite continuous breakdown in the lherzolite follows the reaction: 0.59 phlogopite + 0.52 clinopyroxene + 0.18 garnet = 0.06 olivine + 0.23 orthopyroxene + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.93 phlogopite + 0.46 garnet = 0.25 olivine + 0.14 orthopyroxene + 1.00 melt. Melts from phlogopite-peridotite sources at 3 GPa are silica-undersaturated and are foiditic to trachybasaltic in composition from very low (0.8 wt%) to high (25.6 wt%) degrees of melting. As observed at 1 GPa, the potassium content of primary mantle melts is buffered by the presence of phlogopite, but the buffering values are higher, from 6.0 to 8.0 wt% depending on the source fertility. We finally show that phlogopite garnet-peridotite melts are very close to the composition of the most primitive post-collisional lavas described worldwide.
Contributions to Mineralogy and Petrology, Vol. 171, pp. 95-106.
Mantle
Melting
Abstract: Melting experiments have been performed at 3 GPa, between 1150 and 1450 °C, on a phlogopite-peridotite source in the garnet stability field. We succeeded to extract and determine the melt compositions of both phlogopite-bearing lherzolite and harzburgite from low to high degrees of melting (? = 0.008-0.256). Accounting for the presence of small amounts of F in the mantle, we determined that phlogopite coexists with melt >150 °C above the solidus position (1150-1200 °C). Fluorine content of phlogopite continuously increases during partial melting from 0.2 to 0.9 wt% between 1000 and 1150 °C and 0.5 to 0.6 wt% between 1150 and 1300 °C at 1 and 3 GPa, respectively. The phlogopite continuous breakdown in the lherzolite follows the reaction: 0.59 phlogopite + 0.52 clinopyroxene + 0.18 garnet = 0.06 olivine + 0.23 orthopyroxene + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.93 phlogopite + 0.46 garnet = 0.25 olivine + 0.14 orthopyroxene + 1.00 melt. Melts from phlogopite-peridotite sources at 3 GPa are silica-undersaturated and are foiditic to trachybasaltic in composition from very low (0.8 wt%) to high (25.6 wt%) degrees of melting. As observed at 1 GPa, the potassium content of primary mantle melts is buffered by the presence of phlogopite, but the buffering values are higher, from 6.0 to 8.0 wt% depending on the source fertility. We finally show that phlogopite garnet-peridotite melts are very close to the composition of the most primitive post-collisional lavas described worldwide.
International Geology Review, Vol. 57, 11-12, pp. 1341-1348.
Mantle
Supercontinents
Abstract: There is a correlation of global large igneous province (LIP) events with zircon age peaks at 2700, 2500, 2100, 1900, 1750, 1100, and 600 and also probably at 3450, 3000, 2000, and 300?Ma. Power spectral analyses of LIP event distributions suggest important periodicities at 250, 150, 100, 50, and 25?million years with weaker periodicities at 70-80, 45, and 18-20?Ma. The 25?million year periodicity is important only in the last 300?million years. Some LIP events are associated with granite-forming (zircon-producing) events and others are not, and LIP events at 1900 and 600?Ma correlate with peaks in craton collision frequency. LIP age peaks are associated with supercontinent rifting or breakup, but not dispersal, at 2450-2400, 2200, 1380, 1280, 800-750, and ?200?Ma, and with supercontinent assembly at 1750 and 600?Ma. LIP peaks at 2700 and 2500?Ma and the valley between these peaks span the time of Neoarchaean supercraton assemblies. These observations are consistent with plume generation in the deep mantle operating independently of the supercontinent cycle and being controlled by lower-mantle and core-mantle boundary thermochemical dynamics. Two processes whereby plumes can impact continental assembly and breakup are (1) plumes may rise beneath supercontinents and initiate supercontinent breakup, and (2) plume ascent may increase the frequency of craton collisions and the rate of crustal growth by accelerating subduction.
Abstract: Zircon age peaks are commonly interpreted either as crustal production peaks or as selective preservation peaks of subduction-produced crust selectively preserved during continent-continent collision. We contribute to this ongoing debate, using the Nd isotopic compositions of felsic igneous rocks and their distribution during the accretionary and collisional phases of orogens. The proportion of juvenile input into the continental crust is estimated with a mixing model using arc-like mantle and reworked continental crust end members. Orogen length and duration proxies for juvenile crustal volume show that the amount of juvenile crust produced and preserved at zircon age peaks during the accretionary phase of orogens is ?3 times that preserved during the collisional phase of orogens. The fact that most juvenile crust is both produced and preserved during the accretionary phase of orogens does not require craton collisions for its preservation, thus favoring the interpretation of zircon age peaks as crustal production peaks. Most juvenile continental crust older than 600 Ma is produced and preserved before final supercontinent assembly and does not require supercontinent assembly for its preservation. Episodic destabilization of a compositionally heterogeneous layer at the base of the mantle may produce mantle plume events leading to enhanced subduction and crustal production. Our Nd isotope model for cumulative continental growth based on juvenile crust proxies for the past 2.5 b.y. suggests a step-like growth curve with rapid growth in accretionary orogens at the times of zircon age peaks.
Geochimica et Cosmochimica Acta, Vol. 213, pp. 47-62.
Mantle
tectonics
Abstract: The distribution of high field strength incompatible element ratios Zr/Nb, Nb/Th, Th/Yb and Nb/Yb in terrestrial oceanic basalts prior to 2.7 Ga suggests the absence or near-absence of an enriched mantle reservoir. Instead, most oceanic basalts reflect a variably depleted mantle source similar in composition to primitive mantle. In contrast, basalts from hydrated mantle sources (like those associated with subduction) exist from 4 Ga onwards. The gradual appearance of enriched mantle between 2 and 3 Ga may reflect the onset and propagation of plate tectonics around the globe. Prior to 3 Ga, Earth may have been in a stagnant-lid regime with most basaltic magmas coming from a rather uniform, variably depleted mantle source or from a non-subduction hydrated mantle source. It was not until the extraction of continental crust and accompanying propagation of plate tectonics that “modern type” enriched and depleted mantle reservoirs developed. Consistent with the absence of plate tectonics on the Moon is the near absence of basalts derived from depleted (DM) and enriched (EM) mantle reservoirs as defined by the four incompatible element ratios of this study. An exception are Apollo 17 basalts, which may come from a mixed source with a composition similar to primitive mantle as one end member and a high-Nb component as the other end member. With exception of Th, which requires selective enrichment in at least parts of the martian mantle, most martian meteorites can be derived from sources similar to terrestrial primitive mantle or by mixing of enriched and depleted mantle end members produced during magma ocean crystallization. Earth, Mars and the Moon exhibit three very different planetary evolution paths. The mantle source regions for Mars and the Moon are ancient and have HFS element signatures of magma ocean crystallization well-preserved, and differences in these signatures reflect magma ocean crystallization under two distinct pressure regimes. In contrast, plate tectonics on Earth has destroyed most or all of the magma ocean crystallization geochemical record, or less likely, the terrestrial magma ocean may not have been strongly fractionated during crystallization. The rather uniform incompatible element ratio record in pre-2 Ga oceanic terrestrial basalts requires vigorous mixing of most of the mantle between magma ocean crystallization and about 4 Ga, the onset of the preserved greenstone record.
The 1.75 Ga Iron King volcanics in west central Arizona: a remnant of an accreted oceanic plateau derived from a mantle plume with a deep depleted component.
Lithos, Vol. 64, 1-2, pp. 49-62.
Arizona
Mantle plume - not specific to diamonds, Component
Condie, K.C., Cox, J., O'Reilly, S.Y., Griffin, W.L., Kerrich, R.
Definition of high field strength and rare elements in mantle and lower crustal xenoliths from the SE United States: the role of grain boundary phases.
Geochimica et Cosmochimica Acta, Vol. 68, 19, pp. 3919-3942.
Using xenoliths to explore variations in upper mantle composition and the relation of composition to seismic velocity structure beneath the Colorado Plateau.
Geological Society of America Annual Meeting 2007, Denver Oct. 28, 1p. Abstract
Abstract: Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga. Among the more important of these are the following: (1) increasing proportion of basalts with “arc-like” mantle sources; (2) an increasing abundance of basalts derived from enriched (EM) and depleted (DM) mantle sources; (3) onset of a Great Thermal Divergence in the mantle; (4) a decrease in degree of melting of the mantle; (5) beginning of large lateral plate motions; (6) appearance of eclogite inclusions in diamonds; (7) appearance and rapid increase in frequency of collisional orogens; (8) rapid increase in the production rate of continental crust as recorded by zircon age peaks; (9) appearance of ophiolites in the geologic record, and (10) appearance of global LIP (large igneous province) events some of which correlate with global zircon age peaks. All of these changes may be tied directly or indirectly to cooling of Earth's mantle and corresponding changes in convective style and the strength of the lithosphere, and they may record the gradual onset and propagation of plate tectonics around the planet. To further understand the changes that occurred between 3 and 2 Ga, it is necessary to compare rocks, rock associations, tectonics and geochemistry during and between zircon age peaks. Geochemistry of peak and inter-peak basalts and TTGs needs to be evaluated in terms of geodynamic models that predict the existence of an episodic thermal regime between stagnant-lid and plate tectonic regimes in early planetary evolution.
Abstract: Before 2.7?Ga, 14 igneous and detrital zircon age peaks and 9 large igneous province (LIP) age peaks are robust and statistically significant. Correlation analysis indicates a synchronous association among these peaks and power spectral analysis shows 91, 114-127 and 182-Myr cycles. These age cycles may be related to mantle plume or mantle overturn events, and to the time it takes to reach threshold temperature gradients for thermo-chemical destabilization in the lowermost mantle. Most zircon age peaks are transferred into younger detrital sediments, which does not favor an origin of the peaks by selective erosion. Correlation of eight pre-2.7-Ga LIP age peaks with zircon age peaks is consistent with a genetic relationship between mantle melting events and felsic crustal production and supports an interpretation of pre-2.7-Ga age peaks as growth rather than preservation peaks produced during craton collisions. Also consistent with the growth peak interpretation is the apparent absence of collisional orogens older than 2.7?Ga. An increasing number of geographic age peak sites from 4 to 2.8?Ga suggests production and survival of only small volumes of continental crust during this time and supports an episodic model for continental crustal growth.
Geochimica et Cosmochimica Acta, in press available, 32p.
Asia, Tibet, Andes
melting
Abstract: The lithospheric mantle beneath Archean cratons is conspicuously refractory and thick compared to younger continental lithosphere (Jordan, 1988, Boyd, 1989; Lee and Chin, 2014), but how such thick lithospheres formed is unclear. Using a large global geochemical database of Archean igneous crustal rocks overlying these thick cratonic roots, we show from Gd/Yb- and MnO/FeOT-SiO2 trends that crustal differentiation required continuous garnet fractionation. Today, these signatures are only found where crust is anomalously thick (60-70?km), as in the Northern and Central Andes and Southern Tibet. The widespread garnet signature in Archean igneous suites suggests that thickening occurred not only in the lithospheric mantle but also in the crust during continent formation in the late Archean. Building thick crust requires tectonic thickening or magmatic inflation rates that can compete against gravitational collapse through lower crustal flow, which would have been enhanced in the Archean when geotherms were hotter and crustal rocks weaker. We propose that Archean crust and mantle lithosphere formed by thickening over mantle downwelling sites with minimum strain rates on the order of 10?13-10?12 s?1, requiring mantle flow rates associated with late Archean crust formation to be 10-100 times faster than today.
Abstract: Igneous and detrital zircons have six major U/Pb isotopic age peaks in common (2700 Ma, 1875 Ma, 1045 Ma, 625 Ma, 265 Ma and 90 Ma). For igneous rocks, each age peak is comprised of subpeaks with distinct geographic distributions and a subpeak age range per age peak ?100 Myr. There are eight major LIP age peaks (found on ? 10 crustal provinces) of which only four are in common to major detrital zircon age peaks (2715 Ma, 1875 Ma, 825 Ma, 90 Ma). Of the whole-rock Re depletion ages, 58% have corresponding detrital zircon age peaks and 55% have corresponding LIP age peaks. Ten age peaks are found in common to igneous zircon, detrital zircon, LIP, and Re depletion age time series (3225 Ma, 2875 Ma, 2145 Ma, 2085 Ma, 1985 Ma, 1785 Ma, 1455 Ma, 1175 Ma, 825 Ma, and 90 Ma), and these are very robust peaks on a global scale as recorded in both crustal and mantle rocks. About 50% of the age peaks in each of these time series correspond to predicted peaks in a 94-Myr mantle cycle, including four of the ten peaks in common to all four time series (2875 Ma, 1785 Ma, 825 Ma and 90 Ma). Age peak widths and subpeak ranges per age peak suggest that mantle events responsible for age peaks are <100 Myr and many <50 Myr in duration. Age peak geographic distributions show three populations (?1000 Ma, 2500-1000 Ma, ?2500 Ma), with the number of new provinces in which age peaks are represented decreasing with time within each population. The breaks between the populations (at 2.5 Ga and 1 Ga) fall near the onsets of two transitions in Earth history. The First Transition may represent a change from stagnant-lid tectonics into plate tectonics and the Second Transition, the onset of subduction of continental crust. The major factor controlling geographic distribution of age peaks is the changing locations of orogeny. Before ?2 Ga, age subpeaks and peaks are housed in orogens within or around the edges of crustal provinces, mostly in accretionary orogens, but beginning at 1.9 Ga, collisional orogens become more important. The coincidence in duration between magmatic flare-ups in Phanerozoic arcs and duration of age subpeaks (10-30 Myr) is consistent with subpeaks representing periods of enhanced arc-related magmatism, probably caused by increased subduction flux. The correlation of isotopic age peaks between time series supports a cause and effect relationship between mantle plume activity, continental magma production at convergent margins, and crustal deformation. Correlation of over half of the detrital zircon age peaks (and six of the nine major peaks) with Re depletion age peaks supports an interpretation of the zircon peaks as crustal growth rather than selective preservation peaks.
Abstract: In this study, seven isotopic databases are presented and analyzed to identify mantle and crustal episodes on a global scale by focusing on periodicity ranging from 70 to 200 million years (Myr). The databases are the largest, or among the largest, compiled for each type of data - with an objective of finding some samples from every region of every continent, to make each database as global as conceivably possible. The databases contain zircon Lu/Hf isotopic data, whole-rock Sm/Nd isotopic data, U/Pb detrital zircon ages, U/Pb igneous zircon ages, U/Pb non-zircon ages, whole-rock Re/Os isotopic data, and large igneous province ages. Part I of this study focuses on the periodicities of age histograms and geochemical averages developed from the seven databases, via spectral and cross-correlation analyses. Natural physical cycles often propagate in exact integer multiples of a fundamental cycle, referred to as harmonics. The tests show that harmonic geological cycles of ?93.5 and ?187 Myr have persisted throughout terrestrial history, and the cyclicities are statistically significant for U/Pb igneous zircon ages, U/Pb detrital zircon ages, U/Pb zircon-rim ages, large igneous province ages, mean ?Hf(t) for all samples, mean ?Hf(t) values for igneous-only samples, and relative abundance of mafic rocks. Equally important, cross-correlation analyses show these seven time-series are nearly synchronous (±7 Myr) with a model consisting of periodicities of 93.5 and 187 Myr. Additionally, the similarities between peaks in the 93.5 and 187 Myr mantle cycles and terminal ages of established and suspected superchrons provide a framework for predicting and testing superchron periodicity.
Geochimica et Cosmochimica Acta, Vol. 278, pp. 6-15.
Mantle
craton
Abstract: The lithospheric mantle beneath Archean cratons is conspicuously refractory and thick compared to younger continental lithosphere ( Jordan, 1988 , Boyd, 1989 ; Lee and Chin, 2014), but how such thick lithospheres formed is unclear. Using a large global geochemical database of Archean igneous crustal rocks overlying these thick cratonic roots, we show from Gd/Yb- and MnO/FeOT-SiO2 trends that crustal differentiation required continuous garnet fractionation. Today, these signatures are only found where crust is anomalously thick (60-70?km), as in the Northern and Central Andes and Southern Tibet. The widespread garnet signature in Archean igneous suites suggests that thickening occurred not only in the lithospheric mantle but also in the crust during continent formation in the late Archean. Building thick crust requires tectonic thickening or magmatic inflation rates that can compete against gravitational collapse through lower crustal flow, which would have been enhanced in the Archean when geotherms were hotter and crustal rocks weaker. We propose that Archean crust and mantle lithosphere formed by thickening over mantle downwelling sites with minimum strain rates on the order of 10?13-10?12 s?1, requiring mantle flow rates associated with late Archean crust formation to be 10-100 times faster than today.
Abstract: Many of the peculiar features of the Mesoproterozoic (1.6-1.0 Ga) are related to the assembly of the first supercontinent Nuna, and some may not be distinctive of this time period. A high frequency of A-type granites at 1.6-1.4 Ga may be due, in part, to sampling biases. The overall increase in frequency in A-type granites beginning at 1.9 Ga may track the propagation of plate tectonics especially in the Great Proterozoic Accretionary Orogen. Increases in alkali and related elements in granitoids at 2.4-2.0 Ga may reflect widespread propagation of subduction as plate tectonics spread around the globe, and increases in HREE, Nb, Ti and Sc in granitoids may be related to decreasing importance of restitic garnet in granitoid sources as Earth transitioned from TTG to calc-alkaline magmatic regimes. Related to possible global mantle events at 1.9, 1.0 and 0.6 Ga are peaks in frequency of LIP and zircon ages related to supercontinent assembly. Mesoproterozoic paleomagnetic data that require at least 12 passive margins during this time. An increase in plate speed with time since 2 Ga may reflect a decreasing viscosity contrast across the lithosphere-asthenosphere boundary in response to a cooling mantle or/and thinning of the lithosphere and corresponding increases in heat flux with time. A relatively constant 87Sr/86Sr of seawater between 1.9 and 1.0 Ga may have resulted from a balance between weathered juvenile and reworked crustal sources as well as enhanced seafloor weathering of oceanic basalts. Mostly positive eHf(t) in detrital zircons and eNd(t) granitoids during the Mesoproterozoic suggest significant juvenile crustal input. However, the sparsity of crust of this age today probably results from recycling of large volumes of continental crust into the mantle in peripheral accretionary orogens during the breakup of Nuna at 1.4-1.2 Ga.
Abstract: The thermal evolution of subducting slabs controls a range of subduction processes, yet we lack a robust understanding of how thermal structure develops over a subduction zone's lifetime. We investigate the time-dependence of slab thermal structure using dynamically consistent, time evolving models. Pressure-temperature (P-T) conditions along the slab Moho and slab top exhibit substantial variability throughout the various phases of subduction: initiation, free sinking, and mature subduction. This variability occurs in response to time-dependent subduction properties (e.g., fast vs. slow convergence) and thermal structure inherited from previous phases (e.g., due to upper plate aging). At a given depth, the slab cools rapidly during initiation, after which slower cooling occurs. In the case of the Moho, additional cooling occurs during the free sinking phase. We explore the implications of time-dependent thermal structure on exhumed rocks and slab dehydration. Modeled slab top P-T paths span much of the P-T space associated with exhumed rocks, suggesting a significant component of recorded variability may have dynamic origins. Coupling our P-T profiles with thermodynamic models of oceanic lithosphere, we show that dehydrating ultramafic rocks at the slab Moho provide the bulk of hydrous fluid at subarc depths during the earliest phases. Over subsequent phases, these rocks carry fluids into the deeper mantle, and it is mafic crust along the slab top that releases water at subarc depths. We conclude that varying subduction conditions, and non-steady-state thermal structure, challenge the utility of kinematically driven models with constant subduction parameters, particularly for investigating thermal structure in the geological past.
Journal of South American Earth Sciences, Vol. 79, pp. 431-442.
South America, Brazil
craton
Abstract: The study of the crust using receiver functions can provide valuable geological information, such as average crustal composition, its formation dynamics and the tectonic evolution of a region, as well as serve as an initial reference for the generation of seismic wave velocity models to improve earthquake location. To fill in gaps in information on the crust of the Amazonian Craton and adjacent provinces in Brazil, we used receiver functions and H-k stacking to estimate crustal thicknesses and the VP/VS ratios. The results indicate that the crust of the study region is predominantly felsic, with an average VP/VS around 1.73 and an average thickness of 38.2 km, with a range of 27.4-48.6 km. Minimum curvature interpolation of the crustal thickness values has made it possible to delimitate of the Amazonian Craton, which corresponds to the area with an average thickness equal to or greater than 39 km. In addition, it was possible to identify its potential cratonic blocks, as well as the Paranapanema Block of Paraná Basin. The geometry of the craton, defined by its crustal thickness, is corroborated by the distribution of natural seismicity that accompanies its edges. These are related to suture zones between the Amazonian, São Francisco/Congo and Paranapanema paleocontinents. The sedimentary basins that have undergone rifting processes have a thinner crust, usually less than 37 km thick. Due to the great variability of the results, it was not possible to determine a characteristic value of c
Abstract: Megacrysts are mineral grains of garnet, clinopyroxene, orthopyroxene, ilmenite, olivine, phlogopite and zircon larger than 10 mm frequently observed in kimberlite occurrences across the world, with reported sizes commonly exceeding 10 cm. Despite their common occurrence and decades of research into their origin, megacryst petrogenesis is still a debated topic amongst petrologists. A strictly phenocrystal origin is doubted, with recent research suggesting a multi-stage model involving isobaric formation over a wide temperature range, followed by metasomatism of a protokimberlite fluid that replaces mantle minerals. Our project aims to contribute to ongoing research by modeling the metasomatism of the ambient peridotitic mantle affected by the fluid using major and trace element data obtained from megacrysts from the Jurassic Muskox kimberlite pipe of the Slave province of Canada. We report major element compositions of 24 megacryst samples of garnet, olivine, clinopyroxene and ilmenite and employ DEW (Deep Earth Water) modelling to establish the composition of the potential metasomatizing agent and mineral trends that result from the mantle metasomatism. This project has important implications for not only constraining the composition of the source fluids, but also understanding the reactions in the cratonic mantle leading to the kimberlite melt formation.
Journal of the Geological Society, doi.org/10.1144 /jgs2020-184 53p. Pdf
Canada, Northwest Territories
deposit - Muskox
Abstract: Low-Cr and high-Cr clinopyroxene, garnet, olivine, and ilmenite megacrysts from the Muskox kimberlite (Canada) have been analyzed for major and trace elements, as well as Sr, Nd, and Pb isotopes. Samples display compositional overlap with respective phases in websterite, while clinopyroxene isotope systematics reveal similarities with both websteritic and metasomatic clinopyroxene in peridotites from the same kimberlite, in addition to Muskox and Jericho kimberlite. All lithologies may represent the products of mixing between EM1 mantle, relic Proterozoic enriched mantle and HIMU carbonatitic fluid. Equilibrium melts calculated from clinopyroxene trace element data using experimental distribution coefficients for feasible proto-kimberlitic melts yield a range of possible metasomatic agents. Conclusion on the carbonate-bearing nature of the metasomatism was based on the presence of a HIMU isotopic signature and results obtained from thermodynamic modeling using the Deep Earth Water model. The latter shows that mineral compositions analogous to megacrysts cannot be produced by metasomatism of mantle peridotite by H2O-rich kimberlitic fluids, or fluids in equilibrium with either asthenospheric or eclogitic mantle. Isotope systematics argue against a strictly cognate relationship between megacrysts and their host kimberlite, instead suggesting megacrysts and websterites may represent products of regional metasomatism by carbonatitic HIMU fluids shortly predating kimberlite magmatism.
Cobden, L., Goes, S., Ravenna, M., Styles, E., Cammarano, F., Gallagher, K., Connolly, J.
Thermochemical interpretation of 1-D seismic dat a for the lower mantle: the significance of nonadiabiatic thermal gradients and compositional heterogeneity.
Journal of Geophysical Research, Vol. 114, B 11, B11309
Computation of phase equilibration temperatures by linear programming: a tool for geodynamic modeling and its application to subduction zone decarbonation.
Earth and Planetary Science Letters, Advanced in press,
Contributions to Mineralogy and Petrology, in press available, 17p.
Mantle
Peridotite
Abstract: Natural moissanite (SiC) is reported from mantle-derived samples ranging from lithospheric mantle keel diamonds to serpentinites to podiform chromitites in ophiolites related to suprasubduction zone settings (Luobusa, Dongqiao, Semail, and Ray-Iz). To simulate ultra-reducing conditions and the formation of moissanite, we compiled thermodynamic data for alloys (Fe-Si-C and Fe-Cr), carbides (Fe3C, Fe7C3, SiC), and Fe-silicides; these data were augmented by commonly used thermodynamic data for silicates and oxides. Computed phase diagram sections then constrain the P-T-fO2 conditions of SiC stability in the upper mantle. Our results demonstrate that: Moissanite only occurs at oxygen fugacities 6.5-7.5 log units below the iron-wustite buffer; moissanite and chromite cannot stably coexist; increasing pressure does not lead to the stability of this mineral pair; and silicates that coexist with moissanite have X Mg > 0.99. At upper mantle conditions, chromite reduces to Fe-Cr alloy at fO2 values 3.7-5.3 log units above the moissanite-olivine-(ortho)pyroxene-carbon (graphite or diamond) buffer (MOOC). The occurrence of SiC in chromitites and the absence of domains with almost Fe-free silicates suggest that ultra-reducing conditions allowing for SiC are confined to grain scale microenvironments. In contrast to previous ultra-high-pressure and/or temperature hypotheses for SiC origin, we postulate a low to moderate temperature mechanism, which operates via ultra-reducing fluids. In this model, graphite-/diamond-saturated moderately reducing fluids evolve in chemical isolation from the bulk rock to ultra-reducing methane-dominated fluids by sequestering H2O into hydrous phases (serpentine, brucite, phase A). Carbon isotope compositions of moissanite are consistent with an origin of such fluids from sediments originally rich in organic compounds. Findings of SiC within rocks mostly comprised by hydrous phases (serpentine + brucite) support this model. Both the hydrous phases and the limited diffusive equilibration of SiC with most minerals in the rocks indicate temperatures below 700-800 °C. Moissanite from mantle environments is hence a mineral that does not inform on pressure but on a low to moderate temperature environment involving ultra-reduced fluids. Any mineral in equilibrium with SiC could only contain traces of Fe2+ or Cr3+.
Fullea, J.J.C., Afonso, L.A.D., Connolly, M., et al.
LitMod3D: an interactive 3-D software to model the thermal, compositional, density, seismological, and rheological structure of the lithosphere and sublithos
Geochemistry, Geophysics, Geosystems: G3, Vol. 16, 2, pp. 366-379.
United States, Colorado Plateau
Convection
Abstract: Although volcanism in the southwestern United States has been studied extensively, its origin remains controversial. Various mechanisms such as mantle plumes, upwelling in response to slab sinking, and small-scale convective processes have been proposed, but have not been evaluated within the context of rapidly shearing asthenosphere that is thought to underlie this region. Using geodynamic models that include this shear, we here explore spatiotemporal patterns of mantle melting and volcanism near the Colorado Plateau. We show that the presence of viscosity heterogeneity within an environment of asthenospheric shearing can give rise to decompression melting along the margins of the Colorado Plateau. Our models indicate that eastward shear flow can advect pockets of anomalously low viscosity toward the edges of thickened lithosphere beneath the plateau, where they can induce decompression melting in two ways. First, the arrival of the pockets critically changes the effective viscosity near the plateau to trigger small-scale edge-driven convection. Second, they can excite shear-driven upwelling (SDU), in which horizontal shear flow becomes redirected upward as it is focused within the low-viscosity pocket. We find that a combination of “triggered” edge-driven convection and SDU can explain volcanism along the margins of the Colorado Plateau, its encroachment toward the plateau's southwestern edge, and the association of volcanism with slow seismic anomalies in the asthenosphere. Geographic patterns of intraplate volcanism in regions of vigorous asthenospheric shearing may thus directly mirror viscosity heterogeneity of the sublithospheric mantle.
Geophysical Research Letters, Vol. 45, 15, pp. 7523-7532.
Mantle
bridgemanite
Abstract: Seismic images of the Earth's mantle show two anomalous continent?sized regions close to the core?mantle boundary. The inferred properties of these regions suggest that they have a different composition than the surrounding mantle. Two possible candidate materials have been proposed: accumulated oceanic crust from the Earth's surface or an iron?rich residue remaining from Earth's original magma ocean. Although both materials are denser than the surrounding mantle, it remains unclear whether piles of these chemical heterogeneities can survive at the core?mantle boundary beneath vigorous mantle convection. Numerical models show that the excess density required to preserve these structures is typically larger than indicated by seismological and gravitational observations. In this study, we show that the excess density used in numerical models can be reduced toward the observed values if the pile material is also stiffer than the surrounding mantle. Furthermore, we show that piles must be denser and/or stiffer to avoid destruction during episodes of strong deformation. Because pile formation probably includes vigorous deformation, we expect long?term survival of the piles after their formation is completed.
Earth and Planetary Science Letters, Vol. 543, 116358 14p. Pdf
Mantle
mantle plumes
Abstract: Two large areas of anomalously low seismic velocities are visible in all tomographic models of the lowermost mantle. Depending on the density structure of these Large Low Shear Velocity Provinces (LLSVPs), the core-mantle boundary (CMB) will deform upwards or downwards due to isostatic and dynamic topography, the latter being sensitive to the viscosity structure of the lowermost mantle. Heterogeneities in the viscosity structure, although difficult to constrain, might be especially important if the LLSVPs are thermochemical piles with elevated intrinsic viscosity as suggested by mineral physics. Based on numerical models, we identify a short-wavelength (about 80-120 km wide, up to a few km deep) topographic depression that forms around the pile edges if the pile is more viscous than the surrounding mantle. The depression forms when a wedge of thermal boundary layer material becomes compressed against the viscous pile, and is enhanced by relative uplift of the CMB beneath the pile by plumes rising above it. The depth and asymmetry of the depression constrain the magnitude of the viscosity contrast between pile and the surrounding mantle. Furthermore, (periodic) plume initiation and pile collapse at the pile margin systematically modify the characteristic depression, with a maximum in asymmetry and depth at the time of plume initiation. Core-reflected waves or scattered energy may be used to detect this topographic signature of stiff thermochemical piles at the base of the mantle.
Geochemistry, Geophysics, Geosystems, 10.1029/ 2020GC009159 22p. Pdf
Mantle
olivine
Abstract: The uppermost layer of Earth's mantle, the asthenosphere, experiences large deformations due to a variety of tectonic processes. During deformation, grains of olivine, the main rock?forming mineral in the asthenosphere, rotate into a preferred direction parallel to the deformation, developing a texture that can affect the response of the asthenosphere to tectonic stresses. Laboratory measurements show that the deformation rate depends on the orientation of the shear stress relative to the olivine texture. We use numerical models to apply the findings of the laboratory measurements to geodynamic situations that are difficult to simulate in a laboratory. These models track the development of olivine texture and its directional response to shear stress, which are highly coupled. Our results suggest that anisotropic viscosity in the asthenosphere can significantly affect the motions of tectonic plates, as plate motion in a continuous direction should become faster, while abrupt changes in the direction of plate motion should meet high resistance in the underlying asthenosphere. We suggest that olivine textures in the asthenosphere play a critical role in upper mantle dynamics.
Abstract: We propose to study the Kipawa peralkaline complex, a rare-earth deposit principally composed of eudialyte, mosandrite and britholite. The Kipawa complex is situated in the Parautochton zone of the Grenville Province in the Tesmiscamingue region of Quebec, 55 km south of contact with Superior Province. The complex consists of peralkaline syenites, amphibolites, gneisses that are intercalated with calc-silicate rocks and marble, and overlain by a peralkaline gneissic granite. The Kipawa complex differs geochemically and petrologically from other well-known peralkaline complexes such as the Illimausaq, Lovozero, Thor Lake or Strange Lake complexes. Classic peralkaline complexes are large, circular igneous complexes, with or without volcanism and have an isotopic signature reflecting mantle origin with different degrees of assimilation and crustal contamination (for example Illimausaq is reported with ?Nd values of 0.4 and -5.7). The Kipawa Complex is a thin, folded stack of sheet imbricates between Kikwissi Suite rocks, McKillop Lake sequence and Red Pine Chute gneiss, suggesting a regional tectonic control. Isotopic analyses carried out by other teamsindicate a strong crustal signature (?Nd = -8.7). Several hypotheses are possible: crustal contamination, hydrothermal activity, fluids alteration during formation, metamorphism or dominant crustal origin. Our objective is to characterize the geochemical and isotopic composition of the Kipawa complex in order to improve our understanding of the age and formation of the complex. Analyses of both whole rocks, eudialytes and zircons will be made to obtain isotopic signatures and determine formation ages and/or post-formation processes.
International Journal of Earth Sciences, Vol. 108, pp. 89-113.
Africa, Angola
thermochronology
Abstract: There are two main points of view regarding how continental margins evolve. The first one argues that the present-day margins have been developed by long-term denudation since a major exhumation episode, probably driven by rifting or another relevant tectonic event. The second one argues that continental margins underwent alternating burial and exhumation episodes related to crustal tectonic and surface uplift and subsidence. To demonstrate that the proximal domain of the southwestern Angolan margin has evolved in a polycyclic pattern, we present a review of geological and thermochronological information and integrate it with new combined apatite fission-track and (U-Th)/He data from Early Cretaceous volcanic and Precambrian basement samples. We also provide hypotheses on the possible mechanisms able to support the vertical crustal movements of this margin segment, which are also discussed based on some modern rifting models proposed for Central South Atlantic. The central apatite fission-track ages range from 120.6?±?8.9 to 272.9?±?21.6 Ma, with the mean track lengths of approximately 12 µm. The single-grain apatite (U-Th)/He ages vary between 52.2?±?1 and 177.2?±?2.6 Ma. The integration of the thermochronological data set with published geological constraints supports the following time-temperature evolution: (1) heating since the Carboniferous-Permian, (2) cooling onset in the Early Jurassic, (3) heating onset in the Early Cretaceous, (4) cooling onset in the Mid- to Late Cretaceous, (5) heating onset in the Late Cretaceous, and (6) cooling onset in the Oligocene-Miocene. The thermochronological data and the geological constraints, support that the proximal domain of the southwestern Angolan margin was covered in the past by pre-, syn-, and post-rift sediments, which were eroded during succeeding exhumation events. For this margin segment, we show that a development based on long-term denudation is less realistic than one based on burial and exhumation episodes during the last 130 Myr.
Geochimica et Cosmochimica Acta, Vol. 306, pp. 143-170. pdf
Europe, Italy
subduction
Abstract: The volatile transfer in subduction zones and the role of sulfate as a vector for the mobilization of oxidized components from down-going slabs remain hotly debated issues. Orogenic spinel and garnet peridotite lenses from the Ulten Zone (Eastern Alps, Italy), exhumed as part of felsic metamorphic terranes in continental collision zones, bear witness to mass transfer processes in these pivotal environments. In this study, we carried out a multi-method investigation of mantle sulfides coexisting with four generations of carbonates, indicating coupled sulfur and carbon mobility throughout the peridotites’ metamorphic evolution as part of the Variscan subduction architecture. Detailed petrography, bulk rock measurements, in situ chemical and geochemical analyses of sulfides as well as Sr isotope analyses of associated clinopyroxene and amphibole are combined with the aim to constrain the origin, nature and effect of multiple C-O-H-S-bearing fluids and melts the peridotites interacted with. The first, pre-peak, metasomatic pulse (Stage 1) is represented by an H2S-CO2-bearing melt from the subduction-modified hot mantle wedge, which formed a pyroxenite layer hosting matrix pentlandite with ?34S of +2.77‰. Matrix carbonates occasionally occur in the coarse-grained peridotite under eclogite-facies conditions (Stage 2), with heavier ?34S (up to +3.43‰), radiogenic Sr (87Sr/86Srclinopyroxene > 0.7052) and elevated Pb abundances. These are ascribed to interaction with isotopically heavy melts carrying recycled crustal component, permissive of, but not requiring, involvement of oxidized S species. Conversely, isotopically lighter matrix pentlandite (?34S = ?1.62 to +0.67‰), and radiogenic Sr in amphibole (87Sr/86Sr = 0.7056) and associated dolomite (published data) from fine-grained garnet-amphibole peridotites may point to involvement of H2S-CO2-bearing crustal fluids, which variably equilibrated with the mantle before interacting with the peridotites. The post-peak Stage 3 marks the entrapment of peridotites into a tectonic mélange. Here, kelyphitization of garnet is catalyzed by further ingress of a S-bearing fluid (?34S = ?0.38‰), while carbonate veining with occasional sulfides bear witness to channelized fluid flow. Sulfide and amphibole grains in retrogressed spinel peridotites reveal the highest contents of fluid-mobile elements (As, Sb) and 87Sr/86Sramphibole up to 0.7074, suggesting late interactions with isotopically heavy crustal fluids at high fluid-rock ratios. Textural observations indicate that, during Stage 4, serpentinization of peridotites at low ƒS2 played an active role not only in CO2 release by conversion of dolomite to calcite + brucite intergrowths, but also in local removal of 32S during the final exhumation stage. Late channelized sulfur remobilization is evidenced by the serpentine + magnetite (±millerite ± calcite) vein carrying > 300 ppm S. Overall, the relatively narrow range of sulfur isotope composition (?34S = ?1.62 to +3.76‰) is indicative of limited interaction with isotopically heavy crustal liquids, and points to a subordinate role of subduction-derived sulfate throughout the extended fluid(melt)/rock evolution of the Ulten Zone peridotites, first in the mantle wedge and then as part of a tectonic mélange.
Abstract: In the present work, we report the chemical composition of representative emerald crystals from some of the most important worldwide deposits. Major and trace elements were investigated using Electron Microprobe Analysis (EMPA) and Secondary Ion Mass Spectrometry (SIMS) techniques. Binary, ternary and spider diagrams along with statistical analysis, i.e., Principal Component Analysis (PCA), were used to discriminate each deposit with high reliability. PCA of SiO2, Al2O3, V, Sc, B, Li content identified distinct groups. The use of binary and ternary diagrams contributed to discriminate among emerald crystals from various deposits, which are included in the same clusters of the PCA analysis. In addition, the geochemical features of each group were linked to the geological environment and genetic processes which leaded to emerald formation. In particular, the emeralds related to granitic-pegmatitic intrusions (Type-1) or those occurring in environments controlled by tectonic events (Type-2) were distinguished using the concentrations of major and trace elements. The results of this study can contribute to improve the existing genetic models and classification schemes as well as to identify useful geochemical fingerprints for provenance purposes.-
Canadian Journal of Development Studies, Vol. 41, 3, pp. 432-449. pdf
Africa, Liberia
alluvials
Abstract: Across sub-Saharan Africa, artisanal and small-scale mining (ASM) represents a major source of direct and indirect employment. Yet, despite the livelihood benefits and the growing interest from governments, donors and policy makers to formalise ASM, most artisanal miners still operate informally. Focusing on Liberia, this article critically investigates the question of why formalisation efforts continue to fail and argues that the persistence of informality in the sector needs to first be understood as a rational strategy for those who profit from it. Only then can sustainable mining reforms be linked to broader national and international extractive sector policy frameworks.
Abstract: Micas from mafic ultrapotassic rocks with lamproitic affinity from several localities of the Central Mediterranean region were studied through single-crystal X-ray diffraction (SC-XRD), electron microprobe analysis (EMPA) and Secondary Ion Mass Spectrometry (SIMS); Mössbauer Spectroscopy (MöS), when feasible, was also applied to minimise the number of unknown variables and uncertainties. Lamproitic samples analysed cover the most important Central Mediterranean type localities, from Plan d'Albard (Western Alps) to Sisco (Corsica), Montecatini Val di Cecina and Orciatico (Tuscany, Italy) and Torre Alfina (Northern Latium, Italy). The studied crystals show distinctive chemical and structural features; all of them belong to the phlogopite-annite join and crystallise in the 1 M polytype, except for micas from Torre Alfina, where both 1 M and 2 M1 polytypes were found. Studied micas have variable but generally high F and Ti contents, with Mg/(Mg + Fe) ranging from ~ 0.5 to ~ 0.9; 2M1 crystals from Torre Alfina radically differ in chemical composition, showing high contents of Ti and Fe as well as of Al in both tetrahedra and octahedra, leading to distinctive structural distortions, especially in tetrahedral sites. SIMS data indicate that studied micas are generally dehydrogenated with OH contents ranging from ~ 0.2 apfu (atoms per formula unit) for Orciatico and Torre Alfina to ~ 1.4 for Plan d'Albard crystals; this feature is also testified by the length of the c parameter, which decreases with the loss of hydrogen and/or the increase of the F ? OH substitution. Chemical and structural data suggest that the entry of high charge octahedral cations is mainly balanced by an oxy mechanism and, to a lesser extent, by a M3 +,4 +-Tschermak substitution. Our data confirm that Ti preferentially partitions into the M2 site and that different Ti and F contents, as well as different K/Al values, are both dependant upon fH2O and the composition of magma rather than controlled by P and T crystallisation conditions. The obtained data help to discriminate among lamproite-like rocks formed within a complex geodynamic framework but still related to a destructive tectonic margin and evidence different trends for micas from the youngest Torre Alfina (Northern Latium) lamproites, referred to the Apennine orogeny and those of the older lamproites from Orciatico, Montecatini Val di Cecina (Tuscany), Western Alps, and Corsica, the latter referred to the Alpine orogeny. Phlogopite crystals from the older lamproites fall within the compositional and structural field of worldwide phlogopites from both within-plate and subduction-related settings. Phlogopite from the Plio-Pleistocene lamproite-like occurrence in Tuscany and Northern Latium, despite crystals with low Mg# of the Torre Alfina rock plot well within the general field of the other crystals in less evolved samples, follows a different evolution trend similar to that of shoshonites from Tuscany and Northern Latium. On this basis, we argue that the observed differences are inherited by slight differences in the magma compositions that are related with different genetic and evolution pathways.
Geological Society, London Special Publication, doi.org/10.1144/SP513-2021-36 49p. Pdf
Europe, Italy, France, Spain, Serbia, Macedonia, Turkey
lamproites
Abstract: High-MgO lamproite and lamproite-like (i.e., lamprophyric) ultrapotassic rocks are recurrent in the Mediterranean and surrounding regions. They are associated in space and time with ultrapotassic shoshonites and high-K calc-alkaline rocks. This magmatism is linked with the geodynamic evolution of the westernmost sector of the Alpine-Himalaya collisional margin, which followed the closure of the Tethys ocean. Subduction-related lamproites, lamprophyres, shoshonites and high-K calc-alkaline suites were emplaced in the Mediterranean region in the form of shallow level intrusions (e.g., plugs, dykes, and laccoliths), and small volume lava flows, with very subordinate pyroclastic rocks, starting from the Oligocene, in the Western Alps (Northern Italy), through the Late Miocene in Corsica (Southern France) and in Murcia-Almeria (South-Eastern Spain), to the Plio-Pleistocene in Southern Tuscany and Northern Latium (Central Italy), in the Balkan peninsula (Serbia and Macedonia), and in the Western Anatolia (Turkey). The ultrapotassic rocks are mostly lamprophyric, but olivine latitic lavas with a clear lamproitic affinity are also found, as well as dacitic to trachytic differentiated products. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated composition, have relatively low Al2O3, CaO, and Na2O contents, resulting in plagioclase-free parageneses, and consist of abundant K-feldspar, phlogopite, diopsidic clinopyroxene and highly forsteritic olivine. Leucite is generally absent and it is rarely found only in the groudmasses of Spanish lamproites. Mediterranean lamproites and associated rocks share an extreme enrichment in many incompatible trace elements and depletion in High Field Strength Elements and high, and positively correlated Th/La and Sm/La ratios. They have radiogenic Sr and unradiogenic Nd isotope compositions, high 207Pb over 206Pb and high time integrated 232Th/238U. Their composition requires an originally depleted lithospheric mantle source metasomatised by at least two different agents: i) a high Th/La and Sm/La (i.e., SALATHO) component deriving from lawsonite-bearing, ancient crustal domains likely hosted in mélanges formed during the diachronous collision of the northward drifting continental slivers from Gondwana; ii) a K-rich component derived from a recent subduction and recycling of siliciclastic sediments. These metasomatic melts produced a lithospheric mantle source characterised by network of felsic and phlogopite-rich veins, respectively. Geothermal readjustment during post-collisional events induced progressive melting of the different types of veins and the surrounding peridotite generating the entire compositional spectrum of the observed magmas. In this complex scenario, orogenic Mediterranean lamproites represent rocks that characterise areas that were affected by multiple Wilson cycles, as observed in the the Alpine-Himalayan realm.
Geological Society London Special Publication, doi.org/10.1144/SP513-2021-36. pdf
Mantle
lamproite
Abstract: High-MgO lamproite and lamproite-like (i.e., lamprophyric) ultrapotassic rocks are recurrent in the Mediterranean and surrounding regions. They are associated in space and time with ultrapotassic shoshonites and high-K calc-alkaline rocks. This magmatism is linked with the geodynamic evolution of the westernmost sector of the Alpine-Himalaya collisional margin, which followed the closure of the Tethys ocean. Subduction-related lamproites, lamprophyres, shoshonites and high-K calc-alkaline suites were emplaced in the Mediterranean region in the form of shallow level intrusions (e.g., plugs, dykes, and laccoliths), and small volume lava flows, with very subordinate pyroclastic rocks, starting from the Oligocene, in the Western Alps (Northern Italy), through the Late Miocene in Corsica (Southern France) and in Murcia-Almeria (South-Eastern Spain), to the Plio-Pleistocene in Southern Tuscany and Northern Latium (Central Italy), in the Balkan peninsula (Serbia and Macedonia), and in the Western Anatolia (Turkey). The ultrapotassic rocks are mostly lamprophyric, but olivine latitic lavas with a clear lamproitic affinity are also found, as well as dacitic to trachytic differentiated products. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated composition, have relatively low Al2O3, CaO, and Na2O contents, resulting in plagioclase-free parageneses, and consist of abundant K-feldspar, phlogopite, diopsidic clinopyroxene and highly forsteritic olivine. Leucite is generally absent and it is rarely found only in the groudmasses of Spanish lamproites. Mediterranean lamproites and associated rocks share an extreme enrichment in many incompatible trace elements and depletion in High Field Strength Elements and high, and positively correlated Th/La and Sm/La ratios. They have radiogenic Sr and unradiogenic Nd isotope compositions, high 207Pb over 206Pb and high time integrated 232Th/238U. Their composition requires an originally depleted lithospheric mantle source metasomatised by at least two different agents: i) a high Th/La and Sm/La (i.e., SALATHO) component deriving from lawsonite-bearing, ancient crustal domains likely hosted in mélanges formed during the diachronous collision of the northward drifting continental slivers from Gondwana; ii) a K-rich component derived from a recent subduction and recycling of siliciclastic sediments. These metasomatic melts produced a lithospheric mantle source characterised by network of felsic and phlogopite-rich veins, respectively. Geothermal readjustment during post-collisional events induced progressive melting of the different types of veins and the surrounding peridotite generating the entire compositional spectrum of the observed magmas. In this complex scenario, orogenic Mediterranean lamproites represent rocks that characterise areas that were affected by multiple Wilson cycles, as observed in the the Alpine-Himalayan realm.
Contributions to Mineralogy and Petrology, Vol. 175, 12 pdf
Mantle
peridotites
Abstract: Experiments were performed from 950 to 1250 °C and 1.5-2.4 GPa to determine the effect of pressure (P) on the temperature (T)-dependent partitioning of Al between olivine and spinel, using mixtures of natural spinel, olivine, clino- and ortho-pyroxene. When compared to 100 kPa experiments, the results show that there is no discernible effect of pressure on the Al-in-olivine thermometer at PT conditions relevant to the spinel peridotite facies. In our experiments with high-Cr spinel, we see no change in Al in olivine from starting values, likely due to the refractory nature of high-Cr spinel. Phase boundary flourescence prevented accurate quantification of Ca in olivine in the run products by electron microprobe analysis but measurements by laser ablation are consistent with the Köhler and Brey (Geochim Cosmochim Acta 54:2375-2388, 1990) Ca-in-olivine thermobarometer. The combination of Al (for T) and Ca (for P) in olivine thus has great potential for thermobarometry in spinel facies peridotites. As a test we apply this approach to published high precision Ca and Al data for olivine from the Ray Pic spinel peridotite xenoliths from the Massif Central (De Hoog et al. Chem Geol 270:196-215, 2010). Reassuringly, the calculated PT conditions (1.0-1.8 GPa; 900-1080 °C) for all samples lie beneath the Moho, within the spinel peridotite facies and fall along a geophysically constrained geotherm.
Journal of African Media Studies, Vol. 8, 2, pp. 109-126.
Africa
History
Abstract: This paper argues that the western press often ignores the private sphere of economics in reporting conflict in the developing world. This matters when it comes to resource wars and conflict over natural resources. To explore this concept further I examine American and British press coverage of conflict diamonds in the Angola, Congo and Sierra Leone wars in the pages of four newspapers of record. Overall while conflict diamonds were present in the reporting the press ignored the full extent of private companies and capital in the financing and trading of conflict diamonds. The most common word used was government.
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) , Annual Meeting Abstracts approximately 10 lines, Vol. 86, No. 968, March ABSTRACT p. 70
Products of 2.65 - 2.58 Ga orogenesis in the Slave Province correlated with Slave - Northern Cordillera Lithospheric Evolution ( SNORCLE) seismic reflection
Canadian Journal of Earth Science, Vol. 39,8,August pp. 1189-1200.
Northwest Territories
Geophysics - seismic, Slave Province - tectonics, geodynamics
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.
Distribution and behaviour of platinum in the soils of the Tulameen ultramafic complex, southern BritishColumbia
Geological Association of Canada (GAC)/Mineralogical Association of Canada (MAC) Vancouver 90 Program with Abstracts, Held May 16-18, Vol. 15, p. A27-28. Abstract
Abstract: Variations in the temperature of the mantle drive its convective circulation, a process that links the deep mantle with the atmosphere and oceans through volcanic and tectonic activity. Because of this connection, effective models of Earth’s evolution must incorporate the planet’s thermal history, for which a crucial constraint is the mantle’s current temperature. Researchers look at chemistry of Iceland’s newly erupted lava to analyze the temperature of the mantle below. A false-color backscatter electron image of an olivine crystal from Borgarhraun, a lava field in northern Iceland. The crystal contains a spinel inclusion, set in a fine-grained crystalline groundmass. The chemistry of these crystals records the temperatures at which they crystallized. The image is approximately 1.5 millimeters wide. Credit: S. Matthews. Because the mantle’s temperature cannot be measured directly, scientists have devised a number of creative methods to derive this information, but these have produced widely varying results. Now Matthews et al. offer new constraints on this parameter beneath Iceland, one of the few places on Earth where a divergent plate boundary is subaerially exposed because of an anomalously large amount of melting occurring beneath the island. Using a recently developed mineral thermometry technique, the researchers found that lava flows from four different eruptions along Iceland’s Northern Volcanic Zone crystallized at substantially higher temperatures (maximum 1399°C) than average mid-ocean ridge samples that have experienced little melting (maximum 1270°C). Next, the team developed a thermal model of mantle melting and used it, along with other observations such as the local thickness of the crust, to quantify the uncertainties in deriving mantle temperatures from their data. Researchers look at chemistry of Iceland’s newly erupted lava to analyze the temperature of the mantle below. An analysis of fresh lavas from Iceland indicates the mantle below the island is much hotter than beneath other locations on divergent plate boundaries. Credit: Terri Cook and Lon Abbott. Their results indicate that the mantle below Iceland is at least 140°C hotter than that beneath average mid-ocean ridges. This outcome should shed light on the factors that control the extent of melting beneath Iceland, including the ongoing debate about whether the voluminous melting is due to a deep mantle plume and, if so, whether changes in its magma production reflect variations in the plume’s temperature.
Abstract: Volcanic eruptions of basalt are fed by intrusions of magma, called dikes, which advance through Earth’s crust for a few hours or days before reaching the surface. Although many never make it that far, those that do can pose a serious threat to people and infrastructure, so forecasting when and where a dike will erupt is important to assessing volcanic hazards. However, the migration of magma below a volcano is complex, and its simulation is numerically demanding, meaning that efforts to model dike propagation have so far been limited to models that can quantify either a dike’s velocity or its trajectory but not both simultaneously. To overcome this limitation, Pinel et al. have developed a hybrid numerical model that quantifies both by dividing the simulations into two separate steps, one that calculates a two-dimensional trajectory and a second that runs a one-dimensional propagation model along that path. The results indicate that the migration of magma is heavily influenced by surface loading—the addition or removal of weight on Earth’s surface—such as that caused by the construction of a volcano or its partial removal via a massive landslide or caldera eruption. The team confirmed previous research that showed that increasing surface load attracts magma while also reducing its velocity, whereas unloading diverts much of the magma. To test their approach, the team applied their model to a lateral eruption that occurred on Italy’s Mount Etna in July 2001. The eruption was fed by two dikes, including one that in its final stages clearly slowed down and bent toward the west while still 1-2 kilometers below the surface. The results showed that the two-step model was capable of simulating that dike’s velocity and trajectory and thus offers a new means of constraining the local stress field, which partially controls these properties. In the future, report the authors, more complex versions of this model that incorporate information on local topography and magmatic properties could be integrated with real-time geophysical data to improve forecasts of when and where a propagating dike could erupt at the surface.
Abstract: The theory of plate tectonics, which posits that Earth’s lithosphere is broken into a series of rigid plates that are propelled by currents of heat circulating through the taffy-like rocks below, underpins modern geological sciences. Yet the localized weakening mechanisms that form the plate boundaries remain poorly understood. Because of the presence of regions of very fine grained deformed rocks at many tectonic margins, scientists have argued that a reduction in grain size is a critical step in concentrating deformation into narrow shear zones. Because the grains in these zones are much smaller—and hence weaker—than those in the surrounding material, they appear to provide a pathway for localizing deformation in the lithosphere. But the role the grains’ composition may play in shaping shear zones has been the subject of ongoing debate. To further our understanding of grain-scale weakening processes in different materials, Tasaka et al. conducted a series of torsion laboratory experiments to examine how mixtures of two minerals commonly found in Earth’s mantle, olivine and pyroxene, respond to increasing strain. The results of the first study indicate that at low strain, the mixture responded in the same way as samples of pure olivine but that at higher strain the mixture’s behavior varied, suggesting this deformation occurred via a different grain-reducing mechanism that operates only in samples containing pyroxene. According to the results of the companion study, which analyzed changes in the mixed samples’ microstructures, this mechanism is the nucleation of small grains of olivine and pyroxene along each other’s mineral boundaries. As the new crystals begin to form at moderate strain, argue the authors, they restrict one another’s dimensions, effectively reducing the samples’ average grain size. By permanently inhibiting grain growth, this process eventually creates zones of fine-grained material that remain weak over geologic timescales. These papers provide important insights into the processes that control grain-scale weakening and provide crucial experimental data necessary to test numerical geodynamic models. The mechanism proposed by the researchers may potentially be responsible for forming and perpetuating shear zones and hence the formation of plate boundaries.
Abstract: Real-time tracking during diamond anvil cell experiments indicates reaction rates may control the unusual depth distribution of the extremely rare diamonds that form deep within Earth’s mantle.
Abstract: Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite (more than 8 wt.% MgO). This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios (Zr/Hf = 32-35, Ti/Zr = 107-147), producing flat REE and HFSE profiles [(La/Yb)pm = 0.9-1.3], with abundances of 3-10 times primitive mantle and with minor negative anomalies of Nb [(Nb/La)pm = 0.6-0.8] and Th [(Th/La)pm = 0.6-0.9]. Initial isotope compositions (?Nd) range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping ?50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between ?2830 and ?2820 Ma; are compositionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition.
Contributions to Mineralogy and Petrology, Vol. 171, 2p.
Mantle
Harzburgite
Abstract: Using the Hill et al. (Contrib Mineral Petrol 169:13, 2015. doi:10.1007/s00410-014-1102-7) modeling technique, we have tested the idea of Ivanic et al. (Contrib Mineral Petrol 164:505-520, 2012) that decompression and metamorphic re-equilibration of garnet with spinel causes garnet zoning perpendicular to the Cr-Ca harzburgitic trend in garnet composition. The modeling confirms that garnet zoning across the harzburgitic trend cannot form without spinel buffering. The harzburgitic trend is very rare because it results from extreme compositional heterogeneity of the mantle at the same depth. In contrast, the common lherzolitic trend requires less diversity in the bulk composition of the mantle, as it can be established with only a few samples of metamorphically re-equilibrated mantle peridotite deriving from a variety of depths.
Heaman, L.M., Creaser, R.A., Cookenboo, H.O., Chacko, T.
Multi stage modification of the northern Slave mantle lithosphere: evidence from zircon and diamond bearing eclogite xenoliths entrained in Jericho kimberlite.
Journal of Petrology, Vol. 47, 4, April pp. 821-858.
Diamond populations and diamond associated indicator minerals point to one or more local sources within the Alta Paranaiba diamond province in western Minas Gerais state, Brazil.
GAC-MAC 2013 SS4: from birth to the mantle emplacement in kimberlite., abstract only
Veevers, J.J., Belousova, E.A., Saced, A., Sircombe, K., Cooper, A.F., Read, S.E.
Pan-Gondwanaland detrital zircons from Australia analyzed for Hf isotopes and trace elements reflect an ice covered Antartic provenance 700-500 Ma alkalinity
Veevers, J.J., Belousova, E.A., Saeed, A., Sircombe, K., Cooper, A.F., Read, S.E.
Pan Gondwanaland detrital zircons from Australia analysed for Hf isotopes and trace elements reflect an ice covered Antarctic provenance of 700-500 Ma ...
Earth Science Reviews, Vol. 76, 3-4, June pp. 135-174.
Abstract: The lithospheric mantle beneath West Antarctica has been characterized using petrology, whole-rock and mineral major element geochemistry, whole-rock trace element chemistry and Mössbauer spectroscopy data obtained on a suite of peridotite (lherzolite and harzburgite) and pyroxenite xenoliths from the Mount Morning eruptive centre, Southern Victoria Land. The timing of pyroxenite formation in Victoria Land overlaps with subduction of the Palaeo-Pacific plate beneath the Gondwana margin and pyroxenite is likely to have formed when fluids derived from, or modified by, melting of the subducting, eclogitic, oceanic crustal plate percolated through peridotite of the lithospheric mantle. Subsequent melting of lithospheric pyroxenite veins similar to those represented in the Mount Morning xenolith suite has contributed to the enriched trace element (and isotope) signatures seen in Cenozoic volcanic rocks from Mount Morning, elsewhere in Victoria Land and Zealandia. In general, the harzburgite xenoliths reflect between 20 and 30% melt depletion. Their depleted element budgets are consistent with Archaean cratonization ages and they have mantle-normalized trace element patterns comparable with typical subcontinental lithospheric mantle. The spinel lherzolite mineral data suggest a similar amount of depletion to that recorded in the harzburgites (20-30%), whereas plagioclase lherzolite mineral data suggest <15% melt depletion. The lherzolite (spinel and plagioclase) xenolith whole-rocks have compositions indicating <20% melt depletion, consistent with Proterozoic to Phanerozoic cratonization ages, and have mantle-normalized trace element patterns comparable with typical depleted mid-ocean ridge mantle. All peridotite xenoliths have undergone a number of melt-rock reaction events. Melting took place mainly in the spinel peridotite stability field, but one plagioclase peridotite group containing high-sodium clinopyroxenes is best modelled by melting in the garnet field. Median oxygen fugacity estimates based on Mössbauer spectroscopy measurements of spinel and pyroxene for spinel-facies conditions in the rifted Antarctic lithosphere are -0·6 ?log fO2 at Mount Morning and –1·0 ± 0·1 (1?) ?log fO2 for all of Victoria Land, relative to the fayalite-magnetite-quartz buffer. These values are in good agreement with a calculated global median value of -0·9 ± 0·1 (1?) ?log fO2 for mantle spinel-facies rocks from continental rift systems.
Abstract: Alfred Wegener famously argued that the seemingly puzzle piece–like fit of the Atlantic coastlines was not a mere coincidence, but rather one line of evidence proving that the continents were once arranged as a single, coherent supercontinent (Wegener, 1912,1920). This puzzle piece observation eventually launched a revolution that changed our understanding of the Earth from its deep interior to evolutionary processes. Often, however, we think of the supercontinent puzzle in a two-dimensional sense, neglecting to include or consider how variations of the thickness of the puzzle pieces might also be at play. How do the puzzle pieces fit together at depth, and is there more to learn by including lithospheric thickness in our plate reconstructions? Would thinking three-dimensionally in our plate reconstructions help resolve some of the outstanding questions about supercontinents, continental deformation, and the lithosphere in general? This is the motivation of new research by McKenzie et al. (2015, p. 783 in this issue of Geology).
Abstract: Continental lithosphere houses the oldest and thickest regions of the Earth's surface. Locked within this deep and ancient rock record lies invaluable information about the dynamics that has shaped and continue to shape the planet. Much of that history has been dominated by the forces of plate tectonics which has repeatedly assembled super continents together and torn them apart - the Wilson Cycle. While the younger regions of continental lithosphere have been subject to deformation driven by plate tectonics, it is less clear whether the ancient, stable cores formed and evolved from similar processes. New insight into continental formation and evolution has come from remarkable views of deeper lithospheric structure using enhanced seismic imaging techniques and the increase in large volumes of broadband data. Some of the most compelling observations are that the continental lithosphere has a broad range in thicknesses (< 100 to > 300 km), has complex internal structure, and that the thickest portion appears to be riddled with seismic discontinuities at depths between ~ 80 and ~ 130 km. These internal structural features have been interpreted as remnants of lithospheric formation during Earth's early history. If they are remnants, then we can attempt to investigate the structure present in the deep lithosphere to piece together information about early Earth dynamics much as is done closer to the surface. This would help delineate between the differing models describing the dynamics of craton formation, particularly whether they formed in the era of modern plate tectonics, a transitional mobile-lid tectonic regime, or are the last fragments of an early, stagnant-lid planet. Our review paper (re)introduces readers to the conceptual definitions of the lithosphere and the complex nature of the upper boundary layer, then moves on to discuss techniques and recent seismological observations of the continental lithosphere. We then review geodynamic models and hypotheses for the formation of the continental lithosphere through time and implications for the formation and preservation of deep structure. These are contrasted with the dynamical picture of modern day continental growth during lateral accretion of juvenile crust with reference to examples from the Australian Tasmanides and the Alaskan accretionary margin.
Abstract: Continental lithosphere houses the oldest and thickest regions of the Earth's surface. Locked within this deep and ancient rock record lies invaluable information about the dynamics that has shaped and continue to shape the planet. Much of that history has been dominated by the forces of plate tectonics which has repeatedly assembled super continents together and torn them apart - the Wilson Cycle. While the younger regions of continental lithosphere have been subject to deformation driven by plate tectonics, it is less clear whether the ancient, stable cores formed and evolved from similar processes. New insight into continental formation and evolution has come from remarkable views of deeper lithospheric structure using enhanced seismic imaging techniques and the increase in large volumes of broadband data. Some of the most compelling observations are that the continental lithosphere has a broad range in thicknesses (< 100 to > 300 km), has complex internal structure, and that the thickest portion appears to be riddled with seismic discontinuities at depths between ~ 80 and ~ 130 km. These internal structural features have been interpreted as remnants of lithospheric formation during Earth's early history. If they are remnants, then we can attempt to investigate the structure present in the deep lithosphere to piece together information about early Earth dynamics much as is done closer to the surface. This would help delineate between the differing models describing the dynamics of craton formation, particularly whether they formed in the era of modern plate tectonics, a transitional mobile-lid tectonic regime, or are the last fragments of an early, stagnant-lid planet. Our review paper (re)introduces readers to the conceptual definitions of the lithosphere and the complex nature of the upper boundary layer, then moves on to discuss techniques and recent seismological observations of the continental lithosphere. We then review geodynamic models and hypotheses for the formation of the continental lithosphere through time and implications for the formation and preservation of deep structure. These are contrasted with the dynamical picture of modern day continental growth during lateral accretion of juvenile crust with reference to examples from the Australian Tasmanides and the Alaskan accretionary margin.
Abstract: Earth’s oldest near-surface material, the cratonic crust, is typically underlain by thick lithosphere (>200 km) of Archean age. This cratonic lithosphere likely thickened in a high-compressional-stress environment, potentially linked to the onset of crustal shortening in the Neoarchean. Mantle convection in the hotter Archean Earth would have imparted relatively low stresses on the lithosphere, whether or not plate tectonics was operating, so a high stress signal from the early Earth is paradoxical. We propose that a rapid transition from heat pipe–mode convection to the onset of plate tectonics generated the high stresses required to thicken the cratonic lithosphere. Numerical calculations are used to demonstrate that an existing buoyant and strong layer, representing depleted continental lithosphere, can thicken and stabilize during a lid-breaking event. The peak compressional stress experienced by the lithosphere is 3×-4× higher than for the stagnant-lid or mobile-lid regimes immediately before and after. It is plausible that the cratonic lithosphere has not been subjected to this high stress state since, explaining its long-term stability. The lid-breaking thickening event reproduces features observed in typical Neoarchean cratons, such as lithospheric seismological reflectors and the formation of thrust faults. Paleoarchean "pre-tectonic" structures can also survive the lid-breaking event, acting as strong rafts that are assembled during the compressive event. Together, the results indicate that the signature of a catastrophic switch from a stagnant-lid Earth to the initiation of plate tectonics has been captured and preserved in the characteristics of cratonic crust and lithosphere.
Abstract: We propose that subducting slabs may cause lithospheric removal by directing mantle flow along the craton margin. This process could carve and shape the cratons, leading to conditions that impact the overall (in)stability of the lithosphere. We use three-dimensional geodynamic models to investigate how subduction-driven directed flow interacts with cratonic lithosphere of differing shape, concluding that the margin shape controls both channelization of flow around the craton as well as the potential for destruction. While the simulations show that all craton shapes aid in channelization, the cratons with straight vertical margins are the most resistant to deformation, and the cratons with gradually thickening margins are less resistant to deformation. The dependence on shape could contribute to the progressive removal of cratonic lithosphere along its margin in a runaway process until a more stable vertical margin shape evolves.
Physics of the Earth and Planetary Interiors, doi.org/10.1016/ j.pepi.2-19.106325 54p. Pdf
Mantle
melting
Abstract: It has long been inferred that mantle metasomatism and the incompatible element enrichment of the continents both require movement of melts formed by very low degree melting of the mantle. Yet establishing the presence of these melts and whether this process is on-going and continuous, or spatially and temporally restricted, has proved difficult. Here we report large U-Th-Ra disequilibria in metasomatised, mantle xenoliths erupted in very young lavas from the Newer Volcanics Province in southeastern Australia. The 226Ra-230Th disequilibria appear to require reappraisal of previous estimates for the age of eruption that now seems unlikely to be more than a few kyr at most. We propose that infiltration of carbonatitic melts/fluids, combined with crystallization of pargasite, can account for the first order U-series disequilibria observations. Irrespective of the exact details of the complex processes responsible, the half-lives of the nuclides require that some of the chemical and isotopic disturbance was extremely young (« 8?kyr) and potentially on-going at the time of incorporation into the alkali basalts that transported the xenoliths to the surface. This provides evidence for the presence and possibly continuing migration of small melt fractions (~0.02%) in the upper convecting mantle that may contribute to the seismic low velocity zone. By implication, it appears that the asthenosphere must lie close to its solidus, at least in this region. Pressure-temperature estimates indicate that the small degree melts identified could infiltrate as far as 25?km upwards into the sub-continental lithospheric mantle leading to strong incompatible element enrichment and the recent timing of this event this urges a reappraisal of the meaning of 300-500?Ma Nd model ages in mantle xenoliths from this region. In principle, the resultant metasomatised mantle could provide a component for some ocean island basalts, should the sub-continental lithospheric mantle be returned to the asthenosphere by convective removal at some later time.
Abstract: Crystals within volcanic rocks contain records of the changing chemical and thermal conditions within the magma reservoirs in which they resided before eruption. Observations from these crystal records place fundamental constraints on the processes operating within the reservoirs. Data from volcanic crystals are in accord with recent conceptual models of magma reservoirs being composed dominantly of crystal mushes, with small volumes and/or small fractions of melt present. The implication is that magma reservoirs have differing modes of behavior: magmas are stored over the long term in largely crystalline, quiescent, conditions, punctuated by brief episodes of intense activity during the decades to centuries immediately prior to an eruption.
Deep-seated magmatism, its sources and plumes, Proceedings of XIII International Workshop held 2014., Vol. 2014, pp. 5-21.
Russia, Siberia
Deposit - Murun
Abstract: Vladykinite, ideally Na3Sr4(Fe2+Fe3+)Si8O24, is a new complex sheet silicate occurring as abundant prismatic crystals in a dike of coarse-grained peralkaline feldspathoid syenite in the north-central part of the Murun complex in eastern Siberia, Russia (Lat. 58° 22? 48? N; Long. 119° 03? 44? E). The new mineral is an early magmatic phase associated with aegirine, potassium feldspar, eudialyte, lamprophyllite, and nepheline; strontianite (as pseudomorphs after vladykinite) and K-rich vishnevite are found in the same assemblage, but represent products of late hydrothermal reworking. Vladykinite is brittle, has a Mohs hardness of 5, and distinct cleavage on {100}. In thin section, it is colorless, biaxial negative [a = 1.624(2), b = 1.652(2), g = 1.657(2), 2Vmeas = 44(1)°, 2Vcalc = 45(1)°] and shows an optic orientation consistent with its structural characteristics (X^a = 5.1° in b obtuse, Z^c = 4.7° in b acute, Y = b). The Raman spectrum of vladykinite consists of the following vibration modes (listed in order of decreasing intensity): 401, 203, 465, 991, 968, 915, 348, 167, 129, 264, 1039, and 681 cm–1; O-H signals were not detected. The Mössbauer spectrum indicates that both Fe2+ and Fe3+ are present in the mineral (Fe3+/FeS = 0.47), and that both cations occur in a tetrahedral coordination. The mean chemical composition of vladykinite (acquired by wavelength-dispersive X?ray spectrometry and laser-ablation inductively-coupled-plasma mass-spectrometry), with FeS recast into Fe2+ and Fe3+ in accord with the Mössbauer data, gives the following empirical formula calculated to 24 O atoms: (Na2.45Ca0.56)S3.01(Sr3.81 K0.04Ba0.02La0.02Ce0.01)S3.90(Fe2+0.75Fe3+0.66Mn0.26Zn0.16Al0.12Mg0.05Ti0.01)S2.01(Si7.81Al0.19)S8.00O24. The mineral is monoclinic, space group P21/c, a = 5.21381(13), b = 7.9143(2), c = 26.0888(7) Å, b = 90.3556(7)°, V = 1076.50(5) Å3, Z = 2. The ten strongest lines in the powder X?ray diffraction pattern are [dobs in Å (I) (hkl)]: 2.957 (100) (123, 123); 2.826 (100) (117, 117); 3.612 (58) (114, 114); 3.146 (37) (120); 2.470 (32) (210, 01.10); 4.290 (30) (111, 111); 3.339 (30) (106, 115, 106); 2.604 (28) (200); 2.437 (25) (034); 1.785 (25) (21.10, 234). The structure of vladykinite, refined by single-crystal techniques on the basis of 3032 reflections with Fo > 4sFo to R1 = 1.6%, consists of tetrahedral sheets parallel to (100) and consisting of (Si8O24)16– units incorporating four-membered silicate rings and joined into five- and eight-membered rings by sharing vertices with larger tetrahedra hosting Fe2+, Fe3+, Mn, Zn, Al, Mg, and Ti. Larger cations (predominantly Na, Sr, and Ca) are accommodated in octahedral and square-antiprismatic interlayer sites sandwiched between the tetrahedral sheets. Structural relations between vladykinite and other sheet silicates incorporating four-, five-, and eight-membered rings are discussed. The name vladykinite is in honor of Nikolay V. Vladykin (Vinogradov Institute of Geochemistry, Russia), in recognition of his contribution to the study of alkaline rocks. Holotype and co-type specimens of the mineral were deposited in the Robert B. Ferguson Museum of Mineralogy in Winnipeg, Canada.
Mineralogical Magazine, Vol. 79, 5, pp. 1231-1244.
Russia
Carbonatite
Abstract: Anzaite-(Ce), ideally Formula Fe2+Ti6O18(OH)2, is a new, structurally complex mineral occurring as scarce minute crystals in hydrothermally altered silicocarbonatites in the Afrikanda alkali-ultramafic complex of the Kola Peninsula, Russia. The mineral is a late hydrothermal phase associated with titanite, hibschite, clinochlore and calcite replacing the primary magmatic paragenesis. The rare-earth elements (REE) (dominated by Ce), Ti and Fe incorporated in anzaite-(Ce) were derived from primary Ti oxides abundant in the host rock. Anzaite-(Ce) is brittle and lacks cleavage; the density calculated on the basis of structural data is 5.054(6) g cm?3. The mineral is opaque and grey with a bluish hue in reflected light; its reflectance values range from 15-16% at 440 nm to 13-14% at 700 nm. Its infrared spectrum shows a prominent absorption band at 3475 cm?1 indicative of OH? groups. The average chemical composition of anzaite-(Ce) gives the following empirical formula calculated on the basis of 18 oxygen atoms and two OH? groups: (Ce2.18Nd0.85La0.41Pr0.26Sm0.08Ca0.36Th0.01)?4.15Fe0.97(Ti5.68Nb0.22Si0.04)?5.94O18(OH)2. The mineral is monoclinic, space group C2/m, a = 5.290(2), b = 14.575(6), c = 5.234(2) Å, ? = 97.233(7)°, V = 400.4(5) Å3, Z = 1. The ten strongest lines in the X-ray micro-diffraction pattern are [dobs in Å (I) hkl]: 2.596 (100) 002; 1.935 (18) 170; 1.506 (14) 133; 1.286 (13) 1.11.0; 2.046 (12) 2?41; 1.730 (12) 003; 1.272 (12) 0.10.2; 3.814 (11) 1?11; 2.206 (9) 061; 1.518 (9) 172. The structure of anzaite-(Ce), refined by single-crystal techniques to R1 = 2.1%, consists of alternating layers of type 1, populated by REE (+ minor Ca) in a square antiprismatic coordination and octahedrally coordinated Fe2+, and type 2, built of five-coordinate and octahedral Ti, stacked parallel to (001). This atomic arrangement is complicated by significant disorder affecting the Fe2+, five-coordinate Ti and two of the four anion sites. The order-disorder pattern is such that only one half of these positions in total occupy any given (010) plane, and the disordered (010) planes are separated by ordered domains comprising REE, octahedral Ti and two anion sites occupied by O2?. Structural and stoichiometric relations between anzaite-(Ce) and other REE-Ti (±Nb, Ta) oxides are discussed. The name anzaite-(Ce) is in honour of Anatoly N. Zaitsev of St Petersburg State University (Russia) and The Natural History Museum (UK), in recognition of his contribution to the study of carbonatites and REE minerals. The modifier reflects the prevalence of Ce over other REE in the composition of the new mineral.
Abstract: Zoned crystals of carbocernaite occur in hydrothermally reworked burbankite-fluorapatite-bearing calcite carbonatite at Bear Lodge, Wyoming. The mineral is paragenetically associated with pyrite, strontianite, barite, ancylite-(Ce), and late-stage calcite, and is interpreted to have precipitated from sulfate-bearing fluids derived from an external source and enriched in Na, Ca, Sr, Ba, and rare-earth elements (REE) through dissolution of the primary calcite and burbankite. The crystals of carbocernaite show a complex juxtaposition of core-rim, sectoral, and oscillatory zoning patterns arising from significant variations in the content of all major cations, which can be expressed by the empirical formula (Ca0.43–0.91Sr0.40–0.69REE0.18–0.59Na0.18–0.53Ba0–0.08)?1.96–2.00(CO3)2. Interelement correlations indicate that the examined crystals can be viewed as a solid solution between two hypothetical end-members, CaSr(CO3)2 and NaREE(CO3)2, with the most Na-REE-rich areas in pyramidal (morphologically speaking) growth sectors representing a probable new mineral species. Although the Bear Lodge carbocernaite is consistently enriched in light REE relative to heavy REE and Y (chondrite-normalized La/Er = 500–4200), the pyramidal sectors exhibit a greater degree of fractionation between these two groups of elements relative to their associated prismatic sectors. A sample approaching the solid-solution midline [(Ca0.57Na0.42)?0.99(Sr0.50REE0.47Ba0.01)?0.98(CO3)2] was studied by single-crystal X-ray diffraction and shown to have a monoclinic symmetry [space group P11m, a = 6.434(4), b = 7.266(5), c = 5.220(3) Å, ? = 89.979(17)°, Z = 2] as opposed to the orthorhombic symmetry (space group Pb21m) proposed in earlier studies. The symmetry reduction is due to partial cation order in sevenfold-coordinated sites occupied predominantly by Ca and Na, and in tenfold-coordinated sites hosting Sr, REE, and Ba. The ordering also causes splitting of carbonate vibrational modes at 690–740 and 1080–1100 cm?1 in Raman spectra. Using Raman micro-spectroscopy, carbocernaite can be readily distinguished from burbankite- and ancylite-group carbonates characterized by similar energy-dispersive spectra.
Wyoming craton mantle lithosphere: reconstructions based on xenocrysts from Sloan and Kelsey Lake kimberlites.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 13-27.
Society for Mining, Metallurgy and Exploration (SME)-American Institute of Mining, Metallurgical, and Petroleum, SYMPOSIUM OUTLINE FALL MEETING OCTOBER 24TH. P. 13.
United States, Gulf Coast, Arkansas, California, Virginia, Appalachia
Geology and exploration of the Rose lamproite, southeast Kansas, SOURCE[ Geological Society of Australia Inc. Blackwell Scientific Publishing,SpecialPublication
Geological Society of Australia Inc. Blackwell Scientific Publishing, Special, No. 14, Vol. 2, pp. 1179-1191
Diamondiferous kimberlite at Kelsey Lake, southern Wyoming ArcheanProvince
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 85-88
Abstract: Because of Earth’s dynamic tectonic processes, much of its continental crust has been eroded and recycled and only a fraction of crust older than 2.5 billion years has survived to the present-day. These areas of old crust, known as Archean cratons, have not experienced deformation or magmatism for a billion years or more. This paper investigates whether craton survival is related to their nature, that is, the conditions of their formation, or to nurture, the subsequent events they experienced. Eight case studies are used to evaluate the properties and processes that promote craton stability. Nature is important: surviving Archean cratons tend to be buoyant, viscous, cold, and thick. Some survive because they have not experienced destabilizing geologic processes that introduce heat, magma, and fluids. Others have been modified to various extents by these processes. Some have been weakened and thinned and other, only marginally stable cratons are susceptible to future deformation and destruction. We conclude that both nature and nurture are essential to the survival of Earth’s oldest crust.
Contributions to Mineralogy and Petrology, Vol. 173, 12, pp. 106-
Mantle
metasomatism
Abstract: Multiphase solid inclusions in minerals formed at ultra-high-pressure (UHP) provide evidence for the presence of fluids during deep subduction. This study focuses on barian mica, which is a common phase in multiphase solid inclusions enclosed in garnet from mantle-derived UHP garnet peridotites in the Saxothuringian basement of the northern Bohemian Massif. The documented compositional variability and substitution trends provide constraints on crystallization medium of the barian mica and allow making inferences on its source. Barian mica in the multiphase solid inclusions belongs to trioctahedral micas and represents a solid solution of phlogopite KMg3(Si3Al)O10(OH)2, kinoshitalite BaMg3(Al2Si2)O10(OH)2 and ferrokinoshitalite BaFe3(Al2Si2)O10(OH)2. In addition to Ba (0.24-0.67 apfu), mica is significantly enriched in Mg ( X Mg 0.85 to 0.95), Cr (0.03-0.43 apfu) and Cl (0.04-0.34 apfu). The substitution vector involving Ba in the I-site which describes the observed chemical variability can be expressed as BaFeIVAlClK-1Mg-1Si-1(OH)-1. A minor amount of Cr and VIAl enters octahedral sites following a substitution vector VI(Cr,Al)2?VI(Mg,Fe)-3 towards chromphyllite and muscovite. As demonstrated by variable Ba and Cl contents positively correlating with Fe, barian mica composition is partly controlled by its crystal structure. Textural evidence shows that barian mica, together with other minerals in multiphase solid inclusions, crystallized from fluids trapped during garnet growth. The unusual chemical composition of mica reflects the mixing of two distinct sources: (1) an internal source, i.e. the host peridotite and its garnet, providing Mg, Fe, Al, Cr, and (2) an external source, represented by crustal-derived subduction-zone fluids supplying Ba, K and Cl. At UHP-UHT conditions recorded by the associated diamond-bearing metasediments (c. 1100 °C and 4.5 GPa) located above the second critical point in the pelitic system, the produced subduction-zone fluids transporting the elements into the overlying mantle wedge had a solute-rich composition with properties of a hydrous melt. The occurrence of barian mica with a specific chemistry in barium-poor mantle rocks demonstrates the importance of its thorough chemical characterization.
Canadian Journal of Earth Sciences, Vol. 52, 11, pp. 980-989.
Canada, Nunavut, Baffin Island
Geomorphology
Abstract: Speckle tracking of ALOS PALSAR fine beam data from 2007-2011 are used to determine the surface motion of major ice masses on Baffin Island and Bylot Island in the southern Canadian Arctic Archipelago. Glacier velocities are low overall, with peaks of ?100 m a?1 and means of ?20-60 m a?1 common along the main trunk of many outlet glaciers. Peak velocities on Penny and Bylot Island ice caps tend to occur near the mid-sections of their primary outlet glaciers, while the fastest velocities on all other glaciers usually occur near their termini due to relatively large accumulation areas draining through narrow outlets. Estimates of ice thickness at the fronts of tidewater-terminating glaciers are combined with the velocity measurements to determine a regional dynamic discharge rate of between ?17 Mt a?1 and ?108 Mt a?1, with a mid-point estimate of ?55 Mt a?1, revising downward previous approximations. These velocities can be used as inputs for glacier flow models, and provide a baseline dataset against which future changes in ice dynamics can be detected.
Abstract: The study of sediment cores allows for the reconstruction of past climate and environment through physical-chemical analysis. Nevertheless, this interpretation suffers from many drawbacks that can be overcome with the newest technologies. Hyperspectral imaging is one of these and allows a fast, high resolution, and non-destructive analysis of sediment cores. In this study, we use visible and near-infrared hyperspectral imaging to predict particle size fractions and distribution (PSD) at a resolution of 200??m on a previously well-studied sediment core taken from Lake Bourget (Western Alps, France). These predictions agree with previous studies on this core. Then, the PSD was used to estimate sedimentary deposit sources using the PSD unmixing algorithm AnalySize. It permitted estimation of the contribution of five sources (micrite, small and large bio-induced calcite crystals, diatom frustules, detrital particles), which had previously been characterized. The spatial dimension allowed for laminae to be discretized and counted, in agreement with the age-depth model previously established. We then evaluated the particle size and spectral signatures of each of these annual laminae, hence characterizing their physico-chemical composition. These high-resolution data also allowed for estimation of the accumulation rate (cm/year) of each of the main sources in the laminated unit and inferring the trophic status and the presence of instantaneous events of the lake.
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: 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: The Miaoya carbonatite complex (MCC) is located within the southern edge of the Qinling orogenic belt in central China, and is associated with significant rare earth element (REE) and Nb mineralization. The MCC consists of syenite and carbonatite that were emplaced within Neo- to Mesoproterozoic-aged supracrustal units. The carbonatite intruded the associated syenite as stocks and dikes, and is mainly composed of medium- to fine-grained calcite and abundant REE-bearing minerals. Carbonatite melt generation and emplacement within the MCC occurred during the Silurian (at ~440?Ma), and was subsequently impacted by a late-stage hydrothermal event (~232?Ma) involving REE-rich fluids/melt. This study reports trace element and stable (B, C, and O) and radiogenic (Nd, Pb, and Sr) isotope data for the MCC carbonatite, and these have been subdivided into three groups that represent different REE contents, interpreted as varying degrees of hydrothermal interaction. Overall, the group of carbonatites with the lowest enrichment in LREEs (i.e., least affected by hydrothermal event) is characterized by ?11B values that vary between ?7 (typical asthenospheric mantle) and?+?4‰; ?11B values and B abundances (~0.2 to ~1?ppm) do not correlate with LREE contents. The Sm-Nd and Pb-Pb isotope systems have both been perturbed by the late-stage, REE-rich hydrothermal activity and corroborate open-system behavior. Contrarily, initial 87Sr/86Sr ratios (vary between ~0.70355 and 0.70385) do not correlate significantly with both LREEs and Sr abundances, nor with initial 143Nd/144Nd ratios. The late-stage hydrothermal event overprinted the Nd and Pb isotope compositions for most of the carbonatite samples examined here, whereas a majority of the samples preserve their variable B and Sr isotope values inherited from their mantle source. The B and Sr isotope data for carbonatites exhibiting the least LREE enrichment correlate positively and suggest carbonatite melt generation from a heterogenous upper mantle source that records the input of recycled crustal material. This finding is consistent with those previously reported for young (<300?Ma old) carbonatites worldwide.
Abstract: This study reports the combined major, minor and trace element compositions, and stable (C, O), radiogenic (Nd, Pb, and Sr) isotopic compositions, and first ?11B isotopic data for the Fir, Felix, Gum, and Howard Creek carbonatites from the Blue River Region, British Columbia (Canada). These sill-like occurrences were intruded into Late Proterozoic strata during rifting and extensional episodes during the Late Cambrian and Devonian -Mississippian, and subsequently deformed and metamorphosed to amphibolite grade in relation to a collisional-type tectonic environment. The carbonatites at Fir, Gum, and Felix contain both calcite and dolomite, whereas the carbonatite at Howard Creek contains only calcite. The dolomite compositions reported here are consistent with those experimentally determined by direct partial melting of metasomatized peridotitic mantle. The combined major and trace element compositions and ?13CPDB (?5.37 to ?4.85‰) and ?18OSMOW (9.14 to 9.62‰) values for all the samples investigated are consistent with those for primary igneous carbonate and support their mantle origin. However, these signatures cannot be attributed to closed system melt differentiation from a single parental melt. The initial Nd, Pb, and Sr isotopic ratios are highly variable and suggest generation from multiple, small degree parental melts derived from a heterogeneous mantle source. The ?11B values for carbonates from Felix, Gum, and Howard Creek vary between ?8.67 and ?6.36‰, and overlap the range for asthenospheric mantle (?7.1?±?0.9‰), whereas two samples from Fir yield heavier values of ?3.98 and ?2.47‰. The latter indicate the presence of recycled crustal carbon in their mantle source region, which is consistent with those for young (<300?Ma) carbonatites worldwide. The radiogenic and B isotope results for the Blue River carbonatites are compared to those from contrasting, anorogenic tectonic settings at Chipman Lake, Fen, and Jacupiranga, and indicate that similar upper mantle sources are being tapped for carbonatite melt generation. The pristine, mantle-like ?11B values reported here for the Blue River carbonatites clearly demonstrate that this isotope system is robust and was not perturbed by post-solidification tectono-metamorphic events. This observation indicates that B isotope signatures are a valuable tool for deciphering the nature of the upper mantle sources for carbonates of igneous origin.
Chemical Geology, doi.org/10.1016/j.chemgeo.2019.07.015 59p.
Canada, British Columbia
carbonatite - Blue River
Abstract: This study reports the combined major, minor and trace element compositions, and stable (C, O), radiogenic (Nd, Pb, and Sr) isotopic compositions, and first ?11B isotopic data for the Fir, Felix, Gum, and Howard Creek carbonatites from the Blue River Region, British Columbia (Canada). These sill-like occurrences were intruded into Late Proterozoic strata during rifting and extensional episodes during the Late Cambrian and Devonian -Mississippian, and subsequently deformed and metamorphosed to amphibolite grade in relation to a collisional-type tectonic environment. The carbonatites at Fir, Gum, and Felix contain both calcite and dolomite, whereas the carbonatite at Howard Creek contains only calcite. The dolomite compositions reported here are consistent with those experimentally determined by direct partial melting of metasomatized peridotitic mantle. The combined major and trace element compositions and ?13CPDB (?5.37 to ?4.85‰) and ?18OSMOW (9.14 to 9.62‰) values for all the samples investigated are consistent with those for primary igneous carbonate and support their mantle origin. However, these signatures cannot be attributed to closed system melt differentiation from a single parental melt. The initial Nd, Pb, and Sr isotopic ratios are highly variable and suggest generation from multiple, small degree parental melts derived from a heterogeneous mantle source. The ?11B values for carbonates from Felix, Gum, and Howard Creek vary between ?8.67 and ?6.36‰, and overlap the range for asthenospheric mantle (?7.1?±?0.9‰), whereas two samples from Fir yield heavier values of ?3.98 and ?2.47‰. The latter indicate the presence of recycled crustal carbon in their mantle source region, which is consistent with those for young (<300?Ma) carbonatites worldwide. The radiogenic and B isotope results for the Blue River carbonatites are compared to those from contrasting, anorogenic tectonic settings at Chipman Lake, Fen, and Jacupiranga, and indicate that similar upper mantle sources are being tapped for carbonatite melt generation. The pristine, mantle-like ?11B values reported here for the Blue River carbonatites clearly demonstrate that this isotope system is robust and was not perturbed by post-solidification tectono-metamorphic events. This observation indicates that B isotope signatures are a valuable tool for deciphering the nature of the upper mantle sources for carbonates of igneous origin.
Cimen, O., Corcoran, L., Kuebler, C., Simonetti, S.S., Simonetti, A.
Geochemical, stable ( O, C, and B) and radiogenic ( Sr, Nd, Pb) isotopic data from the Eskisehir-Kizulxaoren ( NW-Anatolia) and the Malatya-Kuluncak ( E- central Anatolia) F-REE-Th deposits, Turkey: implications for nature of carbonate-hosted mineralizati
Turkish Journal of Earth Sciences, Vol. 29, doe:10.3906/yer-2001-7 18p. Pdf
Turkish Journal of Earth Sciences, Vol. 29, pp. 798-814. pdf
Europe, Turkey
REE
Abstract: In Turkey, the largest fluorine (F)-rare earth element (REE)-thorium (Th) deposits are located within the Eski?ehir-K?z?lcaören (north-western Anatolia) and the Malatya-Kuluncak (east-central Anatolia) regions, and these are associated with Oligocene extensional alkaline volcanic and Late Cretecaous-Early Paleocene postcollisional intrusive rocks, respectively. In the K?z?lcaören region, the basement units include the Triassic Karakaya Complex and the Late Cretaceous oceanic units (Neotethyan suture) that are cut and overlain by phonolite and carbonatite intrusions and lava flows. In the Kuluncak region, the plutonic rocks are mainly composed of syenite, quartz syenite, and rare monzonite, and these cut the late-Cretaceous Karap?nar limestone, which hosts the F-REE-Th mineralization in contact zones. A carbonatite sample from the K?z?lcaören region displays both a total rare earth element (TREE) concentration (4795 ppm) and ?11B (-6.83‰) isotope composition consistent with mantle-derived carbonatite; whereas it is characterized by heavier ?13C (+1.43‰) and ?18O (+20.23‰) isotope signatures compared to those for carbonatites worldwide. In contrast, the carbonates which host the F-REE-Th mineralization in the Kuluncak region are characterized by lower TREE concentrations (5.13 to 55.88 ppm), and heavier ?13C (-0.14 to -0.75‰), ?18O (+27.36 to +30.61‰), and ?11B (+5.38 to +6.89‰) isotope ratios compared to mantle-derived carbonatites. Moreover, the combined initial 87Sr/86Sr (0.70584 to 0.70759) and 143Nd/144Nd (0.512238 to 0.512571) isotope ratios for samples investigated here are distinct and much more radiogenic compared to those for carbonatites worldwide, and therefore indicate significant crustal input and/or hydrothermal metasomatic-related alteration. Overall, stable and radiogenic isotope data suggest that the host carbonate rocks for the F-REE-Th mineralization in both the K?z?lcaören and the Kuluncak regions consist of hydrothermally metasomatized carbonatite and limestone, respectively. The mineralization in the K?z?lcaören region may potentially be related to carbonatite magmatism, whereas the mineralization in the Kuluncak region, which most likely formed through interactions between the plutonic rocks and surrounding limestone at contact metamorphism zone, involved hydrothermal/magmatic fluids associated with extensive postcollisional magmatism.
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Magnetic inversion of three airborne dat a sets over the Tli Kwi Cho kimberlite complex.
SEG Annual Meeting Denver, pp. 1790-1794 extended abstract
Fournier, D., Heagy, L., Corcoran, N., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Multi-EM systems inversion - towards a common conductivity model for Tli Kwi Cho complex.
SEG Annual Meeting Denver, pp. 1795-1798. Extended abstract
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO-18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO- 18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Marchant, M., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
Anais Do 15 Simposio Geologia da Amazonia, Belem , Dec. 5p. Abstract pdf
South America, Guiana
craton
Abstract: The Orocaima Igneous Belt (OIB) is a huge plutono-volcanic belt at the central part of Guiana Shield, consisting mainly of 1.99-1.96 Ga volcano-plutonic rocks with high-K calc-alkaline, A-type and shosho-nitic geochemical signatures. Three A-type granitic bodies from the central part of the OIB have been dated using U-Pb SHRIMP and LA-ICPMS methods. A 1985±11 Ma age was calculated for the Macucal Mountain Granite of the Saracura Suite (Brazil) and ages of 1977±3.9 Ma and 1975±5 were calculated for the alkaline riebeckite granites respectively of the Lontra (Brazil) and Makarapan (Guyana) bodies. These ages are in the same range of those reported for the Aricamã A-type granitoids and the results indicate that different A-type magmatism took place in the 1.993-1.975 Ma interval along the OIB, coeval to high-K calc-alkaline and shoshonitic magmatism. This scenario fits well to a post-collisional setting.
Netherlands Journal of Geolsciences, Vol. 95, 4, pp. 491-522.
South America, Suriname
Guiana shield
Abstract: The Proterozoic basement of Suriname consists of a greenstone-tonalite-trondhjemite-granodiorite belt in the northeast of the country, two high-grade belts in the northwest and southwest, respectively, and a large granitoid-felsic volcanic terrain in the central part of the country, punctuated by numerous gabbroic intrusions. The basement is overlain by the subhorizontal Proterozoic Roraima sandstone formation and transected by two Proterozoic and one Jurassic dolerite dyke swarms. Late Proterozoic mylonitisation affected large parts of the basement. Almost 50 new U-Pb and Pb-Pb zircon ages and geochemical data have been obtained in Suriname, and much new data are also available from the neighbouring countries. This has led to a considerable revision of the geological evolution of the basement. The main orogenic event is the Trans-Amazonian Orogeny, resulting from southwards subduction and later collision between the Guiana Shield and the West African Craton. The first phase, between 2.18 and 2.09 Ga, shows ocean floor magmatism, volcanic arc development, sedimentation, metamorphism, anatexis and plutonism in the Marowijne Greenstone Belt and the adjacent older granites and gneisses. The second phase encompasses the evolution of the Bakhuis Granulite Belt and Coeroeni Gneiss Belt through rift-type basin formation, volcanism, sedimentation and, between 2.07 and 2.05 Ga, high-grade metamorphism. The third phase, between 1.99 and 1.95 Ga, is characterised by renewed high-grade metamorphism in the Bakhuis and Coeroeni belts along an anticlockwise cooling path, and ignimbritic volcanism and extensive and varied intrusive magmatism in the western half of the country. An alternative scenario is also discussed, implying an origin of the Coeroeni Gneiss Belt as an active continental margin, recording northwards subduction and finally collision between a magmatic arc in the south and an older northern continent. The Grenvillian collision between Laurentia and Amazonia around 1.2-1.0 Ga caused widespread mylonitisation and mica age resetting in the basement.
Journal of African Earth Sciences, Vol. 129, pp. 366-379.
Africa, Mozambique
Craton, Zimbabwe
Abstract: The eastern margin of the Zimbabwe Craton, along the Mozambique-Zimbabwe border, includes the oldest rocks of west-central Mozambique constituting a large terrain of granite-greenstone type dated between 3000 and 2500 Ma. These rocks consist mainly of gneisses and granitoid rocks of tonalitic-trondhjemitic-granodioritic composition belonging to the Mudzi Metamorphic Complex in the northern part and to the Mavonde Complex in the southern part. The latter is associated with a granite-greenstone terrain, which includes the eastern part of Mutare-Odzi-Manica greenstone belt. A volcano-sedimentary sequences cover, belonging to the apparently Mesoproterozoic and Paleoproterozoic Umkondo and Gairezi groups respectively was deposited along the eastern margin of the craton and is exposed in the territory of Mozambique. The Umkondo minimum age is marked by intrusive dolerite in Zimbabwe dated at 1100 Ma while for the Ghairezi it is still not well established. The Gairezi Group was subjected to progressive metamorphism of Pan-African age. At the margin of the Zimbabwe Craton, in its northern part, metasedimentary units occur representing a passive margin of Neoproterozoic age. They make up the Rushinga Group, which includes felsic metavolcanic rocks dated at ca.800 Ma. Granulites and medium- to high-grade paragneisses, and migmatites of the Chimoio, Macossa and Mungari Groups of Neoproterozoic metamorphic age, overly the ortho-metamorphic pre-existing rock of ca. 1100 Ma, which belongs to the Báruè Magmatic Arc. They characterize the N-S trend Mozambique Belt, which appears to the east of the craton tectonically juxtaposed on the Archean rocks. The maximum age of deposition of these rocks, indicated by U-Pb dating of detrital zircons, is ca. 700 Ma and their minimum age is limited by a few monzonitic Cambrian intrusions dated at ca. 500 Ma. The Neoproterozoic bimodal Guro Suite, dated at ca. 850 Ma and composed of felsic and mafic members characterizes the east-dipping outer rim of the craton margin in the north. The felsic member comprises the Serra Banguatere aplitic granite gneiss-migmatite and the mafic member consists of the Magasso metagabbro and mafic gneiss-migmatite. The geochemical signature and bimodality are all characteristics of anorogenic, A-type granites. The tectono-thermal effects of the Pan-African orogenic event, of approximately 500 Ma, are visible along the margin of the Zimbabwe Craton. Deformation and metamorphism are progressive from the craton towards the belt, from greenschist facies to granulite facies. The main suture in the study area shall be placed along the frontal thrusts of the Mungari and Macossa/Chimoio nappes of Neoproterozoic to Cambrian age. To the west of the suture the rejuvenated margin of the craton occurs, indicated by K-Ar dating. To the east, the Mozambique Belt occurs with its paragneisses of the Neoproterozoic overlaying the Mesoproterozoic granitoids of the Báruè magmatic arc.
Heilbron et al. eds. Sao Francisco Craton, eastern Brazil, Chapter 16, 17p. Researchgate
South America, Brazil
geochronology
Abstract: This chapter, based on paleomagnetic and geologic-geochronological evidence, discusses the position of the São Francisco craton and other South American and African cratonic blocks within paleo-continents, since the formation of Columbia supercontinent in the Paleoproterozoic up to the fragmentation of Pangea in the Mesozoic. In Paleoproterozoic times, between ca. 2.0 and 1.8 Ga, two large independent landmasses were formed. The first one involved several cratonic blocks that were leading to the formation of Laurentia. Later, Laurentia, proto-Amazonia, West Africa and Baltica amalgamated to form the nucleus of the supercontinent Columbia at about 1.78 Ga. The second landmass encompassed the São Francisco-Congo, Kalahari, Rio de la Plata and Borborema-Trans-Sahara, forming the Central African block. For the São Francisco-Congo and Kalahari cratons, two robust Paleoproterozoic poles are available. One is from the Jequié charnockites of Bahia (São Francisco Craton), and the other from the Limpopo high-grade metamorphics in South Africa (Kalahari Craton). They support the possible link between these two cratonic blocks at ca. 2.0 Ga. Columbia may have remained united until 1.25 Ga, when Baltica and Amazonia/West Africa broke apart. Their paleomagnetic record seems to indicate that both executed clockwise rotations, until they collided with Laurentia along the Grenville belt at ca. 1.0 Ga., culminating with the formation of Rodinia. For the Central African block, however, there are no reliable paleomagnetic poles available between 1.78 and 1.27 MA. Nevertheless, during this time interval, the geological-geochronological evidence indicates that no continental collisional episodes affected the São Francisco-Congo craton, where important intra-plate tectonic episodes occurred. Most probably, this large continental block drifted alone since the end of the Paleoproterozoic and did not take part of Columbia or Rodinia. At the end of the Mesoproterozoic, ca. 1100 MA, the robust Umkondo pole of the Kalahari craton, as part of the Central African block, and the equally robust Keweenawan pole of Laurentia at the center of Rodinia, indicated that these landmasses were very far apart. At that time a large oceanic realm, the Goiás-Pharusian Ocean, was indeed separating Amazonia-West Africa from the Central African block. This ocean closed by a continued subduction process that started at ca. 900 MA and ended in a collisional belt with Himalayan-type mountains at ca. 615 MA, as part of the few continental collisions which formed Gondwana. However, the age of the final convergence is still a matter of debate, because paleomagnetic measurements for the Araras Group, which occurs within the Paraguay belt at the eastern margin of the Amazonian craton, would indicate that a large ocean was still in existence between it and São Francisco craton close to the Ediacaran/Cambrian boundary. Consensus about this matter awaits for further paleomagnetic data. Gondwana collided with Laurasia during the late Paleozoic, at about 300 Ma, originating Pangea, which not much later started splitting apart, near the Permian/Triassic boundary. As part of this present-time plate tectonic regime, the São Francisco Craton (in South America) started separation from the Congo craton (in Africa) in Jurassic times, giving rise of the present-day oceanic lithosphere of the Atlantic Ocean.
Abstract: New and compiled data of zircon U-Pb ages and geochemical-isotopic constraints provide new insights into the orogenic evolution of the Rio Apa Terrane (RAT) and its close affinity with the Amazonia throughout the Proterozoic. Two terranes with distinct evolutionary histories built the RAT. The Porto Murtinho (2070-1940 Ma) and Amoguijá (1870-1820 Ma) magmatic arcs generated the Western Terrane which is mainly composed of short-lived crustal components. Granitoid rocks (1870 Ma) in the distal Corumbá Window indicate that the RAT is much larger in extent. The Caracol accretionary arc (1800-1740 Ma) and the associated Alto Tererê back-arc basin formed away from the Amoguijá belt, being roughly coeval with the adjoining Baía das Garças suite (1776 Ma) and Paso Bravo granitoid rocks (1774-1752 Ma). These tectonic units constitute the Eastern Terrane, whilst the NdHf isotopic constraints indicate derivation from a predominantly juvenile magma source with the minor input of crustal-derived contaminants. The youngest detrital zircon grains from the Alto Tererê samples gave 1740-1790 Ma ages and unimodal age spectra were mainly present. The basin infill was, therefore, most likely concomitant with the exhumation of the Caracol belt. Alto Tererê provenance study also included detritus from passive to active margin settings. The RAT underwent regional cooling between 1.35 and 1.27 Ga, documented mainly by 40Ar39Ar and KAr ages. This age pattern matches a collisional episode that formed the accretionary margin of Amazonia, suggesting that the RAT was a close neighbor at Ectasian times. The geodynamic interplay between them lasted until 1.1 Ga ago, highlighted by some shared-components of a LIP event.
Abstract: The Rio Apa cratonic fragment crops out in Mato Grosso do Sul State of Brazil and in northeastern Paraguay. It comprises Paleo-Mesoproterozoic medium grade metamorphic rocks, intruded by granitic rocks, and is covered by the Neoprotero-zoic deposits of the Corumbá and Itapocumi Groups. Eastward it is bound by the southern portion of the Paraguay belt. In this work, more than 100 isotopic determina-tions, including U-Pb SHRIMP zircon ages, Rb-Sr and Sm-Nd whole-rock determina-tions, as well as K-Ar and Ar-Ar mineral ages, were reassessed in order to obtain a complete picture of its regional geological history. The tectonic evolution of the Rio Apa Craton starts with the formation of a series of magmatic arc complexes. The oldest U-Pb SHRIMP zircon age comes from a banded gneiss collected in the northern part of the region, with an age of 1950 23 Ma. The large granitic intrusion of the Alumiador Batholith yielded a U-Pb zircon age of 1839 33 Ma, and from the southeastern part of the area two orthogneisses gave zircon U-Pb ages of 1774 26 Ma and 1721 25 Ma. These may be coeval with the Alto Tererê metamorphic rocks of the northeastern corner, intruded in their turn by the Baía das Garças granitic rocks, one of them yielding a zircon U-Pb age of 1754 49 Ma. The original magmatic protoliths of these rocks involved some crustal component, as indicated by the Sm-Nd T DM model ages, between 1.9 and 2.5 Ga. Regional Sr isotopic homogenization, associated with tectonic deformation and medium-grade metamorphism occurred at approximately 1670 Ma, as suggested by Rb-Sr whole rock reference isochrons. Finally, at 1300 Ma ago, the Ar work indicates that the Rio Apa Craton was affected by widespread regional heating, when the temperature probably exceeded 350°C. Geographic distribution, age and isotopic signature of the lithotectonic units suggest the existence of a major suture separating two different tectonic domains, juxtaposed at about 1670 Ma. From that time on, the unified Rio Apa continental block behaved as one coherent and stable tectonic unit. It correlates well with the SW corner of the Amazonian Craton, where the medium-grade rocks of the Juruena-Rio Negro tectonic province, with ages between 1600 and 1780 Ma, were reworked at about 1300 Ma. Looking at the largest scale, the Rio Apa Craton is probably attached to the larger Amazonian Craton, and the actual configuration of southwestern South America is possibly due to a complex arrangement of allochthonous blocks such as the Arequipa, Antofalla and Pampia, with different sizes, that may have originated as disrupted parts of either Laurentia or Amazonia, and were trapped during later collisions of these continental masses.
Abstract: We present the first regional in-situ zircon U-Pb-Hf isotopic data from metaigneous and metasedimentary rocks from the Paleo- to Mesoproterozoic Rio Apa Terrane (RAT), a crustal fragment outcropping in the central-western Brazil and north-eastern Paraguay. These new ages and Hf isotopic data delineate three magmatic events, which record the construction of the temporally and isotopically distinct Western and Eastern Terranes of the RAT. The Western Terrane comprises the 2100-1940 Ma Porto Murtinho Complex and the 1900-1840 Ma Amoguijá Belt, which both define a crustal reworking array in ?HfT-time space evolving from a precursor source with Hf TDM age of ca. 2700 Ma. The 1800-1720 Ma Caracol Belt constitutes the Eastern Terrane and yields suprachondritic ?HfT signatures up to +7.1, indicating significant juvenile input. The metasedimentary Amolar Group and Rio Naitaca Formation in the Western Terrane have maximum depositional ages of 1850-1800 Ma and subchondritic ?HfT signatures down to ?5.7, similar to the underlying basement of the Amoguijá Belt. In the Eastern Terrane, the Alto Tererê Formation has a maximum depositional age of 1750 Ma and mostly suprachondritic ?HfT signatures, similar to magmatic rocks of the underlying Caracol Belt. Together, the new igneous and detrital zircon age and Hf isotopic data record a temporal and spatial transition from 2100 to 1840 Ma crustal reworking in the west to more juvenile magmatism at 1800-1720 Ma in the east. This transition is interpreted to reflect convergent margin magmatism associated with periods of subduction zone advance and retreat in an accretionary orogenic setting. Comparison of the ?HfT-time signature of the RAT with the Amazonian Craton suggest penecontemporaneous development, with the Western and Eastern Terranes of the RAT being correlative with the Ventuari-Tapajós and Rio Negro-Juruena Province of the Amazonian Craton, respectively. Our new data also reveal that the ?HfT signatures of the RAT are distinct from the Maz terrane, which refutes the MARA Block hypothesis.
Abstract: The Late Cretaceous Catalão I carbonatite complex consists of ultramafic silicate rocks, phoscorites, nelsonites and carbonatites. The latest stages of the evolution of the complex are characterized by several nelsonite (magnetite-apatite rock) and carbonatite dykes, plugs and veins crosscutting earlier alkaline rocks. The interaction between the latter and late-stage carbonatites and/or carbo-hydrothermal fluids, converted the original dunites and bebedourites to metasomatic phlogopitites. Late-stage nelsonites (N1), pseudonelsonites (N2) and various types of dolomite carbonatites (DC) including norsethite-, magnesite- and/or monazite-bearing varieties show significant whole-rock Nd and Sr isotopic variations. To elucidate whether magmatic or metasomatic processes, or both, were responsible for these isotope variations we characterized the Nd and Sr isotope compositions of major mineral phases (i.e. apatite, dolomite, norsethite, pyrochlore and tetraferriphlogopite) in these late-stage rocks. Mineral isotope data recorded the same differences observed between N1 and N2 whole-rocks with N2 minerals showing more enriched isotopic signatures than minerals from N1. Sr isotopic disequilibrium among minerals from N2 pseudonelsonites and spatially related dolomite carbonatite pockets implies formation from batches of carbonate melts with distinct isotopic compositions. A detailed investigation of Nd and Sr isotopes from whole-rocks and minerals suggests that the most evolved rocks of the Catalão I complex probably derive from two different evolution paths. We propose that an earlier magmatic trend (path A) could be explained by several batches of immiscible and/or residual melts derived from carbonated-silicate parental magma (e.g. phlogopite picrite) contaminated with continental crust to a variable extent, in an AFCLI-like process. A second trend (path B) comprises highly variable 143Nd/144Ndi at nearly constant 87Sr/86Sri coupled with high ?18O in carbonates. This is interpreted here as the result of the interaction of previously-formed dolomite carbonatites with carbo-hydrothermal fluids.
Geophysical studies in central Midcontinent CUSMAP Quadrangles
United States Geological Survey (USGS) Open file, United States Geological Survey (USGS)-Missouri G.S. Symp: Mineral resource potential of, p. 5. (abstract.)
International Journal of Remote Sensing, Vol. 38, 3, pp. 827-849.
Technology
LIDAR
Abstract: In recent years lidar technology has experienced a noticeable increase in its relevance and usage in a number of scientific fields. Therefore, software capable of handling lidar data becomes a key point in those fields. In this article, we present GPU-based viewer lidar (GVLiDAR), a novel web framework for visualization and geospatial measurement of lidar data point sets. The design of the framework is focused on achieving three key objectives: performance in terms of real-time interaction, functionality, and online availability for the lidar datasets. All lidar files are pre-processed and stored in a lossless data structure, which minimizes transfer requirements and offers an on-demand lidar data web framework.
Abstract: Most kimberlites contain abundant dunitic nodules. These are centimetre-sized, rounded and multi-grained assemblages of xenocrystic olivine with a wide range of compositions (Fo83 to Fo94). The absence of orthopyroxene and other mantle minerals and the range of olivine compositions have been attributed to reaction between mantle peridotite and (proto)kimberlitic fluid or melt, but the timing of the reaction is a subject of debate. In a kimberlite from the Kangamiut region of Greenland, nodule cores are surrounded by fine-grained outer margins with near-constant Fo contents (~Fo88) but highly variable minor element contents (e.g. 500-2500 ppm Ni). These margins crystallized from the kimberlite melt and we show that their compositions can be explained by crystallization of olivine alone, if a high partition coefficient for Ni between melt and olivine (DNi > 20) is assumed. Orthopyroxene assimilation is not required, removing the constraint that its dissolution occurred during ascent of the kimberlite magma. Within some nodules, in addition to the usual core-to-margin gradients, we observe asymmetric compositional changes (variable Fo but near-constant minor element contents) across mobile grain boundaries. These changes document fluid percolation at the grain scale that occurred during dynamic recrystallization in the deforming lithospheric mantle. We note that chemical gradients associated with mobile grain boundaries are observed in olivines that cover the entire compositional range of the nodules, and propose that fluid-assisted dynamic recrystallization took place in dunite that was already compositionally heterogeneous. Reaction between peridotite and protokimberlitic melt or fluid and dissolution of orthopyroxene thus occurred within the lithospheric mantle, immediately (a few days) prior to the ascent of the kimberlite melt and the entrainment of the dunite nodules. We propose that the grain boundary zones probably mimic, at a fine scale, the fluid-peridotite interaction that caused, at a larger scale, orthopyroxene dissolution and formation of compositionally diverse olivine in kimberlites.
Abstract: Most kimberlites contain abundant dunitic nodules. These are centimetre-sized, rounded and multi-grained assemblages of xenocrystic olivine with a wide range of compositions (Fo83 to Fo94). The absence of orthopyroxene and other mantle minerals and the range of olivine compositions have been attributed to reaction between mantle peridotite and (proto)kimberlitic fluid or melt, but the timing of the reaction is a subject of debate. In a kimberlite from the Kangamiut region of Greenland, nodule cores are surrounded by fine-grained outer margins with near-constant Fo contents (~Fo88) but highly variable minor element contents (e.g. 500-2500 ppm Ni). These margins crystallized from the kimberlite melt and we show that their compositions can be explained by crystallization of olivine alone, if a high partition coefficient for Ni between melt and olivine (DNi > 20) is assumed. Orthopyroxene assimilation is not required, removing the constraint that its dissolution occurred during ascent of the kimberlite magma. Within some nodules, in addition to the usual core-to-margin gradients, we observe asymmetric compositional changes (variable Fo but near-constant minor element contents) across mobile grain boundaries. These changes document fluid percolation at the grain scale that occurred during dynamic recrystallization in the deforming lithospheric mantle. We note that chemical gradients associated with mobile grain boundaries are observed in olivines that cover the entire compositional range of the nodules, and propose that fluid-assisted dynamic recrystallization took place in dunite that was already compositionally heterogeneous. Reaction between peridotite and protokimberlitic melt or fluid and dissolution of orthopyroxene thus occurred within the lithospheric mantle, immediately (a few days) prior to the ascent of the kimberlite melt and the entrainment of the dunite nodules. We propose that the grain boundary zones probably mimic, at a fine scale, the fluid-peridotite interaction that caused, at a larger scale, orthopyroxene dissolution and formation of compositionally diverse olivine in kimberlites.
Abstract: Moore proposes in his Comment (Moore, 2017) that marginal zones in olivine grains in kimberlites (Fig. 1a) are produced by crystallization from kimberlite melt. He suggests that the chemical zones observed in these marginal zones (inner transition zones and outer margins, illustrated in his fig. 1) result from abrupt changes in distribution coefficients during crystallization. He proposes that the transition zones, characterized by variable Fo at constant and high Ni contents, are produced by crystallization with high KdFe-Mg (= 0•45) and low DNi (= 4) whereas the margins, characterized by a sharp drop in Ni content at nearly constant Fo (Fig. 1b), are produced by crystallization with higher DNi owing to a sudden change in physical conditions of crystallization (P,…
Mainprice, D., Tommasi, A., Ferre, D., Carrez, P., Cordier, P.
Predicted glide systems and crystal preferred orientations of polycrystalline silicate Mg perovskite at high pressure: implications for seismic anisotropy
Earth and Planetary Science Letters, Vol. 271, 1-4, pp. 135-144.
Abstract: To test if hydrogen incorporation by ionic diffusion can occur between a volatile-rich kimberlitic liquid and forsterite, results of high-pressure and high-temperature experiments using a piston-cylinder apparatus at 1200–1300 °C and 1 GPa for durations of 1 min, 5 h, and 23 h, are reported here. Kim-berlitic liquid in the system CaO-MgO-Al 2 O 3-SiO 2-CO 2-H 2 O and synthetic forsterite single crystals were chosen as a first simplification of the complex natural kimberlite composition. Unpolarized Fourier transform infrared spectroscopy was used to quantify the concentrations of OH in the crystallographically oriented forsterite. Scanning electron microscopy, electron backscattered diffraction, electron microprobe analyses, and transmission electron microscopy were performed to identify the run products. After 5 and 23 h, a forsterite overgrowth crystallized with the same orientation as the initial forsterite single crystal. The kimberlitic liquid has crystallized as micrometer-scale euhedral forsterite neocrystals with random crystallographic orientations, as well as a nanoscale aluminous phase and a calcic phase. Despite theoretical water-saturation of the system and long duration, none of the initial forsterite single crystals display signs of hydration such as hydrogen diffusion profile from the border toward the center of the crystal. Most likely, the presence of CO 2 in the system has lowered the H 2 O fugacity to such an extent that there is no significant hydration of the starting forsterite single crystal or its overgrowth. Also, the presence of CO 2 enhances rapid forsterite crystal growth. Forsterite growth rate is around 2 × 10 8 mm 3 /h at 1250 °C. These experimental results suggest a deep mantle origin of the high OH content found in natural mantle-derived xenoliths transported in kimberlites, as reported from the Kaapvaal craton. In agreement with previous studies, it also points out to the fact that significant hydration must take place in a CO 2-poor environment.
Earth and Planetary Science Letters, Vol. 467, pp. 99-107.
Mantle
chlorine
Abstract: We report concentrations of Chlorine (Cl) in synthetic wadsleyite (Wd) and ringwoodite (Rw) in the system NaCl-(Mg,?Fe)2SiO4 under hydrous and anhydrous conditions. Multi-anvil press experiments were performed under pressures (14-22 GPa) and temperatures (1100-1400?°C) relevant to the transition zone (TZ: 410-670 km depth). Cl and H contents were measured using Particle Induced X-ray Emission (PIXE) and Elastic Recoil Detection Analysis (ERDA) respectively. Results show that Cl content in Rw and Wd is significantly higher than in other nominally anhydrous minerals from the upper mantle (olivine, pyroxene, garnet), with up to 490 ppm Cl in anhydrous Rw, and from 174 to 200 ppm Cl in hydrous Wd and up to 113 ppm Cl in hydrous Rw. These results put constrains on the Cl budget of the deep Earth. Based on these results, we propose that the TZ may be a major repository for major halogen elements in the mantle, where Cl may be concentrated together with H2OH2O and F (see Roberge et al., 2015). Assuming a continuous supply by subduction and a water-rich TZ, we use the concentrations measured in Wd (174 ppm Cl) and in Rw (106 ppm Cl) and we obtain a maximum value for the Cl budget for the bulk silicate Earth (BSE) of 15.1 × 1022 g Cl, equivalent to 37 ppm Cl. This value is larger than the 17 ppm Cl proposed previously by McDonough and Sun (1995) and evidences that the Cl content of the mantle may be higher than previously thought. Comparison of the present results with the budget calculated for F (Roberge et al., 2015) shows that while both elements abundances are probably underestimated for the bulk silicate Earth, their relative abundances are preserved. The BSE is too rich in F with respect to heavy halogen elements to be compatible with a primordial origin from chondrites CI-like (carbonaceous chondrites CC) material only. We thus propose a combination of two processes to explain these relative abundances: a primordial contribution of different chondritic-like materials, including EC-like (enstatite chondrites), possibly followed by a distinct fractionation of F during the Earth differentiation due to its lithophile behavior compared to Cl, Br and I.
Physics of the Earth and Planetary Interiors, Vol. 287, pp. 65-75.
Mantle
core-mantle boundary
Abstract: The viscosity of the lower mantle results from the rheological behavior of its two main constituent minerals, aluminous (Mg,Fe)SiO3 bridgmanite and (Mg,Fe)O ferropericlase. Understanding the transport properties of lower mantle aggregates is of primary importance in geophysics and it is a challenging task, due to the extreme time-varying conditions to which such aggregates are subjected. In particular, viscosity is a crucial transport property that can vary over several orders of magnitude. It thus has a first-order control on the structure and dynamics of the mantle. Here we focus on the creep behavior of (Mg,Fe)O at the bottom of the lower mantle, where the presence of thermo-chemical anomalies such as ultralow-velocity zones (ULVZ) may significantly alter the viscosity contrast characterizing this region. Two different iron concentrations of (Mg1-xFex)O are considered: one mirroring the average composition of ferropericlase throughout most of the lower mantle (x?=?0.20) and another representing a candidate magnesiowüstite component of ULVZs near the base of the mantle (x?=?0.84). The investigated pressure-temperature conditions span from 120?GPa and 2800?K, corresponding to the average geotherm at this depth, to core-mantle boundary conditions of 135?GPa and 3800?K. In this study, dislocation creep of (Mg,Fe)O is investigated by dislocation dynamics (DD) simulations, a modeling tool which considers the collective motion and interactions of dislocations. To model their behavior, a 2.5 dimensional dislocation dynamics approach is employed. Within this method, both glide and climb mechanisms can be taken into account, and the interplay of these features results in a steady-state condition. This allows the retrieval of the creep strain rates at different temperatures, pressures, applied stresses and iron concentrations across the (Mg,Fe)O solid solution, providing information on the viscosity for these materials. A particularly low viscosity is obtained for magnesiowüstite with respect to ferropericlase, the difference being around 10 orders of magnitude. Thus, the final section of this work is devoted to the assessment of the dynamic implications of such a weak phase within ULVZs, in terms of the viscosity contrast with respect to the surrounding lowermost mantle.
Earth and Planetary Science Letters, Vol. 547, 10p. Pdf
Mantle
Wadsleyite, ringwoodite, majorite garnet
Abstract: The dynamics of the Earth's mantle is still poorly constrained due to the lack of understanding the transfer of matter between the upper and the lower mantle and their convective vigor. The transition zone (TZ) might play a crucial role as the interface connecting the upper to the lower mantle. Here, we examine the rheology of the main TZ minerals, wadsleyite, ringwoodite and majorite garnet based on a mineral physics approach. Using the results of lattice friction modeling and dislocation glide mobilities together with the available data on self-diffusion in the TZ minerals, we quantify their plastic deformation by diffusion and dislocation creep from theoretical plasticity models. We show that pure climb creep is expected to contribute to the plasticity of the TZ without the need of significant diffusion-related hydrolytic weakening, matching well the geophysical observations. Our model results predict that crystallographic preferred orientations (CPO) might only develop along with stress concentrations as present around cold subducting slabs which can be locally weaker than the surrounding TZ despite their lower temperatures.
Earth and Planetary Letters, Vol. 547, 116438, 10p. Pdf
Mantle
wadsleyite
Abstract: The dynamics of the Earth's mantle is still poorly constrained due to the lack of understanding the transfer of matter between the upper and the lower mantle and their convective vigor. The transition zone (TZ) might play a crucial role as the interface connecting the upper to the lower mantle. Here, we examine the rheology of the main TZ minerals, wadsleyite, ringwoodite and majorite garnet based on a mineral physics approach. Using the results of lattice friction modeling and dislocation glide mobilities together with the available data on self-diffusion in the TZ minerals, we quantify their plastic deformation by diffusion and dislocation creep from theoretical plasticity models. We show that pure climb creep is expected to contribute to the plasticity of the TZ without the need of significant diffusion-related hydrolytic weakening, matching well the geophysical observations. Our model results predict that crystallographic preferred orientations (CPO) might only develop along with stress concentrations as present around cold subducting slabs which can be locally weaker than the surrounding TZ despite their lower temperatures.
Brovarone, A.V., Butch, C.J., Ciappa, A., Cleaves, H.J., Elmaleh, A., Faccenda, M., Feineman, M., Hermann, J., Nestola, F., Cordone, A., Giovannelli., D.
American Mineralogist, Vol. 105, pp. 1152-1160. pdf
Mantle
carbon
Abstract: Water plays a key role in shaping our planet and making life possible. Given the abundance of water on Earth's surface and in its interior, chemical reactions involving water, namely hydration and dehydration reactions, feature prominently in nature and are critical to the complex set of geochemical and biochemical reactions that make our planet unique. This paper highlights some fundamental aspects of hydration and dehydration reactions in the solid Earth, biology, and man-made materials, as well as their connections to carbon cycling on our planet.
Heterogeneity of the subcontinental mantle: uranium-lead (U-Pb) (U-Pb) and Lu-Hf in megacrysts of baddeleyite and zircon from the Mbuji-Mayi kimberlite
Eos Transactions, Vol. 73, No. 14, April 7, supplement abstracts p.339
Age and petrogenesis of two late Archean magmatic suites, northwestern Superior Province, Canada: zircon uranium-lead (U-Pb) (U-Pb) and Lu-Hf isotopic relations
Journal of Petrology, Vol. 34, No. 4, August pp. 817-
Review of Archean supracrustal assemblages of the southern Abitibi Greenstone belt in Ontario, Canada: products of microplate interaction within alarge scale plate
Precambrian Research, Vol. 65, No. 1-4, January pp. 183-206
Abstract: Carbonatites (sensu stricto) are igneous rocks typically associated with continental rifts, being emplaced at relatively shallow crustal levels or as extrusive rocks. Some carbonatites are, however, related to subduction and lithospheric collision zones, but so far no carbonatite has been reported from ultrahigh-pressure (UHP) metamorphic terranes. In this study, we present detailed petrological and geochemical data on carbonatites from the Tromsø Nappe—a UHP metamorphic terrane in the Scandinavian Caledonides. Massive to weakly foliated silicate-rich carbonate rocks, comprising the high-P mineral assemblage of Mg-Fe-calcite?±?Fe-dolomite?+?garnet?+?omphacitic clinopyroxene?+?phlogopite?+?apatite?+?rutile?+?ilmenite, are inferred to be carbonatites. They show apparent intrusive relationships to eclogite, garnet pyroxenite, garnet-mica gneiss, foliated calc-silicate marble and massive marble. Large grains of omphacitic pyroxene and megacrysts (up to 5?cm across) of Cr-diopside in the carbonatite contain rods of phlogopite oriented parallel to the c-axis, the density of rods being highest in the central part of the megacrysts. Garnet contains numerous inclusions of all the other phases of the carbonatite, and, in places, composite polyphase inclusions. Zircon, monazite and allanite are common accessory phases. Locally, veins of silicate-poor carbonatite (up to 10?cm across) occur. Extensive fenitization by K-rich fluids, with enrichment in phlogopite along contacts between carbonatite and silicate country rocks, is common. Primitive mantle-normalized incompatible element patterns for the carbonatite document a strong enrichment of light rare earth elements, Ba and Rb, and negative anomalies in Th, Nb, Ta, Zr and Hf. The carbon and oxygen isotope compositions of the carbonatite are distinctly different from those of the spatially associated calc-silicate marble, but also from mantle-derived carbonatites elsewhere. Neodymium and Sr isotope data coupled with the trace element distribution indicate a similarity of the Tromsø carbonatite to orogenic (off-craton) carbonatites rather than to anorogenic (on-craton) ones. U-Pb dating of relatively U-rich prismatic, oscillatory-zoned zircon gives an age of 454•5?±?1•1?Ma. We suggest that the primary carbonatite magma resulted from partial melting of a carbonated eclogite at UHP, in a deeply subducted continental slab.
Geostandards and Geoanalytical Research, http://orchid.org/0000-0002-2701-4635 80p.
Asia, Sri Lanka
geochronology
Abstract: Here we document a detailed characterization of two zircon gemstones, GZ7 and GZ8. Both stones had the same mass at 19.2 carats (3.84 g) each; both came from placer deposits in the Ratnapura district, Sri Lanka. The U-Pb data are in both cases concordant within the uncertainties of decay constants and yield weighted mean ²??Pb/²³?U ages (95% confidence uncertainty) of 530.26 Ma ± 0.05 Ma (GZ7) and 543.92 Ma ± 0.06 Ma (GZ8). Neither GZ7 nor GZ8 have been subjected to any gem enhancement by heating. Structure?related parameters correspond well with the calculated alpha doses of 1.48 × 10¹? g?¹ (GZ7) and 2.53 × 10¹? g?¹ (GZ8), respectively, and the (U-Th)/He ages of 438 Ma ± 3 Ma (2s) for GZ7 and 426 Ma ± 9 Ma (2s) for GZ8 are typical of unheated zircon from Sri Lanka. The mean U concentrations are 680 ?g g?¹ (GZ7) and 1305 ?g g?¹ (GZ8). The two zircon samples are proposed as reference materials for SIMS (secondary ion mass spectrometry) U-Pb geochronology. In addition, GZ7 (Ti concentration 25.08 ?g g?¹ ± 0.18 ?g g?¹; 95% confidence uncertainty) may prove useful as reference material for Ti?in?zircon temperature estimates.
Abstract: The Siberian Craton was assembled in a Paleoproterozoic episode at about 1.88?Ga by the collision of older blocks, followed at about 1.86?Ga by post-collisional felsic magmatism. We have found a set of extremely fresh mica-bearing lamprophyre-looking rocks within the Sharyzhalgay metamorphic complex of the south-western Siberian Craton. Zircon from these rocks yields a UPb TIMS age of 1864.7?±?1.8?Ma, which coincides perfectly with the peak of the post-collisional granite ages and postdates by ~15?Ma the peak of ages obtained for metamorphism. The same ages were reported earlier for a mafic dyke with ocean island basalt (OIB) geochemical signatures and a Pt-bearing mafic-ultramafic intrusion found in the same region. Mineralogy, major and trace element geochemistry and Sr-Nd-Pb isotopes show that the studied rocks (1) have shoshonitic affinity, (2) are hybrid rocks with mineral assemblages which could not be in equilibrium, (3) where derived by recycling of an Archean crustal source and (4) resemble post-collision Tibetan shoshonitic series. The genesis of these rocks is considered to be due to melting of crustal lithologies and metasomatized lithospheric mantle within a subducted slab. Some of the resulting melts ascended through the lithospheric column and fractionated to low-Mg absarokites, whereas other melts were contaminated by orthopyroxenitic mantle material and attained unusual high-Mg mafic compositions. According to our model, the post-collisional magmatism (shoshonite- and OIB-type) occurred due to upwelling of hot asthenosphere through a slab window, when the active collision ceased as a result of the slab break off and loss of the slab pull force. Overall, our study shows that in the Paleoproterozoic shoshonitic melts were emplaced within a similar tectonic setting as seen today in modern orogenic systems.
Journal of South American Earth Sciences, Vol. 102, 102659, 12p. Pdf
India
geochronology
Abstract: The Dharwar Craton developed progressively over a billion years, through two main stages of crustal growth separated by a few-hundred million year long period of relative quiescence. The first stage between 3.4 and 3.0 Ga developed a proto-craton, which was considerably amplified during the second main stage between 2.7 and 2.4 Ga, through extensive magmatism, tectonism, and crustal consolidation. This paper reports U-Pb dating results obtained in four specific areas of the craton, with the data encompassing key moments in this long development. Rocks formed during the proto-craton stage include a 3089 Ma augen gneiss and a 2973 Ma evolved granite, the latter of which marks the final cratonization event of the proto-craton. The beginning of the second main stage is recorded in this study by 2650 Ma tonalite and trondhjemite, a 2623 Ma granite dyke cutting augen gneiss, and 2614, 2602 and 2588 Ma volcanic rocks. Titanite responded differently to the long evolution, as a function of location and type of overprint. It preserved an original 2973 Ma magmatic age in the west, but was reset and/or crystallized during secondary events in central domains of the craton, yielding ages between 2590 and 2360 Ma. A diorite stock intruded at 2207 Ma in the consolidated crust. It is correlated with the Anantapur-Kunigal mafic dyke swarm, one of a series of such events in the Dharwar Craton between 2.35 and 1.79 Ma. In terms of its overall evolution the Dharwar Craton has an affinity with the Slave clan, which includes the Wyoming and Zimbabwe cratons. It also matches many features in the evolution of the São Francisco Craton, a probable other member of Sclavia. This is in contrast to the Amazonian Craton, which has more affinity with the Superior clan.
Geochemical Perspectives Letters, Vol. 18, pp. 11-15. pdf
Russia, Siberia
carbonatite
Abstract: For the last two decades, the end of the voluminous phase of eruptions of the Siberian Traps large igneous province has been constrained by a U-Pb date of discordant baddeleyite collected from the Guli carbonatite intrusion with the assumption that the discordance resulted from unsupported 207Pb. In this study we have re-analysed baddeleyite from the same intrusion and found two types of discordance: (1) due to 207Pb-excess, and (2) radiogenic lead loss from high U mineral inclusions. The former implies that baddeleyite is an efficient scavenger of protactinium during crystallisation, leaving the magma depleted in this element. Together with a published high precision U-Pb date of 252.24?±?0.08 Ma for the Arydzhansky Formation, our new date of 250.33?±?0.38 Ma for the Guli carbonatite constrains the total duration of the voluminous eruptions of the Siberian Traps LIP at 1.91?±?0.38 million years. The lower intercept of the (231Pa)/(235U) corrected discordance line yields a date of 129.2?±?65.0 Ma, which points to the widespread Early Cretaceous rifting in East and Central Asia.
Geochemical Perspectives Letters, Vol. 18, pp. 11-15. pdf
Russia, Siberia
deposit - Guli
Abstract: For the last two decades, the end of the voluminous phase of eruptions of the Siberian Traps large igneous province has been constrained by a U-Pb date of discordant baddeleyite collected from the Guli carbonatite intrusion with the assumption that the discordance resulted from unsupported 207Pb. In this study we have re-analysed baddeleyite from the same intrusion and found two types of discordance: (1) due to 207Pb-excess, and (2) radiogenic lead loss from high U mineral inclusions. The former implies that baddeleyite is an efficient scavenger of protactinium during crystallisation, leaving the magma depleted in this element. Together with a published high precision U-Pb date of 252.24?±?0.08 Ma for the Arydzhansky Formation, our new date of 250.33?±?0.38 Ma for the Guli carbonatite constrains the total duration of the voluminous eruptions of the Siberian Traps LIP at 1.91?±?0.38 million years. The lower intercept of the (231Pa)/(235U) corrected discordance line yields a date of 129.2?±?65.0 Ma, which points to the widespread Early Cretaceous rifting in East and Central Asia.
Abstract: Experimentally determined major and trace element partition coefficients between majoritic garnet, clinopyroxene, and carbon dioxide-rich liquid are reported at 10 GPa and 1800 °C in a model carbonated peridotite composition in the system CaO-MgO-Al2O3-SiO2-CO2. Besides majoritic garnet, the liquid coexists with forsterite, orthopyroxene, and clinopyroxene, making melting phase relations invariant at fixed pressure and temperature conditions. Partition coefficients span a wide range of values - for instance, Sr, Nb, Ba, La, and Ce are highly incompatible in majoritic garnet, while Ca, Y, Nb, and Ho are moderately incompatible, and Lu, Si, Al, and Mg are compatible. Strong fractionation of light rare earth elements (e.g., La, Ce, Nd, Sm) and high field strength elements (e.g., Nb, Ta, Zr, Hf, Th) is seen between majoritic garnet and liquid. The experimentally determined partitioning values are used to calculate compositions of melts in equilibrium with majoritic garnet inclusions in diamonds from select localities in Brazil and Guinea. The calculated melts largely straddle those between documented carbonatites, kimberlites, and alkali basalts, low-degree mantle melting products from carbonated peridotite. This resemblance firmly suggests that majoritic garnet inclusions in diamonds from Brazil and Guinea can simply be interpreted as precipitates from such melts, thereby offering an alternative to the hypothesis that the element chemistry of such inclusions in diamonds can largely, and sometimes only, be ascribed to subducted oceanic crust, and further that, fusion of this crust may limit the terrestrial 'carbon recycling' at depths much beyond corresponding to those of Earth's transition zone.
Abstract: At temperatures less than ~1500 K, previously published CP data demonstrate that the heat capacities of orthoenstatite, proto-enstatite, diopside, and pseudowollastonite include primarily Debye type vibrational and anharmonic contributions, whereas the alkali chain, sheet, and ring silicates, Na2SiO3, Li2SiO3, K2SiO3, and Na2Si2O5 include a third contribution. The third contribution to CP arises from defect formation due to the mobility Na, K, Li, and O2-. The contribution becomes apparent at temperatures above 700-800 K for Na and K silicates, and above 900-1000 K for Li metasilicate. With strong thermal agitation, alkali-non-bridging oxygen (NBO) bonds are ruptured with the cations exiting their structural sites to occupy interstitial sites, thereby producing intrinsic Frenkel defects, which contribute to the CP of the alkali silicates. The magnitudes of the CP defect contributions correlate inversely with cation-oxygen bond strengths, as measured by bond dissociation energies. K-O and Na-O bond strengths are weak (239 and 257 kJ/mol) and defect contributions are large for these alkali chain, ring, and sheet silicates. The greater bond strength of Li-O (341 kJ/mol) correlates with a weaker defect contribution to the CP of Li2SiO3. Mg-O and Ca-O bonds are stronger still (394 and 464 kJ/mol) and no CP defect contributions are observed for the pyroxenes and pseudowollastonite up to ~1500 K. Above ~800 K a polymerization reaction occurs in Na2SiO3, which produces some Q3 species and free oxygen (O2- or oxide ion). The polymerization reaction annihilates an oxygen structural site so that the O2- produced must reside on non-structural sites thus producing intrinsic anionic defects. The same reactions likely occur in Na2Si2O5 and K2SiO3. Raman spectra of Na2SiO3 indicate >10% of Na+ and ~1.7% of O2- on interstitial sites at 1348 K. Ca- and Mg-bearing mantle minerals subjected to temperature greater than ~1500 K experience the destabilizing effects of disordering (Frenkel defect formation). The minerals may respond either by changing their composition or by changing phase. An abundance of Ca and Na defects in pyroxenes, for example, likely promotes production of new components (e.g., CaAl2SiO6, NaAlSi2O6) in pyroxenes. By their production, Ca and Na defect concentrations are reduced thereby stabilizing the phases. Mg-O bond dissociation and production of intrinsic Mg2+ and O2- point defects within olivine likely destabilize it and promote the phase transition to wadsleyite at the base of the upper mantle.
South African Institute of Mining and Metallurgy, Vol. 115, June pp. 523-531.
Global
Resource estimation, kriging not specific to diamonds
Abstract: Mineral resources are typically informed by multiple data sources of varying reliability throughout a mining project life cycle. Abundant data which are imprecise or biased or both (‘secondary data’) are often excluded from mineral resource estimations (the ‘base case’) under an intuitive, but usually untested, assumption that this data may reduce the estimation precision, bias the estimate, or both. This paper demonstrates that the assumption is often wasteful and realized only if the secondary data are naïvely integrated into the estimation. A number of specialized geostatistical tools are available to extract maximum value from secondary information which are imprecise or biased or both; this paper evaluates cokriging (CK), multicollocated cokriging (MCCK), and ordinary kriging with variance of measurement error (OKVME). Where abundant imprecise but unbiased secondary data are available, integration using OKVME is recommended. This re-appropriates kriging weights from less precise to more precise data locations, improving the estimation precision compared to the base case and to Ordinary Kriging (OK) of a pooled data-set. If abundant secondary data are biased and imprecise, integration through CK is recommended as the biased data are zero-sum weighted. CK consequently provides an unbiased estimate with some improvement in estimation precision compared to the base case.
Earth and Planetary Science Letters, Vol. 434, pp. 129-140.
Technology
Petrology - experimental
Abstract: Transitional melts, intermediate in composition between silicate and carbonate melts, form by low degree partial melting of mantle peridotite and might be the most abundant type of melt in the asthenosphere. Their role in the transport of volatile elements and in metasomatic processes at the planetary scale might be significant yet they have remained largely unstudied. Their molecular structure has remained elusive in part because these melts are difficult to quench to glass. Here we use FTIR, Raman, 13C and 29Si NMR spectroscopy together with First Principle Molecular Dynamic (FPMD) simulations to investigate the molecular structure of transitional melts and in particular to assess the effect of CO2 on their structure. We found that carbon in these glasses forms free ionic carbonate groups attracting cations away from their usual ‘depolymerising’ role in breaking up the covalent silicate network. Solution of CO2 in these melts strongly modifies their structure resulting in a significant polymerisation of the aluminosilicate network with a decrease in NBO/Si of about 0.2 for every 5 mol% CO2 dissolved. This polymerisation effect is expected to influence the physical and transport properties of transitional melts. An increase in viscosity is expected with increasing CO2 content, potentially leading to melt ponding at certain levels in the mantle such as at the lithosphere-asthenosphere boundary. Conversely an ascending and degassing transitional melt such as a kimberlite would become increasingly fluid during ascent hence potentially accelerate. Carbon-rich transitional melts are effectively composed of two sub-networks: a carbonate and a silicate one leading to peculiar physical and transport properties.
Abstract: A magnetotelluric survey comprising 18 broadband stations disposed along a 450 km-long profile was carried out at the transition between the Chaco-Paraná (CPB) and the Paraná (PB) intracratonic basins in northeastern Argentina. Three-dimensional inversions of the responses show that the CPB and southern PB lithospheres are resistive (~103 ? m) down to 120 km, but with distinct crustal and upper mantle electrical properties. Also, Bouguer gravity and density anomalies are positive at CPB, whereas they are negative at PB. We associate the CPB lithosphere with the Paleoproterozoic Rio Tebicuary craton and the southern PB lithosphere with an ancient and buried piece of craton, the Southern Paraná craton. Geochemical data of mantle xenoliths from the Cenozoic alkaline/carbonatitic province within the Rio Tebicuary craton suggest a subcontinental lithospheric mantle affected by metasomatic processes, which explains its lower resistivity (reaching values as low as 300 ? m) and higher density (#Mg = 0.87). In contrast, the Southern Paraná craton is more resistive (>103 ? m) and less dense, suggesting a de-hydrated, depleted, and thicker craton. These cratons are separated by a crustal conductor (15 to 20 km depth; 1-10 ? m) that we interpret as a southward continuation of a linear anomaly (Paraná Axial Anomaly) defined in former induction studies within the PB in Brazil. Hence, we redefined the trace of this conductive lineament: instead of bending towards the Torres Syncline, it continues inside the CPB. We propose the lineament to be an Early Neoproterozoic suture zone that controlled the location of maximum subsidence in the intracratonic basins during the Paleozoic. In the Early Cretaceous, the Paraná Axial Anomaly was the site of maximum extrusion and deposition of Serra Geral basalts. This anomaly separates compositionally distinct cratonic lithospheres along its path. Melting of this heterogeneous and enriched mantle created the Paraná igneous province.
Abstract: The Cana Brava complex is the northernmost of three layered complexes outcropping in the Goiás state (central Brasil). New field and geochemical evidences suggest that Cana Brava underwent hyper- to subsolidus deformation during its growth, acquiring a high-temperature foliation that is generally interpreted as the result of a granulite-facies metamorphic event. The increase along the stratigraphy of the incompatible elements abundances (LREE, Rb, Ba) and of the Sr isotopic composition, coupled with a decrease in ?Nd(790), indicate that the complex was contaminated by the embedded xenoliths from the Palmeirópolis Sequence. The geochemical data suggest that the contamination occurred along the entire magma column during the crystallization of the Upper Mafic Zone, with in situ variations determined by the abundance and composition of the xenoliths. These features of the Cana Brava complex point to an extremely similarity with the Lower Sequence of the most known Niquelândia intrusion (the central of the three complexes). This, together with the evidences that the two complexes have the same age (c.a. 790 Ma) and their thickness and units decrease northwards suggests that Cana Brava and Niquelândia are part of a single giant Brasilia body grown through several melt impulses.
Anais da Academia Brasileira de Ciencas, Vol. 78, 2, pp. 309-315.
Africa, Angola
deposit - Camafuca
Abstract: This work presents a geochemical study of a set of garnets, selected by their colors, from the Camafuca-Camazambo kimberlite, located on northeast Angola. Mantle-derived garnets were classified according to the scheme proposed by Grütter et al. (2004) and belong to the G1, G4, G9 and G10 groups. Both sub-calcic (G10) and Ca-saturated (G9) garnets, typical, respectively, of harzburgites and lherzolites, were identified. The solubility limit of knorringite molecule in G10D garnets suggests they have crystallized at a minimum pressure of about 40 to 45 kbar (4-4.5 GPa). The occurrence of diamond stability field garnets (G10D) is a clear indicator of the potential of this kimberlite for diamond. The chemistry of the garnets suggests that the source for the kimberlite was a lherzolite that has suffered a partial melting that formed basaltic magma, leaving a harzburgite as a residue.
Tectonic framework of a Paleoproterozoic arc continent to continent continent collisional zone, Trans Hudson Orogen, from geological and seismic reflection studies.
Canadian Journal of Earth Sciences, Vol. 42, 4, April pp. 421-434.
Abstract: Data- and knowledge-driven techniques are used to produce regional Au prospectivity maps of a portion of Melville Peninsula, Northern Canada using geophysical and geochemical data. These basic datasets typically exist for large portions of Canada's North and are suitable for a "greenfields" exploration programme. The data-driven method involves the use of the Random Forest (RF) supervised classifier, a relatively new technique that has recently been applied to mineral potential modelling while the knowledge-driven technique makes use of weighted-index overlay, commonly used in GIS spatial modelling studies. We use the location of known Au occurrences to train the RF classifier and calculate the signature of Au occurrences as a group from non-occurrences using the basic geoscience dataset. The RF classification outperformed the knowledge-based model with respect to prediction of the known Au occurrences. The geochemical data in general were more predictive of the known Au occurrences than the geophysical data. A data-driven approach such as RF for the production of regional Au prospectivity maps is recommended provided that a sufficient number of training areas (known Au occurrences) exist.
Abstract: There are lingering questions about how far back in geologic time plate tectonic processes began. In the Paleoproterozoic of eastern Laurentia, accretion of intra-oceanic juvenile terranes along the leading edge of the Superior craton apex (Ungava indenter) during the interval 1.87-1.83 Ga was followed by collision with the Churchill plate at ca. 1.83-1.79 Ga. Orthogonal shortening along the indenter led to early obduction of the juvenile terranes including the ca. 2.0 Ga Watts Group ophiolite, followed by out-of-sequence thrusting at ca. 1.83 Ga of granulite-facies crystalline basement of the Sugluk block (Churchill plate) along the Sugluk suture. Exhumation and erosion of the Sugluk block led to deposition of a foreland/delta fan sequence in the Hudson Bay re-entrant (Omarolluk and Loaf formations of the Belcher Group), with detritus sourced exclusively from the Sugluk block. Continued collision led to critical wedge development and orocline formation in the Hudson Bay re-entrant, forming a strongly arcuate fold-thrust belt. On the other (eastern) side of the indenter, material flow during crustal shortening was accommodated by lateral extrusion of microplates towards a then open ocean basin, in a manner similar to present-day extrusion of Indochina as a response to India - South China craton convergence. In the Churchill plate hinterland W-NW of the indenter, propagating strike-slip faults resulted in the far-field extrusion and oblique exhumation of Archean crustal slices of the Rae crustal block. The 1.83-1.79 Ga Superior-Churchill collision accommodated a minimum of 500 km of continent-continent convergence, with resulting style and mechanisms of orogenic growth and material flow similar to those observed in the Alpine-Himalayan orogenic system.
Coexiting potassium-rich alkaline and shoshonitic magmatism of arc affinities In the Proterozoic: a reassessment of syenitic stocks in the southwestern GrenvilleProvince
Contribution to Mineralogy and Petrology, Vol. 113, pp. 262-279
Journal of South American Earth Science, Vol. 76, pp. 290-305.
South America, Brazil
craton - Sao Francisco
Abstract: The Archean-Paleoproterozoic Jequié (JB) and Itabuna-Salvador-Curaçá (ISCB) blocks and their tectonic transition zone in the Valença region, Bahia, Brazil are potentially important for ore deposits, but the geological knowledge of the area is still meager. The paucity of geological information restricts the knowledge of the position and of the field characteristics of the tectonic suture zone between these two crustal segments JB and ISCB. Therefore, interpretation of geophysical data is necessary to supplement the regional structural and petrological knowledge of the area as well as to assist mining exploration programs. The analysis of the airborne radiometric and magnetic data of the region has established, respectively, five radiometric domains and five magnetic zones. Modeling of a gravity profile has defined the major density contrasts of the deep structures. The integrated interpretation of the geophysical data fitted to the known geological information substantially improved the suture zone (lower plate JB versus upper plate ISCB) delimitation, the geological map of the area and allowed to estimate the thicknesses of these two blocks, and raised key questions about the São Francisco Craton tectonic evolution.
Abstract: We investigate the along-axis variations in architecture, segmentation and evolution of the Main Ethiopian Rift (MER), East Africa, and relate these characteristics to the regional geology, lithospheric structure and surface processes. We first illustrate significant along-axis variations in basin architecture through analysis of simplified geological cross-sections in different rift sectors. We then integrate this information with a new analysis of Ethiopian topography and hydrography to illustrate how rift architecture (basin symmetry/asymmetry) is reflected in the margin topography and has been likely amplified by a positive feedback between tectonics (flexural uplift) and surface processes (fluvial erosion, unloading). This analysis shows that ~70% of the 500 km-long MER is asymmetric, with most of the asymmetric rift sectors being characterized by a master fault system on the eastern margin. We finally relate rift architecture and segmentation to the regional geology and geophysical constraints on the lithosphere. We provide strong evidence that rift architecture is controlled by the contrasting nature of the lithosphere beneath the homogeneous, strong Somalian Plateau and the weaker, more heterogeneous Ethiopian Plateau, differences originating from the presence of pre-rift zones of weakness on the Ethiopian Plateau and likely amplified by surface processes. The data provided by this integrated analysis suggest that asymmetric rifts may directly progress to focused axial tectonic-magmatic activity, without transitioning into a symmetric rifting stage. These observations have important implications for the asymmetry of continental rifts and conjugate passive margins worldwide.
Journal of the Geological Society , https://doi.org/10.1144/jgs2020-171
Europe, Turkey
carbonatite
Abstract: Unusual carbonate dykes, which have a thickness of up to 4 m, cross-cut the amphibolites from the high-grade metamorphic rocks in the Armutlu Peninsula (NW Turkey). They are described as carbonatites on the basis of their petrographic, geochemical and isotope-geochemical characteristics. The carbonatites, which commonly show equigranular texture, are composed of calcite and clinopyroxene with other minor phases of plagioclase, mica, garnet, K-feldspar, quartz, epidote, titanite and opaque minerals. They contain abundant xenoliths of pyroxenite and amphibolite. The geochemical characteristics of the carbonatites are significantly different from those of mantle-derived carbonatites. They have remarkably low incompatible element (e.g. Ba, Th, Nb) and total REE (11-91 ppm) contents compared with mantle-derived carbonatites. The high 87Sr/86Sr(i) (0.70797-0.70924) and low ?Nd(t) (?8.08 to ?9.57) of the carbonatites confirm that they were derived from the continental crust rather than from a mantle source. Mica from carbonatite was dated by the 40Ar/39Ar method, yielding a Late Jurassic-Early Cretaceous age (148-137 Ma). This is significantly younger than the age of adjacent amphibolites (Upper Triassic). All data from field studies, as well as petrographic, geochemical and geochronological observations, suggest that these carbonatites were formed from anatectic melting of a carbonated source area in the continental crust.
Journal of the Geological Society, Vol. 178, 10.1144/jgs2020-171
Asia, Turkey
deposit - Armutlu
Abstract: Unusual carbonate dykes, which have a thickness of up to 4 m, cross-cut the amphibolites from the high-grade metamorphic rocks in the Armutlu Peninsula (NW Turkey). They are described as carbonatites on the basis of their petrographic, geochemical and isotope-geochemical characteristics. The carbonatites, which commonly show equigranular texture, are composed of calcite and clinopyroxene with other minor phases of plagioclase, mica, garnet, K-feldspar, quartz, epidote, titanite and opaque minerals. They contain abundant xenoliths of pyroxenite and amphibolite. The geochemical characteristics of the carbonatites are significantly different from those of mantle-derived carbonatites. They have remarkably low incompatible element (e.g. Ba, Th, Nb) and total REE (11-91 ppm) contents compared with mantle-derived carbonatites. The high 87Sr/86Sr(i) (0.70797-0.70924) and low ?Nd(t) (?8.08 to ?9.57) of the carbonatites confirm that they were derived from the continental crust rather than from a mantle source. Mica from carbonatite was dated by the 40Ar/39Ar method, yielding a Late Jurassic-Early Cretaceous age (148-137 Ma). This is significantly younger than the age of adjacent amphibolites (Upper Triassic). All data from field studies, as well as petrographic, geochemical and geochronological observations, suggest that these carbonatites were formed from anatectic melting of a carbonated source area in the continental crust.
Brod, J.A., Junqueira-Brod, T.C., Gaspar, J.C., Petrinovic, I.A., De Castro Valente, S., Corval, A.
Decoupling of paired elements, crossover REE patterns and mirrored spider diagrams: fingerprinting liquid immiscibility in the Tapira alkaline carbonatite complex, SE Brazil.
Journal of South American Earth Sciences, Vol. 41, pp. 41-56.
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.
The Rio Capim volcanic plutonic sedimentary belt, Sao Francisco craton, Brazil: geological, geochemical and isotopic evidence for oceanic accretion during....
Abstract: Mineral chemistry analysis is a valuable tool in several phases of mineralogy and mineral prospecting studies. This type of analysis can point out relevant information, such as concentration of the chemical element of interest in the analyzed phase and, thus, the predisposition of an area for a given commodity. Due to this, considerable amount of data has been generated, especially with the use of electron probe micro-analyzers (EPMA), either in research for academic purposes or in a typical prospecting campaign in the mineral industry. We have identified an efficiency gap when manually processing and analyzing mineral chemistry data, and thus, we envisage this research niche could benefit from the versatility brought by machine learning algorithms. In this paper, we present Qmin, an application that assists in increasing the efficiency of mineral chemistry data processing and analysis stages through automated routines. Our code benefits from a hierarchical structure of classifiers and regressors trained by a Random Forest algorithm developed on a filtered training database extracted from the GEOROC (Geochemistry of Rocks of the Oceans and Continents) repository, maintained by the Max Planck Institute for Chemistry. To test the robustness of our application, we applied a blind test with more than 11,000 mineral chemistry analyses compiled for diamond prospecting within the scope of the Diamante Brasil Project of the Geological Survey of Brazil. The blind test yielded a balanced classifier accuracy of ca. 99% for the minerals known by Qmin. Therefore, we highlight the potential of machine learning techniques in assisting the processing and analysis of mineral chemistry data.
Jelsma, H.,Krishnan, S.U., Perritt, S.,Kumar, M., Preston, R., Winter, F., Lemotlo, L., Costa, J., Van der Linde, G., Facatino, M., Posser, A., Wallace, C., Henning, A., Joy, S., Chinn, I., Armstrong, R., Phillips, D.
Kimberlites from central Angola: a case stidy of exploration findings.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
Jelsma, H., Krishnan, U., Perritt, S., Preston, R., Winter, F., Lemotlo, L., van der Linde, G., Armstrong, R., Phillips, D., Joy, S., Costa, J., Facatino, M., Posser, A., Kumar, M., Wallace, C., Chinn, I., Henning, A.
Kimberlites from central Angola: a case study of exploration findings.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, pp. 173-190.
Abstract: Madagascar hosts several Paleoproterozoic sedimentary sequences that are key to unravelling the geodynamic evolution of past supercontinents on Earth. New detrital zircon U-Pb and Hf data, and a substantial new database of ?15,000 analyses are used here to compare and contrast sedimentary sequences in Madagascar, Africa and India. The Itremo Group in central Madagascar, the Sahantaha Group in northern Madagascar, the Maha Group in eastern Madagascar, and the Ambatolampy Group in central Madagascar have indistinguishable age and isotopic characteristics. These samples have maximum depositional ages > 1700 Ma, with major zircon age peaks at c. 2500 Ma, c. 2000 Ma and c. 1850 Ma. We name this the Greater Itremo Basin, which covered a vast area of Madagascar in the late Paleoproterozoic. These samples are also compared with those from the Tanzania and the Congo cratons of Africa, and the Dharwar Craton and Southern Granulite Terrane of India. We show that the Greater Itremo Basin and sedimentary sequences in the Tanzania Craton of Africa are correlatives. These also tentatively correlate with sedimentary protoliths in the Southern Granulite Terrane of India, which together formed a major intra?Nuna/Columbia sedimentary basin that we name the Itremo?Muva?Pandyan Basin. A new Paleoproterozoic plate tectonic configuration is proposed where central Madagascar is contiguous with the Tanzania Craton to the west and the Southern Granulite Terrane to the east. This model strongly supports an ancient Proterozoic origin for central Madagascar and a position adjacent to the Tanzania Craton of East Africa.
The influence of carbonatite during petrogenesis of nepheline syenites at the Pocos de Caldas Complex, Brazil: evidence from geochemistry and fluid inclusions
International Mineralogical Association meeting August Budapest, abstract p. 567.
Abstract: Reversed-zoned olivines (Fe-richer cores compared to rims), appear to be ubiquitous in kimberlites with a wide distribution. These olivines generally comprise a subordinate population relative to the dominant normally zoned olivines. However, they are notably more abundant in the megacryst-rich mid-Cretaceous Monastery and early Proterozoic Colossus kimberlites, located on the Kaapvaal and Zimbabwe cratons, respectively. The reverse-zoned olivines at these two localities define compositional fields that are closely similar to those for two olivine megacryst populations of the Cr-poor association which have been documented in the Monastery kimberlite. This points to a genetic link between megacrysts and the reversed zoned olivines. The ubiquitous, occurrence of the Fe-rich (relative to the field for rims) olivines in kimberlites with a wide geographic distribution in turn argues for an intimate link between megacrysts and the host kimberlite. Some large olivines have inclusions of rounded Cr-rich clinopyroxenes, garnets and/or spinel, characterized by fine-scale, erratic internal compositional zoning. Olivines with such chemically heterogeneous Cr-rich inclusions are not derived from disaggregated mantle peridotites, but are rather linked to the Cr-rich megacryst suite. Consequently, they cannot be used as evidence that cores of a majority of kimberlitic olivines are derived from disaggregated mantle peridotites.
Abstract: The 2060 Ma old Palabora Carbonatite Complex (PCC), South Africa, comprises diverse REE mineral assemblages formed during different stages and reflects an outstanding instance to understand the evolution of a carbonatite-related REE mineralization from orthomagmatic to late-magmatic stages and their secondary post-magmatic overprint. The 10 rare earth element minerals monazite, REE-F-carbonates (bastnäsite, parisite, synchysite), ancylite, britholite, cordylite, fergusonite, REE-Ti-betafite, and anzaite are texturally described and related to the evolutionary stages of the PCC. The identification of the latter five REE minerals during this study represents their first described occurrences in the PCC as well as in a carbonatite complex in South Africa. The variable REE mineral assemblages reflect a multi-stage origin: (1) fergusonite and REE-Ti-betafite occur as inclusions in primary magnetite. Bastnäsite is enclosed in primary calcite and dolomite. These three REE minerals are interpreted as orthomagmatic crystallization products. (2) The most common REE minerals are monazite replacing primary apatite, and britholite texturally related to the serpentinization of forsterite or the replacement of forsterite by chondrodite. Textural relationships suggest that these two REE-minerals precipitated from internally derived late-magmatic to hydrothermal fluids. Their presence seems to be locally controlled by favorable chemical conditions (e.g., presence of precursor minerals that contributed the necessary anions and/or cations for their formation). (3) Late-stage (post-magmatic) REE minerals include ancylite and cordylite replacing primary magmatic REE-Sr-carbonates, anzaite associated with the dissolution of ilmenite, and secondary REE-F-carbonates. The formation of these post-magmatic REE minerals depends on the local availability of a fluid, whose composition is at least partly controlled by the dissolution of primary minerals (e.g., REE-fluorocarbonates). This multi-stage REE mineralization reflects the interplay of magmatic differentiation, destabilization of early magmatic minerals during subsequent evolutionary stages of the carbonatitic system, and late-stage fluid-induced remobilization and re-/precipitation of precursor REE minerals. Based on our findings, the Palabora Carbonatite Complex experienced at least two successive stages of intense fluid–rock interaction.
Abstract: Earth’s status as the only life-sustaining planet is a result of the timing and delivery mechanism of carbon (C), nitrogen (N), sulfur (S), and hydrogen (H). On the basis of their isotopic signatures, terrestrial volatiles are thought to have derived from carbonaceous chondrites, while the isotopic compositions of nonvolatile major and trace elements suggest that enstatite chondrite-like materials are the primary building blocks of Earth. However, the C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which rules out volatile delivery by a chondritic late veneer. In addition, if delivered during the main phase of Earth’s accretion, then, owing to the greater siderophile (metal loving) nature of C relative to N, core formation should have left behind a subchondritic C/N ratio in the BSE. Here, we present high pressure-temperature experiments to constrain the fate of mixed C-N-S volatiles during core-mantle segregation in the planetary embryo magma oceans and show that C becomes much less siderophile in N-bearing and S-rich alloys, while the siderophile character of N remains largely unaffected in the presence of S. Using the new data and inverse Monte Carlo simulations, we show that the impact of a Mars-sized planet, having minimal contributions from carbonaceous chondrite-like material and coinciding with the Moon-forming event, can be the source of major volatiles in the BSE.
Contributions to Mineralogy and Petrology, Vol 174, https://doi.org/ 10.1007/s00410-019-1552-z
Africa, South Africa
deposit - Roberts Victor, Jagersfontein
Abstract: Mantle eclogites are commonly accepted as evidence for ancient altered subducted oceanic crust preserved in the subcontinental lithospheric mantle (SCLM), yet the mechanism and extent of crustal recycling in the Archaean remains poorly constrained. In this study, we focus on the petrological and geochemical characteristics of 58 eclogite xenoliths from the Roberts Victor and Jagersfontein kimberlites, South Africa. Non-metasomatized samples preserved in the cratonic root have variable textures and comprise bimineralic (garnet (gt)-omphacite (cpx)), as well as kyanite (ky)- and corundum (cor)-bearing eclogites. The bimineralic samples were derived from a high-Mg variety, corresponding to depths of ~ 100-180 km, and a low-Mg variety corresponding to depths of ~ 180-250 km. The high-Al (ky-, cor-bearing) eclogites originated from the lowermost part of the cratonic root, and have the lowest REE abundances, and the most pronounced positive Eu and Sr anomalies. On the basis of the strong positive correlation between gt and cpx ?18O values (r2 = 0.98), we argue that ?18O values are unaffected by mantle processes or exhumation. The cpx and gt are in oxygen isotope equilibrium over a wide range in ?18O values (e.g., 1.1-7.6‰ in garnet) with a bi-modal distribution (peaks at ~ 3.6 and ~ 6.4‰) with respect to mantle garnet values (5.1 ± 0.3‰). Reconstructed whole-rock major and trace element compositions (e.g., MgO variation with respect to Mg#, Al2O3, LREE/HREE) of bimineralic eclogites are consistent with their protolith being oceanic crust that crystallized from a picritic liquid, marked by variable degrees of partial melt extraction. Kyanite and corundum-bearing eclogites, however, have compositions consistent with a gabbroic and pyroxene-dominated protolith, respectively. The wide range in reconstructed whole-rock ?18O values is consistent with a broadly picritic to pyroxene-rich cumulative sequence of depleted oceanic crust, which underwent hydrothermal alteration at variable temperatures. The range in ?18O values extends significantly lower than that of present-day oceanic crust and Cretaceous ophiolites, and this might be due to a combination of lower ?18O values of seawater in the Archaean or a higher temperature of seawater-oceanic crust interaction.
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 (
Nature Communications, doi:.org/10.1038/ s41467-020-17442 -8 11p. Pdf
Africa, South Africa, Russia, Siberia
water
Abstract: Trace amounts of water dissolved in minerals affect density, viscosity and melting behaviour of the Earth’s mantle and play an important role in global tectonics, magmatism and volatile cycle. Water concentrations and the ratios of hydrogen isotopes in the mantle give insight into these processes, as well as into the origin of terrestrial water. Here we show the presence of molecular H2 in minerals (omphacites) from eclogites from the Kaapvaal and Siberian cratons. These omphacites contain both high amounts of H2 (70 to 460 wt. ppm) and OH. Furthermore, their ?D values increase with dehydration, suggesting a positive H isotope fractionation factor between minerals and H2-bearing fluid, contrary to what is expected in case of isotopic exchange between minerals and H2O-fluids. The possibility of incorporation of large quantities of H as H2 in nominally anhydrous minerals implies that the storage capacity of H in the mantle may have been underestimated, and sheds new light on H isotope variations in mantle magmas and minerals.
Abstract: Models for a xenocryst origin for kimberlite olivines emphasise the similarity between their core compositions and those in mantle peridotites. While this permits a xenocryst origin, it does not provide proof, as magmas generated in equilibrium with mantle olivines could, in principle, crystallize initial olivines matching those in the source region. Further, in several kimberlites, there is a striking disparity between the compositional range of olivine cores and that in associated mantle peridotite xenoliths from the same locality. Olivine-liquid Mg-Fe exchange coefficients and Ni partition coefficients permit equilibrium between Mg-rich mantle olivines (Mg#?~?94-93) and magmas matching kimberlite bulk rock compositions. Glass inclusions in olivine megacrysts from the Monastery kimberlite, with compositions which overlap the range of archetypal Group I kimberlites, were interpreted to represent original liquids trapped at pressures of 4.5-6?GPa. These glass inclusions provide direct petrographic support for primitive melts matching kimberlite bulk chemistry in the lower SCLM. A majority of kimberlitic olivines show normal (decreasing Mg#) core to rim zonation. Cores of normal-zoned kimberlitic olivines are typically homogeneous, but collectively define a field with a range in Mg # and invariant or slightly decreasing Ni towards more Fe-rich compositions. The most Mg-rich cores of normal-zoned olivines typically have Mg# in the range 94-93, but there are marked differences in the Fe-rich extreme of the normal-zoned population between different kimberlite clusters. Olivine rims typically define a field characterized by steeply decreasing Ni, coupled with invariant or slightly increasing or decreasing Mg#, which invariably overlaps the Fe-extreme of core compositions of the relatively Mg-rich, normal-zoned olivines. Consequently, while there is a sharp inflection in chemical gradient between the respective fields of cores and rims, they nevertheless define a continuous compositional field. Trace element modelling demonstrates that these zonation patterns can be explained in terms of a Raleigh crystallization model. Most, if not all kimberlites are characterized by a subordinate group of olivine macrocrysts with cores that are Fe-rich relative to the field for rims, and thus show reverse zonation, which are interpreted to be linked to the Cr-poor megacryst suite. Rare Mg-rich olivines (relative to rims), have high-pressure inclusions of garnet, clinopyroxene and orthopyroxene. When present, such inclusions often show disequilibrium features such as internal chemical zonation. This points to a very short mantle residence time prior to entrainment by the host kimberlite, indicating a link to the Cr-rich megacryst suite rather than mantle peridotites. In addition to a variable, but generally subordinate proportion of olivines derived from Cr-poor and Cr-rich megacrysts, xenocrysts derived from disaggregated mantle peridotites will undoubtedly be present. While their proportions are difficult to quantify, the collective evidence points to a cognate origin for a majority of kimberlitic olivines. A kimberlite magma ascent model is proposed which provides a framework for understanding both olivine compositional variation and apparently enigmatic internal and external olivine morphology.
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.
Earth and Planetary Science Letters, Vol. 432, pp. 159-168.
Europe, Iceland
Mantle plume
Abstract: The presence of a mantle plume beneath Iceland has long been hypothesised to explain its high volumes of crustal volcanism. Practical constraints in seismic tomography mean that thin, slow velocity anomalies representative of a mantle plume signature are difficult to image. However it is possible to infer the presence of temperature anomalies at depth from the effect they have on phase transitions in surrounding mantle material. Phase changes in the olivine component of mantle rocks are thought to be responsible for global mantle seismic discontinuities at 410 and 660 km depth, though exact depths are dependent on surrounding temperature conditions. This study uses P to S seismic wave conversions at mantle discontinuities to investigate variation in topography allowing inference of temperature anomalies within the transition zone. We employ a large data set from a wide range of seismic stations across the North Atlantic region and a dense network in Iceland, including over 100 stations run by the University of Cambridge. Data are used to create over 6000 receiver functions. These are converted from time to depth including 3D corrections for variations in crustal thickness and upper mantle velocity heterogeneities, and then stacked based on common conversion points. We find that both the 410 and 660 km discontinuities are depressed under Iceland compared to normal depths in the surrounding region. The depression of 30 km observed on the 410 km discontinuity could be artificially deepened by un-modelled slow anomalies in the correcting velocity model. Adding a slow velocity conduit of ?1.44% reduces the depression to 18 km; in this scenario both the velocity reduction and discontinuity topography reflect a temperature anomaly of 210 K. We find that much larger velocity reductions would be required to remove all depression on the 660 km discontinuity, and therefore correlated discontinuity depressions appear to be a robust feature of the data. While it is not possible to definitively rule out the possibility of uncorrected velocity anomalies causing the observed correlated topography we show that this is unlikely. Instead our preferred interpretation is that the 660 km discontinuity is controlled by a garnet phase transition described by a positive Clapeyron slope, such that depression of the 660 is representative of a hot anomaly at depth.
Earth and Planetary Science Letters, Vol. 433, pp. 159-168.
Europe, Iceland
Mantle - 660 km
Abstract: The presence of a mantle plume beneath Iceland has long been hypothesised to explain its high volumes of crustal volcanism. Practical constraints in seismic tomography mean that thin, slow velocity anomalies representative of a mantle plume signature are difficult to image. However it is possible to infer the presence of temperature anomalies at depth from the effect they have on phase transitions in surrounding mantle material. Phase changes in the olivine component of mantle rocks are thought to be responsible for global mantle seismic discontinuities at 410 and 660 km depth, though exact depths are dependent on surrounding temperature conditions. This study uses P to S seismic wave conversions at mantle discontinuities to investigate variation in topography allowing inference of temperature anomalies within the transition zone. We employ a large data set from a wide range of seismic stations across the North Atlantic region and a dense network in Iceland, including over 100 stations run by the University of Cambridge. Data are used to create over 6000 receiver functions. These are converted from time to depth including 3D corrections for variations in crustal thickness and upper mantle velocity heterogeneities, and then stacked based on common conversion points. We find that both the 410 and 660 km discontinuities are depressed under Iceland compared to normal depths in the surrounding region. The depression of 30 km observed on the 410 km discontinuity could be artificially deepened by un-modelled slow anomalies in the correcting velocity model. Adding a slow velocity conduit of ?1.44% reduces the depression to 18 km; in this scenario both the velocity reduction and discontinuity topography reflect a temperature anomaly of 210 K. We find that much larger velocity reductions would be required to remove all depression on the 660 km discontinuity, and therefore correlated discontinuity depressions appear to be a robust feature of the data. While it is not possible to definitively rule out the possibility of uncorrected velocity anomalies causing the observed correlated topography we show that this is unlikely. Instead our preferred interpretation is that the 660 km discontinuity is controlled by a garnet phase transition described by a positive Clapeyron slope, such that depression of the 660 is representative of a hot anomaly at depth.
Abstract: The Zambezi rises with considerable modesty in north-west Zambia from a small spring on the gentle upland of the Southern Equatorial Divide - the watershed that separates the river from north-west-flowing tributaries of the Congo. The evolution of the Zambezi River has repeatedly modified the distribution of riverine plant and animal species. The hydrology of the Zambezi is further influenced by water exploitation by different users, along its main channel and tributaries. The dams have had severe ecological impacts on the major floodplains, as a result of the reduction of the supply of water and sediment. The major Early Cretaceous Zambezi-Limpopo River system entered the Mozambique coastal plain via a line of crustal weakness that was exploited by a major west-north-west trending dyke swarm. Drainage evolution of the Palaeo-Chambeshi system has been invoked as the primary cause of the recent evolution of the molerats.
Nature Communications, doi:.org/10.1038/ s41467-020-17442 -8 11p. Pdf
Africa, South Africa, Russia, Siberia
water
Abstract: Trace amounts of water dissolved in minerals affect density, viscosity and melting behaviour of the Earth’s mantle and play an important role in global tectonics, magmatism and volatile cycle. Water concentrations and the ratios of hydrogen isotopes in the mantle give insight into these processes, as well as into the origin of terrestrial water. Here we show the presence of molecular H2 in minerals (omphacites) from eclogites from the Kaapvaal and Siberian cratons. These omphacites contain both high amounts of H2 (70 to 460 wt. ppm) and OH. Furthermore, their ?D values increase with dehydration, suggesting a positive H isotope fractionation factor between minerals and H2-bearing fluid, contrary to what is expected in case of isotopic exchange between minerals and H2O-fluids. The possibility of incorporation of large quantities of H as H2 in nominally anhydrous minerals implies that the storage capacity of H in the mantle may have been underestimated, and sheds new light on H isotope variations in mantle magmas and minerals.
Bulletin de la Societe Geologique de France *eng, Vol. 188, 1-2, 14p.
Russia, Siberia
deposit - Udachnaya
Abstract: Xenoliths brought up by kimberlite magmas are rare samples of otherwise inaccessible lithospheric mantle. Eclogite xenoliths are found in most cratons and commonly show a range of mineral and chemical compositions that can be used to better understand craton formation. This study focuses on five new kyanite-bearing eclogites from the Udachnaya kimberlite pipe (367±5 Ma). They are fine-to coarse-grained and consist mainly of “cloudy” clinopyroxene (cpx) and garnet (grt). The clinopyroxene is Al,Na-rich omphacite while the garnet is Ca-rich, by contrast to typical bi-mineral (cpx+grt) eclogites that contain Fe- and Mg-rich garnets. The Udachnaya kyanite eclogites are similar in modal and major element composition to those from other cratons (Dharwar, Kaapvaal, Slave, West African). The kyanite eclogites have lower REE concentrations than bi-mineral eclogites and typically contain omphacites with positive Eu and Sr anomalies, i.e. a “ghost plagioclase signature”. Because such a signature can only be preserved in non-metasomatised samples, we infer that they were present in the protoliths of the eclogites. It follows that subducted oceanic crust is present at the base of the Siberian craton. Similar compositions and textures are also seen in kyanite eclogites from other cratons, which we view as evidence for an Archean, subduction-like formation mechanism related to craton accretion. Thus, contrary to previous work that classifies all kyanite eclogites as type I (IK), metasomatized by carbonatite/kimberlitic fluids, we argue that some of them, both from this work and those from other cratons, belong to the non-metasomatized type II (IIB). The pristine type IIB is the nearest in composition to protoliths of mantle eclogites because it contains no metasomatic enrichments.
Contributions to Mineralogy and Petrology, Vol 174, https://doi.org/ 10.1007/s00410-019-1552-z
Africa, South Africa
deposit - Roberts Victor, Jagersfontein
Abstract: Mantle eclogites are commonly accepted as evidence for ancient altered subducted oceanic crust preserved in the subcontinental lithospheric mantle (SCLM), yet the mechanism and extent of crustal recycling in the Archaean remains poorly constrained. In this study, we focus on the petrological and geochemical characteristics of 58 eclogite xenoliths from the Roberts Victor and Jagersfontein kimberlites, South Africa. Non-metasomatized samples preserved in the cratonic root have variable textures and comprise bimineralic (garnet (gt)-omphacite (cpx)), as well as kyanite (ky)- and corundum (cor)-bearing eclogites. The bimineralic samples were derived from a high-Mg variety, corresponding to depths of ~ 100-180 km, and a low-Mg variety corresponding to depths of ~ 180-250 km. The high-Al (ky-, cor-bearing) eclogites originated from the lowermost part of the cratonic root, and have the lowest REE abundances, and the most pronounced positive Eu and Sr anomalies. On the basis of the strong positive correlation between gt and cpx ?18O values (r2 = 0.98), we argue that ?18O values are unaffected by mantle processes or exhumation. The cpx and gt are in oxygen isotope equilibrium over a wide range in ?18O values (e.g., 1.1-7.6‰ in garnet) with a bi-modal distribution (peaks at ~ 3.6 and ~ 6.4‰) with respect to mantle garnet values (5.1 ± 0.3‰). Reconstructed whole-rock major and trace element compositions (e.g., MgO variation with respect to Mg#, Al2O3, LREE/HREE) of bimineralic eclogites are consistent with their protolith being oceanic crust that crystallized from a picritic liquid, marked by variable degrees of partial melt extraction. Kyanite and corundum-bearing eclogites, however, have compositions consistent with a gabbroic and pyroxene-dominated protolith, respectively. The wide range in reconstructed whole-rock ?18O values is consistent with a broadly picritic to pyroxene-rich cumulative sequence of depleted oceanic crust, which underwent hydrothermal alteration at variable temperatures. The range in ?18O values extends significantly lower than that of present-day oceanic crust and Cretaceous ophiolites, and this might be due to a combination of lower ?18O values of seawater in the Archaean or a higher temperature of seawater-oceanic crust interaction.
Abstract: Rare earth element (REE) ore-bearing carbonatite dikes and a stock at Mountain Pass, California, are spatially associated with a suite of ultrapotassic plutonic rocks, and it has been proposed that the two are genetically related. This hypothesis is problematic, given that existing geochronological constraints indicate that the carbonatite is ?15-25 Myr younger than the ultrapotassic rocks, requiring alternative models for the formation of the REE ore-bearing carbonatite during a separate event and/or via a different mechanism. New laser ablation split-stream inductively coupled plasma mass spectrometry (LASS-ICP-MS) petrochronological data from ultrapotassic intrusive rocks from Mountain Pass yield titanite and zircon U-Pb dates from 1429?±?10 to 1385?±?18?Ma, expanding the age range of the ultrapotassic rocks in the complex by ?20 Myr. The ages of the youngest ultrapotassic rocks overlap monazite Th-Pb ages from a carbonatite dike and the main carbonatite ore body (1396?±?16 and 1371?±?10?Ma, respectively). The Hf isotope compositions of zircon in the ultrapotassic rocks are uniform, both within and between samples, with a weighted mean ?Hfi of 1•9?±?0•2 (MSWD?=?0•9), indicating derivation from a common, isotopically homogeneous source. In contrast, in situ Nd isotopic data for titanite in the ultrapotassic rocks are variable (?Ndi?=?-3•5 to -12), suggesting variable contamination by an isotopically enriched source. The most primitive ?Ndi isotopic signatures, however, do overlap ?Ndi from monazite (?Ndi?=?-2•8?±?0•2) and bastnäsite (?Ndi?=?-3•2?±?0•3) in the ore-bearing carbonatite, suggesting derivation from a common source. The data presented here indicate that ultrapotassic magmatism occurred in up to three phases at Mountain Pass (?1425, ?1405, and ?1380?Ma). The latter two stages were coeval with carbonatite magmatism, revealing previously unrecognized synchronicity in ultrapotassic and carbonatite magmatism at Mountain Pass. Despite this temporal overlap, major and trace element geochemical data are inconsistent with derivation of the carbonatite and ultrapotassic rocks by liquid immiscibility or fractional crystallization from common parental magma. Instead, we propose that the carbonatite was generated as a primary melt from the same source as the ultrapotassic rocks, and that although it is unique, the Mountain Pass ultrapotassic and carbonatite suite is broadly similar to other alkaline silicate-carbonatite occurrences in which the two rock types were generated as separate mantle melts.
Abstract: Combined zircon geochronology and Hf isotopes of plutonic rocks from eastern Marie Byrd Land and Thurston Island, Antarctica, provide a detailed record of Phanerzoic arc magmatism along the paleo-Pacific margin of Gondwana. Magmatism along the Antarctic margin initiated in a dominantly contractional arc setting with an isotopically enriched lithospheric mantle source during the Ross Orogeny (c. 540-485?Ma). After termination of the Ross Orogeny through the Cretaceous, detrital zircon and zircon from igneous rocks record relative increases in zircon ?Hfi inferred to represent episodes of lithospheric-scale extension and relative decreases during inferred contractional episodes along the Antarctic margin. Comparison of this secular isotopic evolutionary trend with similar data from along the paleo-Pacific margin of Gondwana demonstrates a shared history among Marie Byrd Land, Australia, and Zealandia that contrasts with the shared record of Thurston Island, Antarctic Peninsula, and South America. These two contrasting histories highlight an early Permian along arc geochemical and inferred geodynamic switch from an isotopically enriched contractional arc system in South America, Antarctic Peninsula, and Thurston Island to an isotopically depleted extensional arc system in Marie Byrd Land, Zealandia, and Australia. Despite differences in timing, all segments of the paleo-Pacific margin underwent a similar secular isotopic evolution with dramatic shifts from enriched to juvenile isotopic compositions during extensional collapse.
Contributions to Mineralogy and Petrology, Vol. 175, 97, 30p. Pdf
Africa, Tanzania
craton
Abstract: U-Pb petrochronology of deep crustal xenoliths and outcrops across northeastern Tanzania track the thermal evolution of the Mozambique Belt and Tanzanian Craton following the Neoproterozoic East African Orogeny (EAO) and subsequent Neogene rifting. At the craton margin, the upper-middle crust record thermal quiescence since the Archean (2.8-2.5 Ga zircon, rutile, and apatite in granite and amphibolite xenoliths). The lower crust of the craton documents thermal pulses associated with Neoarchean ultra-high temperature metamorphism (ca. 2.64 Ga,?>?900 °C zircon), the EAO (600-500 Ma rutile), and fluid influx during rifting (?650 °C (above Pb closure of rutile and apatite) at the time of eruption. Zoned titanite records growth during cooling of the lower crust at 550 Ma, followed by fluid influx during slow cooling and exhumation (0.1-1 °C/Myr after 450 Ma). Permissible lower-crustal temperatures for the craton and orogen suggest variable mantle heat flow through the crust and reflect differences in mantle lithosphere thickness rather than advective heating from rifting.
Abstract: The decay of short-lived iodine (I) and plutonium (Pu) results in xenon (Xe) isotopic anomalies in the mantle that record Earth’s earliest stages of formation1,2,3,4,5,6,7,8. Xe isotopic anomalies have been linked to degassing during accretion2,3,4, but degassing alone cannot account for the co-occurrence of Xe and tungsten (W) isotopic heterogeneity in plume-derived basalts9,10 and their long-term preservation in the mantle. Here we describe measurements of I partitioning between liquid Fe alloys and liquid silicates at high pressure and temperature and propose that Xe isotopic anomalies found in modern plume rocks (that is, rocks with elevated 3He/4He ratios) result from I/Pu fractionations during early, high-pressure episodes of core formation. Our measurements demonstrate that I becomes progressively more siderophile as pressure increases, so that portions of mantle that experienced high-pressure core formation will have large I/Pu depletions not related to volatility. These portions of mantle could be the source of Xe and W anomalies observed in modern plume-derived basalts2,3,4,9,10. Portions of mantle involved in early high-pressure core formation would also be rich in FeO11,12, and hence denser than ambient mantle. This would aid the long-term preservation of these mantle portions, and potentially points to their modern manifestation within seismically slow, deep mantle reservoirs13 with high 3He/4He ratios.
Earth and Planetary Science Letters, Vol. 494, pp. 172-189.
Mantle
peridotites
Abstract: The oxygen fugacity (fO2) of the oceanic upper mantle has fundamental implications for the production of magmas and evolution of the Earth's interior and exterior. Mid-ocean ridge basalts and peridotites sample the oceanic upper mantle, and retain a record of oxygen fugacity. While fO2 has been calculated for mid-ocean ridge basalts worldwide (>200 locations), ridge peridotites have been comparatively less well studied (33 samples from 11 locations), and never in the same geographic location as basalts. In order to determine whether peridotites and basalts from mid-ocean ridges record congruent information about the fO2 of the Earth's interior, we analyzed 31 basalts and 41 peridotites from the Oblique Segment of the Southwest Indian Ridge. By measuring basalts and peridotites from the same ridge segment, we can compare samples with maximally similar petrogenetic histories. We project the composition and oxygen fugacity of each lithology back to source conditions, and evaluate the effects of factors such as subsolidus diffusion in peridotites and fractional crystallization in basalts. We find that, on average, basalts and peridotites from the Oblique Segment both reflect a source mantle very near the quartz-fayalite-magnetite (QFM) buffer. However, peridotites record a significantly wider range of values (nearly 3 orders of magnitude in fO2), with a single dredge recording a range in fO2 greater than that previously reported for mid-ocean ridge peridotites worldwide. This suggests that mantle fO2 may be heterogeneous on relatively short length scales, and that this heterogeneity may be obscured within aggregated basalt melts. We further suggest that the global peridotite fO2 dataset may not provide a representative sample of average basalt-source mantle. Our study motivates further investigation of the fO2 recorded by ridge peridotites, as peridotites record information about the fO2 of the Earth's interior that cannot be gleaned from analysis of basalts alone.
Abstract: Peridotites dredged from mid-ocean ridges and glassy mid-ocean ridge basalts (MORB) transmit information about the oxygen fugacity (fO2) of Earth's convecting upper mantle to the surface. Equilibrium assemblages of olivine+orthopyroxene+spinel in abyssal peridotites and Fe3+/?Fe ratios in MORB glasses measured by X-ray absorption near-edge structure (XANES) provide independent estimates of MORB source region fO2, with the former recording fO2 approximately 0.8 log units lower than the latter relative to the quartz-fayalite-magnetite (QFM) buffer. To test cross-compatibility of these oxybarometers and examine the compositional effects of changing fO2 on a peridotite plus melt system over a range of Earth-relevant fO2, we performed a series of experiments at 0.1 MPa and fO2 controlled by CO-CO2 gas mixes between QFM-1.87 and QFM+2.23 in a system containing basaltic andesite melt saturated in olivine, orthopyroxene, and spinel. Oxygen fugacities recorded by each method are in agreement with each other and with the fO2 measured in the furnace. Measurements of fO2 from the two oxybarometers agree to within 1? in all experiments. These results demonstrate that the two methods are directly comparable and differences between fO2 measured in abyssal peridotites and MORB result from geographic sampling bias, petrological processes that change fO2 in these samples after separation of melts and residues, or abyssal peridotites may not be residues of MORB melting. As fO2 increases, spinel Fe3+ concentrations increase only at the expense of Cr from QFM-1.87 to QFM-0.11. Above QFM, Al is also diluted in spinel as the cation proportion of Fe3+ increases. None of the three spinel models tested, MELTS (Ghiorso and Sack 1995), SPINMELT (Ariskin and Nikolaev 1996), and MELT_CHROMITE (Poustovetov and Roeder 2001), describe these compositional effects, and we demonstrate that MELTS predicts residues that are too oxidized by >1 log unit to have equilibrated with the coexisting liquid phase. Spinels generated in this study can be used to improve future thermodynamic models needed to predict compositional changes in spinels caused by partial melting of peridotites in the mantle or by metamorphic reactions as peridotites cool in the lithosphere. In our experimental series, where the ratio of Fe2O3/FeO in the melt varies while other melt compositional parameters remain nearly constant, experimental melt fraction remains constant, and Fe3+ becomes increasingly compatible in spinel as fO2 increases. Instead of promoting melting, increasing the bulk Fe3+/?Fe ratio in peridotite drives reactions analogous to the fayalite-ferrosilite-magnetite reaction. This may partly explain the absence of correlation between Na2O and Fe2O3 in fractionation-corrected MORB.
Experimental investigation and peridotite oxybarometers: implications for spinel thermodynamic models and Fe3+ compatibility during generation of upper mantle melts.
IN: Deep carbon: past to present, Orcutt, Daniel, Dasgupta eds., pp. 237-275.
Mantle
carbon
Abstract: This chapter provides a summary of the flux of carbon through various oceanic volcanic centers such as mid-ocean ridges and intraplate settings, as well as what these fluxes indicate about the carbon content of the mantle. By reviewing methods used to measure the carbon geochemistry of basalts and then to estimate fluxes, the chapter provides insight into how mantle melting and melt extraction processes are estimated. The chapter discusses how the flux of carbon compares with other incompatible trace elements and gases. From there, the chapter discusses whether the budget of carbon in the ocean mantle can be explained by primordial carbon or whether carbon recycling is required to balance the budget.
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.
Earth and Planetary Letters, Vol. 553, 116515, 12p. Pdf
Mantle
nitrogen
Abstract: Nitrogen is a major and essential component of Earth's atmosphere, yet relative to other volatile elements, there are relatively few experimental constraints on the pathways by which nitrogen cycles between Earth's interior and exterior. We report mineral-melt and mineral-fluid partitioning experiments to constrain the behavior of nitrogen during slab dehydration and sediment melting processes. Experiments reacted rhyolitic melts with silicate and oxide minerals, in the presence of excess aqueous fluid, over temperatures between 725-925 °C and pressures between 0.2 and 2.3 GPa. Oxygen fugacity ranged between iron metal saturation (?NNO-5) to that in excess of primitive arc basalts (?NNO+2). Our experiments demonstrate that hydrous fluid is the preferred phase for nitrogen over minerals (biotite, K-feldspar, and amphibole) and rhyolitic melts across all conditions explored. Relatively large effects of pressure (?log()/?(GPa/K) = 761 ± 68 (1?), ?log()/?(GPa/K) = 462 ± 169) and moderate effects of oxygen fugacity (NNO = -0.20 ± 0.04, ?logNNO = -0.10 ± 0.04) modulate partitioning of nitrogen. We further document negligible partitioning effects related to mineral composition or Cl content of hydrous fluid. Of the minerals investigated, biotite has the largest affinity for N and should control the retention of N in slabs where present. Application of partitioning data to slab dehydration PT paths highlights the potential for highly incompatible behavior ( < 0.1) from the slab along warmer and oxidized (NNO+1) subduction geotherms, whereas dehydration along reduced and cooler geotherms will extract moderate amounts of nitrogen ( > 0.1). We find that slab melting is less effective at extracting N from slabs than fluid loss, at least under oxidized conditions (NNO+1). Ultimately, the conditions under which slabs lose fluid strongly affect the distribution of nitrogen between Earth's interior and exterior.
Abstract: Knowing when the geodynamo started is important for understanding the evolution of the core, the atmosphere, and life on Earth. We report full-vector paleointensity measurements of Archean to Hadean zircons bearing magnetic inclusions from the Jack Hills conglomerate (Western Australia) to reconstruct the early geodynamo history. Data from zircons between 3.3 billion and 4.2 billion years old record magnetic fields varying between 1.0 and 0.12 times recent equatorial field strengths. A Hadean geomagnetic field requires a core-mantle heat flow exceeding the adiabatic value and is suggestive of plate tectonics and/or advective magmatic heat transport. The existence of a terrestrial magnetic field before the Late Heavy Bombardment is supported by terrestrial nitrogen isotopic evidence and implies that early atmospheric evolution on both Earth and Mars was regulated by dynamo behavior.
Abstract: We performed melting experiments on Fe-O alloys up to 204 GPa and 3500 K in a diamond-anvil cell (DAC) and determined the liquidus phase relations in the Fe-FeO system based on textural and chemical characterizations of recovered samples. Liquid-liquid immiscibility was observed up to 29 GPa. Oxygen concentration in eutectic liquid increased from >8 wt% O at 44 GPa to 13 wt% at 204 GPa and is extrapolated to be about 15 wt% at the inner core boundary (ICB) conditions. These results support O-rich liquid core, although oxygen cannot be a single core light element. We estimated the range of possible liquid core compositions in Fe-O-Si-C-S and found that the upper bounds for silicon and carbon concentrations are constrained by the crystallization of dense inner core at the ICB.
Abstract: The composition of calcite and dolomite from several carbonatite complexes (including a large set of petrographically diverse samples from the Aley complex in Canada) was studied by electron-microprobe analysis and laser-ablation inductively-coupled-plasma mass-spectrometry to identify the extent of substitution of rare-earth and other trace elements in these minerals and the effects of different igneous and postmagmatic processes on their composition. Analysis of the newly acquired and published data shows that the contents of rare-earth elements (REE) and certain REE ratios in magmatic calcite and dolomite are controlled by crystal fractionation of fluorapatite, monazite and, possibly, other minerals. Enrichment in REE observed in some samples (up to ~2000 ppm in calcite) cannot be accounted for by coupled substitutions involving Na, P or As. At Aley, the REE abundances and chondrite-normalized (La/Yb)cn ratios in carbonates decrease with progressive fractionation. Sequestration of heavy REE from carbonatitic magma by calcic garnet may be responsible for a steeply sloping "exponential" pattern and lowered Ce/Ce* ratios of calcite from Magnet Cove (USA) and other localities. Alternatively, the low levels of Ce and Mn in these samples could result from preferential removal of these elements by Ce4+- and Mn3+-bearing minerals (such as cerianite and spinels) at increasing f(O2) in the magma. The distribution of large-ion lithophile elements (LILE = Sr, Ba and Pb) in rock-forming carbonates also shows trends indicative of crystal fractionation effects (e.g., concomitant depletion in Ba + Pb at Aley, or Sr + Ba at Kerimasi), although the phases responsible for these variations cannot be identified unambiguously at present. Overall, element ratios sensitive to the redox state of the magma and its complexing characteristics (Eu/Eu*, Ce/Ce* and Y/Ho) are least variable and in both primary calcite and dolomite, approach the average chondritic values. In consanguineous rocks, calcite invariably has higher REE and LILE levels than dolomite. Hydrothermal reworking of carbonatites does not produce a unique geochemical fingerprint, leading instead to a variety of evolutionary trends that range from light-REE and LILE enrichment (Turiy Mys, Russia) to heavy-REE enrichment and LILE depletion (Bear Lodge, USA). These differences clearly attest to variations in the chemistry of carbonatitic fluids and, consequently, their ability to mobilize specific trace elements from earlier-crystallized minerals. An important telltale indicator of hydrothermal reworking is deviation from the primary, chondrite-like REE ratios (in particular, Y/Ho and Eu/Eu*), accompanied by a variety of other compositional changes depending on the redox state of the fluid (e.g., depletion of carbonates in Mn owing to its oxidation and sequestration by secondary oxides). The effect of supergene processes was studied on a single sample from Bear Lodge, which shows extreme depletion in Mn and Ce (both due to oxidation), coupled with enrichment in Pb and U, possibly reflecting an increased availability of Pb2+ and (UO2)2+ species in the system. On the basis of these findings, several avenues for future research can be outlined: (1) structural mechanisms of REE uptake by carbonates; (2) partitioning of REE and LILE between cogenetic calcite and dolomite; (3) the effects of fluorapatite, phlogopite and pyrochlore fractionation on the LILE budget of magmatic carbonates; (4) the cause(s) of coupled Mn-Ce depletion in some primary calcite; and (5) relations between fluid chemistry and compositional changes in hydrothermal carbonates.
Abstract: The Late Cretaceous (ca. 100 Ma) diamondiferous Fort à la Corne (FALC) kimberlite field in the Saskatchewan (Sask) craton, Canada, is one of the largest known kimberlite fields on Earth comprising essentially pyroclastic kimberlites. Despite its discovery more than two decades ago, petrological, geochemical and petrogenetic aspects of the kimberlites in this field are largely unknown. We present here the first detailed petrological and geochemical data combined with reconnaissance Nd isotope data on drill-hole samples of five major kimberlite bodies. Petrography of the studied samples reveals that they are loosely packed, clast-supported and variably sorted, and characterised by the presence of juvenile lapilli, crystals of olivine, xenocrystal garnet (peridotitic as well as eclogitic paragenesis) and Mg-ilmenite. Interclast material is made of serpentine, phlogopite, spinel, carbonate, perovskite and rutile. The mineral compositions, whole-rock geochemistry and Nd isotopic composition (Nd: + 0.62 to ? 0.37) are indistinguishable from those known from archetypal hypabyssal kimberlites. Appreciably lower bulk-rock CaO (mostly < 5 wt%) and higher La/Sm ratios (12-15; resembling those of orangeites) are a characteristic feature of these rocks. Their geochemical composition excludes any effects of significant crustal and mantle contamination/assimilation. The fractionation trends displayed suggest a primary kimberlite melt composition indistinguishable from global estimates of primary kimberlite melt, and highlight the dominance of a kimberlite magma component in the pyroclastic variants. The lack of Nb-Ta-Ti anomalies precludes any significant role of subduction-related melts/fluids in the metasomatism of the FALC kimberlite mantle source region. Their incompatible trace elements (e.g., Nb/U) have OIB-type affinities whereas the Nd isotope composition indicates a near-chondritic to slightly depleted Nd isotope composition. The Neoproterozoic (~ 0.6-0.7 Ga) depleted mantle (TDM) Nd model ages coincide with the emplacement age (ca. 673 Ma) of the Amon kimberlite sills (Baffin Island, Rae craton, Canada) and have been related to upwelling protokimberlite melts during the break-up of the Rodinia supercontinent and its separation from Laurentia (North American cratonic shield). REE inversion modelling for the FALC kimberlites as well as for the Jericho (ca. 173 Ma) and Snap Lake (ca. 537 Ma) kimberlites from the neighbouring Slave craton, Canada, indicate all of their source regions to have been extensively depleted (~ 24%) before being subjected to metasomatic enrichment (1.3-2.2%) and subsequent small-degree partial melting. These findings are similar to those previously obtained on Mesozoic kimberlites (Kaapvaal craton, southern Africa) and Mesoproterozoic kimberlites (Dharwar craton, southern India). The striking similarity in the genesis of kimberlites emplaced over broad geological time and across different supercontinents of Laurentia, Gondwanaland and Rodinia, highlights the dominant petrogenetic role of the sub-continental lithosphere. The emplacement of the FALC kimberlites can be explained both by the extensive subduction system in western North America that was established at ca. 150 Ma as well as by far-field effects of the opening of the North Atlantic ocean during the Late Cretaceous.
Proxima Diamonds Corp.: exploring for diamonds in the fertile Slave craton.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 33.
Canada, Northwest Territories
Prospect - Sancy
Abstract: Proxima Diamonds Corp. is a private Canadian diamond exploration company exploring diamond targets in the heart of the diamond producing region of the Slave Geological Province. The company holds 17 target-rich properties that were selected based on a review of publicly available data, a proprietary kimberlite indicator mineral sample database and a wealth of experience exploring for diamonds in Canada's north. Focused kimberlite indicator mineral (KIM) sampling conducted by Proxima in 2014 identified a potential source area on the Sancy Property, located near the northern boundary of the Ekati Diamond Mine. Follow-up ground geophysical surveys completed over the area in spring 2015 have returned compelling results. Ground gravity, total magnetic field and capacitively coupled resistivity surveys identified a large, new target approximately 300 m from the diamondiferous T-10 kimberlite pipe. On this and other Proxima properties, focused KIM till sampling is defining likely source areas which will be surveyed with ground geophysical methods this winter.
Dyke Swarms of the World: a modern perspective, Srivastava et al. eds. Springer , pp. 263-314.
Africa, West Africa, South America
geochronology
Abstract: Eight different generations of dolerite dykes crosscutting the Paleoproterozoic basement in West Africa and one in South America were dated using the high precision U-Pb TIMS method on baddeleyite. Some of the individual dykes reach over 300 km in length and they are considered parts of much larger systems of mafic dyke swarms representing the plumbing systems for large igneous provinces (LIPs). The new U-Pb ages obtained for the investigated swarms in the southern West African Craton (WAC) are the following (oldest to youngest): 1791?±?3 Ma for the N010° Libiri swarm, 1764?±?4 Ma for the N035° Kédougou swarm, 1575?±?5 for the N100° Korsimoro swarm, ~1525-1529 Ma for the N130° Essakane swarm, 1521?±?3 Ma for the N90° Sambarabougou swarm, 915?±?7 Ma for the N070° Oda swarm, 867?±?16 Ma for the N355° Manso swarm, 202?±?5 Ma and 198?±?16 Ma for the N040° Hounde swarm, and 200?±?3 Ma for the sills in the Taoudeni basin. The last ones are related to the Central Atlantic Magmatic Province (CAMP) event. The Hounde swarm is oblique to the dominant radiating CAMP swarm and may be linked with the similar-trending elongate Kakoulima intrusion in Guinea. In addition, the N150° Käyser swarm (Amazonian craton, South America) is dated at 1528?±?2 Ma, providing a robust match with the Essakane swarm in a standard Amazonia-West African craton reconstruction, and resulting in a combined linear swarm >1500 km by >1500 km in extent. The Precambrian LIP barcode ages of c. 1790, 1765-1750, 1575, 1520, 915. 870 Ma for the WAC are compared with the global LIP record to identify possible matches on other crustal blocks, with reconstruction implications. These results contribute to the refinement of the magmatic ‘barcode’ for the West African and Amazonian cratons, representing the first steps towards plausible global paleogeographic reconstructions involving the West African and Amazonian cratons.
Eocene shoshonitic mafic dykes intruding the Monashee Complex, British Columbia: a petrogenetic relationship with the Kam loops Group volcanic sequence.
Canadian Journal of Earth Sciences, Vol. 42, 1, pp. 11-24.
El Bahat, A., Ikenne, M., Soderlund, U., Cousens, B., Youbi, N., Ernst, R., Soulaimani, A., El Janati, M., Hafid, A.
U PB baddeleyite ages and geochemistry of dolerite dykes in the Bas Draa In lier of the Anti-Atlas of Morocco: newly identified Ma event in the West African craton.
Kilian, T.M., Bleeker, W., Chamberlain. K., Evans, D.A.D., Cousens, B.
Paleomagnetism, geochronology and geochemistry of the Paleoproterozoic Rabbit Creek and Powder River dyke swarms: implications for Wyoming in supercraton Superia.
Geological Society of London Special Publication Supercontinent Cycles through Earth History., Vol. 424, pp. 15-45.
Abstract: Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America's assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.
Abstract: Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America's assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.
Abstract: Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America’s assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.
Canadian Journal of Earth Sciences, Vol. 54, pp. 203-232.
Canada, Northwest Territories
kimberlite
Abstract: The Wopmay orogen is a Paleoproterozoic accretionary belt preserved to the west of the Archean Slave craton, northwest Canada. Reworked Archean crystalline basement occurs in the orogen, and new bedrock mapping, U–Pb geochronology, and Sm–Nd isotopic data further substantiate a Slave craton parentage for this basement. Detrital zircon results from unconformably overlying Paleoproterozoic supracrustal rocks also support a Slave craton provenance. Rifting of the Slave margin began at ca. 2.02 Ga with a second rift phase constrained between ca. 1.92 and 1.89 Ga, resulting in thermal weakening of the Archean basement and allowing subsequent penetrative deformation during the Calderian orogeny (ca. 1.88–1.85 Ga). The boundary between the western Slave craton and the reworked Archean basement in the southern Wopmay orogen is interpreted as the rifted cratonic margin, which later acted as a rigid backstop during compressional deformation. Age-isotopic characteristics of plutonic phases track the extent and evolution of these processes that left penetratively deformed Archean basement, Paleoproterozoic cover, and plutons in the west, and “rigid” Archean Slave craton to the east. Diamond-bearing kimberlite occurs across the central and eastern parts of the Slave craton, but kimberlite (diamond bearing or not) has not been documented west of ?114°W. It is proposed that while the crust of the western Slave craton escaped thermal weakening, the mantle did not and was moved out of the diamond stability field. The Paleoproterozoic extension–convergence cycle preserved in the Wopmay orogen provides a reasonable explanation as to why the western Slave craton appears to be diamond sterile.
Abstract: Magmatism of the Povungnituk Group of the Cape Smith Belt, northern Superior craton, was formed in three stages: (i)early alkaline magmatism and associated carbonatites (undated), (ii) a main flood basalt sequence (Beauparlant Formation) (constrained between 2040 and 1991?Ma), and (iii) a late stage alkaline pulse (Cecilia Formation) (ca. 1959?Ma). We suggest that the main stage of magmatic activity (middle pulse) was of short duration. A new UPb baddeleyite age of 1998?±?6?Ma is obtained from a dolerite sill intruding the uppermost section of the Beauparlant Formation. This age has regional significance because it matches the previously obtained 1998?±?2?Ma age for the Watts Group (Purtuniq) ophiolite of the northern Cape Smith Belt and the 1998?±?2?Ma?U-Pb age of the Minto dykes intruding the craton to the south. These coeval units, along with additional units correlated on paleomagnetic grounds (Eskimo Formation), are interpreted to define a large igneous province (LIP), extending over an area of >400,000?km2, which we herein define as the Minto-Povungnituk LIP. Geochemical comparison between the Watts Group ophiolite, Minto dykes and the mafic Povungnituk Group shows significant differences allowing these data to be divided into two groups and domains within the LIP. A northern domain, comprising the Povungnituk and Watts groups, shows mixing between a depleted mantle source and a more enriched mantle plume-sourced melt. A southern domain comprising the Minto dykes and the paleomagnetically linked Eskimo Formation shows signs of an even more enriched source, while these magmas also show the effect of crustal contamination. Two distinct source mechanisms can be responsible for the observed geochemical differences between the two domains. First, a difference in lithospheric sources, where melting of different portions of Superior craton lithosphere caused the different melt signatures in the interior of the craton. In this case magmatism in the two domains is only related by having the same heat source (e.g.,a mantle plume) interpreted to be located on the northwestern side of the northern Superior craton. Second, two distinct deep mantle sources that remained separated within the ascending plume. This is analogous to some current hotspots interpreted to sample both large low shear velocity provinces (LLSVP) and adjacent ambient deep mantle. This latter interpretation would allow for the use of bilateral chemistry in LIPs as a potential tool for the recognition and mapping of the LLSVP boundaries throughout Earth's history.
Abstract: Rifting, breakup, and subsequent collision related to the ca. 1.92-1.79?Ga Svecofennian orogeny fragmented and deformed the western margin of the Archean Karelia-Kola craton into four crustal blocks: Pudasjärvi, Iisalmi, Kuhmo, and Taivalkoski. Detailed quantification of Svecofennian deformation is limited due to poorly exposed basement geology and an as yet incomplete dyke swarm record. New U-Pb ID-TIMS geochronological results on baddeleyite and zircon are presented for three key mafic dykes from the Pudasjärvi block, namely the Uolevinlehto, Myllykangas, and Sipojuntti dykes. The age of the 325°-trending Uolevinlehto dyke is estimated at ca. 2400?±?12?Ma from discordant multigrain baddeleyite fractions, showing it to be younger than ca. 2450?Ma dykes across Karelia. The 350°-trending Myllykangas dyke has a minimum age of 2135.2?+?3.6/?3.7?Ma based on chemically abraded zircon. Results from single baddeleyite grains provide a precise upper intercept age of 2128.9?±?1.2?Ma for the 320°-trending Sipojuntti dyke. Our new U-Pb ages are integrated with those from the literature to define six major dyke swarms in the Pudasjärvi block: the WNW-trending ca. 2.45?Ga Pääjärvi, NW-trending ca. 2.40?Ga Uolevinlehto, NW-trending ca. 2.13-2.10?Ga Tohmajärvi, WNW-trending ca. 2.07?Ga Palomaa, NNW-trending ca. 1.98?Ga Paukkajanvaara and undated"East-West" dykes. Trends of contemporaneous dyke swarms in the Taivalkoski and Kuhmo blocks, however, are systematically offset by 35°. With subvertical dips, offset dyke swarms record 35° clockwise vertical-axis rotation of the Pudasjärvi block relative to the interior of Karelia, consistent with dextral transpression during the Svecofennian orogeny. Structural restoration of the Pudasjärvi blocks improves the constraints on regional dyke swarm patterns, and these are used to revise the position of the Karelia-Kola craton within the context of the paleogeographic reconstruction of supercraton Superia.
American Economic Review, Vol. 107, 6, pp. 1564-1610. pdf
Africa
legal
Abstract: We combine georeferenced data on mining extraction of 14 minerals with information on conflict events at spatial resolution of 0.5 degree x 0.5 degree for all of Africa between 1997 and 2010. Exploiting exogenous variations in world prices, we find a positive impact of mining on conflict at the local level. Quantitatively, our estimates suggest that the historical rise in mineral prices (commodity super-cycle) might explain up to one-fourth of the average level of violence across African countries over the period. We then document how a fighting group's control of a mining area contributes to escalation from local to global violence. Finally, we analyze the impact of corporate practices and transparency initiatives in the mining industry.
Abstract: Geodynamics of crustal growth and evolution consist in one of the thorniest questions of the early Earth. In order to solve it, Archean cratons are intensively studied through geophysical, geochemical and geochronological investigations. However, timing and mechanisms leading to accretion and stabilization of crustal blocks are still under question. In this study, new information on the evolution of Archean cratons is provided through complementary approaches applied to the northern margin of the Archean Kaapvaal craton (KC). The study area comprises the Pietersburg Block (PB) and the terrane immediately adjacent to the North: the Southern Marginal Zone of the Limpopo Complex (SMZ). We present a comprehensive petro-metamorphic study coupled with LA-ICP-MS U-Pb isotope examination of both Na- and K-rich granitoids from the two areas. This dataset points toward a new interpretation of the northern KC (PB?+?SMZ). Two significant magmatic events are newly recognized: (i) a ca. 3.2?Ga event, and (ii) a protracted magmatic event between ca. 2.95–2.75?Ga. These events affected in both investigated areas and are unrelated to the ca. 2.7?Ga-old event usually attributed to the SMZ. More importantly, phase equilibrium modelling of several lithologies from the SMZ basement points to middle-amphibolite facies conditions of equilibration instead of granulite-facies conditions historically assumed. This study has both important regional and global implications. Firstly, the presence of a continuous basement from the Thabazimbi-Murchison Lineament to the Palala Shear Zone, different than Central Zone of the Limpopo Complex basement, implies a complete reviewing of the whole Limpopo Complex concept. Secondly, the geometry observed in the northern Kaapvaal craton is assumed to testify for a complete accretionary orogenic sequence with formation of both mafic and TTG lithologies through arc-back arc geodynamic. This was followed by a long-lived lateral compression triggering partial melting of the lower continental crust and emplacement of Bt-granitoids bodies that stabilizes the continental crust. Lastly, partial melting of the underlying enriched mantle stabilized the entire lithosphere allowing long-term preservation of the crustal block.
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Magnetic inversion of three airborne dat a sets over the Tli Kwi Cho kimberlite complex.
SEG Annual Meeting Denver, pp. 1790-1794 extended abstract
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO-18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO- 18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Marchant, M., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
Condie, K.C., Cox, J., O'Reilly, S.Y., Griffin, W.L., Kerrich, R.
Definition of high field strength and rare elements in mantle and lower crustal xenoliths from the SE United States: the role of grain boundary phases.
Geochimica et Cosmochimica Acta, Vol. 68, 19, pp. 3919-3942.
Relationships between crustal contamination and crystallization in continental flood basalt magmas with special reference to the Deccan Traps Of the Westrn Ghats, In
Earth and Planetary Science Letters, Vol. 84, No. 1, June pp. 59-68
Karoo igneous activity, and the early stages of the break-up SOURCE[ Geological Society Special Publication Magmatism and the causes of the continentalBreak-up, editor Storey, B.C. et al.
Geological Society Special Publication Magmatism and the causes of the continental, No. 68, pp. 137-148
Abstract: Impact cratering is a dynamic process that is violent and fast. Quantifying processes that accommodate deformation at different scales during central uplift formation in complex impact structures is therefore a challenging task. The ability to correlate mineral deformation at the microscale with macroscale processes provides a critical link in helping to constrain extreme crustal behavior during meteorite impact. Here we describe the first high-pressure-phase-calibrated chronology of shock progression in zircon from a central uplift. We report both shock twins and reidite, the high-pressure ZrSiO4 polymorph, in zircon from shocked granitic gneiss drilled from the center of the >60-km-diameter Woodleigh impact structure in Western Australia. The key observation is that in zircon grains that contain reidite, which forms at >30 GPa during the crater compression stage, the reidite domains are systematically offset by later-formed shock deformation twins (?20 GPa) along extensional planar microstructures. The {112} twins are interpreted to record crustal extension and uplift caused by the rarefaction wave during crater excavation. These results provide the first physical evidence that relates the formation sequence of both a high-pressure phase and a diagnostic shock microstructure in zircon to different cratering stages with unique stress regimes that are predicted by theoretical and numerical models. These microstructural observations thus provide new insight into central uplift formation, one of the least-understood processes during complex impact crater formation, which can produce many kilometers of vertically uplifted bedrock in seconds.
Morphological, chemical and geochronological techniques for characterizing detrital zircon.
Geochemistry of sediments and sedimentary rocks: evolutionary considerations mineral deposits, D.R.Lenz, Geological Association of Canada GEOtext 4, pp. 105-119
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 66.
Canada, Northwest Territories
Ice road
Abstract: The Inuvik to Tuktoyaktuk Highway (ITH) has been an idea for over 50 years. It was only in the past decade that the project gained traction towards becoming a reality. Once constructed, the ITH will be an all-weather link between these two communities in the Mackenzie Delta region of the Northwest Territories. Working collaboratively with our aboriginal partnership company KAVIK-STANTEC in Inuvik, Stantec has completed a variety of tasks ranging from baseline environmental assessment (wildlife, vegetation, and terrain), regulatory support and civil engineering design services. Crews hit the ground in the winter of 2013/2014 and the project is now entering its third and final winter construction season. The project team (designers, constructors and the Owner) contended with several challenges in bringing the project to fruition. Some of these included: • Fast track schedule • Weather constraints • Data refinements • Climate change considerations • Large complexity of the project (requiring collaborative approach) When completed, the ITH will span over 140 km and provide a vital access route for industry and the public. Stantec will discuss the challenges and triumphs in working on this diverse project working in this remote landscape.
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.
Compressional reworking of the East African Orogen in the Uluguru Mountains of eastern Tanzania at c. 550Ma: implications for the final assembly of Gondwana.
Indicator mineral and geochemistry dat a for a till and alluvium sampling survey in the McFaulds Lake ( Ring of Fire) area, northern Ontario. Mentions KIMS.
Ontario Geological Survey Report and Data, Report 6309, Data release 322.
International Journal of remote sensing, Vol. 39, 23, pp. 8387-8427.
Global
Remote sensing
Abstract: This editorial has its origins in a keynote presentation entitled ‘The Evolution of the Development of Remote Sensing Technologies - the Last 40 years’ which I gave at the 9th International Conference and Exhibition on Geospatial and Remote Sensing (9 IGRSM 2018) in Kuala Lumpur 24-25 April 2018 ‘Geospatial Enablement’. The editorial is not intended to be a definitive history of remote sensing from the beginning up to the day of its submission for publication. Rather it represents a personal account to try to enable present-day practitioners of remote sensing to gain a slight appreciation of what went before the time when they were introduced to the subject. The fun in our group in the 1980s was being able to explore many possible new applications of remote sensing, some of which turned out to be successful and some of which turned out to be failures - for various reasons. At a first glance it may seem that the list of references is woefully inadequate. However this is not an encyclopaedic review of remote sensing as it now is, but an attempt to recall some of the history of how we got here. The references are only meant to document some of the things that are said. For other information we assume that readers will consult whatever search engine, Google, etc., that they commonly use. I chose 40 years because it seemed to me that 1978 was a landmark year for remote sensing. In that year three very important new satellite systems were launched into space, the TIROS-N satellite with the AVHRR (Advanced Very High Resolution Radiometer) on board, the SEASAT satellite and the NIMBUS-7 satellite with the CZCS (Coastal Zone Colour Scanner) on board. In addition to all these, the third satellite in the Landsat programme (Landsat 3) was launched in March 1978. Of rather less importance, it was the year of my very first remote sensing project which involved attempting to use CZCS data to study water quality parameters; we learned the hard way about the difficulties involved in conducting field experiments on a rapidly changing environmental system simultaneously with satellite overflights. 1978 was also just before the launch of the International Journal of Remote Sensing (IJRS) in 1980 and so the initial work on the start up of the IJRS was being done in 1978. This editorial is therefore divided into three parts (a) Part 1 remote sensing before 1978, (b) Part 2 1978, the year of the launch of three very important polar-orbiting satellites and (c) Part 3 remote sensing since 1978. Textbooks sometimes define remote sensing to mean the observation of, or gathering of information about, a target by a device separated from it by some distance. In practice it is usually taken to be more restricted than that. It is sometimes claimed that the expression ‘remote sensing’ was coined by geographers at the U.S. Office of Naval Research in the 1960s at about the time that the use of ‘spy’ satellites was beginning to move out of the military sphere and into the civilian sphere. Remote sensing is often regarded as being synonymous with the use of artificial satellites, but there is an ongoing history of air photos that preceded the satellites and goes right up to the recent development of UAVs (drones) which are likely to supersede satellites in some areas
Abstract: Strata in the Huron (2.5-2.0 Ga) and Animikie (2.2-1.85 Ga) basins were deposited on the southern margin of the Archean Superior province. These rocks were deformed during the Penokean orogeny (?1850 Ma) followed by subsequent accretionary orogens to the south at 1750 Ma (Yavapai) and 1630 Ma (Mazatzal). Strain patterns are unique to each orogenic belt with no far-field effect: Archean Wawa terrane rocks in the Penokean foreland preserve deformation associated with Archean accretion with no younger Penokean, Yavapai or Mazatzal strain overprint. The Penokean orogeny deformed Huron-Animikie basin sediments into a north-vergent fold-and-thrust belt with no Yavapai or Mazatzal strain overprint. Yavapai orogen strains (SW-NE margin-parallel shortening) are unique when compared to the younger Mazatzal shortening (N20°W) shortening, with no strain overprint. Penokean deformation is characterized by shortening from the south including uplifted Archean gneisses and a northerly thin-skinned fold-and-thrust belt, with north-vergent nappes and a gently-dipping foreland. Our study of finite and calcite twinning strains (n=60) along (?1500 km) and across (?200 km) the Penokean belt indicate that this orogeny was collisional as layer-parallel shortening axes are parallel across the belt, or parallel to the tectonic transport direction (?N-S). Penokean nappe burial near the margin resulted in vertical shortening strain overprints, some of which are layer-normal. The Sudbury impact layer (1850 Ma) is found across the Animikie basin and provides a widespread deformation marker with many local, unique strain observations. We also report new geochronology (U-Pb zircon and apatite) for the gneiss-mafic dike rocks at Wissota (Chippewa Falls, WI) and Arbutus (Black River Falls, WI) dams, respectively, which bears on Penokean-Yavapai deformation in the Archean Marshfield terrane which was accreted during the Penokean orogen. Pseudotachylite formation was common in the Superior province Archean basement rocks, especially along terrane boundaries reactivated by contemporaneous Penokean, Trans-Hudson, Cape Smith and New Quebec deformation. In the hinterland (south), the younger Yavapai orogen (1750 Ma; n=8) deformation is preserved as margin-parallel horizontal shortening (?SW-NE) in Yavapai crust and up to 200 km to the north in the Penokean thrust belt as a strain and Barrovian metamorphic overprint. Mazatzal deformation (1630 Ma; n=16) is preserved in quartzites on Yavapai and Penokean crust with layer-parallel and layer-normal shortening strains oriented N20°W.
Geochimica et Cosmochinica Acta, Vol. 213, pp. 346-374.
United States, Colorado
volatiles
Abstract: The Colorado Plateau hosts several large accumulations of naturally occurring, non-hydrocarbon gases, including CO2, N2, and the noble gases, making it a good field location to study the fluxes of these gases within the crust and to the atmosphere. In this study, we present a compilation of 1252 published gas-composition measurements. The data reveal at least three natural gas associations in the field area, which are dominated by hydrocarbons, CO2, and N2 + He + Ar, respectively. Most gas accumulations of the region exhibit compositions that are intermediate between the three end members. The first non-hydrocarbon gas association is characterized by very high-purity CO2, in excess of 75 mol% (hereafter, %). Many of these high-purity CO2 fields have recently been well described and interpreted as magmatic in origin. The second non-hydrocarbon gas association is less well described on the Colorado Plateau. It exhibits He concentrations on the order of 1-10%, and centered log ratio biplots show that He occurs proportionally to both N2 and Ar. Overall ratios of N2 to He to Ar are ?100:10:1 and correlation in concentrations of these gases suggests that they have been sourced from the same reservoir and/or by a common process. To complement the analysis of the gas-composition data, stable isotope and noble-gas isotope measurements are compiled or newly reported from 11 representative fields (previously published data from 4 fields and new data from 7 fields). Gas sampled from the Harley Dome gas field in Utah contains nearly pure N2 + He + Ar. The various compositional and stable and noble gas isotopic data for this gas indicate that noble gas molecule/isotope ratios are near crustal radiogenic production values and also suggest a crustal N2 source. Across the field area, most of the high-purity N2 + He + Ar gas accumulations are associated with the mapped surface trace of structures or sutures in the Precambrian basement and are often accumulated in lower parts of the overlying Phanerozoic sedimentary cover. The high-purity gas association mostly occurs in areas interior to the plateau that are characterized by a narrow range of elevated, moderate heat flow values (53-74 mW/m2) in the ancient (1.8-1.6 Ga) basement terranes of the region. Collectively, the geochemical and geological data suggest that (1) the N2 + He + Ar gas association is sourced from a crustal reservoir, (2) the gas association migrates preferentially along structures in the Precambrian basement, and (3) the sourcing process relates to heating of the crust. Prospecting for noble-gas accumulations may target areas with elevated Cenozoic heat flow, ancient crust, and deep crustal structures that focus gas migration. High-purity CO2 gas may also migrate through regional basement structures, however, there is not always a clear spatial association. Rather, CO2 accumulations are more clearly associated with zones of high heat flow (>63 mW/m2) that sit above hot upper mantle and are proximal to Cenozoic volcanic rocks near the plateau margins. These observations are consistent with previous interpretations of a magmatic gas source, which were based on geochemical measurements.
Aulbach, S., Stachel, T., Craeser, R.A., Heaman, L.M., Shirey, S.B., MUehlenbachs, K., Eichenberg, D., Harris
Sulphide survival and diamond genesis during formation and evolution of Archean subcontinental lithosphere: a comparison between the Slave and Kaapvaal cratons.
Craig, T.J., Jackson, J.A., Priestley, K., McKenzie, D.
Earthquake distribution patterns in Africa: their relationship to variations in lithospheric and geological structure, and their rheological implication
Geophysical Journal International, Vol. 185, 1, pp. 403-404.
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.
Progress in Earth and Planetary Science, Open access
Mantle
Convection, geodynamics
Abstract: Subduction initiation is a key in understanding the dynamic evolution of the Earth and its fundamental difference to all other rocky planetary bodies in our solar system. Despite recent progress, the question about how a stiff, mostly stagnant planetary lid can break and become part in the global overturn of the mantle is still unresolved. Many mechanisms, externally or internally driven, are proposed in previous studies. Here, we present the results on subduction initiation obtained by dynamically self-consistent, time-dependent numerical modelling of mantle convection. We show that the stress distribution and resulting deformation of the lithosphere are strongly controlled by the top boundary formulation: A free surface enables surface topography and plate bending, increases gravitational sliding of the plates and leads to more realistic, lithosphere-scale shear zones. As a consequence, subduction initiation induced by regional mantle flow is demonstrably favoured by a free surface compared to the commonly applied, vertically fixed (i.e. free-slip) surface. In addition, we present global, three-dimensional mantle convection experiments that employ basal heating that leads to narrow mantle plumes. Narrow mantle plumes impinging on the base of the plate cause locally weak plate segments and a large topography at the lithosphere-asthenosphere boundary. Both are shown to be key to induce subduction initiation. Finally, our model self-consistently reproduces an episodic lid with a fast global overturn due to the hotter mantle developed below a former stagnant lid. We conclude that once in a stagnant-lid mode, a planet (like Venus) might preferentially evolve by temporally discrete, global overturn events rather than by a continuous recycling of lid and that this is something worth testing more rigorously in future studies.
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.
Park visitor from Arkansas finds 3.69 carat white, teardrop-shaped diamond yesterday ( April 23) at Arkansas Crater of Diamonds State Park. Hallelujah name
Crystallization of mela-aillikites of the Narsaq region, Gardar alkaline province, south Greenland and relationships to other aillikitic carbonatitic assoc.
Crystallisation of mela-allikites of the Narsaq region, Gardar alkaline province, south Greenland and relationships to other allikitic carbonatitic associate
Geochemistry, Geophysics, Geosystems: G3, Vol. 16, 10, pp. 3555-3574.
Canada, Saskatchewan, Alberta
Rae Craton
Abstract: Reconstruction of the 3-dimensional tectonic assembly of early continents, first as Archean cratons and then Proterozoic shields, remains poorly understood. In this paper, all readily available geophysical and geochemical data are assembled in a 3-D model with the most accurate bedrock geology in order to understand better the geometry of major structures within the Rae craton of central Canada. Analysis of geophysical observations of gravity and seismic wave speed variations revealed several lithospheric-scale discontinuities in physical properties. Where these discontinuities project upward to correlate with mapped upper crustal geological structures, the discontinuities can be interpreted as shear zones. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can also be correlated to surface rocks. The 3.6-2.6 Ga Rae craton comprises at least three smaller continental terranes, which "cratonized" during a granitic bloom. Cratonization probably represents final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho. The peak thermotectonic event at 1.86-1.7 Ga was associated with the Hudsonian orogeny that assembled several cratons and lesser continental blocks into the Canadian Shield using a number of southeast-dipping megathrusts. This orogeny metasomatized, mineralized, and recrystallized mantle and lower crustal rocks, apparently making them more conductive by introducing or concentrating sulfides or graphite. Little evidence exists of thin slabs similar to modern oceanic lithosphere in this Precambrian construction history whereas underthrusting and wedging of continental lithosphere is inferred from multiple dipping discontinuities.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 110.
Canada, Northwest Territories
Deposit - Jay
Abstract: The Ekati property is located above an eastward-dipping Archean suture in the central part of the Slave Structural Province of the Canadian Shield. The bedrock geology comprises supracrustal rocks (metamorphosed greywacke-mudstone turbidites) of the Neoarchean post-Yellowknife Supergroup that are intruded by syn to post-tectonic plutons, made up predominantly of granite, granodiorite, and tonalite. In addition, five mafic Proterozoic dyke swarms, ranging in age from ca. 2.23 to 1.27 Ga, intrude the area. The area is intersected by several mafic dykes, belonging mainly to the Malley, MacKenzie, and Lac de Gras dyke swarms. To date, approximately 150 kimberlites have been discovered at Ekati ranging in age from ca. 45 to 75 Ma, intruding Archean metasediments and granitoids of the Salve Craton. In addition to the 150 kimberlites on the Ekati property, more than 240 confirmed kimberlites have been discovered to date in the region known as the Lac de Gras kimberlite field. The kimberlites represent the only evidence for Phanerozoic igneous activity within the area. Kimberlites on the Ekati property show an apparent bias in the type of host rock they intrude and are commonly associated with faults or dykes of various orientations. The Jay kimberlite pipe is located in the southeastern quadrant of the Ekati property. It is approximately 25 km southeast of the Koala cluster (including Panda, Koala, Koala North and Beartooth kimberlite pipes), and 7 km north-northeast of the Misery Main pipe. Based on available geological data consisting of geophysical surveys, geological maps and borehole data, the Jay kimberlite pipe appears to be hosted within post- Yellowknife Supergroup granitic rocks, ranging from granite to granodiorite in composition. It is interpreted to be emplaced along a regional lithological contact between granitoid rocks and Yellowknife Supergroup metasedimentary rocks that were covered by a now eroded veneer of poorly consolidated muddy sediments. A diabase dyke trending approximately east-west occurs to the north of the Jay kimberlite pipe. Despite the available data, geological and structural settings of the Jay host rocks were still not well understood. This work represents the first comprehensive geological interpretation of the host rocks within the Jay pipe setting. The proposed interpretation will be based on the following: • a detailed review, compilation, and interpretation of previously published geological work in the area; • interpretation of high-resolution light detection and ranging (LiDAR) data; • high-resolution orthophotos and airborne geophysical data; • geological data from delineation and geotechnical boreholes drilled between 2005 and 2007, and the recent 2014 and 2015 drilling programs at the Jay pipe area. An implicit modelling approach has been used to develop a three dimensional geological and structural model of the Jay pipe host rocks based on the preliminary interpretation. Ongoing studies aim to decode the geological and structural controls on the Jay kimberlite emplacement, along with its relationship with the nearby Misery kimberlite cluster.
In vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement: an example from the Fox kimberlite, Ekati diamond mine.
Journal of Volcanology and Geothermal Research, Vol. 174, 1-3, pp. 90-102. reply in press 17p.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0631-6 13p.
Canada, Northwest Territories
deposit - Kelvin
Abstract: The early Cambrian to late Neoproterozoic Kelvin kimberlite pipe is located in the southeast of the Archean Slave Craton in northern Canada, eight km northeast of the Gahcho Kué diamond mine. Kelvin was first discovered in 2000 by De Beers Canada. Subsequent exploration undertaken by Kennady Diamonds Inc. between 2012 and 2016 resulted in the discovery of significant thicknesses of volcaniclastic kimberlite that had not previously been observed. Through extensive delineation drilling Kelvin has been shown to present an atypical, steep-sided inclined L-shaped pipe-like morphology with an overall dip of 15 to 20°. With a surface expression of only 0.08 ha Kelvin dips towards the northwest before turning north. The body (which remains open at depth) has been constrained to a current overall strike length of 700 m with varying vertical thickness (70 to 200 m) and width (30 to 70 m). Detailed core logging, petrography and microdiamond analysis have shown that the pipe infill comprises several phases of sub-horizontally oriented kimberlite (KIMB1, KIMB2, KIMB3, KIMB4, KIMB7 and KIMB8) resulting from multiple emplacement events. The pipe infill is dominated by Kimberley-type pyroclastic kimberlite or “KPK”, historically referred to as tuffisitic kimberlite breccia or “TKB”, with less common hypabyssal kimberlite (HK) and minor units with textures transitional between these end-members. An extensive HK sheet complex surrounds the pipe. The emplacement of Kelvin is believed to have been initiated by intrusion of this early sheet system. The main pipe-forming event and formation of the dominant KPK pipe infill, KIMB3, was followed by late stage emplacement of additional minor KPK and a hypabyssal to transitional-textured phase along the upper contact of the pipe, cross-cutting the underlying KIMB3. Rb-Sr age dating of phlogopite from a late stage phase has established model ages of 531 ± 8 Ma and 546 ± 8 Ma. Texturally and mineralogically, the Kelvin kimberlite is similar to other KPK systems such as the Gahcho Kué kimberlites and many southern African kimberlites; however, the external morphology, specifically the sub-horizontal inclination of the pipe, is unique. The morphology of Kelvin and the other kimberlites in the Kelvin-Faraday cluster defines a new type of exploration target, one that is likely not unique to the Kennady North Project area. Extensive evaluation work by Kennady Diamonds Inc. has resulted in definition of a maiden Indicated Mineral Resource for Kelvin of 8.5 million tonnes (Mt) of kimberlite at an average grade of 1.6 carats per tonne (cpt) with an average diamond value of US$ 63 per carat (ct).
Heaman, L.M., Creaser, R.A., Cookenboo, H.O., Chacko, T.
Multi stage modification of the northern Slave mantle lithosphere: evidence from zircon and diamond bearing eclogite xenoliths entrained in Jericho kimberlite.
Journal of Petrology, Vol. 47, 4, April pp. 821-858.
The origin of Triassic/Jurassic kimberlite magmatism, Canada: two mantle sources revealed from the Sr-Nd isotopic composition of groundmass perovskite.
Minette bodies and cognate mica-clinopyroxenite xenoliths from the Milk River area, southern Alberta: records of a complex history of the northern most part of the Archean Wyoming craton.
Canadian Journal of Earth Sciences, Vol. 37, 11, pp. 1629-1650.
U-Pb geochronology and Sr/Nd isotope compositions of groundmass perovskite from the newly discovered Jurassic Chidliak kimberlite field, Baffin Island, Canada.
Earth and Planetary Science Letters, Vol. 415, April pp. 183-189.
Abstract: Due to the acute scarcity of very ancient rocks, the composition of Earth’s embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today’s oceanic crust1, 2, 3, 4, or included significant amounts of continental crust5, 6, 7, 8 remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth—a 4,019.6 ± 1.8?Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.
Abstract: Due to the acute scarcity of very ancient rocks, the composition of Earth’s embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today’s oceanic crust1, 2, 3, 4, or included significant amounts of continental crust5, 6, 7, 8 remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth—a 4,019.6 ± 1.8?Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.
Earth Planetary Science Letters, Vol. 490, pp. 77-87.
Canada, Ontario
deposit - Victor
Abstract: The central Superior Craton hosts both the diamondiferous 1.1 Ga Kyle Lake and Jurassic Attawapiskat kimberlites. A major thermal event related to the Midcontinent Rift at ca. 1.1 Ga induced an elevated geothermal gradient that largely destroyed an older generation of diamonds, raising the question of when, and how, the diamond inventory beneath Attawapiskat was formed. We determined Re-Os isotope systematics of sulphides included in diamonds from Victor by isotope dilution negative thermal ionisation mass spectrometry in order to obtain insights into the age and nature of the diamond source in the context of regional tectonothermal evolution. Regression of the peridotitic inclusion data (n = 14 of 16) yields a 718 ± 49 Ma age, with an initial 187Os/188Os ratio of 0.1177 ± 0.0016, i.e. depleted at the time of formation (?Os -3.7 ± 1.3). Consequently, Re depletion model ages calculated for these samples are systematically overestimated. Given that reported 187Os/188Os in olivine from Attawapiskat xenoliths varies strongly (0.1012-0.1821), the low and nearly identical initial Os of sulphide inclusions combined with their high 187Re/188Os (median 0.34) suggest metasomatic formation from a mixed source. This was likely facilitated by percolation of amounts of melt sufficient to homogenise Os, (re)crystallise sulphide and (co)precipitate diamond; that is, the sulphide inclusions and their diamond host are synchronous if not syngenetic. The ?720 Ma age corresponds to rifting beyond the northern craton margin during Rodinia break-up. This suggests mobilisation of volatiles (C, N, S) and Os due to attendant mantle stretching and metasomatism by initially oxidising and S-undersaturated melts, which ultimately produced lherzolitic diamonds with high N contents compared to older Kyle Lake diamonds. Thus, some rift-influenced settings are prospective with respect to diamond formation. They are also important sites of hidden, intra-lithospheric volatile redistribution that can be revealed by diamond studies. Later emplacement of the Attawapiskat kimberlites, linking the carbon cycle to the surface, was associated with renewed disturbance during passage of the Great Meteor Hotspot. Lherzolitic diamond formation from oxidising small-volume melts may be the expression of an early and deep stage of the lithospheric conditioning required for the successful eruption of kimberlites, which complements the late and shallow emplacement of volatile-rich metasomes after upward displacement of a redox freezing front.
Physics of the Earth and Planetary Interiors, in press available, 51p. Pdf
United States, Canada
geophysics - seismics
Abstract: We examined SKS-SKKS splitting intensity discrepancies for phases that sample the lowermost mantle beneath North America, which has previously been shown to exhibit seismic anisotropy using other analysis techniques. We examined data from 25 long-running seismic stations, along with 244 stations of the temporary USArray Transportable Array, located in the eastern, southeastern and western U.S. We identified 279 high-quality SKS-SKKS wave pairs that yielded well-constrained splitting intensity measurements for both phases. Of the 279 pairs, a relatively small number (15) exhibited discrepancies in splitting intensity of 0.4 s or greater, suggesting a contribution to the splitting of one or both phases from anisotropy in the lowermost mantle. Because only a small minority of SK(K)S phases examined in this study show evidence of being affected by lowermost mantle anisotropy, the traditional interpretation that splitting of these phases primarily reflects anisotropy in the upper mantle directly beneath the stations is appropriate. The discrepant pairs exhibited a striking geographic trend, sampling the lowermost mantle beneath the southern U.S. and northern Mexico, while other regions were dominated by non-discrepant pairs. We carried out ray theoretical modeling of simple anisotropy scenarios that have previously been suggested for the lowermost mantle beneath North America, invoking the alignment of post-perovskite due to flow induced by the impingement of the remnant Farallon slab on the core-mantle boundary. We found that our measurements are generally consistent with this model and with the idea of slab-driven flow, but relatively small-scale lateral variations in the strength and/or geometry of lowermost mantle anisotropy beneath North America are also likely present.
Physics of the Earth and Planetary Interiors, Vol. 305, 106504, 15p. Pdf
United States
geophysics - seismics
Abstract: We examined SKS-SKKS splitting intensity discrepancies for phases that sample the lowermost mantle beneath North America, which has previously been shown to exhibit seismic anisotropy using other analysis techniques. We examined data from 25 long-running seismic stations, along with 244 stations of the temporary USArray Transportable Array, located in the eastern, southeastern and western U.S. We identified 279 high-quality SKS-SKKS wave pairs that yielded well-constrained splitting intensity measurements for both phases. Of the 279 pairs, a relatively small number (15) exhibited discrepancies in splitting intensity of 0.4 s or greater, suggesting a contribution to the splitting of one or both phases from anisotropy in the lowermost mantle. Because only a small minority of SK(K)S phases examined in this study show evidence of being affected by lowermost mantle anisotropy, the traditional interpretation that splitting of these phases primarily reflects anisotropy in the upper mantle directly beneath the stations is appropriate. The discrepant pairs exhibited a striking geographic trend, sampling the lowermost mantle beneath the southern U.S. and northern Mexico, while other regions were dominated by non-discrepant pairs. We carried out ray theoretical modeling of simple anisotropy scenarios that have previously been suggested for the lowermost mantle beneath North America, invoking the alignment of post-perovskite due to flow induced by the impingement of the remnant Farallon slab on the core-mantle boundary. We found that our measurements are generally consistent with this model and with the idea of slab-driven flow, but relatively small-scale lateral variations in the strength and/or geometry of lowermost mantle anisotropy beneath North America are also likely present.
South African Institute of Mining and Metallurgy, Vol. 116, 8, pp. 737-745.
Technology
microdiamond
Abstract: Estimating the size frequency distribution of the macrodiamonds on a new deposit is important for both economic reasons and for the design of the processing plant. Millions of dollars can be lost due to incorrectly sized comminution circuits. This report analyses an alternative methodology for macrodiamond grade estimation using the cumulative results from small parcels of microdiamonds and plotting them on a log-log scale. The method was first evaluated mathematically for diamond populations to assess the confidence for data extrapolation. Macrodiamond size distributions and grades were predicted using microdiamond data from three kimberlites, and the actual macrodiamond grades compared to the the predicted grades. The predicted grades were found to replicate the actual grades closely, showing that a high degree of confidence can be ascribed to the results from this method of analysis. This analysis can be used both for resource estimates and for predicting the diamond size distribution information needed for designing a new operation.
The utility of clinopyroxene in diamond exploration.
2018 Yellowknife Geoscience Forum , p. 13. abstract
Global
thermobarometry
Abstract: Clinopyroxene single-crystal thermo-barometry is an essential tool in the identification and evaluation of prospective kimberlites. The paleogeothermal gradient preserved by clinopyroxene xenocrysts elucidates the thermal structure of the underlying lithospheric mantle; indicates the depth to and thickness of the “diamond window”. The widely used clinopyroxene thermometer-barometer of Nimis and Taylor (2000) requires that clinopyroxene equilibrated with both garnet and orthopyroxene. With the rare exception of wehrlites, equilibration with orthopyroxene is nearly a given for the majority of chrome-diopside clinopyroxene xenocrysts. Demonstrating equilibration with garnet, however, is a major obstacle for clinopyroxene-based thermobarometry. The most commonly used method for clinopyroxene discrimination is an Al2O3-Cr2O3 diagram proposed by Ramsay and Thompkins in 1994 supplemented with an additional MgO-Al2O3 from Nimis (1998) and an additional 1-dimensional filter based on chemical composition. Despite the aggressiveness of the filtering method, single-clinopyroxene pressure-temperature results have large scatter that can obscure the true paleogeothermal gradient. This is especially true of areas where the lithospheric mantle has undergone chemical modification by melt/fluid influx. Using a database of clinopyroxenes derived from kimberlite-borne mantle-derived lherzolites, we have developed a simple and effective discrimination plot that identifies clinopyroxene from garnet lherzolites and simultaneously removes clinopyroxene from metaosomatised peridotites. Calculated paleogeothermal gradients from clinopyroxene xenocrysts cut across model conductive geotherms which can complicate the interpretation of thermobarometry data. Grütter (2009) presented a solution to the problem by way of relative reference geotherms. He used xenocryst data from three Canadian locations with different thermal structures as references in comparison to the dataset under investigation. Taking a cue from this earlier work, we have developed a new set of relative reference geotherms that are based on single-clinopyroxene thermobarometry data for xenoliths from well-characterized regions - Somerset Island, Kaapvaal on-craton, and the Central Slave. A simple linear fit through the data produces sub-parallel clinopyroxene reference geotherms that are simpler to use and easier to visualize compared to the xenocrysts reference geotherms. Using these two new and simple tools will greatly help maximize the utility of clinopyroxene data in large exploration databases.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 343-358.
Abstract: The South American platform is the stable part of the South American plate, unaffected by the orogenesis of the Andes and the Caribbean. Its basement is composed of Archean and Proterozoic cratonic blocks amalgamated by mobile belts, and can be separated in two large domains or continental masses: 1) The Amazonian, Northwest-west portion, including the Amazonian craton, related to the Laurentia supercontinent; and 2) the extra-Amazonian, Central-southeast or Brasiliano domain, related to West Gondwana, formed of several paleocontinental fragments, where the São Francisco and Rio de La Plata cratons and the Paranapanema block are the largest. It has been suggested that these two domains are separated by the Transbrasiliano Lineament to the south and the Araguaia Fold Belt to the north. Teleseismic P waves from 4,989 earthquakes recorded by 339 stations operated mainly in Brazil in the last 25 years have been used for relative-time tomography. The Amazonian domain is predominantly characterized by higher velocities. The SW (extra-Amazonian) domain is characterized by several blocks with high velocities, such as in and around the Sao Francisco Craton, and the Paranapanema block. Results of P-wave travel time tomography allowed to observe a strong low-velocity anomaly near 100-200 km depth following the Araguaia-Paraguay fold belt. This strong low-velocity anomaly could be considered the limit between these two domains, reaching lithospheric depths, and does not necessarily follow the Transbrasiliano lineament, especially in its southern portion.
Journal of Structural Geology, Vol. 86, pp. 47-61.
Africa, South Africa
Deposit - Venetia
Abstract: The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3-3.1 Ga, 2.7-2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo provides an unprecedented insight into CZ ductile deformation and sheath folding. D1 juxtaposed gneisses against metasediments. D2 produced a pervasive axial planar foliation (S2) to isoclinal F2 folds. Sheared lithological contacts and S2 were refolded into regional, open, predominantly southward-verging, E-W trending F3 folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S2. Constrictional-prolate D4 shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D4 overlaps with a: 1) 2.03-2.01 Ga regional M3 metamorphic overprint; b) transpressional deformation at 2.2-1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.
Journal of Structural Geology, Vol. 86, pp. 47-61.
Africa, South Africa
deposit - Venetia
Abstract: The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3-3.1 Ga, 2.7-2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo™ provides an unprecedented insight into CZ ductile deformation and sheath folding. D1 juxtaposed gneisses against metasediments. D2 produced a pervasive axial planar foliation (S2) to isoclinal F2 folds. Sheared lithological contacts and S2 were refolded into regional, open, predominantly southward-verging, E-W trending F3 folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S2. Constrictional-prolate D4 shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D4 overlaps with a: 1) 2.03-2.01 Ga regional M3 metamorphic overprint; b) transpressional deformation at 2.2-1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.
Journal of African Earth Sciences, Vol. 137, pp. 9-21.
Africa, Botswana
deposit - Jwaneng
Abstract: Country rock at Jwaneng Diamond Mine provides a rare insight into the deformational history of the Transvaal Supergroup in southern Botswana. The ca. 235 Ma kimberlite diatremes intruded into late Archaean to Early Proterozoic, mixed, siliciclastic-carbonate sediments, that were subjected to at least three deformational events. The first deformational event (D1), caused by NW-SE directed compression, is responsible for NE-trending, open folds (F1) with associated diverging, fanning, axial planar cleavage. The second deformational event (D2) is probably progressive, involving a clockwise rotation of the principal stress to NE-SW trends. Early D2, which was N-S directed, involved left-lateral, oblique shearing along cleavage planes that developed around F1 folds, along with the development of antithetic structures. Progressive clockwise rotation of far-field forces saw the development of NW-trending folds (F2) and its associated, weak, axial planar cleavage. D3 is an extensional event in which normal faulting, along pre-existing cleavage planes, created a series of rhomboid-shaped, fault-bounded blocks. Normal faults, which bound these blocks, are the dominant structures at Jwaneng Mine. Combined with block rotation and NW-dipping bedding, a horst-like structure on the northwestern limb of a broad, gentle, NE-trending anticline is indicated. The early compressional and subsequent extensional events are consistent throughout the Jwaneng-Ramotswa-Lobatse-Thabazimbi area, suggesting that a large area records the same fault geometry and, consequently, deformational history. It is proposed that Jwaneng Mine is at or near the northernmost limit of the initial, northwards-directed compressional event.
Journal of African Earth Sciences, Vol. 137, pp. 9-21.
Africa, Botswana
deposit - Jwaneng
Abstract: Country rock at Jwaneng Diamond Mine provides a rare insight into the deformational history of the Transvaal Supergroup in southern Botswana. The ca. 235 Ma kimberlite diatremes intruded into late Archaean to Early Proterozoic, mixed, siliciclastic-carbonate sediments, that were subjected to at least three deformational events. The first deformational event (D1), caused by NW-SE directed compression, is responsible for NE-trending, open folds (F1) with associated diverging, fanning, axial planar cleavage. The second deformational event (D2) is probably progressive, involving a clockwise rotation of the principal stress to NE-SW trends. Early D2, which was N-S directed, involved left-lateral, oblique shearing along cleavage planes that developed around F1 folds, along with the development of antithetic structures. Progressive clockwise rotation of far-field forces saw the development of NW-trending folds (F2) and its associated, weak, axial planar cleavage. D3 is an extensional event in which normal faulting, along pre-existing cleavage planes, created a series of rhomboid-shaped, fault-bounded blocks. Normal faults, which bound these blocks, are the dominant structures at Jwaneng Mine. Combined with block rotation and NW-dipping bedding, a horst-like structure on the northwestern limb of a broad, gentle, NE-trending anticline is indicated. The early compressional and subsequent extensional events are consistent throughout the Jwaneng-Ramotswa-Lobatse-Thabazimbi area, suggesting that a large area records the same fault geometry and, consequently, deformational history. It is proposed that Jwaneng Mine is at or near the northernmost limit of the initial, northwards-directed compressional event.
Abstract: The transition zone (TZ) is believed to be the primary destination of subducted water [1], with the main TZ minerals (wadsleyite and ringwoodite) capable of holding up to ~ 3 wt.% H2O in their structures’. Observations of high attenuation and elevated conductivity suggest some areas of the transition zone are hydrated [2,3]. Combined with the observation of ~ 1.4 wt% H2O in a diamond-hosted ringwoodite inclusion [4], it is probable that the transition zone is at least regionally, if not globally, “wet”. The presence of water can induce partial melting, alter chemical partitioning and drastically change the strength of rocks. The detailed effect of water’s presence in the TZ will strongly depend on hydrogen’s incorporation mechanism, i.e. exchange with Si4+, Mg2+, Fe2+ cations or coupled substitution with Fe3+ in ringwoodite. Recent developments in neutron single-crystal Laue diffraction now allow measurements on crystals smaller than 0.1 mm3 [5]. Here we quantitatively study the incorporation of hydrogen in a synthetic iron-bearing ringwoodite. A multi-technique approach, with independent determination of chemistry, ferric iron content, water content and structure via x-ray and neutron diffraction allows a detailed study of the hydrous ringwoodite structure and the incorporation mechanism of water throughout Earth’s TZ.
Geochimica et Cosmochimica Acta, Vol. 222, Feb 1, pp. 421-435.
Mantle
geobarometry
Abstract: We have performed an experimental cross calibration of a suite of mineral equilibria within mantle rock bulk compositions that are commonly used in geobarometry to determine the equilibration depths of upper mantle assemblages. Multiple barometers were compared simultaneously in experimental runs, where the pressure was determined using in-situ measurements of the unit cell volumes of MgO, NaCl, Re and h-BN between 3.6 and 10.4?GPa, and 1250 and 1500?°C. The experiments were performed in a large volume press (LVPs) in combination with synchrotron X-ray diffraction. Noble metal capsules drilled with multiple sample chambers were loaded with a range of bulk compositions representative of peridotite, eclogite and pyroxenite lithologies. By this approach, we simultaneously calibrated the geobarometers applicable to different mantle lithologies under identical and well determined pressure and temperature conditions. We identified discrepancies between the calculated and experimental pressures for which we propose simple linear or constant correction factors to some of the previously published barometric equations. As a result, we establish internally-consistent cross-calibrations for a number of garnet-orthopyroxene, garnet-clinopyroxene, Ca-Tschermaks-in-clinopyroxene and majorite geobarometers.
Rosenthal, A., Yaxley, G.M., Crichton, W.A., Kovacs, I.J., Spandler, C., Hermann, J., Sandorne, J.K., Rose-Koga, E., Pelleter, A-A.
Phase relations and melting of nominally 'dry' residual eclogites with variable CaO/Na2O from 3 to 5 Gpa and 1250 to 1500C; implications for refertilisation of upwelling heterogeneous mantle.
Abstract: Seismology records the presence of various heterogeneities throughout the lower mantle1,2, but the origins of these signals—whether thermal or chemical—remain uncertain, and therefore much of the information that they hold about the nature of the deep Earth is obscured. Accurate interpretation of observed seismic velocities requires knowledge of the seismic properties of all of Earth’s possible mineral components. Calcium silicate (CaSiO3) perovskite is believed to be the third most abundant mineral throughout the lower mantle. Here we simultaneously measure the crystal structure and the shear-wave and compressional-wave velocities of samples of CaSiO3 perovskite, and provide direct constraints on the adiabatic bulk and shear moduli of this material. We observe that incorporation of titanium into CaSiO3 perovskite stabilizes the tetragonal structure at higher temperatures, and that the material’s shear modulus is substantially lower than is predicted by computations3,4,5 or thermodynamic datasets6. When combined with literature data and extrapolated, our results suggest that subducted oceanic crust will be visible as low-seismic-velocity anomalies throughout the lower mantle. In particular, we show that large low-shear-velocity provinces (LLSVPs) are consistent with moderate enrichment of recycled oceanic crust, and mid-mantle discontinuities can be explained by a tetragonal-cubic phase transition in Ti-bearing CaSiO3 perovskite.
Skeleton, A., Whitmarsh, R., Arghe, F., Crill, P., Koyi, H.
Constraining the rate and extent of mantle serpentinization from seismic and petrological data: implications for chemosynthesis and tectonic processes.
Skelton, A., Whitmarsh, R., Arghe, F., Crill, P., Koyi, H.
Constraining the rate and extent of mantle serpentinization from seismic and petrological data: implications for chemosynthesis and tectonic processes.
Abstract: The chemical and physical processes operating during subduction-zone metamorphism can profoundly influence the cycling of elements on Earth. Deep-Earth carbon (C) cycling and mobility in subduction zones has been of particular recent interest to the scientific community. Here, we present textural and geochemical data (CO, Sr isotopes and bulk and in-situ trace element concentrations) for a suite of ophicarbonate rocks (carbonate-bearing serpentinites) metamorphosed over a range of peak pressure-temperature (P-T) conditions together representing a prograde subduction zone P-T path. These rocks, in order of increasing peak P-T conditions, are the Internal Liguride ophicarbonates (from the Bracco unit, N. Apennines), pumpellyite- and blueschist-facies ophicarbonates from the Sestri-Voltaggio zone (W. Ligurian Alps) and the Queyras (W. Alps), respectively, and eclogite-facies ophicarbonates from the Voltri Massif. The Bracco oceanic ophicarbonates retain breccia-like textures associated with their seafloor hydrothermal and sedimentary origins. Their trace element concentrations and ?18OVSMOW (+15.6 to +18.2‰), ?13CVPDB (+1.1 to +2.5‰) and their 87Sr/86Sr (0.7058 to 0.7068), appear to reflect equilibration during Jurassic seawater-rock interactions. Intense shear deformation characterizes the more deeply subducted ophicarbonates, in which prominent calcite recrystallization and carbonation of serpentinite clasts occurred. The isotopic compositions of the pumpellyite-facies ophicarbonates overlap those of their oceanic equivalents whereas the most deformed blueschist-facies sample shows enrichments in radiogenic Sr (87Sr/86Sr?=?0.7075) and depletion in 13C (with ?13C as low as ?2.0‰). These differing textural and geochemical features for the two suites reflect interaction with fluids in closed and open systems, respectively. The higher-P-metamorphosed ophicarbonates show strong shear textures, with coexisting antigorite and dolomite, carbonate veins crosscutting prograde antigorite foliation and, in some cases, relics of magnesite-nodules enclosed in the foliation. These rocks are characterized by lower ?18O (+10.3 to 13.0‰), enrichment in radiogenic Sr (87Sr/86Sr up to 0.7096) and enrichment in incompatible and fluid-mobile element (FME; e.g., As, Sb, Pb). These data seemingly reflect interaction with externally-derived metamorphic fluids and the infiltrating fluids likely were derived from dehydrating serpentinites with hybrid serpentinite-sediment compositions. The interaction between these two lithologies could have occurred prior to or after dehydration of the serpentinites elsewhere. We suggest that decarbonation and dissolution/precipitation processes operating in ancient subduction zones, and resulting in the mobilization of C, are best traced by a combination of detailed field and petrographic observations, C, O and Sr isotope systematics (i.e., 3D isotopes), and FME inventories. Demonstration of such processes is key to advancing our understanding of the influence of subduction zone metamorphism on the mobilization of C in subducting reservoirs and the efficiency of delivery of this C to depths beneath volcanic arcs and into the deeper mantle.
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.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 24.
Canada, Northwest Territories
Deposit - Ekati
Abstract: How would a large open pit mine on caribou range (e.g., the Ekati Diamond Mine in the Bathurst caribou’s summer range) have influenced caribou? A traditional knowledge study on the cumulative impacts on the Bathurst caribou herd qualitatively described how mining activities might have influenced the herd (Mackenzie et al. 2013): caribou migration routes deflected away from the mines probably due to seeing mining activities or hearing the noises; and skinny caribou or abnormal smells and materials in caribou meat, liver, or the hide linings probably related to changes in caribou forage and quality of water and air. In other words, the potential influences of mining operations on caribou were most likely through altering what caribou can see, hear, smell (e.g., dusts and fine particle matter < 2.5 ?m (PM2.5) in the air, and from acidity in the soil), and taste (e.g., dust on foliage, vegetation composition change). Boulanger et al. (2012) estimated the size of a zone of influence (ZOI) of the Ekati-Diavik mining complex in the Bathurst caribou summer range, using caribou presence dataset. They also explored the mechanisms of ZOI using the spatial distribution of the total suspended particles, which was simulated with an atmospheric transport and dispersion model (Rescan, 2006). While these studies have added to our understanding of the possible impacts of mining operations on caribou, knowledge gaps remain. One outstanding gap is the lack of direct measurements about the caribou relevant environmental changes caused by mining operations. For example, exactly from how far away can caribou clearly see the vehicles driving on a mining road, or the buildings and the elevated waste piles in a camp? From how far away might caribou hear the noise caused by mining operations? To what spatial extent had the dusts and PM2.5 from mining operations influenced the tundra ecosystems? And how the dusts and PM2.5 from mining operations might have influenced caribou forage quality? Potentially these questions can be answered by in-situ measurements and satellite remote sensing. For example, studies have showed that it is possible to remotely sense PM2.5 distribution using twice-daily MODIS data at a spatial resolution of 1 km (Lyapustin et al., 2011; Chudnovsky et al., 2013; Hu et al., 2014). The objective of this study is thus to quantitatively measure these changes around the Ekati Diamond Mine, by means of in-situ surveys and satellite remote sensing. We conducted field surveys at more than 100 sites around the Ekati Diamond Mine during August 14-23, 2015, a collaborative effort of the NWT CIMP project entitled “Satellite Monitoring for Assessing Resource Development’s Impact on Bathurst Caribou (SMART)”, and the Dominion Diamond Ekati Corporation. In this presentation, we will report preliminary results and lessons learned from our first year’s study.
Abstract: The origin of diamond-forming carbon in the Earth is unclear [1-3]; sources include subducted organic sediment and primordial mantle carbon. For example, some diamonds contain eclogitic silicate + sufide inclusions and have depleted ?13C (-10 to -30‰), enriched ?15N (+3 to +35‰) values, consistent with subducted crustal material [2-3]. However, some diamonds show mantle-like ?15N (<-5‰) and depleted ?13C values (-10 to -30‰ ) which have been cited as evidence of enstatite chondrite-like primordial C-N sources [1]. The helium isotope composition of mantle rocks are powerful tracers,of Earth’s volatile history because primordial 3He is not recycled back into the mantle. However, there are few He isotope studies of diamond fluids. The 3He/4He of garnetbearing diamondites from the Orapa mine (Botswana) range from 0.1 to 3 Ra [4-5], consistent with a recycled origin. However, our recent work has identified a suite of diamondites with 3He/4He = 0.06 to 8.2 Ra which correlates negatively with ?13C, suggesting that the subduction-related C is associated with mantle 3He/4He ratios. To unravel this complexity we are combining He, C and N isotope analyses in polycrystalline diamond from garnetbearing diamondites from the Orapa mine. These data will also be used to assess the extent to which carbon and nitrogen isotopes are decoupled during diamond-formation [3].
Abstract: The origin of the diamond-forming fluids are routinely addressed with the stable isotopes of carbon and nitrogen, where average ?13C and ?15N values are -5 ± 3‰ and -5 ± 4‰, respectively. Because these values differ from crustal sources the application of C-N stable isotopes are applied as tracers of recycled crustal volatiles into the mantle. Additionally, fluid inclusions in fast-growing diamonds provides a unique opportunity to further examine the origin of diamond-forming fluids using noble gas geochemistry. Here we combine C-N isotopes, N concentrations from the diamond with He isotopes released from trapped fluids by in vacuo crushing of mg-sized polycrystalline diamonds. The samples examined are dominantly eclogitic to websteritic abd originate from Southern Africa. ?13C values range from -4.3 to -22.2 ‰ and ?15N values from -4.9 to +23.2 ‰. These data require a significant contribution of material that is 13Cdepleted and 15N-enriched relative to mantle, akin to altered oceanic crust or deep ocean sediments. 3He/4He ratios range from typical mantle values (8.5 Ra) to those dominated by radiogenic He (< 0.1 Ra). These new data show 3He/4He correlates with 3He concentration, suggesting that the low 3He/4He are, at least in part, the result of ingrowth of radiogenic 4He in He-poor diamonds after their formation. 13C-depleted and 15N-enriched diamonds dominate the population studied here. This indicates that subducted altered oceanic crust is essential for diamondite-formation within the SCLM beneath southern Africa. However, the fluids trapped in the low ?13C diamondites (< -15 ‰) have 3He/4He ratios that indicate an origin in the convective upper asthenospheric mantle. Ergo, helium reveals what carbon and nitrogen cannot. When the carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamondformation, helium isotopes reveal an unambiguos mantle component hidden within strongly 13C-depleted diamond.
Geochemical Perspectives Letters, Vol. 11, pp. 39-43. pdf
Africa, Botswana, South America, French Guiana
deposit - Dachine, Orapa
Abstract: Fluid inclusions trapped in fast-growing diamonds provide a unique opportunity to examine the origin of diamonds, and the conditions under which they formed. Eclogitic to websteritic diamondites from southern Africa show 13C-depletion and 15N-enrichment relative to mantle values (?13C = -4.3 to -22.2 ‰ and ?15N = -4.9 to +23.2 ‰). In contrast the 3He/4He of the trapped fluids have a strong mantle signature, one sample has the highest value so far recorded for African diamonds (8.5 ± 0.4 Ra). We find no evidence for deep mantle He in these diamondites, or indeed in any diamonds from southern Africa. A correlation between 3He/4He ratios and 3He concentration suggests that the low 3He/4He are largely the result of ingrowth of radiogenic 4He in the trapped fluids since diamond formation. The He-C-N isotope systematics can be best described by mixing between fluid released from subducted altered oceanic crust and mantle volatiles. The high 3He/4He of low ?13C diamondites reflects the high 3He concentration in the mantle fluids relative to the slab-derived fluids. The presence of post-crystallisation 4He in the fluids means that all 3He/4He are minima, which in turn implies that the slab-derived carbon has a sedimentary organic origin. In short, although carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamond-formation, helium isotopes reveal an unambiguous mantle component hidden within a strongly 13C-depleted system.
Chemie der Erde, doi.org/10.1016/j ,chemer.2018.09.002 30MB
South America, Brazil
meteorite
Abstract: In the first part of this review of the impact record of South America, we have presented an up-to-date introduction to impact processes and to the criteria to identify/confirm an impact structure and related deposits, as well as a comprehensive examination of Brazilian impact structures. The current paper complements the previous one, by reviewing the impact record of other countries of South America and providing current information on a number of proposed impact structures. Here, we also review those structures that have already been discarded as not being formed by meteorite impact. In addition, current information on impact-related deposits is presented, focusing on impact glasses and tektites known from this continent, as well as on the rare K-Pg boundary occurrences revealed to date and on reports of possible large airbursts. We expect that this article will not only provide systematic and up-to-date information on the subject, but also encourage members of the South American geoscientific community to be aware of the importance of impact cratering and make use of the criteria and tools to identify impact structures and impact deposits, thus potentially contributing to expansion and improvement of the South American impact record.
Abstract: The Tapajós mineral province (TMP), in the Brazilian Amazon Craton, comprises NW-SE Paleoproterozoic insular magmatic arcs accreted to the Carajás Archean Province (CAP). We present new geological and geophysical data pointing toward a different evolutionary model for the TMP. Results obtained from magnetic data indicate that NNW-SSE trending structures occur at shallow crustal levels. Furthermore, an E-W structural framework shows up at 15.4 km depth, in disagreement with the accreted island arc orientation. These E-W structures are associated with north-dipping blocks, reflecting ductile compressive tectonics, similar to the tectonic setting found in the CAP. We interpret these E-W structures of the TMP as the continuity westwards of similar structures from the CAP, under the Paleoproterozoic volcanic rocks of the Uatumã Supergroup. Based on this evidence, we propose that Paleoproterozoic arcs have been formed in an Archean active continental margin, instead of in island arcs. This novel tectonic setting for the TMP has significant implications for the tectonic evolution and the metallogenic potential of the southern portion of the Amazon craton, particularly for Paleoproterozoic magmatic-hydrothermal (epithermal and porphyry) precious and base metal systems.
Abstract: In the first part of this review of the impact record of South America, we have presented an up-to-date introduction to impact processes and to the criteria to identify/confirm an impact structure and related deposits, as well as a comprehensive examination of Brazilian impact structures. The current paper complements the previous one, by reviewing the impact record of other countries of South America and providing current information on a number of proposed impact structures. Here, we also review those structures that have already been discarded as not being formed by meteorite impact. In addition, current information on impact-related deposits is presented, focusing on impact glasses and tektites known from this continent, as well as on the rare K-Pg boundary occurrences revealed to date and on reports of possible large airbursts. We expect that this article will not only provide systematic and up-to-date information on the subject, but also encourage members of the South American geoscientific community to be aware of the importance of impact cratering and make use of the criteria and tools to identify impact structures and impact deposits, thus potentially contributing to expansion and improvement of the South American impact record.
Abstract: The structural depression that occupies the Okavango Basin in southern Africa comprises a depo?centre within the intracratonic Kalahari Basin where sediments of the Cenozoic Kalahari Group have accumulated. The Okavango Basin has been formed due to stretching and subsidence at an area of diffused deformation, southwestwards to the main East African Rift System (EARS). Sediments from two full Kalahari Group sequences, located on opposite sides of the Gumare Fault that forms a major fault within the Okavango Basin, were studied to determine their provenance and chronology. Terrestrial Cosmogenic Nuclide (TCN) 26Al/10Be burial dating was used to constrain a chronostratigraphical framework, and Pb, Sr, and Nd isotopic ratios combined with geochemical and sedimentological analyses were applied to track the source areas of the sediments.Results indicate the following sequence of basin filling: (a) Accumulation between ca. 4-3 Ma during which the currently downthrown (southern) block received a mixture of sediments mostly from the Choma?Kalomo, Ghanzi?Chobe, and Damara terranes, and possibly from the Lufilian Belt and/or Karoo basalts during earlier stages of deposition. Simultaneously, the upthrown (northern) block received sediments from more distant Archean sources in the Zimbabwe and/or Kasai cratons, (b) Hiatus in sedimentation occurred at both sites between ca. 3-2 Ma, (c) Sediments on both sides of the Gumare Fault share a similar source (Angolan Shield) with minor distinct contributions to the downthrown block from the Kasai Craton and local sources input to the upthrown block, and (d) Regional distribution of aeolian sand since at least 1 Ma. The change in source areas is attributed to rearrangements of the drainage systems that were probably linked to vertical crustal movements on the margins of the Okavango Basin. The tectonically induced morphodynamics controlled the landscape evolution of the endorheic basin where vast lakes, wetlands and salt pans have developed through time.
Direct discovery of concealed kimberlites with microbial community fingerprinting.
2018 Yellowknife Geoscience Forum , p. 36. abstract
Canada, Northwest Territories
mineral chemistry
Abstract: Mineral exploration in Canada is becoming increasingly complex as the majority of undiscovered commodities are likely deeply buried beneath significant glacial overburden and bedrock, reducing the effectiveness of many existing tools. The development of innovative exploration protocols and techniques is imperative to the continuation of discovery success. Preliminary experimentation has demonstrated the potential viability of microbial fingerprinting through genetic sequencing to directly identify the projected subcrop of mineralization in addition to the more distal entrained geochemical signatures in till. With the advent of inexpensive modern sequencing technology and big-data techniques, microbiological approaches to exploration are becoming more quantitative, cost effective, and efficient. The integration of microbial community information with soil chemistry, mineralogy and landscape development coupled with geology and geophysics propagates the development of an improved decision process in mineral exploration. Soils over porphyry, kimberlite, and VMS deposits have undergone microbial community profiling. These community-genome derived datasets have been integrated with trace metal chemistry, mineralogy, surface geology and other environmental variables including Eh and pH. Analyses of two kimberlites in the Northwest Territories show significant microbial community shifts that are correlated with subsurface mineralization, with distinctive microbial community profiles present directly above the kimberlite. The relationship between microbial profiles and mineralization leads to the use of microbial fingerprinting as a method for more accurately delineating ore deposits in glacially covered terrain. As databases are developed, there is potential for application as a field based technique, as sequencing technology is progressively developed into portable platforms.
Yellowknife Forum NWTgeoscience.ca, abstract volume p. 42-43.
Canada, Northwest Territories
kimberlite
Abstract: Mineral exploration in northern latitudes is challenging in that undiscovered deposits are likely buried beneath significant glacial overburden. The development of innovative exploration strategies and robust techniques to see through cover is imperative to future discovery success. Microbial communities are sensitive to subtle environmental fluctuations, reflecting these changes on very short timescales. Shifts in microbial community profiles, induced by chemical differences related to geology, are detectable in the surficial environment, and can be used to vector toward discrete geological features. The modernization of genetic sequencing and big-data evaluation allows for efficient and cost-effective microbial characterization of soil profiles, with the potential to see through glacial cover. Results to date have demonstrated the viability of microbial fingerprinting to directly identify the surface projection of kimberlites in addition to entrained geochemical signatures in till. Soils above two kimberlites in the Northwest Territories, have undergone microbial community profiling. These community-genome derived datasets have been integrated with chemistry, mineralogy, surface geology, vegetation type and other environmental variables including Eh and pH. Analyses show significant microbial community shifts, correlated with the presence of kimberlites, with a distinct community response at the species level directly over known deposits. Diversity of soil bacteria is also depressed in the same regions of the microbial community response. The relationship between microbial profiles and buried kimberlites has led to the application of microbial fingerprinting as a method to accurately delineate potential ore deposits in covered terrain. The integration of microbial community information with soil chemistry and landscape development coupled with geology and geophysics significantly improves the drill / no-drill decision process and has proven to be far more accurate than traditional surficial exploration methods. There is high potential for application as a field-based technique as microbial databases for kimberlites in northern regions are refined, and as sequencing technology is progressively developed into portable platforms.
Testing the model of late Archean terrane accretion in southern West Greenland: a comparison of the timing of geological events across the Qarlit nunaat fault.
Precambrian Research, Vol. 116, No.1-2, pp. 57-79.
U Pb Hf isotopic analysis of zircon in lower crustal xenoliths from the Navajo volcanic field: 1.4 Ga mafic magmatism and metamorphism beneath Colorado Plateau.
Contributions to Mineralogy and Petrology, Vol. 151, 3, pp. 313-330.
Abstract: The Nonacho Group comprises six formations of continental clastic rocks that were deposited between 1.91 and 1.83?Ga. The Nonacho Group is part of a broader assemblage of conglomerate and sandstone that was deposited atop the Rae craton in response to the amalgamation of Laurentia and supercontinent Nuna, but the details of its tectonic setting are contentious. This paper documents an outlier of Nonacho Group rocks ?50?km east of the main Nonacho basin. Field observations and LA-ICPMS (laser ablation inductively coupled plasma mass spectrometry) U-Pb detrital zircon geochronology are integrated with previous studies of the main basin to better understand the group’s depositional history, provenance and tectonic setting. The lithology and detrital zircon age spectra of the outlier allow for its correlation to the upper two formations of the Nonacho Group. CA-ID-TIMS (chemical abrasion isotope dilution thermal ionization mass spectrometry) analyses of two fragments of the youngest detrital zircon provide a maximum depositional age of 1901.0?±?0.9?Ma. A felsic volcanic cobble dated at ca. 2.38?Ga provides evidence of volcanism during the Arrowsmith orogeny. Detrital zircon dates recovered from the outlier (ca. 3.4-3.0, 2.7, 2.5-2.3 and 2.0-1.9?Ga) are consistent with derivation from topography of the Taltson and/or Thelon orogens on the western margin of the Rae craton. Taltson-Thelon (2.0 to 1.9?Ga) aged detritus is only abundant in the upper two formations of the Nonacho Group, marking a change in provenance from the lower formations. This change in provenance may have coincided with a period of renewed uplift and the unroofing of Taltson-Thelon plutons. The detrital zircon provenance and depositional age of the Nonacho Group is consistent with models that link its deposition to the Taltson and/or Thelon orogens. However, tectonism associated with the 1.9 to 1.8?Ga Snowbird and Trans-Hudson orogens to the east could also have affected basin formation or the change in provenance from the lower to upper Nonacho Group. This study highlights the importance of CA-ID-TIMS in establishing accurate and precise maximum depositional ages for sedimentary successions.
Simandl, G.J., Paradis, S., Stone, R.S., Fajber, R., Kressall, R.D., Grattan, K., Crozier, J., Simandl, L.J.
Applicablity of handheld X-ray fluroescence spectrometry in the exploration and development of carbonatite related niobium deposits: a case study of the Aley carbonatite, British Columbia, Canada.
Geochemistry: Exploration, Environment, Analysis, Vol. 14, 3, pp. 211-221.
Melilite-type and melilite related compounds: structural variations along the join Sr2a, Bax, MgS2iO7 and high pressure behaviour of the two end members.
Physics and Chemistry of Minerals, Vol. 39, 3, pp.199-211.
Abstract: Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry, and electromagnetic data can identify contemporary melt zones, magma reservoirs, and, or, crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs), and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community.
Abstract: Microcontinents occur outboard of passive margins and stranded in ocean basins. Three-dimensional analogue laboratory experiments of continental rifting demonstrate that microcontinent formation at passive margins requires a combination of preexisting linear weaknesses in the lithosphere and rotational extension. Our results suggest that separation of microcontinents from passive margins occurs during the latest stages of continental breakup, before the onset of seafloor spreading, and that preexisting lithospheric weaknesses are a first-order control on where they form. These findings suggest that microcontinent formation may be restricted to localized regions along passive margins associated with zones of lithospheric weakness, providing a new structural and tectonic framework for the interpretation of microcontinents in the geological record.
Abstract: Alkaline lavas occur globally in subduction-related volcanic arcs. Conventional models for the origin of these lavas typically invoke a multi-stage process in which mantle wedge peridotite, enriched in phlogopite and/or amphibole due to prior metasomatism, partially melts during infiltration by fluids and melts derived from subducted oceanic lithosphere. However, geochemical systematics in the majority of subduction-related alkaline lavas require physical mixing of subducted components and peridotite prior to partial melting. This can be explained by the mélange diapir model, which predicts the generation of arc magmas during advection of buoyant material from the slab-wedge interface into the mantle wedge below arcs. Here we report results from experiments in which natural mélange materials were partially melted at upper mantle conditions to produce alkaline magmas. Partial melts produced in our experiments have trace-element abundance patterns that are typical of alkaline arc lavas, such as enrichment in large ion lithophile elements (LILEs) and depletion in Nb and Ta. These results favor generation of alkaline magmas in the arc and backarc regions of subduction zones by partial melting of mélange materials rather than previously metasomatized peridotite.
Abstract: Empirical and experimental calibration of single element solubility thermometers, such as Zr-in-rutile, Zr-in-titanite, Ti-in-zircon, and Ti-in-quartz, within the past 13 years has greatly expanded our ability to assess the pressure and temperature conditions of individual minerals associated with specific textures in metamorphic rocks. Combined with advances in in situ techniques for analyzing trace concentrations, this has led to an increase in the combined use of single element thermometers, geochronometers, and isotope ratios, often simultaneously, in metamorphic minerals. Here we review the calibration and application of single element thermometers at the pressure and temperature conditions of interest in metamorphic rocks. We discuss to what extent accessory phase equilibrium and trace element equilibrium are attained in metamorphic systems, and the thermodynamic and kinetic framework within which trace element equilibrium is assessed. As an example, we present a comprehensive study of trace element distribution during rutile replacement by titanite in rocks that experienced high-temperature amphibolite-facies overprinting and those that underwent low-temperature blueschist-facies overprinting from a variety of subduction-related terranes worldwide. We find that trace element distributions approach equilibrium partition coefficients in rocks from amphibolite-facies overprinted terranes, whereas trace element distributions do not approach equilibrium in rocks that experienced blueschist-facies overprinting. We caution that single element thermometers that rely upon slow-diffusing high field strength elements should not be applied to rocks equilibrated at <600 °C unless attainment of trace element equilibrium can be demonstrated.
Abstract: The Ampasindava alkaline province consists of a series of circular and elliptical intrusions, lava flows, dyke swarms and plugs of Cenozoic age emplaced into the Mesozoic-Cenozoic sedimentary rocks of the Antsiranana basin (NW Madagascar) and above the crystalline basement. The magmatism in the Ampasindava region is linked to a NW-SE trending extensional tectonic setting. New 40Ar/39Ar age determinations on feldspar separate of alkali granites and basaltic dykes yielded ages of 18.01?±?0.36 Ma and 26?±?7 Ma, respectively. Alkali basalts and basanites, nepheline syenites and phonolites, and silica saturated-to-oversaturated syenites, trachytes, granites and rhyolites are the main outcropping lithologies. These rocks have sodic affinity. The felsic rocks are dominant, and range from peraluminous to peralkaline. The mantle-normalized incompatible element patterns of the mafic lavas match those of Na-alkaline lavas in within-plate rift settings. The patterns are identical in shape and absolute concentrations to those of the Bobaomby (Cap d’Ambre) and Massif d’Ambre primitive volcanic rocks. These geochemical features are broadly compatible with variable degrees of partial melting of incompatible element-enriched mantle sources. The mineralogical and geochemical variations are consistent with fractional crystallization processes involving removal of olivine, feldspar, clinopyroxene, amphibole, Fe-Ti oxides and apatite. Removal of small amount of titanite explains the concave upward lanthanide pattern in the evolved nepheline syenites and phonolites, which are additionally rich in exotic silicates typical of agpaitic magmas (eudialyte, F-disilicates).
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.
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.
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.
Episodic growth of zircon in UHP orthogneisses from the North Dabie Terrane of east central China: implications for crustal architecture of a collisional orogen.
Journal of Metamorphic Geology, In press available,
Ventura Santos, R., Souza de Alvarenga, C.J., Babinski, M., Ramos, M.L.S., Cukrov, N., Fonsec, M.A., Da Norbrega
Carbon isotopes of Mesoproterozoic Neoproterozoic sequences from southern Sao Francisco craton and Aracuai Belt, Brazil: paleogeorgraphic implications.
Journal of South American Earth Sciences, Vol. 18, 1, Dec. 30, pp. 27-39.
A zoned low pressure-high T complex at the level of anatexis- structural and plutonic patterns in metasediments of the Archean Yellowknife Supergroup, near Bathurst Inlet
Yellowknife Forum NWTgeoscience.ca, abstract volume p. 9-10.
Canada, Northwest Territories
eskers
Abstract: Eskers are long ridges of glaciofluvial sand and gravel frequently sampled during mineral exploration campaigns. Sampling of the 700 km long Exeter Lake esker by Chuck Fipke and Stu Blusson in the 1980s led directly to the discovery of the Lac De Gras kimberlite field and establishment of the diamond industry in the Northwest Territories. Despite their significant role in mineral exploration, the details surrounding eskers formation remain controversial (e.g. long-conduit vs. short-conduit models). In my coming research I will use a combination of geomorphological and provenance data to gain insight into the nature of the Exeter Lake esker and the origin of its sediment to help further define the parameters surrounding esker formation and their application in the mining industry. The geomorphology of the esker will be characterized in ArcMap using (1) the new Arctic DEM (2 m resolution), supplemented by (2) aerial imagery (3) GoPro footage of the entire esker collected during a low-level fly-over, (4) ground observations and short foot traverses made at regularly spaced intervals, and (5) locally collected drone footage. Morpho-sedimentary building block elements of the esker system will be identified and interpreted. Esker provenance will be studied using two sample suites. The first suite (112 samples) was collected at coarsely spaced intervals (15-20 km) along the entire length of the esker and contains pared till and esker-ridge samples from both the pebble and finer fractions. These samples will be used to ascertain whether dispersal trains—such as those emanating from the Dubawnt Supergroup—extend the entire length of the esker, considerably overshooting the till dispersal trains from which they were sourced, or whether they are more local in scale. Mud fractions (<63 microns) will be analyzed geochemically; this fraction has never been analyzed previously in similar studies, but could be more indicative of subglacial stream length. Zircon grains from the sand fraction will be analyzed using uranium-lead dating and correlated to diversely aged rock units along the esker system. Finally, the lithology of the pebbles will be analyzed and compared against previously mapped bedrock lithologies along the esker transect. The second suite (62 samples) was collected at closely spaced intervals (300-600 m) from various geomorphological expressions of the greater esker system near the edge of the Lac de Gras kimberlite indicator mineral (KIM) plume, as defined in the KIDD database. KIM concentrations from the samples will be compared with one another, and if the KIM train in the esker considerably overshoots that in the till, a long-conduit model may be more likely. Additionally, by comparing multiple expressions of the esker system any bias in the concentration of KIMs should be detected. Due to the novel approach and large dataset this study has the potential to provide considerable insight into the nature of esker systems and how they are deposited. With this knowledge, mining and exploration companies will be able reassess their esker datasets backed by a scientifically robust exploration model.
Journal of Maps ( Taylor & Francis) on linkedin, Vol. 14, 2, pp. 476-485. pdf
United States, Canada
Glaciation, geomorphology, map
Abstract: A map depicting glacial dispersal trains in North America has been compiled from published sources. It covers the Canadian Shield, the Arctic Islands, the Cordillera and Appalachian mountains, and Phanerozoic sedimentary basins south of the Shield. In total, 140 trains are portrayed, including those emanating from major mineral-deposit types (e.g. gold, base metal, diamondiferous kimberlite, etc.). The map took 10 years of on-and-off work to generate, and it culls data from over 150 years of work by government, industry, and academia. It provides a new tool to help companies find ore deposits in Canada: the trains are generally a better predictor of dispersal distance and direction than striations and streamlined landforms, the data typically depicted on surficial-geology maps, including the Glacial Map of Canada. It also gives new insight into sedimentation patterns and processes beneath ice sheets, a sedimentary environment that, because of its inaccessibility, remains poorly understood and controversial.
Constraints on the current rate of deformation and surface uplift of the Australian continent from a new seismic database and low T thermochronological data.
Australian Journal of Earth Sciences, Vol. 56, 2, pp. 99-110.
Abstract: Continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle. When two continents collide, the mass balance for the continental crust is therefore assumed to be maintained. Here we use estimates of pre-collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the India-Asia collisional system. Using the current best estimates for the timing of the diachronous onset of collision between India and Eurasia, we find that about 50% of the pre-collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at Earth’s surface today—represented by the mass preserved in the thickened crust that makes up the Himalaya, Tibet and much of adjacent Asia, as well as southeast Asian tectonic escape and exported eroded sediments. This implies large-scale subduction of continental crust during the collision, with a mass equivalent to about 15% of the total oceanic crustal subduction flux since 56 million years ago. We suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent-continent collisions is reflected in some ocean crust and ocean island basalt geochemistry. The subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry.
Lithos, doi.org/10.1016/ j.lithos.2020.105889 13p. Pdf
Canada, Northwest Territories
deposit - Diavik A-154
Abstract: Ambient Moho temperatures and lower crustal heat production are surprisingly poorly constrained in cratons. Here we address these problems using 15 lower crustal xenoliths from the Diavik A-154 kimberlite, Slave craton, Canada. Iron?magnesium exchange geothermometry on small biotite and amphibole inclusions in garnet indicates that the Slave craton lower crust was at a temperature of ?500 °C at the time of kimberlite eruption (~55 Ma). The ambient lower crustal temperature was likely lower than 500 °C because the thermometers record the closure temperature of diffusional Fe2+-Mg exchange between touching mineral pairs. New measurements of K, U and Th concentrations in the constituent minerals, together with xenolith modes, allow reconstruction of the heat-producing element (HPE) K, U, and Th budget of the Slave craton lower crust. Metasedimentary granulites have an average heat production of 0.29 ± 0.01 ?W/m3 (n = 3) whereas mafic granulites have an average heat production of 0.13 ± 0.03 ?W/m3 (n = 12). Our new data clearly show that plagioclase abundance in both lithologies has a major influence on overall lower crustal heat production, being an important reservoir of all three HPE. Combining the heat production of mafic and metasedimentary granulites in their observed 80:20 proportions results in an average heat production value for the Slave craton lower crust of 0.16 ± 0.03 ?W/m3. Using these heat production estimates, modeled Moho temperatures beneath Diavik of ~450-470 °C are broadly consistent with maximum lower crustal temperatures indicated by geothermometry. The low HPE contents predicted for cratonic lower crust must result in lower temperatures in the deep crust and mantle lithosphere, and in turn higher estimates for the thickness of mantle lithosphere. This effect becomes larger as the thickness of the low-HPE lower crustal layer increases. In the specific case of the central Slave craton, we find that model estimates of the diamond potential of the mantle lithosphere, as judged by the proportion of lithospheric mantle in the diamond stability field, are not strongly affected by small variations in lower crustal heat production and Moho temperature.
Geochemistry, Geophysics, Geosystems, 10.1029/2021GC009691 55p. Pdf
Africa, South Africa, Lesotho, Europe, Greenland
craton
Abstract: Understanding the rate at which temperature changes with increasing depth (geothermal gradients) within ancient continental crust and its underlying mantle (cratonic lithosphere) is essential for understanding the internal structure of Earth. However, understanding geothermal gradients requires a chemical and physical understanding of deep cratonic lithosphere (up to ?200 km depth) and samples from such depths are only available as fragments hosted in melts that originate there (e.g., kimberlites). This limited sample availability of the cratonic mantle roots has resulted in some properties of this domain, used in geothermal modeling, to be poorly constrained. Here we use samples of cratonic mantle lithosphere to determine one critical and poorly constrained parameter used in modeling geothermal gradients—the heat produced from the radiogenic decay of K, U, and Th to their daughter isotopes. We measure these elements in the samples via in situ laser ablation methods to quantify their potential heat production. Comparing our results to previous estimates of heat production, our new estimates produce differences in the thicknesses of cratonic lithosphere calculated from modeled geothermal gradients by >10 km depending on the chosen lithological model. The results from this study provide an important new data set for constraining heat production in cratonic mantle peridotites.
Abstract: Lateral variations in lithosphere thickness are observed in many continental regions, especially at the boundary between the ancient cratonic core and the adjacent more juvenile lithosphere. In some places, such as the North America craton margin in western Canada and the Sorgenfrei-Tornquist Zone in northern Europe, the transition in lithosphere thickness has a steep gradient (>45°) and it appears to be a long-lived feature (at least 50 Ma). We use thermal-mechanical numerical models to address the dynamics of lithospheric thickness changes on timescales of 100 Ma. Models start with the juxtaposition of 60 km thick lithosphere ("mobile belt") and 160 km thick lithosphere ("craton"). In the reference model, all mantle materials have a damp olivine rheology and a density comparable to primitive mantle. With this configuration, edge-driven mantle convection occurs at the craton boundary, resulting in a lateral smoothing of the thickness transition. The density and rheology of the craton mantle lithosphere are then varied to approximate changes in composition a
