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Sheahan Diamond Literature Technical Reference Compilation 2018
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: The continental basement in the Eastern Mediterranean represents the northern edge of Gondwana, which has been the site of repeated crustal accretion and has subsequently been modified by consecutive rifting events. We investigated the geologic and thermal history of the North Gondwana lower crust by examining the U-Pb-Hf isotope systematics in zircons within 6 mafic granulite xenoliths from Pliocene lava cone in North Israel. The lava cone protrudes through the platform cover that seals the late Neoproterozoic junction between the Arabian-Nubian basement to the South and the Cadomian basement exposed in the Taurides to the North. The mafic granulite xenoliths are composed of plagioclase + orthopyroxene + clinopyroxene ± garnet ± spinel ± secondary amphibole. U-Pb zircon ages from the granulites vary among the different samples with distinct zircon age populations at 400-1200 Ma, 170-350 Ma, and 3.6-4.2 Ma, attesting the lower crust preserves a prolonged thermal and igneous history. While 400-550 Ma U-Pb ages are interpreted to be the result of Pb loss, the wide scatter of zircon grains aged between 550 and 1200 Ma, alongside their diverse ?Hf(t) values (?25-+10), is an extraordinary evidence for the accretion of Neoproterozoic sediments into the North Gondwana arc root lower crust. The U-Pb-Hf signature of these zircons resembles Cadomian sediments of the Tauride block to the north, indicating southward (present coordinates) subduction under North Gondwana and possible accretion of fore-arc sediments to the lower crust through relamination in the latest Neoproterozoic. One xenolith contained metamorphic-shaped zircons aged 170-350 Ma with positive ?Hf values and Hf-TDM of 0.85 Ga interpreted to reflect Paleozoic recycling of the Neoproterozoic juvenile Arabian basement, which we consider to form a major component of the lower crust in the region. An overwhelming cluster of Carboniferous zircons concentrating at 305 Ma with exclusively negative ?Hf values around ?6, was retrieved from three xenoliths. Some of these zircons portrayed igneous textures and shape. While Carboniferous igneous activity is the hallmark of Western Europe's Variscan orogeny, the latter did not affect the southern rifted edge of Neo-Tethys where our xenoliths were retrieved. The Paleozoic age-Hf composition in our xenoliths is therefore interpreted to result from syn-Variscan recycling of Neoproterozoic sedimentary remains in the lower crust, and some degree of melting in a non-orogenic environment. Rather than with horizontal plate motions and orogeny, the Carboniferous zircon ages in the xenoliths appear to coalesce with significant vertical movements that created continental scale unconformities and a broad basin and swell architecture known to develop over the entire North Gondwana margin at that time. The Carboniferous aged zircons in northern Israel lower crustal xenoliths are therefore a unique gauge of the thermal perturbation that accompanied the large-scale mantle dynamics below the then passive North African margin of Gondwana, while Variscan orogenic accretion occurred on the Eurasian margin. These lower crustal granulites xenoliths therefore contain important information with respect to the nature of the lower crust under Israel, with implications on the geodynamic setting during the Cadomian and Variscan cycles.
Abstract: The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5?m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7?±?4.4?Ma and 86.5?±?2.6?Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster. The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections. The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration. Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn?/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.
Abstract: The petrologically unique Udachnaya-East kimberlite (Siberia, Russia) is characterised by unserpentinised and H2O-poor volcaniclastic and coherent units that contain fresh olivine, along with abundant alkali-rich carbonates, chlorides, sulphides and sulphates in the groundmass. These mineralogical and geochemical characteristics have led to two divergent models that advocate different origins. It has been suggested that the unserpentinised units from Udachnaya-East are representative of pristine unaltered kimberlite. Conversely, the alkali-chlorine-sulphur enrichment has been attributed to interactions with crustal materials and/or post-emplacement contamination by brines. The mineralogical and geochemical features and the compositions of melt inclusions in unserpentinised and serpentinised Udachnaya-East kimberlite varieties are compared in this study. Both varieties of kimberlite have similar major, compatible and incompatible trace element concentrations and primitive mantle normalised trace element patterns, groundmass textures and silicate, oxide and sulphide mineral compositions. However, these two kimberlite varieties are distinguished by: (i) the presence of unaltered olivine, abundant Na-K-Cl-S-rich minerals (i.e. chlorides, S-bearing alkali-carbonates, sodalite) and the absence of H2O-rich phases (i.e. serpentine, iowaite (Mg4Fe3+(OH)8OCl•3(H2O)) in unserpentinised samples, and (ii) the absence of alkali- and chlorine-enriched phases in the groundmass and characteristic olivine alteration (i.e. replacement by serpentine and/or iowaite) in serpentinised samples. In addition, melt inclusions hosted in olivine, monticellite, spinel and perovskite from unserpentinised and serpentinised kimberlite contain identical daughter phase assemblages that are dominated by alkali-carbonates, chlorides and sulphates/sulphides. This enrichment in alkalis, chlorine and sulphur in melt inclusions demonstrates that these elements were an intrinsic part of the parental magma. The paucity of alkali-carbonates and chlorides in the groundmass of serpentinised Udachnaya-East kimberlite is attributed to their instability and removal during post-emplacement alteration. All evidence previously used in support of crustal and brine contamination of the Udachnaya-East kimberlite is thoroughly evaluated. We demonstrate that ‘contamination models’ are inconsistent with petrographic, geochemical and melt inclusion data. Our combined data suggest that the Udachnaya-East kimberlite crystallised from an essentially H2O-poor, Si-Na-K-Cl-S-bearing carbonate-rich melt.
Abstract: The abundance and distribution of halogens (F, Cl) are rarely recorded in kimberlites and therefore their petrogenetic significance is poorly constrained. Halogens are usually present in kimberlite rocks in the structure of phlogopite and apatite, but their original concentrations are never fully retained due to the effects of alteration. To provide new constraints on the origin and evolution of halogens in kimberlites and their melts, we present a detailed study of the petrography and geochemistry of the late-Cretaceous Group-I (or archetypal) Roger kimberlite (Ekati cluster, Canada). The studied samples contain abundant anhedral-to-euhedral olivine which is set in a crystalline groundmass of monticellite, phlogopite, apatite, spinel (i.e. magnesian ulvöspinel-magnetite (MUM), Mg-magnetite, pleonaste, Cr-spinel), and perovskite along with abundant secondary alteration phases (i.e. serpentine, garnet (andradite-schlorlomite), amakinite ((Fe2 +, Mg, Mn)(OH)2), calcite). The Roger kimberlite is characterised by the highest recorded F-content (up to 2688 ppm) of the Ekati cluster kimberlites, which is reflected by the preservation of F-rich phases, where bultfonteinite (Ca4(Si2O7)(F, OH)2) and fluorite commonly replace olivine. In order to examine the composition and evolution of the kimberlite melt prior to post-magmatic processes, we studied melt inclusions in olivine, Cr-spinel, monticellite and apatite. Primary multiphase melt inclusions in Cr-spinel, monticellite and apatite and secondary inclusions in olivine are shown to contain a diversity of daughter phases and compositions that are dominated by alkali/alkali-earth (Na, K, Ba, Sr)-enriched Ca-Mg-carbonates ± F, Na-K-chlorides and sulphates, phosphates ± REE, spinel, silicates (e.g. olivine, phlogopite, (clino)humite), and sulphides. Although alkali/alkali-earth- and halogen-bearing phases are abundant in melt inclusions, they are generally absent from the kimberlite groundmass, most likely due to ubiquitous effects of syn- and/or post-magmatic alteration (i.e. serpentinisation). Comparisons between halogens and other trace elements of similar compatibility (i.e. F/Nd and Cl/U) in the Roger kimberlite and their respective estimated primitive mantle abundances show that halogens should be a more significant component in kimberlites than typically measured. We propose that fluorine in the Roger kimberlite was magmatic and was redistributed during hydrothermal alteration by Ca-bearing serpentinising fluids to produce the observed bultfonteinite/fluorite assemblages. Based the compositions and daughter mineral assemblages in primary melt inclusions and reconstructed halogen abundances, we suggest that Cr-spinel, monticellite and apatite crystallised from a variably differentiated Si-P-Cl-F-bearing carbonate melt that was enriched in alkalis/alkali-earths and highly incompatible trace elements.
Abstract: Monticellite is a magmatic and/or deuteric mineral that is often present, but widely varying in concentrations in Group-I (or archetypal) kimberlites. To provide new constraints on the petrogenesis of monticellite and its potential significance to kimberlite melt evolution, we examine the petrography and geochemistry of the minimally altered hypabyssal monticellite-rich Leslie (Canada) and Pipe 1 (Finland) kimberlites. In these kimberlites, monticellite (Mtc) is abundant (25-45 vol%) and can be classified into two distinct morphological types: discrete and intergrown groundmass grains (Mtc-I), and replacement of olivine (Mtc-II). Primary multiphase melt inclusions in monticellite, perovskite and Mg-magnetite contain assemblages dominated by alkali (Na, K, Ba, Sr)-enriched Ca-Mg-carbonates, chlorides, phosphates, spinel, silicates (e.g. olivine, phlogopite) and sulphides. These melt inclusions probably represent snapshots of a variably differentiated kimberlite melt that evolved in-situ towards carbonatitic and silica-poor compositions. Although unconstrained in their concentration, the presence of alkali-carbonates and chlorides in melt inclusions suggests they are a more significant component of the kimberlite melt than commonly recorded by whole-rock analyses. We present petrographic and textural evidence showing that pseudomorphic Mtc-II resulted from an in-situ reaction between olivine and the carbonate component of the kimberlite melt in the decarbonation reactio. This reaction is supported by the preservation of abundant primary inclusions of periclase and to a lesser extent Fe-Mg-oxides in monticellite, perovskite and Mg-magnetite. Based on the preservation of primary periclase inclusions, we infer that periclase also existed in the groundmass, but was subsequently altered to brucite. We suggest that CO2 degassing in the latter stages of kimberlite emplacement into the crust is largely driven by the observed reaction between olivine and the carbonate melt. For this reaction to proceed, CO2 should be removed (i.e. degassed), which will cause further reaction and additional degassing in response to this chemical system change (Le Chatelier's principle). Our study demonstrates that these proposed decarbonation reactions may be a commonly overlooked process in the crystallisation of monticellite and exsolution of CO2, which may in turn contribute to the explosive eruption and brecciation processes that occur during kimberlite magma emplacement and pipe formation.
Ackerman, L., Magna, T., Rapprich, V., Upadhyay, D., Kratky, O., Cejkova, B., Erban, V., Kochergina, Y.V., Hrstka, T.
Contrasting petrogenesis of spatially related carbonatites from Samalpatti and Sevattur, Tamil Nadu, India: insights from trace element and isotopic geochemistry.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 31-33.
India
deposit - Samalpatti, Sevattur
Abstract: The Tamil Nadu region in southern India hosts several carbonatite bodies (e.g., Hogenakal, Samalpatti, Sevattur, Pakkanadu-Mulakkadu) which are closely associated with alkaline silicate rocks such as syenites, pyroxenites or dunites (e.g, Kumar et al., 1998; Schleicher et al., 1998; Srivastava, 1998). This is in contrast to the carbonatite occurrences in north-western India associated with the Deccan Trap basalts (e.g., Amba Dongar) or Proterozoic Newania dolomitic carbonatites. We have studied two, spatially related, Neoproterozoic carbonatite-silico(carbonatite) suites in association with alkaline silicate rocks (e.g., pyroxenite, gabbro) from Sevattur and Samalpatti in terms of petrography, chemistry and radiogenic-stable isotopic compositions in order to provide constraints on their genesis and evolution. In these two bodies, several different carbonatite types have been reported previously with striking differences in their trace element and isotopic compositions (Srivastava, 1998; Viladkar and Subramanian, 1995; Schleicher et al., 1998; Pandit et al., 2002). Collected data for previously poorly studied calcite carbonatites from the Sevattur representing the first carbonatite magmas on this locality, indicate similar geochemical characteristics to those of dolomitic carbonatites, such as high LREE/HREE ratios, very high Sr and Ba contents, large amounts of apatite and magnetite, identical Sr-Nd-C-O isotopic compositions. Thus, they were derived from an enriched mantle source without significant post-emplacement modifications through crustal contamination and hydrothermal overprint, in agreement with previous studies (e.g., Schleicher et al., 1998). Detailed microprobe analyses revealed that high levels of some incompatible elements (e.g., REE, Y, Sr, Ba) cannot be accounted by matrix calcite hosting only significant amounts of SrO (~0.6-1.2 wt.%). On the other hand, abundant micro- to nano-scale exsolution lamellae and/or inclusions of mckelveyite-(Nd) appear to host a significant fraction of LREE in parallel with apatite. Distribution of Sr is most likely influenced also by common but heterogeneously dispersed barite and strontianite. Newly acquired as well as detailed inspection of available geochemical data permits distinguish two different types of carbonatites in Samalpatti: (1) Type I similar to Sevattur carbonatites in terms of mineralogy, trace element and radiogenic-stable isotopic compositions and (2) Type II with remarkably low concentrations of REE and other incompatible trace elements, more radiogenic Sr isotopic compositions and extremely variable C–O isotopic values. The petrogenesis of the Type II seems to be intimately associated with the presence of silicocarbonatites and abundant silicate mineral domains. Instead of liquid immiscible separation from a silicate magma, elevated SiO2 contents observed in silico-carbonatites may have resulted from the interaction of primary carbonatitic melts and crustal rocks prior to and/or during magma emplacement. Arguments for such hypothesis include variable, but radiogenic Sr isotopic compositions correlated with SiO2 and other lithophile elements (e.g., Ti, Y, Zr, REE). Calc-silicate marbles present in the Samalpatti area could represent a possible evolved crustal end member for such process due to the inability of common silicate rocks (pyroxenites, granites, diorites) to comply with radiogenic isotopic constraints. The wide range of C-O isotopic compositions found in Samalpatti carbonatites belong to the highest values ever reported for magmatic carbonates and can be best explained by massive hydrothermal interaction with carbonated fluids. Unusual high-Cr silicocarbonatites were discovered at Samalpatti forming centimetre to decimetre-sized enclaves enclosed in pyroxenites with sharp contacts at hand specimen scale. Detailed microprobe analyses revealed peculiar chemical compositions of the Mgamphibole with predominantly sodic composition embaying and replacing Na-Cr-rich pyroxene (kosmochlor), accompanied by the common presence of Cr-spinel and titanite. Such association have been reported for hydrous metasomatism by Na-rich carbonatitic melts at upper mantle conditions (Ali and Arai, 2013). However, the mineralogy and the mode of occurrence of Samalpatti Mg–-r-rich silicocarbonatites argue against such origin. We explain the petrogenesis of these rocks through the reaction of pyroxenites with enriched mantle-derived alkali-CO2-rich melts, as also evidenced by mantle-like O and Hf isotopic compositions.
Journal of Appled Mathematics and Mechanics, Vol. 81, pp. 206-213.
Russia
deposit - Mir
Abstract: A linear analysis of the stability of the course of serpentization, that is, of the exothermic hydration reaction, due to the flow of water in a kimberlite pipe is carried out, taking both the heat conduction and the convective heat transfer by the fluid saturating the pipe rocks into account. It is shown that two different serpentization processes exist: a homogeneous process and an inhomogeneous process associated with a loss of stability by the homogeneous process and a non-uniform reaction rate distribution. Dimensionless similarity parameters that determine the course of the reaction are proposed. It is shown that convective heat transfer promotes a stabilization of the flow and the formation of a homogeneous serpentinite distribution. Other conditions being equal, an increase in the convective heat flux leads to an increase in the wavelengths of the unstable perturbations and to a decrease in their amplitude. A critical value of the flow rate exists, and, when this is exceeded, instability does not develop and serpentinization takes place under homogeneous conditions.
Abstract: Short- and long-term processes at or close to the subduction plate interface (e.g.,mineral transformations, fluid release, seismicity and more generally deformation) might be more closely related than previously thought. Increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle. This contribution uses rocks recovered (episodically) from subduction zones, together with insights from thermomechanical modelling, to provide a new dynamic vision of the nature, structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones (e.g.,coupling inferred from geophysical monitoring) and fossil ones (e.g.,coupling required to detach and recover subducted slab fragments). Based on critical observations and an exhaustive compilation of worldwide subducted oceanic units (for which the presence near the plate interface, rock types, pressure, temperature, T/P gradients, thickness and timing of detachment can be assessed), the present study demonstrates how long-term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery. Critical assessment of rock T/P characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to ~2.8?GPa. Rock clusters are identified at depths of 30, 5560 and 80?km, with some differences between rock types. Data also reveal a first-order evolution with subduction cooling (in the first ~5?Myr), which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling, after which subduction is lubricated and mostly inhibits rock recovery. This contribution places bounds on the plate interface constitution, regular thickness (<300?m; i.e. where/when there is no detachment), changing geometry and effective viscosity. The concept of ‘coupled thickness' is used here to capture subduction interface dynamics, notably during episodes of strong mechanical coupling, and to link long- and short-term deformation. Mechanical coupling depends on mantle wedge rheology, viscosity contrasts and initial structures (e.g.,heterogeneous lithosphere, existence of décollement horizons, extent of hydration, asperities) but also on boundary conditions (convergence rates, kinematics), and therefore differs for warm and cold subduction settings. Although most present-day subduction zone segments (both along strike and downdip) are likely below the detachment threshold, we propose that the most favorable location for detachment corresponds to the spatial transition between coupled and decoupled areas. Effective strain localization involves dissolution-precipitation and dislocation creep but also possibly brittle fractures and earthquakes, even at intermediate depths.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-06174 12p.
Russia, Siberia
deposit - Mirny
Abstract: Here we present new data from a systematic Sr, Nd, O, C isotope and geochemical study of kimberlites of Devonian age Mirny field that are located in the southernmost part of the Siberian diamondiferous province. Major and trace element compositions of the Mirny field kimberlites show a significant compositional variability both between pipes and within one diatreme. They are enriched in incompatible trace elements with La/Yb ratios in the range of (65-00). Initial Nd isotope ratios calculated back to the time of the Mirny field kimberlite emplacement (t?=?360 ma) are depleted relative to the chondritic uniform reservoir (CHUR) model being 4 up to 6 ?Nd(t) units, suggesting an asthenospheric source for incompatible elements in kimberlites. Initial Sr isotope ratios are significantly variable, being in the range 0.70387-0.70845, indicating a complex source history and a strong influence of post-magmatic alteration. Four samples have almost identical initial Nd and Sr isotope compositions that are similar to the prevalent mantle (PREMA) reservoir. We propose that the source of the proto-kimberlite melt of the Mirny field kimberlites is the same as that for the majority of ocean island basalts (OIB). The source of the Mirny field kimberlites must possess three main features: It should be enriched with incompatible elements, be depleted in the major elements (Si, Al, Fe and Ti) and heavy rare earth elements (REE) and it should retain the asthenospheric Nd isotope composition. A two-stage model of kimberlite melt formation can fulfil those requirements. The intrusion of small bodies of this proto-kimberlite melt into lithospheric mantle forms a veined heterogeneously enriched source through fractional crystallization and metasomatism of adjacent peridotites. Re-melting of this source shortly after it was metasomatically enriched produced the kimberlite melt. The chemistry, mineralogy and diamond grade of each particular kimberlite are strongly dependent on the character of the heterogeneous source part from which they melted and ascended.
Lithos, DOI:10.1016 /j.lithos.2018 .05.027 available 52p.
Russia
deposit - Udachnaya
Abstract: A suite of seventeen unique, large, and fresh eclogite xenoliths from the Udachnaya pipe have been studied for their whole-rock and mineral major- and trace-element compositions. Based on their major-element compositions, the Udachnaya eclogites can be subdivided in two groups: high magnesian (Mg# 68.8-81.9) and low magnesian (Mg# 56.8-59). The two eclogite groups are clearly different in the style of correlation between major elements. Positive correlations of FeO and CaO with MgO are observed in the low-magnesian group, whereas these correlations are negative in the high-magnesian group. In terms of trace element composition, the Udachnaya eclogites are enriched over Primitive Mantle, but comparable to mid-ocean-ridge basalt composition, except for significant enrichment in large-ion lithophile elements (LILE; Rb, Ba, K, Sr). Most of the samples show a positive Eu anomaly, irrespective of group. Reconstructed whole-rock composition from clinopyroxene and garnet modal abundances contains much less incompatible elements (LILE, light rare earth elements, high field strength elements) than measured composition. Approximately 60 to 100% of the middle rare earth elements, Zr, and Hf, and nearly 100% of the heavy rare earth elements, Co, V, and Sc of the whole-rock budget are concentrated in Gar and Cpx. Variations in major element compositions cover a full section of the modern and Archaean oceanic crust, from troctolite, through gabbroic rocks, to basalts. The low-Mg# eclogites could have formed from upper oceanic crust protoliths, being a mixture of basalts and gabbro, whereas the high-Mg# eclogites are originated from gabbro-troctolite section of the lower oceanic crust. Concordant variations of Eu anomaly with the Lu/Sr ratio and the V and Ni contents in the eclogite compositions are in agreement with the fractionation of plagioclase, clinopyroxene, and olivine in their low-pressure precursor rocks. Negative correlations of SiO2 and MgO, and a low Nd/YbNMORB ratio, in the low-Mg# eclogites are in agreement with partial melt loss, but the presence of accessory quartz limits the degree of melting to 13%. Major and trace element compositions suggest that the high-Mg# eclogites, and, consequently, the lower oceanic crust, could not have experienced significant melt loss, and subduction in the Archaean may have been essentially dry, compared to the present day.
Finland Geological Survey, http://tupa.gtk.fi /julkaisu/ specialpaper/ sp_058.pdf
Finland
geophysics
Abstract: In this Special Paper volume, the review of physical properties of ore deposit types or mineralization styles is mainly based on published information, in particular on the key note speeches and presentations that were given at two geosciences conferences: Exploration07 held in Toronto in 2007 (proceedings by Milkereit (ed.) 2007), and the SGA meeting held in Uppsala in 2013 (proceedings by Johnsson et al. (eds.) 2013). Diamonds referred to on p. 13.
Akaogi, M., Kawahara, A., Kojitani, H., Yoshida, K., Anegawa, Y., Ishii, T.
High pressure phase transitions in MgCr2O4 MgSiO4 composition: reactions between olivine and chromite with implications for ultrahigh pressure chromitites.
Abstract: The article reports basic design parameters of open pit mines of ALROSA, as well as criteria and factors that govern the choice of production capacity of an open pit diamond mine under conditions of permafrost. The analytical relations and tables to calculate open pit mine life are presented, and the influence of the rate of the downward advance of an open pit mine on its capacity is demonstrated. The authors formulate key provisions for a paragraph of the national standard of RF enabling systematization of approaches to optimization of open diamond mining parameters.
2018 Yellowknife Geoscience Forum , p. 91-92. abstract
Africa, South Africa
deposit - Roberts Victor
Abstract: It is well known that mechanical disaggregation, such as jaw crushing, can cause irreversible damage to valuable gemstones hosted in crystalline rocks. The SELFRAG Lab device uses electrical pulses at high voltages - typically between 150 and 200 kV - to separate material into individual grains along natural boundaries. The purpose of this research is to assess the viability of the SELFRAG as a tool to disaggregate diamond-bearing eclogites, and to assess if this method preserves grains that would otherwise be damaged through mechanical disaggregation. In order to test the applicability of the SELFRAG to diamond recovery from mechanically strong diamond-bearing lithologies, we studied its effects on a diamondiferous eclogite, RV09, from Roberts Victor mine. The Roberts Victor mine is located in South Africa and is renowned for its unusually high abundance of mantle-derived eclogite xenoliths1. Before the eclogite was disaggregated, we bisected the sample and used a CT scan to determine its constituent minerals and the spatial distribution of diamond. One half of the sample was then placed into the SELFRAG, where it was subjected to ~100 shots of 200 kV electrical discharges that segregated the sample into individual grains of similar sizes. The other half was jaw crushed, using a steel jaw crusher which produced non-uniform composite grains and abundant fine material. The varying sizes and aggregate pieces made it difficult to pick diamonds, and after no diamonds were found, the jaw-crushed portion underwent further disaggregation in the SELFRAG. After exerting the same time and effort picking through both portions of the RV09 sample, ten diamonds were recovered from the electronically disaggregated portion, while no diamonds were found in the conventionally disaggregated sample. The diamonds released from the SELFRAG were then imaged with a scanning electron microscope (SEM) to determine the extent to which the diamonds were damaged. Most of the released diamonds showed no evidence of breakage, but a few showed signs of damage that may have occurred prior to kimberlite eruption. The dramatic disparity between the number of diamonds recovered with the SELFRAG and the lack of diamonds in the jaw crushed portion indicates that electrical disaggregation is a superior method compared to the conventional mechanical comminution technique. There are little to no signs of breakage in the SELFRAG-liberated diamonds, whereas, the damage caused by jaw crushing was extensive enough to produce small fragments not readily visible via optical microscopy. The SELFRAG is a promising alternative to conventional disaggregation and offers a practical solution for lessening damage to valuable stones in rocks such as eclogites and kimberlites.
Abstract: Carbonatites are rare igneous rocks that have a high content of carbonate minerals and nearly no silica. Carbonatitic magmas are derived from carbonated mantle sources but the origin of the carbonates (recycling of surface material or primary mantle source) is still debated. While mafic igneous rocks present a ?44/40Ca around 0.8-1.2‰ normalised to SRM, surface carbonates have ?44/40Ca ~ 0‰. Ca isotopes are therefore well suited to study the origin of Ca in carbonatites. We analysed the Ca isotopic composition of 25 carbonatites from continental and oceanic locations and from different ages (from 2 Ga to present day). The large majority of the carbonatites are isotopically light (?44/40Ca down to 0.07‰) compared to mantle derived rocks. On the other hand, the natrocarbonatite from Oldoinyo Lengai is isotopically heavier (?44/40Ca =0.82‰), similarly to basalts. Three mechanisms can explain the very light isotopic composition of the calciocarbonatites i) A very low degree of partial melting of the mantle may enrich the melt in light isotopes, yet there is no evidence of such large isotopic fractionation during partial melting. ii) The mantle source for the calciocarbonatites is enriched in light Ca likely due to recycling of surface material. iii) aqueous alteration has enriched the calciocarbonatites in the lighter isotopes. On the other hand, the natrocarbonatite from Oldoinyo Lengai have a MORB-like Ca isotopic composition. The difference of ?44/40Ca between natro- and calcio-carbonatite would then suggest that they either have different mantle sources, were formed from different degree of partial melting and/or that aqueous alteration has modified the Ca isotopic composition of calciocarbonatites.
Abstract: Experimental data reveal that Earth’s mantle melts more readily than previously thought, and may have remained mushy until two to three billion years ago.
Physics and Chemistry of Minerals, Vol. 45, 2, pp. 95-131.
Mantle
olivines
Abstract: Elasticity is a key property of materials, not only for predicting volumes and densities of minerals at the pressures and temperatures in the interior of the Earth, but also because it is a major factor in the energetics of structural phase transitions, surface energies, and defects within minerals. Over the 40 years of publication of Physics and Chemistry of Minerals, great progress has been made in the accuracy and precision of the measurements of both volumes and elastic tensors of minerals and in the pressures and temperatures at which the measurements are made. As an illustration of the state of the art, all available single-crystal data that constrain the elastic properties and pressure–volume–temperature equation of state (EoS) of mantle-composition olivine are reviewed. Single-crystal elasticity measurements clearly distinguish the Reuss and Voigt bulk moduli of olivine at all conditions. The consistency of volume and bulk modulus data is tested by fitting them simultaneously. Data collected at ambient pressure and data collected at ambient temperature up to 15 GPa are consistent with a Mie–Grünesien–Debye thermal-pressure EoS in combination with a third-order Birch–Murnaghan (BM) compressional EoS, the parameter V 0 = 43.89 cm3 mol?1, isothermal Reuss bulk modulus KTR,0=126.3(2) GPa, K?TR,0=4.54(6), a Debye temperature ?D=644(9)K, and a Grüneisen parameter ? 0 = 1.044(4), whose volume dependence is described by q = 1.9(2). High-pressure softening of the bulk modulus at room temperature, relative to this EoS, can be fit with a fourth-order BM EoS. However, recent high-P, T Brillouin measurements are incompatible with these EoS and the intrinsic physics implied by it, especially that (?K?TR?T)P>0. We introduce a new parameterisation for isothermal-type EoS that scales both the Reuss isothermal bulk modulus and its pressure derivative at temperature by the volume, KTR(T,P=0)=KTR,0[V0V(T)]?T and K?TR(T,P=0)=K?TR,0[V(T)V0]??, to ensure thermodynamic correctness at low temperatures. This allows the elastic softening implied by the high-P, T Brillouin data for mantle olivine to be fit simultaneously and consistently with the same bulk moduli and pressure derivatives (at room temperature) as the MGD EoS, and with the additional parameters of ? V0 = 2.666(9) × 10?5 K?1, ?E=484(6), ?T = 5.77(8), and ?? = ?3.5(1.1). The effects of the differences between the two EoS on the calculated density, volume, and elastic properties of olivine at mantle conditions and on the calculation of entrapment conditions of olivine inclusions in diamonds are discussed, and approaches to resolve the current uncertainties are proposed.-
Abstract: “Super-deep” diamonds are thought to have a sub-lithospheric origin (i.e., below ~300 km depth) because some of the mineral phases entrapped within them as inclusions are considered to be the products of retrograde transformation from lower-mantle or transition-zone precursors. CaSiO3-walstromite, the most abundant Ca-bearing mineral inclusion found in super-deep diamonds, is believed to derive from CaSiO3-perovskite, which is stable only below ~600 km depth, although its real depth of origin is controversial. The remnant pressure (Pinc) retained by an inclusion, combined with the thermoelastic parameters of the mineral inclusion and the diamond host, allows calculation of the entrapment pressure of the diamond-inclusion pair. Raman spectroscopy, together with X-ray diffraction, is the most commonly used method for measuring the Pinc without damaging the diamond host. In the present study we provide, for the first time, a calibration curve to determine the Pinc of a CaSiO3-walstromite inclusion by means of Raman spectroscopy without breaking the diamond. To do so, we performed high-pressure micro-Raman investigations on a CaSiO3-walstromite crystal under hydrostatic stress conditions within a diamond-anvil cell. We additionally calculated the Raman spectrum of CaSiO3-walstromite by ab initio methods both under hydrostatic and non-hydrostatic stress conditions to avoid misinterpretation of the results caused by the possible presence of deviatoric stresses causing anomalous shift of CaSiO3-walstromite Raman peaks. Last, we applied single-inclusion elastic barometry to estimate the minimum entrapment pressure of a CaSiO3-walstromite inclusion trapped in a natural diamond, which is ~9 GPa (~260 km) at 1800 K. These results suggest that the diamond investigated is certainly sub-lithospheric and endorse the hypothesis that the presence of CaSiO3-walstromite is a strong indication of super-deep origin.
Physics of the Earth and Planetary Interiors, Vol. 269, 1, pp. 112-132.
Mantle
subduction
Abstract: The present study aims at better deciphering the different mechanisms involved in the functioning of the subduction interplate. A 2D thermo-mechanical model is used to simulate a subduction channel, made of oceanic crust, free to evolve. Convergence at constant rate is imposed under a 100 km thick upper plate. Pseudo-brittle and non-Newtonian behaviours are modelled. The influence of the subduction channel strength, parameterized by the difference in activation energy between crust and mantle () is investigated to examine in detail the variations in depth of the subduction plane down-dip extent, . First, simulations show that numerical resolution may be responsible for an artificial and significant shallowing of if the weak crustal layer is not correctly resolved. Second, if the age of the subducting plate is 100 Myr, subduction occurs for any . The stiffer the crust is, that is, the lower is, the shallower is (60 km depth if kJ/mol) and the hotter the fore-arc base is. Conversely, imposing a very weak subduction channel ( J/mol) leads there to an extreme mantle wedge cooling and inhibits mantle melting in wet conditions. Partial kinematic coupling at the fore-arc base occurs if kJ/mol. If the incoming plate is 20 Myr old, subduction can occur under the conditions that the crust is either stiff and denser than the mantle, or weak and buoyant. In the latter condition, cold crust plumes rise from the subduction channel and ascend through the upper lithosphere, triggering (1) partial kinematic coupling under the fore-arc, (2) fore-arc lithosphere cooling, and (3) partial or complete hindrance of wet mantle melting. then ranges from 50 to more than 250 km depth and is time-dependent if crust plumes form. Finally, subduction plane dynamics is intimately linked to the regime of subduction-induced corner flow. Two different intervals of are underlined: 80-120 kJ/mol to reproduce the range of slab surface temperature inferred from geothermometry, and 10-40 kJ/mol to reproduce the shallow hot mantle wedge core inferred from conditions of last equilibration of near-primary arc magmas and seismic tomographies. Therefore, an extra process controlling mantle wedge dynamics is needed to satisfy simultaneously the aforementioned observations. A mantle viscosity reduction, by a factor 4-20, caused by metasomatism in the mantle wedge is proposed. From these results, I conclude that the subduction channel down-dip extent, , should depend on the subduction setting, to be consistent with the observed variability of sub-arc depths of the subducting plate surface.
Abstract: Recent studies conclude that plate tectonics started 3 b.y. ago in the mid Archean. A transition from a "presubduction" regime to modern plate tectonics is said to be marked by changes in trace-element or isotopic ratios, the appearance of eclogitic inclusions in diamonds, or an apparent change in upper crust composition. Behind these arguments is the notion that subduction was intermittent or impossible early in Earth history when the mantle was hotter. If so, a mechanism other than subduction must have created granitoids of Archean continental crust. In the sagduction model, the base of thick oceanic crust converts to eclogite, founders, and melts to generate granitic magma. Here we evaluate two crucial constraints on the sagduction process: to generate granitic magma requires that water and basalt is taken deep into the mantle; thick oceanic crust is internally differentiated into uppermost layers of hydrated basalt and lower mafic-ultramafic cumulates. Our numerical modelling shows that any deformation within thick, differentiated crust is restricted to the lower cumulates that lack ingredients essential to generate granitic magma. Emplacement of hot intrusions heats the lower crust which was hot and anhydrous. We conclude that the sagduction model is flawed. Recent re-evaluation gives temperatures in ambient Archean upper mantle only moderately higher than in modern mantle, which deflates arguments that subduction was impossible in the Archean. We conclude that Archean continental crust was generated in subduction zones and that plate tectonics started in the early Archean.
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: New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites, pyroxenites and eclogites, and also garnet barometer for eclogites and peridotites were developed. They were checked using large experimental data sets for eclogitic (?530) and peridotitic systems (>650). The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermakite method produced by Nimis and Taylor (2000). Cpx barometer was transformed by the substitution of major multiplier for KD by the equations dependent from Al-Na-Fe. Obtained equation in combination with the thermometer of Nimis and Taylor (2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites. These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks. They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals. They are useful for PT estimates for diamond inclusions. The positions of eclogite groups in mantle sections are similar to those determined with new Gar–Cpx barometer produced by C. Beyer et al. (2015). The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle (SCLM) at 3–4 GPa marking pyroxenite eclogites layer. Ca-rich eclogites and especially grospydites in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts. The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5–6 GPa. Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch. The Mg- and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums.
Journal of Asian Earth Sciences, Vol. 157, pp. 218-234.
India
Craton
Abstract: The Pitepani volcanic suite of the Dongargarh Supergroup, central India comprises of a calc-alkaline suite and a tholeiitic suite, respectively. The rare earth element (REE) patterns, mantle normalized plots and relict clinopyroxene chemistry of the Pitepani calc-alkaline suite are akin to high-Mg andesites (HMA) and reveal remarkable similarity to the Cenozoic Setouchi HMA from Japan. The Pitepani HMAs are geochemically correlated with similar rocks in the Kotri-Dongargarh mobile belt (KDMB) and in the mafic dykes of the Bastar Craton. The rationale behind lithogeochemical correlations are that sanukitic HMAs represent fore-arc volcanism over a very limited period of time, under abnormally high temperature conditions and are excellent regional and tectonic time markers. Furthermore, the tholeiitic suites that are temporally and spatially associated with the HMAs in the KDMB and in the mafic dykes of the Bastar Craton are classified into: (a) a continental back-arc suite that are depleted in incompatible elements, and (b) a continental arc suite that are more depleted in incompatible elements, respectively. The HMA suite, the continental back-arc and continental arc suites are lithogeochemically correlated in the KDMB and in the mafic dykes of the Bastar Craton. The three geochemically distinct Neoarchaean magmatic suites are temporally and spatially related to each other and to an active continental margin. The identification of three active continental margin magmatic suites for the first time, provides a robust conceptual framework to unravel the Neoarchaean geodynamic evolution of the Bastar Craton. We propose an active continental margin along the Neoarchaen KDMB with eastward subduction coupled with slab roll back or preferably, ridge-subduction along the Central Indian Tectonic Zone (CITZ) to account for the three distinct magmatic suites and the Neoarchean geodynamic evolution of the Bastar Craton.
Abstract: Cratons represent modern Earth’s thickest, coldest and most depleted lithospheres. Reintroduction of volatiles led to gradual re-oxidation and refertilisation of initially highly refractory and reducing cratonic mantle, enabling the eventual deposition of carbonates and hydrous minerals, which can cause seismic velocity reductions appearing as mid-lithospheric discontinuities (MLDs). Ubiquitous small-volume potassic magmas erupted since at least the Palaeoproterozoic testify to the presence of such metasomes. Attendant rheological weakening and densification led to reworking up to complete loss of deep cratonic roots, i.e. meta-cratonisation. The depths of meta-cratonic lithosphere-asthenosphere boundaries (LABs),concentrated at ~80 to 150 km, strikingly overlap those of MLDs, which may have provided lithospheric weak zones along which the deep root could be removed during continent collision, flat subduction or rifting. Since most cratonic lithospheres are too cold for melt to be presently stable, some MLDs associated with reduced resistivities may point to a role for mantle brines, which bears further investigation. If small volumes of melt are required to generate the rarely observed seismic discontinuities at the depths of intact cratonic LABs (180-260 km), their presence places lower bounds on the amount of solidus-depressing volatiles in the underlying mantle volume, whereas their absence places upper bounds if temperature can be independently constrained.
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.
Abstract: Strontium and Pb isotopic compositions of clinopyroxene (cpx) in selected samples from three well-characterised eclogite suites with oceanic crustal protoliths (Lace/Kaapvaal craton, Orapa/Zimbabwe craton and Koidu/West African craton) were acquired by high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and in situ multicollector-laser ablation-inductively-coupled plasma mass spectrometry (MC-LA-ICPMS). The aims of this study are twofold: (1) assess their utility to obtain formation or resetting age constraints and identify elemental signatures that enhance the chances of successful age dating, and (2) to confirm the veracity and utility of results obtained by novel MC-LA-ICPMS techniques. Strontium-Pb isotope systematics of eclogitic cpx measured in this study are decoupled and may reflect addition of unsupported radiogenic Sr during seawater alteration or interaction with oceanic sediments in subduction mélanges, and/or disturbance due to mantle metasomatism, to which the more incompatible Pb is more susceptible. Despite a complex history, subsets of samples yield meaningful model dates. Clinopyroxene fractions from Lace with high Pb contents (36?ppm), unradiogenic Pb isotopic compositions (206Pb/204Pb?=?13.5713.52) and low 238U/204Pb (1.01.5) give single-stage model Pb dates of 2.902.84?Ga. In contrast, samples from Orapa plot to the right of the Geochron and do not yield meaningful Pb model ages. However, these data do define secondary isochrons that can be modelled to yield minimum age constraints on major events affecting the cratonic lithosphere. Within the uncertainties, the resultant 2.18?±?0.45?Ga age obtained for Koidu eclogites reflect disturbance of the Pb isotope system due to subduction beneath the craton linked to the Eburnean orogeny, while they retained their unradiogenic 87Sr/86Sr (0.7016). Similarly, the age for samples from Orapa (2.20?±?0.54?Ga) is interpreted as an overprint age related to Palaeoproterozoic accretion at the western craton margin. Gabbroic eclogites (Eu/Eu*?>?1) with plagioclase-rich protoliths having low time-integrated Rb/Sr and U/Pb retain the least radiogenic Sr and, in part, Pb. High model ? (9.0 to 9.1) for several eclogites from Lace with elevated LREE, Th and Pb abundances reflects ca. 3.0?Ga addition of a sedimentary component, possibly derived from reworking of a high-? basaltic protocrust, as observed on other cratons. We suggest that sample targeting can be usefully guided by fast-throughput in situ LA-ICPMS techniques, which largely yield results identical to ID-TIMS, albeit at lower precision, and which can further help identify kimberlite contamination in the mineral separates used for solution work.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp.319-342.
Geochimica et Cosmochimica Acta, Vol. 213, 1, pp. 574-592.
Africa, Botswana
deposit - Orapa
Abstract: Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and ?18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy ?18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu?, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like ?18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ?3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
Abstract: The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterization of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, to unravel the timing and nature of mantle metasomatism, and its link to the formation of low-volume melts (e.g. kimberlites) and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognized. The first type comprises lherzolites, harzburgites and dunites, some phlogopite-bearing, which occur from ?100-170?km depth. They form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to c. 150?Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet breakdown, texturally evident by pyroxene-spinel assemblages occupying former coarse grains and compositionally evident by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, heavy rare earth elements) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidizing melts percolated upwards and metasomatized the shallower lithospheric mantle, which is composed of phlogopite-bearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene-breakdown. This is the second type of lithology, which occurs at ?90-110?km depth and is inferred to have highly depleted protoliths. This type is compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. Whereas low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidizing mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590-550?Ma extended to >210?km depth, and suggests loss of ?40?km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval (?90-110?km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton.
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.-
Geochimica et Cosmochimica Acta, Vol. 232, pp. 82-100.
Mantle
geochemistry
Abstract: We have analyzed ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3?Ga) to Paleozoic (404?Ma) rocks in an attempt to document the evolution of the elemental composition and isotopic signature of the atmosphere with time. Doing so, we aimed at understanding how physical and chemical processes acted over geological time to shape the modern atmosphere. Modern atmospheric xenon is enriched in heavy isotopes by 30-40‰ u?1 relative to Solar or Chondritic xenon. Previous studies demonstrated that, 3.3?Ga ago, atmospheric xenon was isotopically fractionated (enriched in the light isotopes) relative to the modern atmosphere, by 12.9?±?1.2 (1?) ‰ u?1, whereas krypton was isotopically identical to modern atmospheric Kr. Details about the specific and progressive isotopic fractionation of Xe during the Archean, originally proposed by Pujol et al. (2011), are now well established by this work. Xe isotope fractionation has evolved from 21‰ u?1 at 3.5?Ga to 12.9‰ u?1 at 3.3?Ga. The current dataset provides some evidence for stabilization of the Xe fractionation between 3.3 and 2.7?Ga. However, further studies will be needed to confirm this observation. After 2.7?Ga, the composition kept evolving and reach the modern-like atmospheric Xe composition at around 2.1?Ga ago. Xenon may be the second atmospheric element, after sulfur, to show a secular isotope evolution during the Archean that ended shortly after the Archean-Proterozoic transition. Fractionation of xenon indicates that xenon escaped from Earth, probably as an ion, and that Xe escape stopped when the atmosphere became oxygen-rich. We speculate that the Xe escape was enabled by a vigorous hydrogen escape on the early anoxic Earth. Organic hazes, scavenging isotopically heavy Xe, could also have played a role in the evolution of atmospheric Xe. For 3.3?Ga-old samples, Ar-N2 correlations are consistent with a partial pressure of nitrogen (pN2) in the Archean atmosphere similar to, or lower than, the modern one, thus requiring other processes than a high pN2 to keep the Earth's surface warm despite a fainter Sun. The nitrogen isotope composition of the atmosphere at 3.3?Ga was already modern-like, attesting to inefficient nitrogen escape to space since that time.
Abstract: The Cuiabá Group is the basal part of the sequence of passive margin sediments that unconformably overly the Amazonian Craton in central Brazil. Despite these rock's importance in understanding Brazil's path in the supercontinent cycle from Rodinia to Gondwana and their potential record of catastrophic glaciation their internal stratigraphy and relationship to other units is still poorly understood. The timing of deposition and source areas for the subunits of the Cuiabá Group sedimentary rocks are investigated here using integrated U-Pb and Sm-Nd isotope data. We sampled in the northern Paraguay Belt, a range that developed in response to the collision between the Amazonian Craton, the Rio Apa Block, the São Francisco Craton and the Paranapanema Block. 1125 detrital zircon LA-ICPMS U-Pb ages were calculated and 22 whole rock samples were used for Sm-Nd isotope analysis. The U-Pb ages range between Archean and Neoproterozoic and the main source is the Sunsás Province. Moving up stratigraphy there is a subtle increase in slightly younger detritus with the youngest grain showing an age of 652?±?5 Ma, found at the top of the sequence. The age spectra are similar across each of the sampled units and when combined with the Sm-Nd data, indicate that the source of the detritus was mostly similar throughout deposition. This is consistent with the analysis here that indicates sedimentation occurred in a passive margin environment on the southern margin of the Amazonian Craton. The maximum depositional age of 652?±?5 Ma along with the age of the overlying cap carbonate of the Mirassol d’Oeste Formation suggests that part of this section of sediments were deposited in the purportedly global ?636 Ma Marinoan glaciation, although we give no sedimentological evidence for glaciation in the study area. Compared to the southern Paraguay Belt where no direct age constraints exist, the glacial epoch could be either Cryogenian or Ediacaran. In addition, available data in the literature indicates a diachronous evolution between the northern and southern arms of the Paraguay Belt in the final stages of deposition and deformation.
Abstract: Using IR-Fourier spectrometry (FTIR) and simultaneous thermal analysis combined with quadrupole mass spectrometry of thermal decomposition products (STA + QMS), olivines and clinopyroxene from xenolites of spinel and garnet lherzolites contained in kimberlites and alkaline basalts were studied to confirm the occurrence of hydrogen and carbon within the structure of the minerals, as well as to specify the forms of H and C. The presence of hydroxyl ions (OH-) and molecules of crystal hydrate water (H2Ocryst) along with CO2, CH, CH2, and CH3 groups was detected, which remained within the structures of mantle minerals up to 1300°C (by the data of both techniques). The total water (OH-and H2Ocryst) was the prevailing component of the C-O-H system.
Russian Journal of Pacific Geology, Vol. 12, 5, pp. 408-418.
Russia, Kamchatka
picrites
Abstract: The mineral and chemical compositions of the layered subvolcanic ultrabasic rocks formed through fluid-silicate (liquid) separation of the ultrabasic magma into high-potassium and low-potassium fractions are characterized by the example of the layered picritic sill from the Late Cretaceous ultrabasic volcanic complex of Eastern Kamchatka. It is determined that the main potassium concentrator in the picrites from the high-potassium layers is a residual volcanic glass containing up to 8-9 wt % K2O, which is unique for ultrabasic melts.
Geochemistry, Geophysics, Geosystems, Vol. 18, 12, pp. 4469-4486.
Mantle
subduction
Abstract: The classical Wilson Cycle concept, describing repeated opening and closing of ocean basins, hypothesizes spontaneous conversion of passive continental margins into subduction zones. This process, however, is impeded by the high strength of passive margins, and it has never occurred in Cenozoic times. Here using thermomechanical models, we show that additional forcing, provided by mantle flow, which is induced by neighboring subduction zones and midmantle slab remnants, can convert a passive margin into a subduction zone. Models suggest that this is a long-term process, thus explaining the lack of Cenozoic examples. We speculate that new subduction zones may form in the next few tens of millions of years along the Argentine passive margin and the U.S. East Coast. Mantle suction force can similarly trigger subduction initiation along large oceanic fracture zones. We propose that new subduction zones will preferentially originate where subduction zones were active in the past, thus explaining the remarkable colocation of subduction zones during at least the last 400 Myr.
Journal of African Earth Sciences, Vol. 147, pp. 374-382.
Africa, Tanzania
deposit - Oldoinyo Lengai
Abstract: The northern Tanzanian sector of the Gregory Rift is an area of an active continental rifting, in which sedimentation processes are strongly affected by volcanism. Due to limited stratigraphic exposure, the volcanic record of the region is rather sparse, and assigning volcanic centres for the individual eruptions is difficult. This study presents new data on the tephrostratigraphy of the sedimentary sequence of Lake Emakat, Empakaai Crater, northern Tanzania. Seven volcanic ash layers are identified and described from a 1.1-m core of lake sediments. Geochemical, mineralogical, petrographic and magnetic analyses show that: (1) all ash layers are products of highly explosive eruptions of melilite-bearing magmas; (2) most of the eruptions originate from a complex magmatic system; (3) all ash horizons are very well preserved in the lake environment; and (4) there are significant fluctuations of the bulk magnetic susceptibility of the lacustrine sediments which is related to microtephra from additional eruptions, the result of detritus, washed from the shore during periods of strong lake level fluctuations or periods of high erosion rates, or simply by the contamination by the material from the ash layers. Based on geochemistry and mineralogy of the seven identified ash layers in Lake Emakat, combined with the eruption ages from ¹?C datings, we can pinpoint Oldoinyo Lengai volcano as the source of these specific layers. The combination of this new data with existing chronological data from Ryner et al. (2007), retrieved from the same core, provides precise ages of the voluminous highly explosive eruptions in this region of East Africa during the Pleistocene-Holocene transition.
Balasubramani, S., Sahoo, P., Bhattacharya, D., Rengarajan, M., Thangavel, S., Bhatt, A.K., Verma, M.B., Nanda, L.K.
A note on anomalous concentration of scandium in the Pakkanadu alkaline complex, Salem District, Tamil Nadu, India.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 46.
India
alkaline rocks
Abstract: Pakkanadu Alkaline complex (PAC) of Neoproterozoic age is located at the southwestern end of Dharmapuri rift/shear zone on the northern part of southern granulitic terrain in Tamil Nadu, India. PAC mainly comprises carbonatite-syenitepyroxenite suite of rocks. Syenite is the predominant rock exposed on the eastern and western part of the explored area with enclaves of pyroxenite and dunite. The carbonatite (sovite) occurs as thin veins/bands and discontinuous lenticular bodies intrusive into highly deformed biotite schist that is considered as the fenitised product of pyroxenite traceable over a strike length of 1.5 km. Petromineralogical study of the biotite schist, pyroxenite containing carbonatite rock and carbonatite indicated presence of monazite, allanite, sphene and betafite as the main radioactive minerals occurring as inclusion within biotite or as discrete mineral grains. Other ore minerals are apatite, thorite, titanite, rutile and barite. Chloritisation, hematitisation, silicification and calcitisation are the main wall rock alteration observed in pyroxenite and syenite. Sub-surface exploration carried out by Atomic Minerals Directorate (AMD) in PAC revealed that biotite schist (n=166) contains anomalously high concentration of Scandium (11-1275 ppm, av.161 ppm), REE (67-58275 ppm, av. 14836 ppm,) and V (5-620 ppm, av. 127 ppm, with carbonatite veins and syenite (n=149) contain scandium (10-462 ppm, av.71 ppm,), REE (18-57510 ppm, av. 4106 ppm) and V (1-285 ppm, av. 48 ppm). In these rocks, LREE (12.5-57670 ppm, av. 9617 ppm, n=315) shows enrichment over HREE (7.1-774 ppm, av. 173 ppm, n=315). The concentration of Scandium (Av. 166 and 71 ppm in biotite schist and syenite respectively) is anomalous as compared to its crustal abundance (22 ppm). Geochemical analyses of the rock indicate that the radioactive biotite schist, pyroxenite containing carbonatite veins generally shows higher Sc and REE concentrations as compared to those of the other rocks (syenite). However, there is no significant correlation between REE and Sc. The higher concentration of scandium in PAC is possibly due to selective partitioning of it into minerals like apatite, pyrochlore, allanite, monazite and other REE bearing phases, apart from its concentration in the ferromagnesian minerals. Scandium rarely concentrates in nature as independent ore mineral. The demand for the metal is very high due to multiple high value commercial uses as an alloy with aluminum, specifically in aerospace and automobile industry, besides, in solid oxide fuel cells (SOFC) in electrical industries. Eight boreholes drilled as part of the preliminary subsurface exploration in PAC, covering an area of 0.05 sq. km, indicated an elevated Scandium content of about 6 times that of the average crustal abundance.
Major, trace element compositions and Nd, Sr and stable Ca isotopic compositions of carbonatites and alkaline silicate rocks of the Amba Dongar carbonatite complex, India: role of mantle mineralogy and subducted carbonates.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 18.
India
deposit - Amba Dongar
Abstract: Carbonatites, with limited spatial but wide temporal occurrences, are unique igneous rocks with more than 50% modal carbonate. Geochemically carbonatites are characterized by high La/Yb(N) and enrichment in the Ba, Sr concentrations and superchondritic whole-rock Nb/Ta (~35) and Zr/Hf ratios (~60). Most of the global carbonatites are associated with the alkaline silicate rocks and their origin is highly debated. To understand the petrogenesis of carbonatites, we present geochemical and isotopic data, including the first ever measurements of Ca stable isotopes in the ~65 Ma old carbonatites and associated silicate rocks occurring in the Amba Dongar carbonatitic complex in Western India We have performed a detail geochemical and isotopic investigation of the carbonatites and associated silicate rocks occurring in the Amba Dongar carbonatitic complex in Western India. The analysed carbonatites are primarily calcitic and ankeritic in nature. The associated silicate rocks can be classified into two groups, the first being highly Krich and alkaline while the second group is tholeiitic in nature. The high La/Yb(N) ratio of carbonatites are suggestive of low degree partial melting while the enrichment in large ion lithophile elements (LILE) of the silicate rocks suggest that these rocks are derived from a LILE enriched upper mantle source. The carbonatites and the associated alkaline silicate rocks from the Amba Dongar carbonatite complex show overlapping Nd-Sr isotopic compositions with the tholeiitic rocks from the Phenai Mata complex, located approximately 16 km NW of Amba Dongar. Variability in ? 44/40Ca in Hawaiian shield stage tholeiites have been interpreted as evidence of subducted ancient marine carbonates, with very low -44/40Ca, into the Hawaiian plume (Huang et al., 2010). Boron isotopic composition of global carbonatites suggests that subducted crustal components contributed to the mantle source of relatively young carbonatites (<300 Ma old) (Hulett et al., 2016), a signature which should potentially be traceable using Ca isotopes. We report -44/40Ca of carbonatites and associated alkaline silicate rocks from the Amba Dongar complex. The samples were analyzed using a 43Ca-48Ca double spike on a Thermo Fischer Triton Plus Thermal Ionization Mass Spectrometer (TIMS) at IISc. ?44/40Ca in these rocks show a significant variation (~0.6 ‰- (w.r.t. SRM 915a), which is much larger than the variation observed in limited analyses of global carbonatites (Amini et al., 2009). Our external reproducibility, estimated from multiple analyses of NIST standards SRM 915a, SRM 915b and seawater (NASS6), is better than 0.1 - (2SD). ?44/40Ca of the ~65 Ma old Amba Dongar carbonatites shows correlations with Nb/Yb, K/Rb as well as with Sr/Nb, Sr/Zr. These variations suggest the role of phlogopite versus amphibole in the mantle source as well sas subducted carbonates in controlling the ?44/40Ca of these carbonatites.
Abstract: MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) xenoliths are transported from the deep-cratonic lithosphere to the Earth's surface by Cretaceous kimberlites emplaced in the Kimberley area of the Kaapvaal Craton. MARID xenoliths have high modal abundances (70-80?vol%) of mica and amphibole, indicating their origin from a hydrous source. The ?18O values (4.7????18O???6.9‰) of phlogopite micas from 14 MARID samples indicate that these minerals are both 18O-enriched and 18O-depleted with respect to the average upper mantle ?18O value of 5.8?±?0.6‰. The range of ?2H values of phlogopites (?83????2H????53‰, n?=?14) of MARID xenoliths are slightly larger than the average mantle ?2H value (?70?±?10‰). The oxygen (?18Ophlogopites-amphibole?=??0.4 and 0.4‰) and hydrogen (?2Hphlogopite-amphibole?=?14 and 36‰) isotopic disequilibrium recorded from two MARID xenoliths suggests the duration of the last isotopic exchange, possibly just before the kimberlite emplacement, between these minerals and metasomatic fluids was too short to reach isotopic equilibrium. Our model calculation indicates that the phlogopites of MARID xenoliths underwent isotopic exchange with fluids of ?18O?=?5.5 to 10‰, ?2H?=??62 to ?90‰. The range of ?18O value of the calculated metasomatic fluids resembles the oxygen isotopic composition of the primary mantle carbonate (~ 6-9‰) suggesting interaction between carbonatic melt and MARID xenoliths in the continental lithospheric mantle. Furthermore, ?18O values of phlogopites together with previously published nitrogen isotope data (?11 ? ?15N ? 9‰; Banerjee et al., 2015) indicates incorporation of inhomogeneously distributed recycled crustal material from subducted crust within their source magma. Therefore, O-H-N isotope data for MARID xenoliths indicates their crystallization from geochemically heterogeneous magma in the upper continental mantle and subsequent metasomatism with mantle fluids.
Russian Geology and Geophysics, Vol. 59, pp. 512-524.
Africa, Australia, South America, India
craton
Abstract: This is a synopsis of available data the on crustal structure and properties of thirteen Archean cratons of Gondwanaland (the cratons of Africa, Australia, Antarctica, South America, and the Indian subcontinent). The data include estimates of surface area, rock age and lithology, Moho depth, thickness of lithosphere and sediments, as well as elevations, all summarized in a table. The cratons differ in size from 0.05 x 106 km2 (Napier craton) to 4 x 106 km2 (Congo craton) and span almost the entire Archean period from 3.8 to 2.5 Ga. Sediments are mostly thin, though reach 7 km in the Congo and West African cratons. Elevations above sea level are from 0 to 2 km; some relatively highland cratons (Kaapvaal, Zimbabwe, and Tanzanian) rise to more than 1 km. On the basis of regional seismic data, the Moho map for cratons has been improved. The Moho diagrams for each craton are constructed. The analysis of the available new data shows that the average Moho depth varies from 33 to 44 km: Pilbara (33 km), Grunehogna (35 km), Sao Francisco (36 km), Yilgarn (37 km), Dharwar (38 km), Tanzanian (39 km), Zimbabwe (39 km), Kaapvaal (40 km), Gawler (40 km), Napier (40 km), West Africa (40 km), Congo (42 km), and Amazon (44 km) cratons. The Moho depth within the cratons is less uniform than it was assumed before: from 28 to 52 km. The new results differ significantly from the earlier inference of a relatively flat Moho geometry beneath Archean cratons. According to the new data, early and middle Archean undeformed crust is characterized by a shallow Moho depth (28-38 km), while late Archean or deformed crust may be as thick as 52 km.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0621-8 16p.
Canada, Northwest Territories
deposit - Kelvin, Faraday
Abstract: The Kennady North Project kimberlites (Northwest Territories of Canada) comprises multiple shallow dipping dykes and several volcaniclastic bodies that have an unusual shallow plunging geometry and complex "pipe" shapes that are termed chonoliths. The detailed exploration of the entire system provides exceptional evidence for subterranean volcanic conduit growth processes. The possible processes leading to the development of the kimberlite bodies are discussed, with emphasis on the importance of the subsurface intrusive system geometry and the local stress tensor. Emplacement into a locally compressive stress regime (i.e. ?1 and ?2 inclined at a low angle to surface) could change the kimberlite emplacement geometries to that observed at Kennady North. Models are proposed for the development of the chonoliths, to emphasize aspects of the growth of kimberlite systems that are not well understood. The conclusions challenge or evolve current emplacement models and should influence kimberlite exploration and resource definition assumptions.
Abstract: The authors propose methods and means to monitor deformation and subsidence of ore crown under mining of open-pit bottom reserves by room-and-pillar system with cemented backfill in Mir Mine, ALROSA. The article describes layout and data of geomechanical monitoring. The mechanism of ore subsidence at the lower boundary of the safety pillar is determined.
Abstract: Placer diamonds from the Proterozoic and Paleoproterozoic terranes of the Guiana Shield in Guyana, have an enigmatic geochemistry and provenance. Diamonds may be derived from kimberlite intrusions, but no outcrops have been identified in the dense tropical terrain. Alternatively, they may be weathered from 1.79 Ga, NE-SW trending mafic dykes of the Avanavero suite or eroded from 1.95 to 1.78 Ga sandstones and conglomerates of the Roraima Formation into recent alluvial river bed deposits [1]. To resolve these uncertainties, we acquired initial samples of 212 placer diamonds from different locations in Guyana for study. Diamonds range in size from 1.1 mm to 1.7 mm with a mean diameter of 1.3±0.2 mm. Diamonds are primarily dodecahedral to cubic, with lesser octahedral and minor macle forms. The diamonds are colourless to brown and most have a green surface skin. Diamond surfaces show diverse textures, including frosting, edge abrasions, network patterns, and ruts. Dissolution features are common and include point bottom trigons, with a diameter of 21±15 ?m. We measured N concentrations using FTIR (measured at 1282 cm-1). Diamonds are Type IaA-IaB with N concentrations of 55 ppm to 210 ppm. Total N ppm vs %NB ratios indicate mantlederived conditions of 1200 ºC and 4.7 GPa. Calculated Shields Parameter shear stresses of 0.0009 dynes/cm2 to 0.0016 dynes/cm2 suggest diamonds could be transported in bedloads derived from medium to very coarse sandstones or coarse- to pebble-sized kimberlitic lithics. Guyana’s diamonds are dissimilar to those from other regions of the Guiana Shield. To further this comparison, we studied 8 diamonds from Eastern Venezuela and Western Colombia on loan from the Smithsonian Museum of Natural History. Compared to Venezuelan and Colombian diamonds, Guyana’s diamonds are dodecahedral, and have a higher degree of dissolution textures, suggestive of higher ƒO2 conditions during kimberlite magma ascent. We will continue to study Guyana’s diamonds using a combination of electron microprobe, ?XRD, and Raman analyses of inclusions. Taken together we hope to infer the provenance of Guyana’s placer diamonds and the petrology of the mantle rocks from which they were derived.
Abstract: Of great importance in the problem of redox evolution of mantle rocks is the reconstruction of scenarios of alteration of Fe0- or Fe3C-bearing rocks by oxidizing mantle metasomatic agents and the evaluation of stability of these phases under the influence of fluids and melts of different compositions. Original results of high-temperature high-pressure experiments (P = 6.3 GPa, T = 13001500°?) in the carbideoxidecarbonate systems (Fe3CSiO2(Mg,Ca)CO3 and Fe3CSiO2Al2O3(Mg,Ca)CO3) are reported. Conditions of formation of mantle silicates with metallic or metalcarbon melt inclusions are determined and their stability in the presence of CO2-fluid representing the potential mantle oxidizing metasomatic agent are estimated. It is established that garnet or orthopyroxene and CO2-fluid are formed in the carbideoxidecarbonate system through decarbonation, with subsequent redox interaction between CO2 and iron carbide. This results in the formation of assemblage of Fe-rich silicates and graphite. Garnet and orthopyroxene contain inclusions of a FeC melt, as well as graphite, fayalite, and ferrosilite. It is experimentally demonstrated that the presence of CO2-fluid in interstices does not affect on the preservation of metallic inclusions, as well as graphite inclusions in silicates. Selective capture of FeC melt inclusions by mantle silicates is one of the potential scenarios for the conservation of metallic iron in mantle domains altered by mantle oxidizing metasomatic agents.
Abstract: Experimental studies in the Fe3C-SiO2-MgO system (P = 6.3 GPa, T = 1100-1500°C, t = 20-40 h) have been carried out. It has been established that carbide-oxide interaction resulted in the formation of Fe-orthopyroxene, graphite, wustite, and cohenite (1100 and 1200°C), as well as a Fe-C-O melt (1300-1500°C). The main processes occurring in the system at 1100 and 1200°C are the oxidation of cohenite, the extraction of carbon from carbide, and the crystallization of metastable graphite, as well as the formation of ferrosilicates. At T ? 1300°C, graphite crystallization and diamond growth occur as a result of the redox interaction of a predominantly metallic melt (Fe-C-O) with oxides and silicates. The carbide-oxide interaction studied can be considered as the basis for modeling a number of carbon-producing processes in the lithospheric mantle at fO2 values near the iron-wustite buffer.
Anais da Academia Brasileira de Ciencas, Vol. 79, pp. 321-332.
South America, Brazil
microdiamonds
Abstract: The origin of diamonds from Serra do Espinhaço in Diamantina region (State of Minas Gerais) and in Chapada Diamantina, Lençóis region (State of Bahia) remains uncertain, even taking into account the ample research carried out during the last decades. The lack of typical satellite minerals in both districts makes a kimberlitic source for these diamonds uncertain. In mid 18th century the occurrence of a metamorphosed igneous rock composed of martite, sericite and tourmaline was described in Diamantina region and named hematitic phyllite, considered by some researchers as a possible diamond source. Similar rocks were found in Lençóis and examined petrographically and their heavy mineral concentration was investigated by means of scanning electron microscopy (SEM). Petrographic analyses indicated an igneous origin for these rocks and SEM analyses showed the discovery of microdiamonds. Geochronological studies using the Ar/Ar technique in muscovites yielded minimum ages of 1515+/-3 Ma, which may correlate with 1710+/-12 Ma from U-Pb method in igneous zircons from the hematitic phyllites. Both rock types also have the same mineral and chemical composition which leads to the conclusion that the intrusive rocks were protolith of the hematitic phyllites. This first discovery of microdiamonds in intrusive rocks opens the possibility of new investigation models for diamond mineralization in Brazilian Proterozoic terrains.
Journal of Volcanology and Geothermal Research, Vol. 358, pp. 262-272.
Africa, Tanzania
carbonatite
Abstract: Hanang volcano is the southern volcano of, the southern area of the east part of the East African Rift (the North Tanzanian Divergence) and represents volcanic activity of the first stage of continental break-up. In this study, we investigate glassy melt inclusions in nepheline phenocrysts to constrain the late stage of Mg-poor nephelinite evolution and the behaviour of volatiles (CO2, H2O, S, F, Cl) during magma storage and ascent during early stage rifting. The melt inclusions have a green silicate glass, a carbonate phase and a shrinkage bubble free of gas phase indicating that carbonatite:silicate (18:82) liquid immiscibility occurred during nephelinite magmatic evolution. The silicate glasses have trachytic composition (Na?+?K/Al?=?1.6-7.2, SiO2?=?54-65.5?wt%) with high CO2 (0.43?wt% CO2), sulfur (0.21-0.92?wt% S) and halogens (0.28-0.84?wt% Cl; 0.35-2.54?wt% F) contents and very low H2O content (<0.1?wt%). The carbonate phase is an anhydrous Ca-Na-K-S carbonate with 33?wt% CaO, 20?wt% Na2O, 3?wt% K2O, and 3?wt% S. The entrapped melt in nepheline corresponds to evolved interstitial CO2-rich phonolitic composition (Na?+?K/Al?=?6.2-6.9) with 6?±?1.5?wt% CO2 at pressure of 800?±?200?MPa after crystallization of cpx (17%), nepheline (40%) garnet (6.5%) and apatite (1.7%) from Mg-rich nephelinitic magma. During ascent, immiscibility in phonolitic melt inclusions leads to Ca-Na carbonate melt with composition within the range of carbonate melt from Oldoinyo Lengai and Kerimasi, in equilibrium with trachytic silicate melt (closed-system, P?
Reviews in Mineralogy & Geochemistry, Vol. 83, Chap. 15, pp. 469-533.
Technology
geochronology
Abstract: Garnet could be the ultimate petrochronometer. Not only can you date it directly (with an accuracy and precision that may surprise some), but it is also a common rock-forming and porphyroblast-forming mineral, with wide ranging—yet thermodynamically well understood—solid solution that provides direct and quantitative petrologic context. While accessory phase petrochronology is based largely upon establishing links to the growth or breakdown of key rock-forming pressure–temperature–composition (P–T–X) indicators (e.g., Rubatto 2002; Williams et al. 2007), garnet is one of those key indicator minerals.
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: The lower plate is the dominant agent in modern convergent margins characterized by active subduction, as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight. This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle. As geological and geochemical data seem inconsistent with the existence of modern-style ridges and arcs in the Archaean, a periodically-destabilized stagnant-lid crust system is proposed instead. Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle, perturbing Earth's heat generation/loss balance, eventually triggering mantle overturns. Archaean basalts were derived from fertile mantle in overturn upwelling zones (OUZOs), which were larger and longer-lived than post-Archaean plumes. Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods, allowing basal crustal cannibalism, garnetiferous crustal restite delamination, and coupled development of continental crust and sub-continental lithospheric mantle. Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB (mid-ocean ridge basalt) mantle. Only after the start of true subduction did sequestration of subducted slabs at the core-mantle boundary lead to the development of the depleted MORB mantle source. During Archaean mantle overturns, pre-existing continents located above OUZOs would be strongly reworked; whereas OUZO-distal continents would drift in response to mantle currents. The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion, imbrication, subcretion and anatexis of unsubductable oceanic lithosphere. As Earth cooled and the background oceanic lithosphere became denser and stiffer, there would be an increasing probability that oceanic crustal segments could founder in an organized way, producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga. Plate tectonics today is constituted of: (1) a continental drift system that started in the Early Archaean, driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons; (2) a subduction-driven system that started near the end of the Archaean.
Earth and Planetary Science Letters, Vol. 502, pp. 166-173.
India
subduction
Abstract: Tectonic plate velocities predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere, and slab-upper plate interface. A range of observations suggest that slabs may be weak, implying a more prominent role for plate interface dissipation than previously thought. The shallow thrust interface is commonly assumed to be weak due to an abundance of fluids and near-lithostatic pore fluid pressures, but little attention has been paid to the influence of the deeper, viscous interface. Here we show that the deep interface viscosity in subduction zones is strongly affected by the relative proportions of sedimentary to mafic rocks that are subducted to depth. Where sediments on the down-going plate are sparse, the deep interface is dominated by mafic lithologies that metamorphose to eclogites, which exhibit viscosities 1-2 orders of magnitude higher than the asthenospheric mantle, and reduce subduction plate speeds. In contrast, where sediments are abundant and subducted to depth, the deep interface viscosity is 1-2 orders of magnitude lower than the asthenospheric mantle, thus allowing significantly faster plate velocities. This correlation between subduction plate speed and deep sediment subduction may help explain dramatic accelerations (or decelerations) in convergence rates, such as the acceleration documented for India-Asia convergence during the mid-Cenozoic.
Abstract: Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising slab agents to melts trapped in the sub-arc mantle. Measurements of sulfur (S) valence state in sub-arc mantle peridotites identify sulfate, both as crystalline anhydrite (CaSO4) and dissolved SO42? in spinel-hosted glass (formerly melt) inclusions. Copper-rich sulfide precipitates in the inclusions and increased Fe3+/?Fe in spinel record a S6+Fe2+ redox coupling during melt percolation through the sub-arc mantle. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and ?34S (+?7 to +?11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between liberated slab components and oxidised arc magmas.
A secular variation of stable isotope record in global carbonatite magma.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p.11.
Global
carbonatites
Abstract: Carbonatites are magmatic rocks, origin of these relates to the involvement of mantle fluid. Thus they provide indirect method to understand the sub-continental upper mantle fluid composition. The first report on carbonatites and the later eruption of the natrocarbonatite paved way for investigating the heterogeneity of the mantle with depth and since then, many other occurrences have been found worldwide, offering suitable samples for probing the mantle. We present record of stable isotopic composition of carbonatites spanning Precambrian, Proterozoic to Phanerozoic to Recent time based on their temporal occurrences and global distribution in the geological record. We consider the various tectonic settings from which carbonatites have been reported, the underlying eruption mechanisms taking into account the tectonic significance of their occurrence and their imprints on surrounding rocks. This account covers carbonatites and associated rocks from different continents with a prime focus on carbon and oxygen isotopes. Carbon and oxygen isotope composition vary significantly within time spans. These variations depend on other factors besides mantle composition i.e. carbonate mineralogy and alteration processes that can cause a shift from original compositions. We envisage the use of stable isotope records to address the secular variation of crustal mixing / contamination process in geological time. Many of these secular variation are abrupt and probably indicate shift in the tectonic forcing - a vital factor responsible for driving the secular trend.
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.
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).
Abstract: Pyrope-almandine garnets (Mg# = 28.3-44.9, Ca# = 15.5-21.3) from a heavy mineral concentrate of diamondiferous kimberlites of the largest diamond deposit, the Yubileinaya pipe, along with kimberlite- like rocks and diamondiferous volcano-sediments of the Laptev Sea coast, have been found to contain polymineral, predominantly acicular inclusions, composed of aggregates of shrilankite (Ti2ZrO6), rutile, ilmenite, clinopyroxene, and apatite. The presence of shrilankite as an inclusion in garnets from assumed garnet-pyroxene rocks of the lower crust, lifted up by diamond-bearing kimberlite, allows it to be considered as an indicator mineral of kimberlite, which expands the possibilities when searching for kimberlite in the Arctic.
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.
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.
Abstract: Interphase partitioning of minor elements was studied experimentally upon partial melting of model pyrolite [1] with addition of 2 wt % H2O, 10, and 15 wt % of multicomponent carbonate at 22-24 GPa and 1300-2200°C. The concentrations of minor elements were analyzed on an Agilent 7500a mass spectrometer. Phase associations included quenched melt (L), bridgmanite (Brd), CaSiO3- perovskite (CaPrv), ringwoodite (Rwd), ferropericlase (Fp), and majoritic garnet (Maj). The sequence of phase assemblages in our runs is consistent to that reported in [2] for melting of anhydrous pyrolite at 24 GPa: Fp+L, Fp+Maj+Brd(Rwd)+L, Fp+Maj+Brd(Rwd)+CaPrv. Most of minor elements, except for Sc, Cr, and Ni, are incompatible for Brd and show slight increase in partitioning coefficients from LREEs to HREEs in the H2O-bearing system. Pyrolite with carbonate is characterized by slightly higher LREE partitioning coefficients. Monovalent elements (Li, K, Cs, Rb), as well as Sr and Pb, are strongly incompatible for Brd in all systems. The similar features are observed for Fp enriched in HREEs and depleted in LREEs; all minor elements show redistribution into Fp with pressure. CaPrv is enriched in LREEs and depleted in HREEs. We applied the lattice strain model [3] for interpretation of the analytical data, which allowed us to study the behavior of minor elements as a function of P-T parameters. Our data and some previous results [4] were used for estimation of the composition of melts in equilibrium with inclusions in diamonds from the transition zone and lower mantle.
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.
Abstract: Flash laser heating in diamond anvil cells has been performed to melt diamond up to 37.5 GPa and 4500K using three different methods and three different starting materials: graphite, glassy carbon and diamond. In these experiments molten diamonds were confirmed by FIB/SEM images of the quenched samples. The melting slope of diamond is strongly negative, in contrast to all theoretical predictions. This is the first direct measurement of diamond melting temperatures at high pressure supporting early predictions based on analogies in the phase behavior of the group IV elements carbon, silicon and germanium. For diamond, these analogies had been dismissed for over 30 years based on theoretical grounds. The results imply that, at very high pressure, diamond, seemingly stable in all static and shock experiments, must be outside its thermodynamic stability field. This could be comparable to its behavior at ambient pressures, where diamond exhibits remarkable stability when heated to several thousand degrees even though the thermodynamically stable form of carbon is graphite.
Doklady Earth Sciences, Vol. 482, 1, pp. 1130-1133.
Russia
deposit - Lake Kimozero
Abstract: Syngenetic and epigenetic ore mineralization was studied in Paleoproterozoic metakimberlites in the area of Kimozero Lake. In the Kimozero structure, redeposited ore mineralization is constrained to fracture and shear zones and consists of Fe-vaesite, Fe-Co-polydymite, millerite, Ni-pyrrhotite, pentlandite, chalcopyrite, Zn-bearing copper, galena, and Ni-pyrite. The composition of this mineralization is analogous to that of syngenetic mineralization in pyroclastic and coherent kimberlite, and its likely source was the kimberlite itself.
Abstract: We report on the quantum yield (?) and decay time (?) measurements at room temperature for the bright red-orange (602 nm) luminescence from new germanium-vacancy (Ge-V) centers in nano- and microcrystalline diamonds synthesized at high pressure and high temperature. The values ? = 3 ± 1% and ? = 6.2±0.2 ns were found. The Stokes shift measured as the energy difference between the maxima of the luminescence and luminescence excitation spectra is negligible. The relative intensity of the zero-phonon line constitutes up to 70% from the total intensity of the luminescence. Results of our ab initio DFT calculations for the ground-state electronic and vibrational structure of (Ge-V)? in diamond are presented and discussed.
European Journal of Mineralogy, Vol. 30, 2, pp. 333-348.
Mantle
pyroxenite
Abstract: Pyroxenites are a diffuse heterogeneity in the upper mantle and represent key lithologies in melting processes and mantle deformation. Mantle peridotites exposed in ultramafic massifs are commonly veined by pyroxenites. The latter experienced the same metamorphic evolution as host peridotite and may develop substantially different phase assemblages in response to the different bulk composition. Although several experimental studies focused on melting relations in pyroxenites, subsolidus phase relations are still poorly known. We provide new experimental constraints on phase stability and mineral chemistry for a natural mantle pyroxenite. Piston-cylinder experiments were conducted from 0.7 to 1.5?GPa, 1100-1250?°C. Al-rich spinel, clinopyroxene, orthopyroxene and olivine are ubiquitous phases within the whole pressure range investigated. At 1100?°C, plagioclase is stable up to 0.9?GPa; anorthite content [An?=?Ca/(Ca?+?Na)] decreases as a function of pressure from 0.70 at 0.7?GPa to 0.61 at 0.9?GPa. Maximum plagioclase modal abundance of 14?wt% forms at 0.7?GPa; this amount is more than twice as experimentally determined at the same P-T conditions in fertile lherzolite (5-6?wt%). At intermediate pressure (1.0-1.4?GPa), modal spinel is almost constant (4-5?wt%). A pyrope-rich garnet is stable at 1.5?GPa and its modal abundance increases from 5 to 10 wt% when temperature decreases from 1230?°C to 1150?°C, from 1230?°C to 1150?°C. The Al content in pyroxenes varies significantly across the plagioclase-out and garnet-in transitions and is not pressure-dependent in the spinel-pyroxenite field. At 1100?°C, the plagioclase-out boundary occurs at comparable pressures in the pyroxenite and in fertile lherzolites. On the contrary, the garnet-in curve is located at significantly lower pressure than for mantle peridotites.
Mineral Processing and Extractive Metallurgy Review, Vol. 39, pp. 136-144.
Africa, Angola
mineral processing
Abstract: Kimberlite is the host rock of diamonds and varies widely in geological and mineralogical features as well as color, processing capability, and dewatering characteristics. This study investigated the dewatering behavior of problematic Angolan kimberlites. The presence of clay minerals in kimberlite causes difficulties in dewatering due to high flocculant demand, poor supernatant clarity, and low settling rates. Identifying critical parameters governing the settling behavior will assist in managing the settling behavior of different kimberlite slurries. The influence of particle size, pH of the kimberlite slurry, cation exchange capacity, exchangeable sodium percentage, and smectite content of the kimberlite on the settling rate were investigated for 18 different African kimberlite samples. The settling rate and slurry bed compaction during natural settling were also measured for the kimberlite slurries. Seventeen different Angolan clay-rich kimberlites and one South African clay-rich kimberlite were tested, and, except for two kimberlites, colloidal stability was experienced during natural settling. The pH values of the kimberlite slurries ranged between 9 and 11, which is similar to the pH band where colloidal stability was found during earlier research. The results indicate that colloidal stable slurries were experienced with kimberlites that had exchangeable sodium percentages as low as 0.7%. The cation exchange capacity of the various kimberlites differentiated more distinctly between colloidal stability and instability. A new model is proposed whereby clay-rich kimberlites with a cation exchange capacity of more than 10cmol/kg will experience colloidal stability if the pH of the solvent solution is within the prescribed pH range of 9-11.
Journal of African Earth Sciences, Vol. 149, pp. 215-234.
Africa, Cameroon
subduction
Abstract: High- to ultrahigh-pressure metamorphic assemblages consisting of garnet-omphacitic clinopyroxene bearing mafic rocks have been identified within the Paleoproterozoic Nyong Group in SW Cameroon, at the northwestern margin of the Archean Congo craton. These rocks were investigated in detail and for the first time evidence for eclogite facies metamorphism at ca 25?kbar and 850?°C is provided. A clockwise P-T path with nearly isothermal decompression (ITD) is deduced from mineral zoning and textural relationships characterized by mineral recrystallization and multi-layered coronitic overgrowths of plagioclase and clinopyroxene surrounding garnet porphyroblasts. These P-T conditions imply a burial depth greater than 90?km, at lower geothermal gradient of ca 10?°C/km. The geochemical signature of ten representative rock samples show that two groups of eclogite facies rocks genetically originate from mostly basaltic and basaltic andesite compositions, with a characteristic upper mantle-derived tholeiitic trend. Moreover, their chondrite and MORB normalized REE and trace element concentrations are characterized by nearly flat REE patterns with very little to no Eu anomaly, (La/Sm)N???1 and Zr/Nb???10, as well as a gradual depletion from LREE to HREE with also very little to no Eu anomaly, but (La/Sm)N < 1, Zr/Nb > 10 and negative anomalies in Th, K, Nb, Ta, Sr, Zr and Ti consistent with mid-ocean ridge basalt (MORB) contaminated by a subduction component or by a crustal component. Previous available geochronological data coupled with our new petrological, mineralogical and geochemical findings clearly indicate that the eclogite facies metabasites from the Eburnean Nyong Group between 2100 and 2000 Ma represent one of the oldest subducted oceanic slab or trace of a suture zone so far recorded within the West Central African Fold Belt (WCAFB). The geodynamic implications of these eclogites suggest a subduction-related process followed by a rapid exhumation of their protoliths, therefore, providing critical information corroborating that plate tectonic processes operated during the Paleoproterozoic.
Abstract: With the withdrawal of Soviet troops from Afghanistan, villagers in the Pani- & shir Valley are Lurning their attention to the emerald riches of the nearby Hindu Kush Mountains. Large, dark green crystals have been found in the hundreds of tunnels and shafts dug there. Teams of miners use explosives and drills to remove the limestone that hosts the emerald-bearing quartz and onkerite veins. The gemological properties of Panjshir emeralds are consisrent with those of emeralds from other localities; chemically, they are most similar to emeralds from the Muzo mine in Colombia. "Nodules," previously reported only in tourmaline and morganite, have been found in Panjshir emeralds as well. Approximntely $1 0 million in emeralds were produced in 1990; future prospects ore excellent.
Abstract: Kimberlite is the host rock of diamonds and varies widely in geological and mineralogical features as well as color, processing capability, and dewatering characteristics. This study investigated the dewatering behavior of problematic Angolan kimberlites. The presence of clay minerals in kimberlite causes difficulties in dewatering due to high flocculant demand, poor supernatant clarity, and low settling rates. Identifying critical parameters governing the settling behavior will assist in managing the settling behavior of different kimberlite slurries. The influence of particle size, pH of the kimberlite slurry, cation exchange capacity, exchangeable sodium percentage, and smectite content of the kimberlite on the settling rate were investigated for 18 different African kimberlite samples. The settling rate and slurry bed compaction during natural settling were also measured for the kimberlite slurries. Seventeen different Angolan clay-rich kimberlites and one South African clay-rich kimberlite were tested, and, except for two kimberlites, colloidal stability was experienced during natural settling. The pH values of the kimberlite slurries ranged between 9 and 11, which is similar to the pH band where colloidal stability was found during earlier research. The results indicate that colloidal stable slurries were experienced with kimberlites that had exchangeable sodium percentages as low as 0.7%. The cation exchange capacity of the various kimberlites differentiated more distinctly between colloidal stability and instability. A new model is proposed whereby clay-rich kimberlites with a cation exchange capacity of more than 10cmol/kg will experience colloidal stability if the pH of the solvent solution is within the prescribed pH range of 9-11.The Trans-Saharan Belt is one of the most important orogenic systems constitutive of the Pan-African cycle, which, at the end of the Neoproterozoic, led to the formation of the Gondwana Supercontinent. It is marked by the opening and closing of oceanic domains, collision of continental blocks and the deformation of thick synorogenic sedimentary basins. It extends from north to south over a distance of 3000?km in Africa, including the Nigerian Shield and the Tuareg Shield as well as their counterparts beneath the Phanerozoic oil-rich North- and South-Saharan sedimentary basins. In this study, we take advantage of potential field methods (magnetism and gravity) to analyze the crustal-scale structures of the Tuareg Shield terranes and to track these Pan-African structures below the sedimentary basins, offering a new, >1000?km extent. The map interpretations are based on the classical potential field transforms and two-dimensional forward modeling. We have identified geophysical units and first-order bounding lineaments essentially defined owing to magnetic and gravimetric anomaly signatures. In particular, we are able to highlight curved terminations, which in the Trans-Saharan context have been still poorly documented. We provide for the first time a rheological map showing a categorization of contrasted basement units from the south of the Tuareg Shield up to the Atlas Belt. These units highlight the contrasted rheological behavior of the Tuareg tectonostratigraphic terranes during (i) the northerly Pan-African tectonic escape characteristic of the Trans-Saharan Belt and (ii) the North Sahara basin development, especially during intraplate reworking tied to the Variscan event. The discovery of a relatively rigid E-W oriented unit to the south of the Atlas system, and on which the escaping Pan-African terranes were blocked, offers a new perspective on the structural framework of the north-Gondwana margin. It will help to understand how occurred the rendezvous of the N-S oriented Pan-African terranes and the E-W oriented Cadomian peri-Gondwanan terranes.
Abstract: Among fancy-color diamonds, natural-color green stones with saturated hues are some of the rarest and most sought after. These diamonds are colored either by simple structural defects produced by radiation exposure or by more complex defects involving nitrogen, hydrogen, or nickel impurities. Most of the world’s current production of fine natural green diamonds comes from South America or Africa. Laboratory irradiation treatments have been used commercially since the late 1940s to create green color in diamond and closely mimic the effects of natural radiation exposure, causing tremendous difficulty in gemological identification. Compounding that problem is a distinct paucity of published information on these diamonds due to their rarity. Four different coloring mechanisms—absorption by GR1 defects due to radiation damage, green luminescence from H3 defects, and absorptions caused by hydrogen- and nickel-related defects—can be identified in green diamonds. Careful microscopic observation, gemological testing, and spectroscopy performed at GIA over the last decade allows an unprecedented characterization of these beautiful natural stones. By leveraging GIA’s vast database of diamond information, we have compiled data representative of tens of thousands of samples to offer a look at natural green diamonds that has never before been possible.
Abstract: The high amount of Fe-rich ferropericlase inclusions found in diamonds of a potential super-deep origin questions the bulk chemical model of the Earth [e.g., 1]. Although this might be due to a biased sampling of the lower mantle, it is worth to further address this discrepancy. A limiting factor of the Fe-content of the Earth´s deep mantle (TZ and lower mantle) is a correlation of the depths of the observed main mantle discontinuities with the (Fe,Mg)SiO4 phase diagram. In particular, the 520 kmdiscontinuity is related to the phase transformation of wadsleyite (assuming Fa10) to ringwoodite. The existing phase diagrams suggest a stability limit of wadsleyite ?Fa40 [e.g., 2,3], which limits the Fe-content of the Earth´s transition zone. Here we report on a discovery of Fe-rich wadsleyite grains (up to Fa56) in the high-pressure silicate melt droplets within Fe,Ni-metal in shock veins of the CB (Bencubbin-like) metal-rich carbonaceous chondrite QC 001 [4], which were identified using HR-EDX, nano-EBSD and TEM. Although the existence of such Fe-rich wadsleyite in shock veins may be due to the kinetic reasons, new theoretical and experimental studies of the stability of (Fe,Mg)SiO4 at high temperature (> 1800 K) are clearly needed. This may have significant impact on the temperature and chemical estimates of the Earth´s transition zone.
Petrology of the White River Diamondiferous Paleoproterozoic intrusive rocks and constraining the timing of destruction of the southern Superior cratonic rocks.
Vancouver Kimberlite Cluster, May 3, 1p. Abstract
Canada, Ontario
Wawa
Abstract: Diamond-bearing kimberlitic rocks have been identified as occurring within the Oskabukuta property,15km west of the town of White River, Northwestern Ontario. These rocks were emplaced within Neo-to-Mesoarchean (2.5 to 3.4 Ga) crystalline rock of the Wawa Subprovince, located within the Superior Province of North America. The emplacement age of the dyke is dated at 1945.3 ± 1.9 Ma (1?) (U-Pb in perovskite). The diamond-bearing, kimberlitic intrusion has been mapped at surface for over a 900 m strike, and is referred to as the Rabbit Foot occurrence. Geothermobarometry of the nearby Proterozoic (2.7 Ga) aged diamondiferous metaconglomerate in Wawa (90 km SE) reported a maximum geothermal gradient range between 39 and 41 mW/m2 corresponding to a minimum lithospheric thickness of the Superior Craton of 190-220 km (Miller et al., 2012). In contrast, the study highlight that younger kimberlite (e.g. ~1.1 Ga Wawa kimberlite) within the Southern Superior record a substantially warmer conductive geotherm (46 mW/m2; Kaminsky et al., 2002) and maximum depth of garnet sampled of 150 km. Miller et al. (2012) interpret the apparent heating of the mantle is likely to have resulted from the Midcontinent Rift, which is broadly coeval with the Wawa kimberlite age. Pressure-Temperature estimates calculated using garnet and clinopyroxene xenocryst mineral compositions extracted from Rabbit Foot Model are consistent with model conductive heat flow of between 38-41 mW-m-2. These data support the interpretation of Miller et al. (2012) and further constrain the presence of a cool and thick Southern Superior keel at 1945 Ma. In fact, several of our garnet compositions support a minimum lithosphere-asthenosphere boundary of 250 km in depth and suggest (along with the presence of diamond) that the Rabbit Foot intrusion transected and sampled a significant portion of depleted and diamond stable lithospheric mantle at ~1945 Ma. A later thermal event, likely related to the Mid-continental rift, has subsequently heated and thinned the Southern Superior Craton, thereby constraining timing of the cessation of diamond fertile sublithospheric mantle in the region.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 253-266.
Mineralogy and Petrology, 10.1007/ s00710-018- 0577-8, 19p.
Africa
metasomatism, subduction, geobarometry
Abstract: The Kaapvaal craton and its underlying mantle is probably one of the best studied Archean entity in the world. Despite that, discussion is still vivid on important aspects. A major debate over the last few decades is the depth of melting that generated the mantle nuclei of cratons. Our new evaluation of melting parameters in peridotite residues shows that the Cr2O3/Al2O3 ratio is the most useful pressure sensitive melting barometer. It irrevocably constrains the pressure of melting (melt separation) to less than 2 GPa with olivine (ol), orthopyroxene (opx) and spinel (sp) as residual phases. Garnet (grt) grows at increasing pressure during lithosphere thickening and subduction via the reaction opx?+?sp ? grt?+?ol. The time of partial melting is constrained by Re-depletion model ages (TRD) mainly to the Archean (Pearson and Wittig 2008). However, only 3% of the ages are older than 3.1 Ga while crustal ages lie mainly between 3.1 to 2.8 Ga for the W- and 3.7 to 2.8 Ga for the E-block. Many TRD-ages are probably falsified by metasomatism and the main partial melting period was older than 3.1 Ga. Also, Nd- and Hf- model ages of peridotitic lithologies from the W-block are 3.2 to 3.6 Ga old. The corresponding very negative ?Nd (?40) and ?Hf values (?65) signal the presence of subducted crustal components in these old mantle portions. Subducted components diversify the mantle in its chemistry and thermal structure. Adjustment towards a stable configuration occurs by fluid transfer, metasomatism, partial melting and heat transfer. Ages of metasomatism from the Lu-Hf isotope system are 3.2 Ga (Lace), 2.9 Ga (Roberts Victor) and 2.62 Ga (Finsch) coinciding with the collision of cratonic blocks, the growth of diamonds, metamorphism of eclogites and of Ventersdoorp magmatism. The cratonic lithosphere was stabilized thermally by the end of the Archean and cooled since then with a rate of 0.07 °C/Ma.
Nature Geoscience, 10.1038/s41561-018-0215-4 pp. 682-687.
Russia, Siberia
subduction
Abstract: Magmatic volatile release to the atmosphere can lead to climatic changes and substantial environmental degradation including the production of acid rain, ocean acidification and ozone depletion, potentially resulting in the collapse of the biosphere. The largest recorded mass extinction in Earth’s history occurred at the end of the Permian, coinciding with the emplacement of the Siberian large igneous province, suggesting that large-scale magmatism is a key driver of global environmental change. However, the source and nature of volatiles in the Siberian large igneous province remain contentious. Here we present halogen compositions of sub-continental lithospheric mantle xenoliths emplaced before and after the eruption of the Siberian flood basalts. We show that the Siberian lithosphere is massively enriched in halogens from the infiltration of subducted seawater-derived volatiles and that a considerable amount (up to 70%) of lithospheric halogens are assimilated into the plume and released to the atmosphere during emplacement. Plume-lithosphere interaction is therefore a key process controlling the volatile content of large igneous provinces and thus the extent of environmental crises, leading to mass extinctions during their emplacement.
Geochemical Perspectives, Vol. 7, no. 2, pp. 117-196. doi: 10.7185/geochempersp.7.2
Mantle
carbon
Abstract: As we struggle to cope with the ongoing buildup of CO2 produced by burning fossil fuels, can we acquire guidance from the geologic record? Although our ability to reconstruct past atmospheric CO2 content reliably is currently confined to the last 800 thousand years, we do have compelling evidence that this greenhouse gas played a key role throughout the Earth’s history. It certainly compensated for the young Sun’s lower luminosity. There is no question that it bailed us out of two snowball episodes or that it led to a brief 5 °C warming at the onset of the Eocene. Less certain is that diminishing atmospheric CO2 content was responsible for the global cooling that began 50 million years ago when the Indian subcontinent collided with Asia. Finally, it colluded with changing seasonality, ocean circulation re-organisation and iron fertilisation to generate the 100 thousand year glacial cycles that dominated the last half-million years.
Abstract: After travelling in Earth’s interior for up to billions of years, recycled material once injected at subduction zones can reach a subridge melting region as pyroxenite dispersed in the host peridotitic mantle. Here we study genetically related crustal basalts and mantle peridotites sampled along an uplifted lithospheric section created at a segment of the Mid-Atlantic Ridge through a time interval of 26 million years. The arrival of low-solidus material into the melting region forces the elemental and isotopic imprint of the residual peridotites and of the basalts to diverge with time. We show that a pyroxenite-bearing source entering the subridge melting region induces undercooling of the host peridotitic mantle, due to subtraction of latent heat by melting of the low-T-solidus pyroxenite. Mantle undercooling, in turn, lowers the thermal boundary layer, leading to a deeper cessation of melting. A consequence is to decrease the total amount of extracted melt, and hence the magmatic crustal thickness. The degree of melting undergone by a homogeneous peridotitic mantle is higher than the degree of melting of the same peridotite but veined by pyroxenites. This effect, thermodynamically predicted for a marble-cake-type peridotite-pyroxenite mixed source, implies incomplete homogenization of recycled material in the convective mantle.
Abstract: After travelling in Earth’s interior for up to billions of years, recycled material once injected at subduction zones can reach a subridge melting region as pyroxenite dispersed in the host peridotitic mantle. Here we study genetically related crustal basalts and mantle peridotites sampled along an uplifted lithospheric section created at a segment of the Mid-Atlantic Ridge through a time interval of 26 million years. The arrival of low-solidus material into the melting region forces the elemental and isotopic imprint of the residual peridotites and of the basalts to diverge with time. We show that a pyroxenite-bearing source entering the subridge melting region induces undercooling of the host peridotitic mantle, due to subtraction of latent heat by melting of the low-T-solidus pyroxenite. Mantle undercooling, in turn, lowers the thermal boundary layer, leading to a deeper cessation of melting. A consequence is to decrease the total amount of extracted melt, and hence the magmatic crustal thickness. The degree of melting undergone by a homogeneous peridotitic mantle is higher than the degree of melting of the same peridotite but veined by pyroxenites. This effect, thermodynamically predicted for a marble-cake-type peridotite–pyroxenite mixed source, implies incomplete homogenization of recycled material in the convective mantle.
Abstract: n this study, we identify a giant circumferential mafic dyke swarm associated with the 135-75 Ma High Arctic Large Igneous Province (HALIP). Previously, a HALIP giant radiating mafic dyke swarm, with portions scattered across the Canadian high Arctic islands, northern Greenland, Svalbard and Franz Josef Land, was recognized in a pre-drift plate tectonic reconstruction of the Arctic region. The radiating swarm has been interpreted to focus above a mantle plume responsible for HALIP magmatism. The newly-recognized HALIP giant circumferential swarm has a centre that is near the focus of the HALIP radiating system, and hence, is likely related to the HALIP plume. Elements of the circumferential swarm are located in each of the four regions where the radiating system is found. The circumferential swarm has a quasi-circular or slightly elliptical geometry, an outer diameter of ~1600 km and an arc of ~220°. It is one of the largest giant circumferential dyke swarms recognized on Earth, and could be linked to the outer edge of the flattening plume head. It is also the first such swarm to have been identified by means of a plate tectonic reconstruction. Although giant circumferential dyke swarms appear to be relatively rare on Earth, possible analogues are common on Venus and are also found on Mars. On Venus giant circular or elliptical tectono-magmatic features, termed coronae, are characterized by an annulus of graben or fissures and prominent topography. Some coronae include a radiating graben-fissure system. Both radiating and circumferential graben may be underlain by dykes. If so, coronae could be analogues for terrestrial giant circumferential dyke swarms such as observed in the case of the HALIP.
Earth and Planetary Science Letters, Vol. 498, pp. 77-87.
Mantle
geophysics - seismic
Abstract: Earth's transition zone at depths between 410 km and 660 km plays a key role in Earth's deep water cycle since large amounts of hydrogen can be stored in the nominally anhydrous minerals wadsleyite and ringwoodite, . Previous mineral physics experiments on iron-free wadsleyite proposed low seismic velocities as an indicative feature for hydration in the transition zone. Here we report simultaneous sound wave velocity and density measurements on iron-bearing wadsleyite single crystals with 0.24 wt-% . By comparison with earlier studies, we show that pressure suppresses the velocity reduction caused by higher degrees of hydration in iron-bearing wadsleyite, ultimately leading to a velocity cross-over for both P-waves and S-waves. Modeling based on our experimental results shows that wave speed variations within the transition zone as well as velocity jumps at the 410-km seismic discontinuity, both of which have been used in previous work to detect mantle hydration, are poor water sensors. Instead, the impedance contrast across the 410-km seismic discontinuity that is reduced in the presence of water can serve as a more robust indicator for hydrated parts of the transition zone.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 425-
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 169-190.
Bulbuc, K.M., Galarneau, M., Stachel, T., Stern, R.A., Kong, J., Chinn, I.
Contrasting growth conditions for sulphide-and garnet-included diamonds from the Victor mine ( Ontario).
2018 Yellowknife Geoscience Forum , p. 97-98. abstract
Canada, Ontario, Attawapiskat
deposit - Victor
Abstract: The Victor Diamond Mine, located in the Attawapiskat kimberlite field (Superior Craton), is known for its exceptional diamond quality. Here we study the chemical environment of formation of Victor diamonds. We imaged eight sulphide-included diamond plates from Victor using cathodoluminescence (CL). Then, along core-rim transects, we measured nitrogen content and aggregation state utilizing Fourier Transform Infrared (FTIR) spectroscopy, and the stable isotope compositions of carbon (?13C) and nitrogen (?15N), using a multi-collector ion microprobe (MC-SIMS). We compare the internal growth features and chemical characteristics of these sulphide inclusion-bearing diamonds with similar data on garnet inclusion-bearing diamonds from Victor (BSc thesis Galarneau). Using this information, possible fractionation processes during diamond precipitation are considered and inferences on the speciation of the diamond forming fluid(s) are explored. Sulphide inclusion-bearing diamonds show much greater overall complexity in their internal growth features than garnet inclusion-bearing diamonds. Two of the sulphide-included samples have cores that represent an older generation of diamond growth. Compared to garnet inclusion-bearing diamonds, the sulphide-included diamonds show very little intra-sample variation in both carbon and nitrogen isotopic composition; the inter-sample variations in carbon isotopic composition, however, are higher than in garnet included diamonds. For sulphide-included diamonds, ?13C ranges from -3.4 to -17.5 and ?15N ranges from -0.2 to -9.2. Garnet inclusion-bearing diamonds showed ?13C values ranging from -4.6 to -6.0 and ?15N ranging from -2.8 to -10.8. The observation of some 13C depleted samples indicates that, unlike the lherzolitic garnet inclusion-bearing diamonds, the sulphide inclusion-bearing diamonds are likely both peridotitic and eclogitic in origin. The total range in N content across sulphide inclusion-bearing diamonds was 2 to 981 at ppm, similar to the garnet-included samples with a range of 5 to 944 at ppm. The very limited variations in carbon and nitrogen isotopic signatures across growth layers indicate that sulphide-included Victor diamonds grew at comparatively high fluid:rock ratios. This is contrasted by the garnet inclusion-bearing diamonds that commonly show the effects of Rayleigh fractionation and hence grew under fluid-limited conditions.
Abstract: Two recent co-ordinated research programs - the SAMPLE (South Atlantic Margin Processes and Links with onshore Evolution) program of the German Science Foundation and the French Topo-Africa program - have focused attention on the interaction of the lithosphere with sublithospheric processes. With a main thrust on the West-Gondwana break up and the subsequent post-rift evolution of the South Atlantic passive margins and their hinterlands, SAMPLE and Topo-Africa made concerted efforts to advance models and observations of vertical motions (MoveOn) in the South Atlantic region as a probe into mantle convection/lithosphere interaction. In this special issue of Gondwana Research we assemble a set of contributions that stem from these programs aimed to gain insights on rifting in a geodynamic context with a particular focus on models and observations of the vertical motions of the lithosphere induced by mantle flow. Anderson (1982) suggested that breakup of the supercontinent Gondwana owed to forces in the sublithospheric mantle. However, despite much progress in mantle flow modeling (see Zhong and Liu, 2016 for a recent review), linking mantle convection forces and motion of the lithosphere in quantitative terms has remained elusive. It is generally accepted that plate tectonics is a surface expression of mantle convection and that mantle flow drives horizontal plate motion (Davies, 1999). However, plate tectonic motion reflects a balance of poorly known sublithospheric forces related to mantle flow, and of shallow plate-boundary forces (see Iaffaldano and Bunge, 2015 for a recent review). The latter involve topographic loads from mountain belts and fault friction along convergent plate boundaries (Iaffaldano and Bunge, 2009). Rates of change of plate velocities connect to changes in orogenic topography (Iaffaldano et al., 2006; Austermann and Iaffaldano, 2013) or plate boundary strength (Iaffaldano, 2012), making it possible to reduce some uncertainty on plate boundary forces from the analysis of plate motion changes. But the superposition of sublithospheric forces and shallow plate-boundary forces inhibits interpretations of horizontal plate motions solely in terms of mantle flow related forces. It is also believed that substantial vertical deflections of the earth's surface are induced by viscous stresses from the mantle (e.g., Pekeris, 1935). Such deflections were recognized early on in the sedimentary record through unconformities and missing sections (e.g., Stille, 1919, 1924). Termed ‘Dynamic Topography’ by Hager et al. (1985) > 30 years ago, this topic has received much attention lately (see Braun, 2010 for a recent review). The essential role of dynamic topography in dynamic earth models is well understood, because the mass anomalies associated with surface deflections yield gravity anomalies of comparable amplitude to the flow inducing mantle density variations. Therefore, Geoid interpretations have long been performed with dynamic earth models that account for dynamic topography as well as mantle density heterogeneity (e.g., Ricard et al., 1984; Richards and Hager, 1984; Forte and Mitrovica, 2001). The dynamic topography response of earth models to internal loads (e.g., hot rising plumes or cold sinking slabs) is commonly expressed through kernels (see Colli et al., 2016, for a recent review). They imply that the earth's surface sustains deflections on the order of ± 1 km. For a plume rising through a uniform viscosity mantle the kernels predict the deflections to grow continuously during plume ascend. This is borne out in laboratory models of isoviscous mantle flow (Griffith et al., 1989). However, in the presence of a weak upper mantle much of the surface deflection develops in the final phase of the plume ascend, in a time span of a few million years (Myrs) associated with vertical transit of the plume through the low viscosity upper mantle (Fig. 1). This makes rapid surface uplift events geodynamically plausible.
Abstract: Natural diamonds contain mineral and fluid inclusions that record diamond growth conditions. Replicating the growth of inclusion-bearing diamonds in a laboratory is therefore a novel diagnostic tool to constrain the conditions of diamond formation in Earth’s lithosphere. By determining the carbon isotopic fractionation during diamond growth in fluids or melts, our laboratory experiments revealed that lithospheric monocrystalline and fibrous and coated diamonds grow similarly from redox reactions at isotopic equilibrium in water and carbonate-rich fluids or melts, and not from native carbon. These new results explain why most of the lithospheric diamonds are characterized by a common carbon isotopic fingerprint, inherited from their common parent fluids and not from the mantle assemblage.
Bulk organic geochemistry and U-Pb zircon geochronology of the Wombat sedimentary fill.
2018 Yellowknife Geoscience Forum , p. 98-99. abstract
Canada, Northwest Territories
deposit - Wombat
Abstract: The Wombat locality (64.73°N, 110.59°W) is a diamondiferous kimberlite in the Lac de Gras kimberlite field of Northwest Territories. Two drill cores, CH 93-29 and DDH 0-005, intersect the Wombat crater facies and include 195 m of well preserved, undisturbed lake sediment fill. Bulk sediment elemental analysis, C isotope composition, and Rock-Eval pyrolysis, together with inferences from microfossils, are used to characterize conditions of sedimentation and paleoenvironment in the maar lake. Bulk sediment C/N, hydrogen index (HI), and ?13C indicate material derived from C3 land plants dominates the sedimentary organic matter, with a minor algal contribution. The ?13C values range from -25.3 ‰ to -30.2 ‰ (average -26.6 ‰) and are typical for C3 land plants, with fluctuations in ?13C likely related to shifts in the proportions of land-derived material and algal organic matter. An overall trend of higher ?13C towards the top of the core suggests increasing autochthonous organic matter production. 18 samples analyzed by Rock-Eval pyrolysis all plot in the Type III kerogen field for HI vs. Tmax,with average Tmax values ~425 °C indicative of the low thermal maturity of organic matter. Total organic carbon (TOC) averages 3.6 wt.% and average total carbonate content is 14.1 wt.%, indicating bottom water anoxia and substantial carbonate input from weathering of overlying carbonate cover rocks, respectively. Together with well-preserved freshwater microfossils (e.g. diatoms, chrysophytes, synurophytes), the results indicate deposition in a non-marine setting. The age of the Wombat maar lake sediments is determined using MC-LA-ICP-MS U-Pb zircon geochronology from two distal rhyolitic tephra beds found in the core DDH 0-005, yielding a date of 82.97±0.60 Ma (MSWD = 1.7, n=18 of 33 grains analyzed). This minimum age suggests that Wombat kimberlite pipe emplacement occurred during the Late Cretaceous, with sedimentation in the maar beginning shortly thereafter. Though our geochronology is preliminary at this point, our findings from the Wombat pipe post-eruptive lake sediment fill provide direct evidence for a non-marine environment in the Lac De Gras area during the Late Cretaceous. Furthermore, microfossils in the Wombat pipe sediment fill likely include the oldest-known occurrence of freshwater diatoms.
Mineralogy and Petrology, 10.1007/s00710 -018-0599-2, 14p.
Canada, Northwest Territories
deposit - Diavik, Ekati
Abstract: Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.
Gems & Gemology Lab Notes, Vol. 53, 4, pp. 479-481.
Canada, Northwest Territories
diamond notable - Foxfire
Abstract: The largest gem-quality rough diamond found in Canada, reported earlier in Gems & Gemology (Summer 2016 GNI, pp. 188-189), has revealed remarkable responses to excitation with long- and mid-wave UV light. This 187.63 ct diamond (figure 1) was extracted from the Diavik mine in the Canadian Arctic in the spring of 2015. Aptly named for the aurora borealis, the “Foxfire” displays unusual fluorescence and phosphorescence behavior upon exposure to ultraviolet light. As previously reported, this type Ia diamond has a high concentration of nitrogen impurities, a weak hydrogen-related absorption at 3107 cm-1, and typical "cape" absorption lines.
Abstract: Geologists have a new window onto the early Earth: zircon crystals from South Africa that could be as much as 4.1 billion years old. Ancient crystals of zircon - a durable mineral found in rock that has been squeezed and heated - from Western Australia have revealed some of the planet’s early secrets, such as clues to the chemistry of its primordial crust. But researchers have had little in the way of other records for this period of Earth’s infancy. Now, Benjamin Byerly at Louisiana State University in Baton Rouge and his colleagues report their discovery of a second rich trove of zircons. The crystals lie east of Pretoria, in a rock formation known as the Barberton greenstone belt. The African zircons are important because they have been subject to less heating and squeezing than the Australian samples. As a result, the African crystals may have richer stories to tell about the chemistry of Earth’s early years.
Abstract: To date, the eponymous color-changing behavior of chameleon diamonds lacks an explanation in terms of an identified diamond defect structure or process. Well known, however, is that this color-change is driven by the influence of both light and heat. In this paper, we present observations of how luminescence emission in chameleon diamonds responds to temperature changes and optical pumping. Fluorescence, phosphorescence, and thermoluminescence experiments on a suite of natural chameleon diamonds reveal that a specific emission band, peaking near 550 nm, may be stimulated by several different mechanisms. We have observed thermal quenching of the 550 nm emission band with an activation energy of 0.135 eV. The 550 nm band is also observed in phosphorescence and thermoluminescence. Thermoluminescence spectra suggest the presence of low lying acceptor states at 0.7 eV above the valence band. When excited with 270 nm light, we observe emission of light in two broad spectral bands peaking at 500 and 550 nm. We suggest that the 550 nm emission band results from donor—acceptor pair recombination (DAPR) from low lying acceptor states at ca. 0.7 eV above the valence band and donor states approximately 2.5 to 2.7 eV above the valence band. We do not identify the structure of these defects. We propose a speculative model of the physics of the color change from ‘yellow’ to ‘green’ which results from increased broad-band optical absorption in the near-IR to visible due to transitions from the valence band into un-ionized acceptor states available in the ‘green’ state of the chameleon diamond. We report near-IR absorption spectra confirming the increased absorption of light in the near-IR to visible in the ‘green’ when compared to the ‘yellow’ state with a threshold at ca. 0.65 eV, supporting the proposed model.
Abstract: The purpose of this paper is to elucidate the semantic relation between continental drift and plate tectonics. The numerous attempts to account for this case in either Kuhnian or Lakatosian terms have been convincingly dismissed by Rachel Laudan (PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association. Symposia and Invited Papers, 1978), who nevertheless acknowledged that there was not yet a plausible alternative to explain the so called "geological revolution". Several decades later, the epistemological side of this revolution has received much attention (Ruse in The darwinian paradigm, essays on its history, philosophy and religious implications. London, Routledge, 1981/1989; Thagard in Conceptual revolutions. Princeton University Press, Princeton, 1992; Marvin in Metascience 10:208-217, 2001; Oreskes in Plate tectonics: an insiders’ history of the modern theory of the earth. Westview Press, Boulder, 2003), while the semantic relation between drift theory and plate tectonics has remained mainly unexplored. In studying this case under a new light, the notion of embedding, as distinguished from other sorts of intertheoretical relations (Moulines in Cognitio Humana-Dynamik des Wissens und der Werte. XVII, Institut für Philosophie der Universität Leipzig, Leipzig, 1996, Time, chance, and reduction: philosophical aspects of statistical mechanics. Cambridge University Press, Cambridge, 2010, Metatheoria 1(2):11-27, 2011), will have a particular significance. After formally analyzing the relationship between continental drift and plate tectonics, it will become evident that the models of drift theory are part of the models of plate tectonics, thereby fulfilling the conditions for embedding. All theoretical concepts from drift theory are presupposed in some theoretical concepts from plate tectonics, and all empirical concepts of the former are shared by the latter. Furthermore, all the successful paradigmatic applications of continental drift are also successful applications of plate tectonics. As a consequence, under the label "geological revolution", we actually find a salient historical case of cumulative progress across theory change.
Abstract: On the early Earth, oceanic plateaux similar to present-day Iceland are thought to have evolved into less dense microcontinents as they thickened by continued melt intrusion and crustal fractionation. These earliest continents may have been so weak on a hotter Earth that they collapsed laterally in response to thickening by further magmatic growth or tectonic imbrication. This continental spreading is likely to have resulted in the development of pervasive ductile strain fabrics in the deeper crust, which, if preserved, could generate seismic reflections. Here we present seismic images from the ancient core of the Archaean Yilgarn Craton of Australia that reveal shallowly dipping to horizontal reflections that pervade the middle and lower crust. We interpret these reflective fabrics as the result of widespread lateral crustal flow during the late stage of craton evolution approximately 2.66 to 2.61?billion years ago, which coincided with the widespread intrusion of high-temperature crustal melts, as thickened early continental crust collapsed. The consequent subsidence of large regions of the upper crust, including volcanic and sedimentary greenstone rocks, in the hanging walls of listric mid-lower crustal ductile flow fabrics caused these rocks to drop beneath the granitic melts rising towards the surface, and did not involve Rayleigh-Taylor instabilities within a mostly mobile crust.
Abstract: On the early Earth, oceanic plateaux similar to present-day Iceland are thought to have evolved into less dense microcontinents as they thickened by continued melt intrusion and crustal fractionation. These earliest continents may have been so weak on a hotter Earth that they collapsed laterally in response to thickening by further magmatic growth or tectonic imbrication. This continental spreading is likely to have resulted in the development of pervasive ductile strain fabrics in the deeper crust, which, if preserved, could generate seismic reflections. Here we present seismic images from the ancient core of the Archaean Yilgarn Craton of Australia that reveal shallowly dipping to horizontal reflections that pervade the middle and lower crust. We interpret these reflective fabrics as the result of widespread lateral crustal flow during the late stage of craton evolution approximately 2.66 to 2.61?billion years ago, which coincided with the widespread intrusion of high-temperature crustal melts, as thickened early continental crust collapsed. The consequent subsidence of large regions of the upper crust, including volcanic and sedimentary greenstone rocks, in the hanging walls of listric mid-lower crustal ductile flow fabrics caused these rocks to drop beneath the granitic melts rising towards the surface, and did not involve Rayleigh-Taylor instabilities within a mostly mobile crust.
Applied Radiation and Isotopes, Vol. 126, 1, pp. 66-72.
Global
diamond - green
Abstract: Treated green diamonds can show residual radioactivity, generally due to immersion in radium salts. We report various activity measurements on two radioactive diamonds. The activity was characterized by alpha and gamma ray spectrometry, and the radon emanation was measured by alpha counting of a frozen source. Even when no residual radium contamination can be identified, measurable alpha and high-energy beta emissions could be detected. The potential health impact of radioactive diamonds and their status with regard to the regulatory policy for radioactive products are discussed.
Abstract: Carbonates in fresh hypabyssal kimberlites worldwide have been studied to understand their origin [i.e. primary magmatic (high T) versus deuteric (‘low T’) versus hydrothermal/alteration (‘low T’)] and identify optimal strategies for petrogenetic studies of kimberlitic carbonates. The approach presented here integrates detailed textural characterisation, cathodoluminescence (CL) imaging, in situ major- and trace-element analysis, as well as in situ Sr-isotope analysis. The results reveal a wide textural diversity. Calcite occurs as fine-grained groundmass, larger laths, segregations, veins or as a late crystallising phase, replacing olivine or early carbonates. Different generations of carbonates commonly coexist in the same kimberlite, each one defined by a characteristic texture, CL response and composition (e.g., variable Sr and Ba concentrations). In situ Sr isotope analysis revealed a magmatic signature for most of the carbonates, based on comparable 87Sr/86Sr values between these carbonates and the coexisting perovskite, a robust magmatic phase. However, this study also shows that in situ Sr isotope analysis not always allow distinction between primary (i.e., magmatic) and texturally secondary carbonates within the same sample. Carbonates with a clear secondary origin (e.g., late-stage veins) occasionally show the same moderately depleted 87Sr/86Sr ratios of primary carbonates and coexisting perovskite (e.g., calcite laths-shaped crystals with 87Sr/86Sr values identical within uncertainty to those of vein calcite in the De Beers kimberlite). This complexity emphasises the necessity of integrating detailed petrography, geochemical and in situ Sr isotopic analyses for an accurate interpretation of carbonate petrogenesis in kimberlites. Therefore, the complex petrogenesis of carbonates demonstrated here not only highlights the compositional variability of kimberlites, but also raises concerns about the use of bulk-carbonate C-O isotope studies to characterise the parental melt compositions. Conversely, our integrated textural and in situ study successfully identifies the most appropriate (i.e. primary) carbonates for providing constraints on the isotopic parameters of parental kimberlite magmas.
Abstract: Models of the volume of continental crust through Earth history vary significantly due to a range of assumptions and data sets; estimates for 3?Ga range from <10% to >120% of present day volume. We argue that continental area and thickness varied independently and increased at different rates and over different periods, in response to different tectonic processes, through Earth history. Crustal area increased steadily on a pre-plate tectonic Earth, prior to ca. 3?Ga. By 3?Ga the area of continental crust appears to have reached a dynamic equilibrium of around 40% of the Earth's surface, and this was maintained in the plate tectonic world throughout the last 3?billion?years. New continental crust was relatively thin and mafic from ca. 4-3?Ga but started to increase substantially with the inferred onset of plate tectonics at ca. 3?Ga, which also led to the sustained development of Earth's bimodal hypsometry. Integration of thickness and area data suggests continental volume increased from 4.5?Ga to 1.8?Ga, and that it remained relatively constant through Earth's middle age (1.8-0.8?Ga). Since the Neoproterozoic, the estimated crustal thickness, and by implication the volume of the continental crust, appears to have decreased by as much as 15%. This decrease indicates that crust was destroyed more rapidly than it was generated. This is perhaps associated with the commencement of cold subduction, represented by low dT/dP metamorphic assemblages, resulting in higher rates of destruction of the continental crust through increased sediment subduction and subduction erosion.
Philosphical Transactions of the Royal Society, rsta.royalsociety publishing.org 30p. Available
Mantle
tectonics, geochemistry
Abstract: Plate tectonics, involving a globally linked system of lateral motion of rigid surface plates, is a characteristic feature of our planet, but estimates of how long it has been the modus operandi of lithospheric formation and interactions range from the Hadean to the Neoproterozoic. In this paper, we review sedimentary, igneous and metamorphic proxies along with palaeomagnetic data to infer both the development of rigid lithospheric plates and their independent relative motion, and conclude that significant changes in Earth behaviour occurred in the mid- to late Archaean, between 3.2?Ga and 2.5?Ga. These data include: sedimentary rock associations inferred to have accumulated in passive continental margin settings, marking the onset of sea-floor spreading; the oldest foreland basin deposits associated with lithospheric convergence; a change from thin, new continental crust of mafic composition to thicker crust of intermediate composition, increased crustal reworking and the emplacement of potassic and peraluminous granites, indicating stabilization of the lithosphere; replacement of dome and keel structures in granite-greenstone terranes, which relate to vertical tectonics, by linear thrust imbricated belts; the commencement of temporally paired systems of intermediate and high dT/dP gradients, with the former interpreted to represent subduction to collisional settings and the latter representing possible hinterland back-arc settings or ocean plateau environments. Palaeomagnetic data from the Kaapvaal and Pilbara cratons for the interval 2780-2710?Ma and from the Superior, Kaapvaal and Kola-Karelia cratons for 2700-2440?Ma suggest significant relative movements. We consider these changes in the behaviour and character of the lithosphere to be consistent with a gestational transition from a non-plate tectonic mode, arguably with localized subduction, to the onset of sustained plate tectonics.
Abstract: The formation of transitional agpaitic rocks is not a well understood process as there are few studies of miaskitic to agpaitic transitions. The Mesoproterozoic Sushina Hill complex (India) provides a suitable site to investigate these "transitions" as this complex hosts diverse miaskitic and agpaitic nepheline syenites, together with syenites containing exotic mineral assemblages. In this study, we have used mineralogical and geochemical data to describe the evolution of the transitional agpaitic rocks occurring at Sushina Hill. In common with other occurrences, high field strength elements (HFSE) in miaskitic nepheline syenites are mainly sequestered by primary zircon and magnetite. In contrast, the major HFSE carriers in agpaitic nepheline syenites (agpaitic unit-I) are late-magmatic eudialyte and rinkite-(Ce) - nacareniobsite-(Ce), formed at T between 825° - 784ºC and aSiO2 in the range of 0.41 - 0.44. With decreasing temperature (? 575ºC) and aSiO2(0.30), coupled with an increase in aH2O, this assemblage has undergone extensive subsolidus alteration leading to the decomposition of late-magmatic eudialyte to wöhlerite - marianoite, alkali-zirconosilicates (catapleiite/gaidonnyaite, hilairite), and pectolite - serandite. Decomposition of late-magmatic eudialyte resulted in a more alkaline fluid by increasing the a(Na+)/a(Cl-) ratio, facilitating crystallization of hydrothermal eudialyte replacing late-magmatic eudialyte. Crystallization of hydrothermal eudialyte leads to evolving fluids which are less alkaline, resulting in the crystallization of a transitional agpaitic assemblage of pyrochlore + zircon + niobokupletskite + wadeite in agpaitic unit-II in the temperature range 547º - 455ºC with aSiO2 in the range 0.27 - 0.25. Regional scale deformation contemporaneous with the subsolidus alteration stage leads to separation of the evolving fluid from the system, resulting in extensive albitization, with superposition of a new miaskitic-like assemblage in syenite I in the form of late-stage zircon - magnetite - xenotime - monazite-(Ce) upon the early assemblage of primary zircon and magnetite. During deformation, syenite unit-II composed of eudialyte - albite - aegirine was also formed and considered as a later stage pegmatitic offshoot of agpaitic unit I. The mineralogical changes are also complemented by variations in the bulk-rock composition in which the total REE, Nb, U and Th concentrations increase in order from: miaskitic unit ? agpaitic unit I ? syenite unit II, -I ? agpaitic unit II at constant Zr concentration. This suggests that the REE-Nb are mainly mobilized in agpaitic unit-II during the agpaitic - to - transitional agpaitic assemblage transformation in a relatively less alkaline environment.
Geological Society of London Special Publication, No. 463, pp. 137-169.
India
carbonatite
Abstract: The genetic connection between Large Igneous Province (LIP) and carbonatite is controversial. Here, we present new major and trace element data for carbonatites, nephelinites and Deccan basalts from Amba Dongar in western India, and probe the linkage between carbonatite and the Deccan LIP. Carbonatites are classified into calciocarbonatite (CaO, 39.5-55.9 wt%; BaO, 0.02-3.41 wt%; ?REE, 1025-12 317 ppm) and ferrocarbonatite (CaO, 15.6-31 wt%; BaO, 0.3-7 wt%; ?REE, 6839-31 117 ppm). Primitive-mantle-normalized trace element patterns of carbonatites show distinct negative Ti, Zr-Hf, Pb, K and U anomalies, similar to that observed in carbonatites globally. Chondrite-normalized REE patterns reveal high LREE/HREE fractionation; average (La/Yb)N values of 175 in carbonatites and approximately 50 in nephelinites suggest very-low-degree melting of the source. Trace element modelling indicates the possibility of primary carbonatite melt generated from a subcontinental lithospheric mantle (SCLM) source, although it does not explain the entire range of trace element enrichment observed in the Amba Dongar carbonatites. We suggest that CO2-rich fluids and heat from the Deccan plume contributed towards metasomatism of the SCLM source. Melting of this SCLM generated primary carbonated silicate magma that underwent liquid immiscibility at crustal depths, forming two compositionally distinct carbonatite and nephelinite magmas.
Journal of Mining Science, Vol. 53, 1, pp. 126-132.
Russia
deposit - Mir
Abstract: The structural and chemical surface transformation of basic kimberlite-forming minerals (calcite, olivine, serpentine) under the contact with natural and waste mineralized water and products of electrochemical treatment of the water are studied using X-ray photoelectronic spectroscopy, scanning electron microscopy and X-ray spectral micro-analysis, and atomic force microscopy. It is found that contact with kimberlite extract and recycling water induces chemical modification of calcite surface, which consists in adsorption of hydrocarbon impurities, and chlorine- and silica-bearing compounds, majority of which are removed during interaction with the product of electrochemical treatment of recycling water. The change in the structural and chemical surface properties of rock-forming silicates, aside from adsorption-desorption of organic compound, is also connected with the distortion of nano-size layer structure after leaching of Mg, Fe and Si, and with the carbonatization of the surface.
Abstract: he meta-analysis of about 13,000 analyses of scandium content in garnet grains shows that, below the spinel-garnet transition, this phase carries about three-quarters of the Sc budget of the mantle, indicating its control on Sc mobility. The Sc content of garnets in mafic rocks is low, due to a dilution effect resulting from their high modal content in garnet. Garnets from ultramafic rocks exhibit a wider range of Sc concentrations. We assess the relative influence of thermobarometry, crystal chemistry and fluid-related events on the distribution of Sc in garnet from such rocks to improve the tracking of geochemical processes in the mantle. Pressure and temperature of equilibration in the mantle are second-order factors influencing the Sc content of garnet, while crystal-chemistry, in particular and , is the main parameter controlling the compatibility of Sc. Scandium is incorporated in both X and Y sites of Cr-Ca-rich garnets, resulting in a behaviour intermediate between rare-earth elements, incorporated in the X site, and trivalent transition elements, occupying the Y site. This affinity for both sites results in a mild compatibility of Sc in the garnet stability field of the mantle; hence Sc concentration in garnet increases with melt extraction and can be reduced by silicate-melt metasomatism. In contrast, metasomatism by volatile-rich fluids increases the Sc concentration in garnet. The control of garnet on the compatibility of Sc in deep lithospheric rocks demonstrates the potential of using Sc to track the conditions of formation of magmas and their residual rocks, as well as the origin and nature of metasomatic fluids.
Doklady Earth Sciences, Vol. 481, 2, pp. 1008-1012.
Russia, Aldan shield
lamproite
Abstract: Obtained data shows that high-potassic dyke rocks of the Ryabinoviy massif (Central Aldan) belong to low-titanium lamproite series (Mediterranean type) and are distinct with “classic” high-titanium lamproites. Based on Al-in-olivine thermometer, temperature of olivine-chrome-spinel pair crystallization varies in range between 1100 and 1250°C. This suggests lithospheric mantle source for the parental melt and makes role of mantle plume insignificant. High-precision data on olivine composition and bulk rock traceelement composition imply mixed source for the parental melt, consisted of depleted peridotite and enriched domains, originated during ancient subduction.
Abstract: Carbonatite and related alkaline silicate rocks contain one of the most significant rare earth element (REE) reserves in the world. It is well-known that these REE deposits are characterized by a strong light REE enrichment with a steep fractionation from La to Lu in the chondrite-normalized diagram. However, the origin of their REE enrichment remains debatable. The Shaxiongdong (SXD) carbonatite in the South Qinling orogenic belt hosts one of the most important REE deposits in central China. In this study, in situ chemical and isotopic data have been obtained for carbonate minerals from the complex. Our results show that calcite has variable trace element abundances, especially REEs. In situ Pb isotope data for calcite reveal extreme variations of 206Pb/204Pb (18.05-31.71) and 207Pb/204Pb (15.49-16.36) ratios. Interestingly, Pb isotope variations display positive correlations with REE enrichments [i.e., (La/Yb)N and (La/Nd)N]. Calcite with extreme radiogenic Pb isotopic compositions displays upper mantle C and O isotopic compositions (?13Cavg?=??5.74‰, ?18Oavg?=?7.13‰) and depleted 87Sr/86Sr isotopic ratios (~0.7030). The observed various REE enrichments accompanying the variable Pb isotopic composition within SXD calcite possibly result from a closed-system metasomatic event. The U-bearing mineral (i.e., pyrochlore) accumulating abundant uranogenic lead since their Silurian formation serves as the radiogenic Pb and LREE source for the metasomatism. Alternatively, the chemical and isotopic composition observed might suggest involvement of two mantle sources (PREMA and the distinct radiogenic Pb mantle reservoir).
Abstract: The Western Canada Sedimentary Basin marks a boundary zone between the Precambrian North American craton and the Phanerozoic Cordillera. Its crystalline basement has documented more than 3 billion years of evolution history of western Laurentia. Here we conduct a high?resolution survey of the mantle P and S wave velocities using finite?frequency tomography. Our models show pronounced eastward increases of 4% P and 6% S wave velocities beneath the foreland region, which define a sharp seismic Cordillera?Craton boundary. In the cratonic region, distinctive high? (>2%) velocity anomalies representing depleted mantle lithospheres are well correlated with major Precambrian crustal domains. The largest lithosphere thickness contrast coincides with the Snowbird Tectonic Zone, where the Hearne province extends down to ~300 km, nearly 100 km deeper than the Proterozoic terranes in northern Alberta. In the latter region, a pronounced cylindrical negative velocity anomaly extends subvertically from 75 to ~300?km depth, which potentially results from significant tectonothermal modifications during subduction and/or plume activities. At the basin scale, mantle velocities show no apparent correlations with surface heat flux, suggesting a minimum mantle contribution to the regional thermal variability. Furthermore, the long?wavelength isostatic gravity correlates negatively with the velocities, which confirms that the melt extraction from Precambrian cratons is responsible for the formation of highly depleted mantle lithospheres. Moreover, our model reveals the increased concentrations of kimberlites and lamproites near the zones of high horizontal velocity gradients. The distinct spatial pattern may reflect either preferential formation or eruption of potentially diamondiferous rocks at lithospheric weak zones near the western margin of Laurentia.
Journal of Asian Earth Sciences, Vol. 164, pp. 179-193.
Asia, Myanmar
peridotites
Abstract: Peridotites from the Myitkyina ophiolite are mainly composed of lherzolite and harzburgite. The lherzolites have relatively fertile compositions, with 39.40-43.40?wt% MgO, 1.90-3.17?wt% Al2O3 and 1.75-2.84?wt% CaO. They contain spinel and olivine with lower Cr# (12.6-18.2) and Fo values (88.7-91.6) than those of the harzburgites (24.5-59.7 and 89.6-91.6 respectively). The harzburgites have more refractory compositions, containing 42.40-46.23?wt% MgO, 0.50-1.64?wt% Al2O3 and 0.40-1.92?wt% CaO. PGE contents of the peridotites show an affinity to the residual mantle. Evaluation of petrological and geochemical characteristics of these peridotites suggests that the lherzolites and harzburgites represent residual mantle after low to moderate degrees of partial melting, respectively, in the spinel stability field. The U-shaped, primitive mantle-normalized REE patterns and strong positive Ta and Pb anomalies of the harzburgites suggest melt/fluid refertilization in either a MOR or SSZ setting after their formation at a MOR. Mineral separation of the peridotites has yield a range of exotic minerals, including diamond, moissanite, native Si, rutile and zircon, a collection similar to that reported for ophiolites of Tibet and the Polar Urals. The discovery of these exotic minerals in the Myitkyina ophiolite supports the view that they occur widely in the upper oceanic mantle.
Abstract: The beginning of plate tectonics on Earth remains the subject of fundamental debate. Also, future evolution of plate tectonics has not yet been addressed adequately in the literature. Here I develop models to extrapolate secular trends of plate tectonics closely associated with mantle potential temperature; intensity of magmatic activities to estimate the lifetime of plate tectonics. First a fractal model is utilized to analyze global igneous and detrital zircon U-Pb age datasets to characterize nonlinear intensity and periodicity of peaks in magmatic activities associated with deep-seated avalanche-type events (slab break off, lithospheric root detachments and mantle plumes). The results show descending trends of both nonlinear intensity and duration of peaks in magmatism observed from 3?Ga to the present day, suggesting a general trend of mantle cooling. This relation when extrapolated yields 1.45?Gyr of time for when nonlinear intensity of major magmatic activity would vanish. Further it is demonstrated by an independent polynomial model fitted to the relation between mantle potential temperature and future age of the earth that this result conforms to the time estimated for when mantle potential temperature is reduced to mantle solidus temperature.
Abstract: The contribution of magmatic and hydrothermal processes to rare earth element (REE) mineralization of carbonatites remains an area of considerable interest. With the aim of better understanding REE mineralization mechanisms, we conducted a detailed study on the petrology, mineralogy and C-O isotopes of the Bachu carbonatites, NW China. The Bachu carbonatites are composed predominantly of magnesiocarbonatite with minor calciocarbonatite. The two types of carbonatite have primarily holocrystalline textures dominated by dolomite and calcite, respectively. Monazite-(Ce) and bastnäsite-(Ce), the major REE minerals, occur as euhedral grains and interstitial phases in the carbonatites. Melt inclusions in the dolomite partially rehomogenize at temperatures above 800?°C, and those in apatite have homogenization temperatures (Th) ranging from 645 to 785?°C. Oxygen isotope ratios of the calciocarbonatite intrusions (?18OV-SMOW?=?6.4‰ to 8.3‰), similar to the magnesiocarbonatites, indicate the parental magma is mantle-derived, and that they may derive from a more evolved stage of carbonatite fractionation. The magnesiocarbonatites are slightly enriched in LREE whereas calciocarbonatites have higher HREE concentrations. Both dolomite and calcite have low total REE (TREE) contents ranging from 112 to 436?ppm and 88 to 336?ppm, respectively, much lower than the bulk rock composition of the carbonatites (371 to 36,965?ppm). Hence, the fractional crystallization of carbonates is expected to elevate REE concentrations in the residual magma. Rocks from the Bachu deposit with the highest TREE concentration (up to 20?wt%) occur as small size (2?mm to 3 cm) red rare earth-rich veins (RRV) with barite + celestine + fluorapatite + monazite-(Ce) associations. These rocks are interpreted to have a hydrothermal origin, confirmed by the fluid inclusions in barite with Th in the range 198-267?°C. Hydrothermal processes may also explain the existence of interstitial textures in the carbonatites with similar mineral assemblages. The C-O isotopic compositions of the RRV (?13CV-PDB?=??3.6 to ?4.3‰, ?18OV-SMOW?=?7.6 to 9.8‰) are consistent with an origin resulting from fluid exsolution at the end of the high temperature fractionation trend. A two-stage model involving fractional crystallization and hydrothermal fluids is proposed for the mineralization of the Bachu REE deposit.
Geological Society of London, Special Publication, Vol. 470, doi:10.1144 /SP470.10
Mantle
wilson cycle
Abstract: Although the Wilson cycle is usually considered in terms of wide oceans floored with normal oceanic crust, numerous orogens result from the closure of embryonic oceans. We discuss how orogenic and post-orogenic processes may be controlled by the size/maturity of the inverted basin. We focus on the role of lithospheric mantle in controlling deformation and the magmatic budget. We describe the physical properties (composition, density, rheology) of three types of mantle: inherited, fertilized and depleted oceanic mantle. By comparing these, we highlight that fertilized mantle underlying embryonic oceans is mechanically weaker, less dense and more fertile than other types of mantle. We suggest that orogens resulting from the closure of a narrow, immature extensional system are essentially controlled by mechanical processes without significant thermal and lithological modification. The underlying mantle is fertile and thus has a high potential for magma generation during subsequent tectonic events. Conversely, the thermal state and lithology of orogens resulting from the closure of a wide, mature ocean are largely modified by subduction-related arc magmatism. The underlying mantle wedge is depleted, which may inhibit magma generation during post-orogenic extension. These end-member considerations are supported by observations derived from the Western Europe-North Atlantic region.
From subcalcic pyropes to uvarovites: experimental crystalllization of Cr-rich garnets in ultramafic systems with presence of Ca bearing hydrous fluid.
Journal of Volcanology and Seismology, Vol. 12, 2, pp. 140-149.
Russia
deposit- Nyurbinskaya
Abstract: The study of viscosity in sub-liquidus heterogeneous media, which includes kimberlite magma at the pressures and temperatures that prevail in the mantle, is an urgent task. We have conducted experiments in the serpentine-olivine, serpentine-CaCO3?olivine, and native kimberlite-olivine systems at a pressure of 4 GPa and temperatures of 1400?1600°? in a BARS high-pressure device using the technique of a falling Pt pellet. The samples were examined after experiments to find fine-grained chilled mass of crystals where the Pt pellet was observed at the time of chilling. The concentration of the solid phase was varied in the experiments between 10 and 50 wt %. We showed that when 50 wt % of olivine grains has been introduced, it was not possible to detect the motion of the Pt pellet, while when the concentration of olivine xenocrysts reached 10 wt %, the Pt pellet very rapidly descended to the bottom of the reaction volume. Viscosity was calculated using the Stokes method. We found that the viscosity of a homogeneous kimberlite melt at 4 GPa and 1600°? is below 2 Pa s, with the viscosity of a melt that contained up to 10 wt % of the solid phase being approximately constant. A kimberlite melt that contained 30 wt % of the solid phase had a viscosity on the order of 100 Pa s, while with 50 wt % of the solid phase the relative viscosity of an ultrabasic system increased to reach values over 1000 Pa s.
Geochimica et Cosmochimica Acta, Vol. 232 pp. 206-224.
Technology
diamond - inclusions DFT
Abstract: Diffusivities of helium, deuterium and hydrogen have been characterized in diamond. Polished CVD diamond was implanted with either 3He, 2H, or 1H. Implanted samples were sealed under vacuum in silica glass capsules, and annealed in 1-atm furnaces. 3He, 2H and 1H distributions were measured with Nuclear Reaction Analysis. We obtain these Arrhenius relations: DHe = 4.00?×?10?15 exp(?138?±?14?kJ?mol?1/RT) m2?s?1. D2H = 1.02?×?10?4 exp(?262?±?17?kJ?mol?1/RT) m2?s?1. D1H = 2.60?×?10?4 exp(?267?±?15?kJ?mol?1/RT) m2?s?1. Diffusivities of 1H and 2H agree within experimental uncertainties, indicating little diffusive mass fractionation of hydrogen in diamond. To complement the experimental measurements, we performed calculations using a first-principles quantum mechanical description of diffusion in diamond within the Density Functional Theory (DFT). Differences in 1H and 2H diffusivities from calculations are found to be ?4.5%, reflected in differences in the pre-exponential factor. This small difference in diffusivities, despite the large relative mass difference between these isotopes, is due to the fact that the atomistic process involved in the transition along the diffusion pathway is dictated by local changes to the diamond structures rather than to vibrations involving 1H/2H. This finding is consistent with the experimental results given experimental uncertainties. In contrast, calculations for helium diffusion in diamond indicate a difference of 15% between diffusivities of 3He and 4He. Calculations of diffusion distances for hydrogen using our data yield a distance of 50??m in diamond in 300,000?years at 500?°C and ?30?min at 1400?°C. Diffusion distances for He in diamond are shorter than for H at all temperatures above ?350?°C, but differences increase dramatically with temperature because of the higher activation energy for H diffusion. For example, a 50??m diffusion distance for He would be attained in ?40 Myr at 500?°C and 400?yr at 1400?°C. For comparison, a 50??m diffusion distance for N in diamond would require nearly 1 billion years at 1400?°C. The experimental data indicate that diamonds equilibrate with ambient H and He in the mantle on timescales brief relative to most geological processes and events. However, He diffusion in diamond is slower than in any other mineral measured to date, including other kimberlite-hosted minerals. Under some circumstances, diamond may provide information about mantle He not recoverable from other minerals. One possibility is diamonds entrained in kimberlites. Since the ascent of kimberlite from the mantle to near-surface is very rapid, entrained diamonds may retain most or all of the H and He acquired in mantle environments. Calculations using reasonable ascent rates and T-t paths indicate that He diffusive loss from kimberlite-hosted diamonds is negligible for grains of 1.0-0.2?mm radius, with fractional losses <0.15% for all ascent rates considered. If the host kimberlite magma is effectively quenched in the near-surface (or is erupted), diamonds should contain a faithful record of [He] and He isotopes from the mantle source region. Preservation of H in kimberlite-hosted diamonds is less clear-cut, with model outcomes depending critically upon rates of ascent and cooling.
Earth and Planteray Science Letters, Vol. 489, pp. 258-266.
Mantle
subduction
Abstract: The impact of remotely forced mantle flow on regional subduction evolution is largely unexplored. Here we investigate this by means of 3D thermo-mechanical numerical modeling using a regional modeling domain. We start with simplified models consisting of a 600 km (or 1400 km) wide subducting plate surrounded by other plates. Mantle inflow of ?3 cm/yr is prescribed during 25 Myr of slab evolution on a subset of the domain boundaries while the other side boundaries are open. Our experiments show that the influence of imposed mantle flow on subduction evolution is the least for trench-perpendicular mantle inflow from either the back or front of the slab leading to 10-50 km changes in slab morphology and trench position while no strong slab dip changes were observed, as compared to a reference model with no imposed mantle inflow. In experiments with trench-oblique mantle inflow we notice larger effects of slab bending and slab translation of the order of 100-200 km. Lastly, we investigate how subduction in the western Mediterranean region is influenced by remotely excited mantle flow that is computed by back-advection of a temperature and density model scaled from a global seismic tomography model. After 35 Myr of subduction evolution we find 10-50 km changes in slab position and slab morphology and a slight change in overall slab tilt. Our study shows that remotely forced mantle flow leads to secondary effects on slab evolution as compared to slab buoyancy and plate motion. Still these secondary effects occur on scales, 10-50 km, typical for the large-scale deformation of the overlying crust and thus may still be of large importance for understanding geological evolution.
Contributions to Mineralogy and Petrology, Vol. 173, 22p. Doi.org/10.1007/s00410-018-1512-z
Africa, Tanzania
magmatism
Abstract: Magmatism on Earth is most abundantly expressed by surface volcanic activity, but all volcanism has roots deep in the crust, lithosphere, and mantle. Intraplate magmatism, in particular, has remained enigmatic as the plate tectonic paradigm cannot easily explain phenomena such as large flood basalt provinces and lithospheric rupture within continental interiors. Here, I explore the role of deep crustal magmatic processes and their connection to continental rift volcanism as recorded in deep crustal xenoliths from northern Tanzania. The xenoliths are interpreted as magmatic cumulates related to Cenozoic rift volcanism, based on their undeformed, cumulate textures and whole-rock compositions distinct from melt-reacted peridotites. The cumulates define linear trends in terms of whole-rock major elements and mineralogically, can be represented as mixtures of olivine?+?clinopyroxene. AlphaMELTS modeling of geologically plausible parental melts shows that the end-member cumulates, clinopyroxenite and Fe-rich dunite, require fractionation from two distinct melts: a strongly diopside-normative melt and a fractionated picritic melt, respectively. The former can be linked to the earliest, strongly silica-undersaturated rift lavas sourced from melting of metasomatized lithosphere, whereas the latter is linked to the increasing contribution from the upwelling asthenospheric plume beneath East Africa. Thus, deep crustal cumulate systematics reflect temporal and compositional trends in rift volcanism, and show that mixing, required by the geochemistry of many rift lava suites, is also mirrored in the lavas’ cumulates.
Mineralogy and Petrology, in press available, 11p.
Africa, Botswana
deposit - Orapa
Abstract: Spatially resolved analyses reveal considerable isotopic heterogeneity within and among diamonds ranging in size from 0.15 to 4.75 mm from the Orapa Mine, Botswana. The isotopic data are interpreted in conjunction with nitrogen aggregation state data and growth zone relationships from cathodoluminescence images. The integrated information confirms that a distinct diamond growth event (with low IaAB nitrogen aggregation states, moderately high nitrogen contents and ?13C and ?15N values compatible with average mantle values) is younger than the dominant population(s) of Type IaAB diamonds and cores of composite diamonds with more negative and positive ?13C and ?15N values, respectively. A significant proportion of the older diamond generation has high nitrogen contents, well outside the limit sector relationship, and these diamonds are likely to reflect derivation from subducted organic matter. Diamonds with low ?13C values combined with high nitrogen contents and positive ?15N values have not been previously widely recognised, even in studies of diamonds from Orapa. This may have been caused by prior analytical bias towards inclusion-bearing diamonds that are not necessarily representative of the entire range of diamond populations, and because of average measurements from heterogeneous diamonds measured by bulk combustion methods. Two distinct low nitrogen/Type II microdiamond populations were recognised that do not appear to continue into the macrodiamond sizes in the samples studied. Other populations, e.g. those containing residual singly-substituted nitrogen defects, range in size from small microdiamonds to large macrodiamonds. The total diamond content of the Orapa kimberlite thus reflects a complex assortment of multiple diamond populations.
Physics and Chemistry of Minerals, Vol. 45, pp. 745-758.
Russia, Kola Peninsula
alkaline
Abstract: The new eudialyte-group mineral siudaite, ideally Na8(Mn2+2Na)Ca6Fe3+3Zr3NbSi25O74(OH)2Cl•5H2O, was discovered in a peralkaline pegmatite situated at the Eveslogchorr Mt., Khibiny alkaline massif, Kola Peninsula, Russia. The associated minerals are aegirine, albite, microcline, nepheline, astrophyllite, and loparite-(Ce). Siudaite forms yellow to brownish-yellow equant anhedral grains up to 1.5 cm across. Its lustre is vitreous, and the streak is white. Cleavage is none observed. The Mohs’ hardness is 4½. Density measured by hydrostatic weighing is 2.96(1) g/cm3. Density calculated using the empirical formula is equal to 2.973 g/cm3. Siudaite is nonpleochroic, optically uniaxial, negative, with ??=?1.635(1) and ??=?1.626(1) (??=?589 nm). The IR spectrum is given. The chemical composition of siudaite is (wt%; electron microprobe, H2O determined by HCN analysis): Na2O 8.40, K2O 0.62, CaO 9.81, La2O3 1.03, Ce2O3 1.62, Pr2O3 0.21, Nd2O3 0.29, MnO 6.45, Fe2O3 4.51. TiO2 0.54, ZrO2 11.67, HfO2 0.29, Nb2O5 2.76, SiO2 47.20, Cl 0.54, H2O 3.5, -O?=?Cl ??0.12, total 99.32. According to Mössbauer spectroscopy data, all iron is trivalent. The empirical formula (based on 24.5 Si atoms pfu, in accordance with structural data) is [Na7.57(H2O)1.43]?9(Mn1.11Na0.88Ce0.31La0.20Nd0.05Pr0.04K0.41)?3(H2O)1.8(C a5.46Mn0.54)?6(Fe3+1.76Mn2+1.19)?2.95Nb0.65(T i0.20Si0.50)?0.71(Zr2.95Hf0.04Ti0.01)?3Si24.00Cl0.47O70(OH)2Cl0.47•1.2H2O. The crystal structure was determined using single-crystal X-ray diffraction data. The new mineral is trigonal, space group R3m, with a?=?14.1885(26) Å, c?=?29.831(7) Å, V?=?5200.8(23) Å3 and Z?=?3. Siudaite is chemically related to georgbarsanovite and is its analogue with Fe3+-dominant M2 site. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 6.38 (60) (-114), 4.29 (55) (-225), 3.389 (47) (131), 3.191 (63) (-228). 2.963 (100) (4-15), 2.843 (99) (-444), 2.577 (49) (3-39). Siudaite is named after the Polish mineralogist and geochemist Rafa? Siuda (b. 1975).
Journal of Geophysical Research, Vol. 122, 10.1002/2017JB014501
Mantle
thermodynamics
Abstract: We present a newly developed software framework, MMA-EoS, that evaluates phase equilibria and thermodynamic properties of multicomponent systems by Gibbs energy minimization, with application to mantle petrology. The code is versatile in terms of the equation-of-state and mixing properties and allows for the computation of properties of single phases, solution phases, and multiphase aggregates. Currently, the open program distribution contains equation-of-state formulations widely used, that is, Caloric-Murnaghan, Caloric-Modified-Tait, and Birch-Murnaghan-Mie-Grüneisen-Debye models, with published databases included. Through its modular design and easily scripted database, MMA-EoS can readily be extended with new formulations of equations-of-state and changes or extensions to thermodynamic data sets. We demonstrate the application of the program by reproducing and comparing physical properties of mantle phases and assemblages with previously published work and experimental data, successively increasing complexity, up to computing phase equilibria of six-component compositions. Chemically complex systems allow us to trace the budget of minor chemical components in order to explore whether they lead to the formation of new phases or extend stability fields of existing ones. Self-consistently computed thermophysical properties for a homogeneous mantle and a mechanical mixture of slab lithologies show no discernible differences that require a heterogeneous mantle structure as has been suggested previously. Such examples illustrate how thermodynamics of mantle mineralogy can advance the study of Earth's interior.
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: 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.
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.
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).
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: 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.
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.
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.
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.
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.
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.
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.
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.
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: 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: 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.
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: The dynamics of formation and exhumation of high-pressure (HP) and ultra-high pressure (UHP) metamorphic orogens in double subduction-collision zones remain enigmatic. Here we employ two-dimensional thermo-mechanical numerical models to gain insights on the exhumation of HP-UHP metamorphic rocks, as well as their deformation during the collision of a micro-continent with pro- and retro-continental margins along two subduction zones. A three-stage collisional process with different convergence velocities is tested. In the initial collisional stage, a fold-and-thrust belt and locally rootless superimposed folds are developed in the micro-continent and subduction channel, respectively. In the second (exhumation) stage of HP-UHP rocks, a faster convergence model results in upwelling of the asthenosphere, which further leads to a detachment between the crust and lithospheric mantle of the micro-continent. A slower convergence model results in rapid exhumation of HP-UHP rocks along the north subduction channel and a typical piggy-back thrusting structure in the micro-continent. A non-convergence model produces a slab tear-off, leading to the rebound of residual lithosphere of the micro-continent. In the third and final stage, a series of back and ramp thrusts are formed in the micro-continent with the pro-continent re-subducted. Based on an analogy of our numerical results with the Western Dabie Orogen (WDO), we suggest that: (1) slab tear-off results in a rebound of residual lithosphere, which controls the two-stage syn-collisional exhumation process of HP-UHP rocks in the WDO; and (2) in contrast to the single subduction-collision system, the exhumation range of the partially molten rocks with lower viscosity and density is restricted to a specific region of the micro-continent by the Mianlue and Shangdan subduction zones, which generated the complex deformation features in the WDO.
Earth and Planetary Science Letters, Vol. 458, 1, pp. 141-151.
Mantle
carbon
Abstract: One of the most remarkable observations regarding volatile elements in the solar system is the depletion of N in the bulk silicate Earth (BSE) relative to chondrites, leading to a particularly high and non-chondritic C:N ratio. The N depletion may reflect large-scale differentiation events such as sequestration in Earth's core or massive blow off of Earth's early atmosphere, or alternatively the characteristics of a late-added volatile-rich veneer. As the behavior of N during early planetary differentiation processes is poorly constrained, we determined together the partitioning of N and C between Fe–N–C metal alloy and two different silicate melts (a terrestrial and a martian basalt). Conditions spanned a range of fO2 from ?IW?0.4 to ?IW?3.5 at 1.2 to 3 GPa, and 1400?°C or 1600?°C, where ?IW is the logarithmic difference between experimental fO2 and that imposed by the coexistence of crystalline Fe and wüstite. N partitioning ( ) depends chiefly on fO2, decreasing from to with decreasing fO2. also decreases with increasing temperature and pressure at similar fO2, though the effect is subordinate. In contrast, C partition coefficients () show no evidence of a pressure dependence but diminish with temperature. At 1400?°C, partition coefficients increase linearly with decreasing fO2 from to At 1600?°C, however, they increase from ?IW?0.7 to ?IW?2 ( to ) and decrease from ?IW?2 to ?IW?3.3 . Enhanced C in melts at high temperatures under reduced conditions may reflect stabilization of C–H species (most likely CH4). No significant compositional dependence for either N or C partitioning is evident, perhaps owing to the comparatively similar basalts investigated. At modestly reduced conditions (?IW?0.4 to ?2.2), N is more compatible in core-forming metal than in molten silicate ( ), while at more reduced conditions (?IW?2.2 to ?IW?3.5), N becomes more compatible in the magma ocean than in the metal phase. In contrast, C is highly siderophile at all conditions investigated (). Therefore, sequestration of volatiles in the core affects C more than N, and lowers the C:N ratio of the BSE. Consequently, the N depletion and the high C:N ratio of the BSE cannot be explained by core formation. Mass balance modeling suggests that core formation combined with atmosphere blow-off also cannot produce a non-metallic Earth with a C:N ratio similar to the BSE, but that the accretion of a C-rich late veneer can account for the observed high BSE C:N ratio.
Experimental investigation and peridotite oxybarometers: implications for spinel thermodynamic models and Fe3+ compatibility during generation of upper mantle melts.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 201-222.
American Journal of Science, Vol. 318, 1, pp. 141-165.
Mantle
melting
Abstract: Besides depth and temperature, CO2 and H2O, are the two most important variables in stabilizing partial melts in the Earth's mantle. However, despite decades of experimental studies on the roles of these two volatile species in affecting mantle melting, ambiguity remains in terms of the stability, composition, and proportion of volatile-bearing partial melts at depths. Furthermore, the difference in the influence of H2O versus CO2 in production of mantle melts is often inadequately discussed. Here I first discuss how as a function of depth and concentration of volatiles, the peridotite + H2O versus peridotite + CO2 near-solidus melting conditions differ - discussing specifically the concepts of saturation of volatile-bearing phases and how the mode of storage of ‘water’ and carbon affects the near solidus melting relations. This analysis shows that for the Earth's mantle beneath oceans and continents, deep, volatile-induced melting is influenced mostly by carbon, with water-bearing carbonated silicate melt being the key agent. A quantitative framework that uses the existing experimental data, allows calculation of the loci, extent of melting, and major element compositions of volatile-bearing partial melts beneath oceans and continents. How the domains of volatile-bearing melt stability are affected when possible oxygen fugacity variation at depths in the mantle is taken into account is also discussed. I show that trace amount hydrous carbonated silicate melt is likely stabilized at two or more distinct depths in the continental lithospheric mantle, at depths ranges similar to where mid-lithospheric discontinuity (MLD) and lithosphere-asthenosphere boundary (LAB) have been estimated from seismology. Whereas beneath oceans, hydrous carbonated silicate melt likely remain continuously stable from the base of the thermal boundary layer to at least 200 km or deeper depending on the prevailing oxygen fugacity at depths. Hotter mantles, such as those beneath oceans, prevent sampling strongly silica-undersaturated, carbonated melts such as kimberlites as shallower basaltic melt generation dominates. Thick thermal boundary layers, such as those in cratonic regions, on the other hand allow production of kimberlitic to carbonatitic melt only. Therefore, the increasing frequency of occurrence of kimberlites starting at the Proterozoic may be causally linked to cooling and growth of sub-continental mantles through time.
Abstract: Diamonds represent one of the few witnesses of our planet interior. They are mainly formed in the first 200 km of the lithospheric mantle, and, more rarely from the transition zone to 700 km deep. Diamonds contain a lot of information about global evolution, however their mode of formation remains poorly understood. Recent studies in high-pressure mineralogy suggest that diamonds precipitate from oxidized metasomatic fluids. The study of inclusions trapped in diamonds may provide precise information on composition, pressure, temperature and redox conditions. The aim of this study is to use the inclusions trapped in diamond as probes of the deep cycling of volatiles (C, H, halogens). Therefore, we investigate inclusions in diamonds with a systematic study of diamonds from collections. We selected 73 diamonds from three museums: National Museum of Natural History, School of Mines and Sorbonne University. The selected diamonds are studied with the help of a large range of in situ methods: RAMAN and FTIR spectrometry and X-Ray Diffraction. These analyses allow us to identify the nature of the different inclusions without damaging the gems. First results indicate silicate minerals inclusions as pyrope garnet, olivine and enstatite pyroxene. This assemblage is typical of peridotitic-type diamonds in the lithosphere.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0634-3 12p.
Africa, Botswana
deposit - Jwaneng
Abstract: To fully understand the implications of the compositional information recorded by inclusions in diamond it is vital to know if their growth was syn- or protogenetic and the extent to which they have equilibrated with diamond forming agents. The current paradigm is that the majority of inclusions in diamond are syngenetic but recently this assumption has been questioned. This study presents an integrated cathodoluminescence (CL) and electron backscatter diffraction (EBSD) study of 8 diamonds containing eclogitic inclusions: 19 pyrope-almandine garnets, 12 omphacitic clinopyroxenes, 4 sulphides, 1 coesite and 1 rutile from the Jwaneng diamond mine, Botswana. Diamond plates were sequentially polished to expose inclusions at different levels and CL imaging and EBSD were performed to constrain the relationship between diamond and inclusion growth. Despite complex growth and resorption, individual diamonds are single crystals with a homogeneous crystallographic orientation. All individual inclusions have homogeneous crystallographic orientation and no resolvable compositional zonation. The combined CL and EBSD data suggest that epitaxial inclusion-diamond growth is rare (none of 24 inclusions) and that the imposition of cubo-octahedral faces on inclusions does not necessarily result in epitaxy. Individual diamonds contain inclusions that record evidence of both syngentic and protogenetic relationships with the host diamond and in one case an inclusion appears syngenetic to the diamond core but protogenetic to the growth zone that surrounds 70% of the inclusion. These findings emphasise that inclusions in diamonds have multiple modes of origin and that in order to validate the significance of geochronological studies, further work is needed to establish that there is rapid chemical equilibration of protogenetic inclusions with diamond forming agents at mantle temperatures.
Alteration of Mn ilmentite in Horton area of Lena West.
2018 Yellowknife Geoscience Forum , p. 102-103. abstract
Canada, Northwest Territories
deposit - Lena West
Abstract: Mn-ilmenite was recognized as a kimberlite indicator mineral (KIM) in the Lena West diamond region of the Northwest Territories by Darnley Bay, Talmora and Sanatana. It includes compositions that match those found as inclusions in type IIa diamonds from Brazil and Venezuela that formed in the lower mantle. The recent determination that large, high value type IIa diamonds like the Cullinan, Koh-I-Nor, etc. also formed in the lower mantle increases the importance of Mn-ilmenites not only as a KIM resistant to tropical weathering but as a possible indicator of large high value stones. The Mn-ilmenite alteration products, pseudorutile (Fe2Ti3O9) and ferropseudobrookite (FeTi2O5) may also be used as KIMs and provide useful additional information.
Mn-ilmenites found as inclusions in diamonds range from 51 wt.% TiO2 (total wt.% 100) to 56 wt.% TiO2 (total wt.% 95). The shortfall in wt.% of the high TiO2 grains was ascribed by Kaminsky and Belasouva (2009) to some element not included in the analysis. The shortfall may also occur when some ferric iron is calculated as ferrous iron?
As most Lena West Mn-ilmenite analyses have high totals those with totals less than 96 wt.% have been considered an alteration product (“pseudorutile”). “Pseudorutile” is produced by the oxidation of FeO in ilmenite to Fe2O3 which results in an apparent loss of total weight percent when Fe is calculated as ferrous iron. A range of values approximating “ferropseudobrookite” with totals close to 100 wt.% is another alteration product of ilmenite with a loss of iron but without its oxidation to ferric iron.
The Horton area consists of a cluster of magnetic anomalies averaging ~ 200 m diameter east of a very large magnetic anomaly beneath Seahorse Lake. The Seahorse anomaly is at the focus of a train of kimberlite pathfinder elements coincident with a NNW trending KIM train characterised by Mn-ilmenite, picro-ilmenite and chromite. A parallel train of similar KIMs is focused Mn-ilmenite was recognized as a kimberlite indicator mineral (KIM) in the Lena West diamond region of the Northwest Territories by Darnley Bay, Talmora and Sanatana. It includes compositions that match those found as inclusions in type IIa diamonds from Brazil and Venezuela that formed in the lower mantle. The recent determination that large, high value type IIa diamonds like the Cullinan, Koh-I-Nor, etc. also formed in the lower mantle increases the importance of Mn-ilmenites not only as a KIM resistant to tropical weathering but as a possible indicator of large high value stones. The Mn-ilmenite alteration products, pseudorutile (Fe2Ti3O9) and ferropseudobrookite (FeTi2O5) may also be used as KIMs and provide useful additional information. Mn-ilmenites found as inclusions in diamonds range from 51 wt.% TiO2 (total wt.% 100) to 56 wt.% TiO2 (total wt.% 95). The shortfall in wt.% of the high TiO2 grains was ascribed by Kaminsky and Belasouva (2009) to some element not included in the analysis. The shortfall may also occur when some ferric iron is calculated as ferrous iron? As most Lena West Mn-ilmenite analyses have high totals those with totals less than 96 wt.% have been considered an alteration product (“pseudorutile”). “Pseudorutile” is produced by the oxidation of FeO in ilmenite to Fe2O3 which results in an apparent loss of total weight percent when Fe is calculated as ferrous iron. A range of values approximating “ferropseudobrookite” with totals close to 100 wt.% is another alteration product of ilmenite with a loss of iron but without its oxidation to ferric iron. The Horton area consists of a cluster of magnetic anomalies averaging ~ 200 m diameter east of a very large magnetic anomaly beneath Seahorse Lake. The Seahorse anomaly is at the focus of a train of kimberlite pathfinder elements coincident with a NNW trending KIM train characterised by Mn-ilmenite, picro-ilmenite and chromite. A parallel train of similar KIMs is focused on the cluster of smaller anomalies to the east. “Pseudorutile” is found over the cluster of anomalies that lie within the Horton River drainage and in the area north towards Darnley Bay. It was especially abundant with some unaltered Mn-ilmenite grains in the cuttings of a Packsack drill hole that penetrated a few feet of rusty coloured clay coincident with one of the anomalies. It does not appear to travel far. “Ferropseudobrookite” is found mostly west of the Horton River drainage, about 100 kilometers down-ice in the trains coming off the Seahorse anomaly and the cluster of smaller anomalies respectively. It represents the weathering of Mn-ilmenite in the upper part of anomalies that was carried furthest by glaciation indicating a resistance to mechanical wear. Mn-ilmenite is a useful KIM in areas of tropical weathering and is also an indicator of rare large high value diamonds. The distribution of pseudorutile and ferropseudobrookite suggests that the initial alteration of Mn-ilmenite is to pseudorutile and then ferropseudobrookite. The presence of one or the other is therefore a measure of the distance to the source.
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.
International Engineering Journal, Ouro Preto *** IN: PORT, Vol. 71, 1, pp. 27-35.
South America, Brazil
deposit - Chapada
Abstract: The Chapada Diamantina, located in the central region of the State of Bahia, is of important historical significance due to its diamond occurrences. Discovered in the nineteenth century, comprehensive research about the regional diamonds and their origins are still limited, demanding more investigation in the matter. Looking for insights about their genesis, mineral inclusions in 23 alluvial diamonds from 4 garimpos located in the Chapada Diamantina were analyzed through the use of Raman micro spectroscopy. Additionally, the characteristics of nitrogen aggregation of the host diamonds were measured using Fourier-transform infrared spectroscopy (FTIR). The diamonds from Chapada Diamantina consist mainly of well-formed crystals, with dominant dodecahedral habits, characterized by faint to very light yellow body colors, typically with green and brown radiation spots on their surface. The main surface textures observed are related to processes that took place in the late stage resorption and during the residence of the diamonds in placer environments. The diamonds are predominantly type IaAB, with a significant occurrence of poorly aggregated nitrogen (Type IaA diamond). The main mineral assemblages of the studied peridotitic inclusions refer to a harzburgitic paragenesis.
Mineralogy and Petrology, in press available, 16p.
Africa, Botswana
diamond exploration
Abstract: Bechuanaland/Botswana has a long and colourful history in exploration and mining. Here these activities are subdivided into three phases: pre-historic, historic and modern. Quarrying stone in Botswana was ongoing 500,000 years ago during the Early Stone Age (ESA). Actual mining of stones probably only started during the Middle Stone Age (MSA) i.e. post 250,000 BP, and the first prehistoric hard rock mining of specularite and limonite, likely started during the Late Stone Age (LSA) 20,000 to 2,000 BP. In east Botswana iron and copper were mined from AD 800 onwards; the mining of gold started in the thirteenth century. Historic mining started with the re-discovery of gold close to Francistown in 1865 and lasted until the 1950s. Rumours of diamonds in Bechuanaland had already surfaced in the 1880s, and it was Ngamiland, in the northwest, that was first explored systematically for diamonds and gold between 1896 and 1899. A joint initiative between Anglo American and De Beers started serious prospecting parts of eastern Bechuanaland between 1932 and 1938; and in 1938 the first diamond finds in Bechuanaland were reported. Modern mining and exploration started with the signing of an agreement in 1959, allowing Consolidated African Selection Trust Ltd. (CAST) into the Bamangwato Tribal Reserve. CAST found a few diamonds in the Motloutse River, but concluded that these were reworked and dropped the exploration rights. De Beers believed that these diamonds had come from west of the Motloutse headwaters, across the watershed in the Kalahari. This ultimately led to the discovery of the Orapa kimberlite field in 1967, a year after Botswana became independent. This discovery triggered a major exploration boom across Botswana adding important diamond-bearing kimberlites such as at Letlhakane (1968), Jwaneng (1973), Gope (1981) and Lerala (1991).
Springer Nature , book review in Mineralium Deposita diamonds mentioned p. 6-7. of review
India
diamonds
Abstract: Nonrenewable natural resources - metallic and non-metallic minerals, industrial rocks and energy resources (both organic and inorganic), have been treated in a holistic manner in this book, including two important resources (soil and water), not commonly covered in most books on this topic. For the uninitiated reader, an introductory chapter looks into some basic definitions as well as nature and characteristics of mineral deposits followed by a chapter on the different crustal processes that produce the various ore deposits in the endogenous and exogenous environments. The strength of the book lies in its critical treatment of the genetic processes of the mineral deposits, their classification and the geodynamic context of metallogeny, and coverage of sustainable development of mineral deposits with special reference to various socio-economic as well as regulatory and environmental issues that face the Indian mining industry today. The text is punctuated with examples of Indian deposits, balanced with classical deposits around the world, to cater to the interests of Indian students and the international readership. This is a book for advanced undergraduate and post-graduate students of Geology, Environmental Sciences and Natural Resource Management.
Society of Exploration Geophysicists, Interpretation, August T 299, 13p.
Canada, Northwest Territories
deposit - Tli Kwi Cho
Abstract: The Tli Kwi Cho (TKC) kimberlite complex contains two pipes, called DO-27 and DO-18, which were discovered during the Canadian diamond exploration rush in the 1990s. The complex has been used as a testbed for ground and airborne geophysics, and an abundance of data currently exist over the area. We have evaluated the historical and geologic background of the complex, the physical properties of interest for kimberlite exploration, and the geophysical surveys. We have carried out 3D inversion and joint interpretation of the potential field data. The magnetic data indicate high susceptibility at DO-18, and the magnetic inversion maps the horizontal extent of the pipe. DO-27 is more complicated. The northern part is highly magnetic and is contaminated with remanent magnetization; other parts of DO-27 have a low susceptibility. Low densities, obtained from the gravity and gravity gradiometry data, map the horizontal extents of DO-27 and DO-18. We combine the 3D density contrast and susceptibility models into a single geologic model that identifies three distinct kimberlite rock units that agree with drilling data. In further research, our density and magnetic susceptibility models are combined with information from electromagnetic data to provide a multigeophysical interpretation of the TKC kimberlite complex.
Philosphical Transactions of the Royal Society, http://dx.doi.org/ 10.1098/rsta .2017.0403 12p. Available
Mantle
plate tectonics
Abstract: Less than 25% of the volume of the juvenile continental crust preserved today is older than 3?Ga, there are no known rocks older than approximately 4?Ga, and yet a number of recent models of continental growth suggest that at least 60-80% of the present volume of the continental crust had been generated by 3?Ga. Such models require that large volumes of pre-3?Ga crust were destroyed and replaced by younger crust since the late Archaean. To address this issue, we evaluate the influence on the rock record of changing the rates of generation and destruction of the continental crust at different times in Earth's history. We adopted a box model approach in a numerical model constrained by the estimated volumes of continental crust at 3?Ga and the present day, and by the distribution of crust formation ages in the present-day crust. The data generated by the model suggest that new continental crust was generated continuously, but with a marked decrease in the net growth rate at approximately 3?Ga resulting in a temporary reduction in the volume of continental crust at that time. Destruction rates increased dramatically around 3 billion years ago, which may be linked to the widespread development of subduction zones. The volume of continental crust may have exceeded its present value by the mid/late Proterozoic. In this model, about 2.6-2.3 times of the present volume of continental crust has been generated since Earth's formation, and approximately 1.6-1.3 times of this volume has been destroyed and recycled back into the mantle.
Abstract: We present the joint interpretation of airborne electromagnetic and aeromagnetic data, acquired to study kimberlite pipes. We analyse the data surveyed in 2005 over Drybones Bay, Archean Slave Province of the Northwest Territories, northern Canada. This area hosts a recently discovered kimberlite province with >150 kimberlite pipes. Magnetic and electromagnetic data were each one modelled by 1D inversion. For magnetic data we inverted vertical soundings built through upward continuations of the measured data at various altitudes. The validity of the method was prior verified by tests on synthetic data. Electromagnetic data were processed and inverted using the modified AarhusINV code, with Cole-Cole modelling, in order to take into account induced polarization effects, consisting in negative voltages and otherwise skewed transients. The integrated study of the two kinds of data has led to a better understanding of the structures at depth, even though the comparison between the magnetic and the electromagnetic models shows the different sensitivity of the two methods with respect to the geological structure at Drybones Bay.
The Extractive Industries and Society, Vol. 4, pp. 489-496.
Africa, Guinea
artisanal mining
Abstract: The period of protracted conflict in Sierra Leone and Liberia brought the politics of alluvial diamond mining in West Africa to the forefront of academic and policy-oriented discussions. Using social contract theory, this paper moves away from discussions on how minerals have perpetuated conflict in the region, and interrogates how the governance of diamond mining in Guinea impacts regime stability and social insecurity. More importantly, it attempts to illustrate how artisanal diamond mining contributes to stability. The paper situates this discussion within the broad spectrum of the social contract between state and citizens and an analysis of how these are at play in diamond mining areas. It illustrates how artisanal diamond mining enables specific social contracts to emerge and how this in turn contributes to stability in the regions where they are extracted.
Abstract: discovered a unique type of diamond in the Tolbachik volcano frozen lava- in the Kamchatka Peninsula of far East Russia. Scientists from all over the world are quite impressed with these findings and they now classify the gemstones as completely new & unique types of diamonds. The new types of diamonds are named after the place they were found- Tolbachik Diamonds. According to geologists, the Tolbachik diamonds were born under a very peculiar natural formation where the gemstones crystalized under the direct influence of the electric discharge of lightning and the pressure from the volcanic gases. Geologists also agree that several decades ago experts wanted to create a synthetic type of diamond where the result would have been exactly the one now created naturally in the Tolbachik diamonds. In the early 60s the French wanted to create a synthetic diamond where they would use gas as primary substance and then using electric discharge for the crystallization of the gemstone. At a closer look, the Tolbachik diamonds are similar to the synthetic diamonds found on today’s market. However, the main mineralogical and geological structure of the diamond is unlike any other type of precious stone discovered until now. These diamonds are particularly large in size (200 and up to 700 microns!). According to scientists, this is actually the first time when they discovered lava rocks that show such very dense diamond content. This discovery actually gave way to very intense disputes among specialists regarding if the Lab Grown diamonds should be regarded as ‘synthetic’ any longer. The Tolbachik diamonds resemble almost in every aspect the structure of the lab grown diamonds and they are naturally formed in lava. Therefore, the Lab Grown diamonds should be more highly regarded just as the naturally mined diamonds are. Another important fact to keep in mind is that only a true expert is actually able to tell the difference between the Tolbachik diamonds created naturally and the lab grown diamonds. Someone who is not an expert would simply regard the Tolbachik diamonds as synthetic diamonds, given their different structure from the regular diamonds formed in magmatic melt. Experts found an extremely high quantity of diamonds (several hundreds of pieces) in only a small sample of frozen lava in the Kamchatka Lava in Russia. When or how these gemstones will enter the fascinating diamond industry, and if the price of jewelry featuring Tolbachik diamonds will be more budget friendly is yet to be settled…
Abstract: Lamprophyres represent hydrous alkaline mantle melts that are a unique source of information about the composition of continental lithosphere. Throughout southwest Britain, post-Variscan lamprophyres are (ultra)potassic with strong incompatible element enrichments. Here we show that they form two distinct groups in terms of their Sr and Nd isotopic compositions, occurring on either side of a postulated, hitherto unrecognized terrane boundary. Lamprophyres emplaced north of the boundary fall on the mantle array with ?Nd ?1 to +1.6. Those south of the boundary are enriched in radiogenic Sr, have initial ?Nd values of ?0.3 to ?3.5, and are isotopically indistinguishable from similar-aged lamprophyres in Armorican massifs in Europe. We conclude that an Armorican terrane was juxtaposed against Avalonia well before the closure of the Variscan oceans and the formation of Pangea. The giant Cornubian Tin-Tungsten Ore Province and associated batholith can be accounted for by the fertility of Armorican lower crust and mantle lithosphere.
Abstract: Ophiolitic peridotites represent variously depleted residues of the primitive mantle after multiple episodes of partial melting, melt extraction, and melt-rock interactions. They display a wide range of compositional and geochemical heterogeneities at different scales, and their incompatible bulk-rock compositions and mineral chemistries are commonly inconsistent with their evolution through simple partial melting processes at shallow mantle depths. Approaching these issues from different perspectives, the papers in this volume concentrate on (1) melt evolution and magmatic construction of ophiolites in various tectonic settings, and (2) the occurrence of microdiamonds, ultrahigh-pressure (UHP) minerals, and crustal material as inclusions in ophiolitic chromitites and peridotites. Crustal and mantle rock units exposed in different ophiolites show that the mantle melt sources of ophiolitic magmas undergo progressive melting, depletion, and enrichment events, constantly modifying the melt compositions and the mineralogical and chemical makeup of residual peridotites. Formation and incorporation of microdiamonds and UHP minerals into chromite grains occurs at depths of 350-660 km in highly reducing conditions of the mantle transition zone. Carbon for microdiamonds and crustal minerals are derived from subduction-driven recycling of surface material. Host peridotites with their UHP mineral and diamond inclusions are transported into shallow mantle depths by asthenospheric upwelling, associated with either slab rollback-induced channel flow or superplumes. Decompression melting of transported mantle rocks beneath oceanic spreading centers and their subsequent flux melting in mantle wedges result in late-stage formation of podiform chromitites during the upper mantle petrogenesis of ophiolites. Future studies should demonstrate whether diamonds and UHP minerals also occur in peridotites and chromitites of nonsubduction-related ophiolites.
Optimizing carbon capture and storage in kimberlite tailings for environmental benefit and operational efficiency.
Vancouver Kimberlite Cluster, Nov. 6, 1p. Abstract
Global
carbon
Abstract: Ultramafic mine tailings, including those from kimberlite-hosted diamond mines, offer potential operational and environmental benefit through reaction with carbon dioxide from air and power plant flue gas. The carbon dioxide is sequestered from the environment through the precipitation of carbonate minerals, thus reducing or offsetting the greenhouse gas emissions associated with mining. Additional benefits can include tailings stabilization, dust reduction, acid mine drainage prevention, and toxic metal encapsulation. In this talk I will present an overview of the processes and controls on carbonation reactions within tailings at active mines with a focus on acceleration of carbon sequestration within kimberlite tailings. Carbonation reactions can be limited by transport (rate of CO2 supply) and by reaction kinetics (mineral dissolution or mineral precipitation). Field studies of accidental passive carbonation within tailings at operating mines supplemented with laboratory experiment and reactive transport modelling has been key to identifying the rate limits to carbon sequestration at each mine site. With these limits identified, acceleration approaches can be tailored to the local climate, gangue mineralogy, and mine design, all of which can exert a primary control on carbon sequestration rates. The result is a methodology for evaluating the carbon sequestration potential of a mine site and a toolbox of acceleration strategies which together allow for site selection and project design. In the coming years, these systems will be deployed on site at active mines to further test and advance the technology. I will end with a perspective on the role that mining of ultramafic-hosted deposits can play in achieving net negative CO2 emissions as is projected to be required by the end of this century if we are to avoid net global warming in excess of two degrees centigrade.
Mineralogy and Petrology, 10.1007/s00710-018-0608-5 8p.
Africa, Botswana
indicator minerals
Abstract: The majority of the diamond mines in Botswana were discovered as a direct consequence of soil sampling for indicator minerals such as garnet and picroilmenite. Over the past 60 years the application of soil sampling for indicator minerals as a primary exploration tool has declined while aeromagnetic surveys have increased in popularity. The rate of kimberlite discovery in Botswana has declined significantly. The obvious magnetic kimberlites have been discovered. The future of new kimberlite discoveries is once again dependent on soil sampling for kimberlite indicator minerals. It is essential to have an in depth understanding of the transport mechanism of kimberlite indicator minerals from the kimberlite to the modern day surface of the Kalahari Formation, which is solely via termite bioturbation. Field observations indicate that the concentration of indicator minerals at surface is directly dependent on the physical characteristics and capabilities as well as behavioural patterns of the particular termite species dominant in the exploration area. The discovery of future diamond mines in Botswana will be closely associated with an in depth understanding of the relationship between size and concentration of kimberlite indicator minerals in surface soils and the seasonal behaviour, depth penetration capabilities, earthmoving efficiencies and mandible size of the dominant termite species within the exploration area. Large areas in Botswana, where kimberlite indicator minerals recovered from soil samples have been described as distal from source or background, will require re-evaluation. Without detailed termite studies the rate of discovery will continue to decline.
Contrasting termite transported indicator mineral concentrations in the Kgalafadi of central district Botswana: Macrotermes micaelseni vs Hodotermes mossambicus.
Vancouver Kimberlite Cluster, March 9, 1p. Abstract
Abstract: Here, we present studies of natural SiC that occurs in situ in tuff related to the Miocene alkaline basalt formation deposited in northern part of Israel. Raman spectroscopy, SEM and FIB-assisted TEM studies revealed that SiC is primarily hexagonal polytypes 4H-SiC and 6H-SiC, and that the 4H-SiC polytype is the predominant phase. Both SiC polytypes contain crystalline inclusions of silicon (Sio) and inclusions of metal-silicide with varying compositions (e.g. Si58V25Ti12Cr3Fe2, Si41Fe24Ti20Ni7V5Zr3, and Si43Fe40Ni17). The silicides crystal structure parameters match Si2TiV5 (Pm-3 m space group, cubic), FeSi2Ti (Pbam space group, orthorhombic), and FeSi2 (Cmca space group, orthorhombic) respectively. We hypothesize that SiC was formed in a local ultra-reduced environment at respectively shallow depths (60-100 km), through a "desilification" reaction of SiO2 with highly reducing fluids (H2O-CH4-H2-C2H6) arisen from the mantle "hot spot" and passing through alkaline basalt magma reservoir. SiO2 (melt) interacting with the fluids may originate from the walls of the crustal rocks surrounding this magmatic reservoir. The "desilification" process led to the formation of SiC and the reduction of metal-oxides to native metals, alloys, and silicides. The latter were trapped by SiC during its growth. Hence, interplate "hot spot" alkali basalt volcanism can now be included as a geological environment where SiC, silicon, and silicides can be found.
Earth and Planteray Science Letters, Vol. 489, pp. 84-91.
Mantle
carbonate
Abstract: Carbonate minerals are important hosts of carbon in the crust and mantle with a key role in the transport and storage of carbon in Earth's deep interior over the history of the planet. Whether subducted carbonates efficiently melt and break down due to interactions with reduced phases or are preserved to great depths and ultimately reach the core-mantle boundary remains controversial. In this study, experiments in the laser-heated diamond anvil cell (LHDAC) on layered samples of dolomite (Mg,?Ca)CO3 and iron at pressure and temperature conditions reaching those of the deep lower mantle show that carbon-iron redox interactions destabilize the MgCO3 component, producing a mixture of diamond, Fe7C3, and (Mg,?Fe)O. However, CaCO3 is preserved, supporting its relative stability in carbonate-rich lithologies under reducing lower mantle conditions. These results constrain the thermodynamic stability of redox-driven breakdown of carbonates and demonstrate progress towards multiphase mantle petrology in the LHDAC at conditions of the lowermost mantle.
Journal of Asian Earth Sciences, Vol. 154, pp. 354-368.
Russia, Yakutia
carbonatite -Seligdar
Abstract: The Paleoproterozoic Seligdar magnesiocarbonatite intrusion of the Aldan-Stanovoy shield in Russia underwent extensive postmagmatic hydrothermal alteration and metamorphic events. This study comprises new isotopic (Sr, Nd, C and O) data, whole-rock major and trace element compositions and trace element characteristics of the major minerals to gain a better understanding of the source and the formation process of the carbonatites. The Seligdar carbonatites have high concentrations of P2O5 (up to 18?wt%) and low concentrations of Na, K, Sr and Ba. The chondrite-normalized REE patterns of these carbonatites display significant enrichments of LREE relative to HREE with an average La/Ybcn ratio of 95. Hydrothermal and metamorphic overprints changed the trace element characteristics of the carbonatites and their minerals. These alteration processes were responsible for Sr loss and the shifting of the Sr isotopic compositions towards more radiogenic values. The altered carbonatites are further characterized by distinct 18O- and 13C-enrichments compared to the primary igneous carbonatites. The alteration most likely resulted from both the percolation of crustal-derived hydrothermal fluids and subsequent metamorphic processes accompanied by interaction with limestone-derived CO2. The narrow range of negative ?Nd(T) values indicates that the Seligdar carbonatites are dominated by a homogenous enriched mantle source component that was separated from the depleted mantle during the Archean.
Mineralogical Magazine, Vol. 81, 6, pp. 1367-1395.
Africa, Malawi
carbonatite - Chilwa
Abstract: Carbonatites are enriched in critical raw materials such as the rare earth elements (REE), niobium, fluorspar and phosphate. A better understanding of their fluid regimes will improve our knowledge of how to target and exploit economic deposits. This study shows that multiple fluid phases penetrated the surrounding fenite aureole during carbonatite emplacement at Chilwa Island, Malawi. The first alkaline fluids formed the main fenite assemblage and later microscopic vein networks contain the minerals of potential economic interest such as pyrochlore in high-grade fenite and RE minerals throughout the aureole. Seventeen samples of fenite rock from the metasomatic aureole around the Chilwa Island carbonatite complex were chosen for study (Natural History Museum, London collection BM1968 P37). In addition to the main fenite assemblage of feldspar and aegirine ± arfvedsonite, riebeckite and richterite, the fenite contains micro-mineral assemblages including apatite, ilmenite, rutile, magnetite, zircon, RE minerals and pyrochlore in vein networks. Petrography using SEM-EDX showed that the RE minerals (monazite, bastnäsite and parisite) formed later than the fenite feldspar, aegirine and apatite and provide evidence of REE mobility into all grades of fenite. Fenite apatite has a distinct negative Eu anomaly (determined by LA-ICP-MS) that is rare in carbonatite-associated rocks and interpreted as related to pre-crystallisation of plagioclase and co-crystallisation with K-feldspar in the fenite. The fenite minerals have consistently higher mid REE/light REE ratios (La/Sm = ~1.3 monazite, ~1.9 bastnäsite, ~1.2 parisite) than their counterparts in the carbonatites (La/Sm = ~2.5 monazite, ~4.2 bastnäsite, ~3.4 parisite). Quartz in the low- and medium-grade fenite hosts fluid inclusions, typically a few µm in diameter, secondary and extremely heterogeneous. Single phase, 2- and 3-phase, single solid and multi solid-bearing examples are present, with 2-phase the most abundant. Calcite, nahcolite, burbankite and barite were found in the inclusions. Decrepitation of inclusions occurred at around 200?C before homogenisation but melting temperature data indicate that the inclusions contain relatively pure CO2. A minimum salinity of around 24 wt.% NaCl equivalent was determined. Among the trace elements in whole rock analyses, enrichment in Ba, Mo, Nb, Pb, Sr, Th and Y and depletion in Co, Hf and V are common to carbonatite and fenite but enrichment in carbonatitic type elements (Ba, Nb, Sr, Th, Y, and REE) generally increases towards the inner parts of the aureole. A schematic model contains multiple fluid events, related to first and second boiling of the magma, accompanying intrusion of the carbonatites at Chilwa Island, each contributing to the mineralogy and chemistry of the fenite. The presence of distinct RE mineral micro-assemblages in fenite at some distance from carbonatite could be developed as an exploration indicator of REE enrichment.
Mineralogical Magazine, Vol. 81, 6, pp. 1367-1395.
Africa, Malawi
carbonatite
Abstract: Carbonatites are enriched in critical raw materials such as the rare earth elements (REE), niobium, fluorspar and phosphate. A better understanding of their fluid regimes will improve our knowledge of how to target and exploit economic deposits. This study shows that multiple fluid phases penetrated the surrounding fenite aureole during carbonatite emplacement at Chilwa Island, Malawi. The first alkaline fluids formed the main fenite assemblage and later microscopic vein networks contain the minerals of potential economic interest such as pyrochlore in high-grade fenite and RE minerals throughout the aureole. Seventeen samples of fenite rock from the metasomatic aureole around the Chilwa Island carbonatite complex were chosen for study (Natural History Museum, London collection BM1968 P37). In addition to the main fenite assemblage of feldspar and aegirine ± arfvedsonite, riebeckite and richterite, the fenite contains micro-mineral assemblages including apatite, ilmenite, rutile, magnetite, zircon, RE minerals and pyrochlore in vein networks. Petrography using SEM-EDX showed that the RE minerals (monazite, bastnäsite and parisite) formed later than the fenite feldspar, aegirine and apatite and provide evidence of REE mobility into all grades of fenite. Fenite apatite has a distinct negative Eu anomaly (determined by LA-ICP-MS) that is rare in carbonatite-associated rocks and interpreted as related to pre-crystallisation of plagioclase and co-crystallisation with K-feldspar in the fenite. The fenite minerals have consistently higher mid REE/light REE ratios (La/Sm = ~1.3 monazite, ~1.9 bastnäsite, ~1.2 parisite) than their counterparts in the carbonatites (La/Sm = ~2.5 monazite, ~4.2 bastnäsite, ~3.4 parisite). Quartz in the low- and medium-grade fenite hosts fluid inclusions, typically a few µm in diameter, secondary and extremely heterogeneous. Single phase, 2- and 3-phase, single solid and multi solid-bearing examples are present, with 2-phase the most abundant. Calcite, nahcolite, burbankite and barite were found in the inclusions. Decrepitation of inclusions occurred at around 200?C before homogenisation but melting temperature data indicate that the inclusions contain relatively pure CO2. A minimum salinity of around 24 wt.% NaCl equivalent was determined. Among the trace elements in whole rock analyses, enrichment in Ba, Mo, Nb, Pb, Sr, Th and Y and depletion in Co, Hf and V are common to carbonatite and fenite but enrichment in carbonatitic type elements (Ba, Nb, Sr, Th, Y, and REE) generally increases towards the inner parts of the aureole. A schematic model contains multiple fluid events, related to first and second boiling of the magma, accompanying intrusion of the carbonatites at Chilwa Island, each contributing to the mineralogy and chemistry of the fenite. The presence of distinct RE mineral micro-assemblages in fenite at some distance from carbonatite could be developed as an exploration indicator of REE enrichment.
Abstract: Earth's global magnetic field shields us from the dangers of space, including harmful levels of Solar radiation. This shield is one of the major reasons why life has survived on Earth, and it all starts in the core, where the geomagnetic field is generated by the turbulent motion of liquid iron deep inside the planet. It's the so-called geodynamo. Scientists like DTM's Peter Driscoll devote their career to understanding the nature of such planetary dynamics. In 2016, Driscoll's research suggested that in ancient times Earth may have been significantly different, with prolonged periods of weak field intensity and strange multipolar geometry with many poles in contrast to the modern north-and-south pole orientation. Now, in a recent paper co-authored with DTM's Cian Wilson, Driscoll finds that geologic evidence for such a complex magnetic field will be very difficult to identify. In preparation for his Neighborhood Lecture on November 8, 2018, Driscoll answers some questions about his latest research and why studying the geodynamo is critical to understanding the history of our planet.
Earth and Planetary Science Letters, Vol. 494, 1, pp. 92-98.
Mantle
water
Abstract: In this study, we present new experimental constraints on the phase stability and thermal equation of state of an important hydrous phase, ?-AlOOH, using synchrotron X-ray diffraction up to 142 GPa and 2500 K. Our experimental results have shown that ?-AlOOH remains stable at the whole mantle pressure-temperature conditions above the D? layer yet will decompose at the core-mantle boundary because of a dramatic increase in temperature from the silicate mantle to the metallic outer core. At the bottom transition zone and top lower mantle, the formation of ?-AlOOH by the decomposition of phase Egg is associated with a ?2.1-2.5% increase in density (?) and a ?19.7-20.4% increase in bulk sound velocity (V?). The increase in ? across the phase Egg to ?-AlOOH phase transition can facilitate the subduction of ?-AlOOH to the lower mantle. Compared to major lower-mantle phases, ?-AlOOH has the lowest ? but greatest V?, leading to an anomalous low ? /V? ratio which can help to identify the potential presence of ?-AlOOH in the region. More importantly, water released from the breakdown of ?-AlOOH at the core-mantle boundary could lower the solidus of the pyrolitic mantle to cause partial melting and/or react with Fe in the region to form the low-velocity FeO2Hx phase. The presence of partial melting and/or the accumulation of FeO2Hx phase at the CMB could be the cause for the ultra-low velocity zone. ?-AlOOH is thus an important phase to transport water to the lowermost mantle and helps to understand the origin of the ultra-low velocity zone.
Textural evidences of late stage carbonate dissolution precipitation and stable isotope exchange re-equilibration in the Kangankunde carbonatite complex, Malawi.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 39-40.
Africa, Malawi
deposit - Kangankunde
Abstract: The Kangankunde carbonatite complex, southern Malawi is an eroded remnant of a carbonatite depocentre belonging to the Lower Cretaceous Chilwa Alkaline Province (Garson and Campbell-Smith, 1965; Karmalkar et al., 2010). The carbonatite complex consists of fenites, carbonated agglomeratic breccias and ankeritic-ferron dolomite carbonatite with sporadic patches of REE minerals. Coarse pegmatitic siderite and ankeritic carbonatite hosts exotic cavities and vugs that contain RE minerals like monazite, synchysite, bastnasite, and florencite-goyazite along with barite and strontianite. Bright green monazite occurs with thick quartz veins and as disseminations within host carbonatites (Garson and Campbell-Smith, 1965; Holt, 1965). Texturally, the host carbonatite exhibits euhedral to subhedral rhombohedra of ankerite-calcite and dolomite set in a fine groundmass. However samples close to mineralized zones show streaky textures, streaming effects and patterns that resemble remobilization and fluidization. Textural evidences such as presence of pseudomorphs containing the REE mineral assemblage, veinlets and drusy cavities indicate that REE mineralization replaced earlier formed carbonates (Wall et al. 1994). SEM micromorphology of such carbonatites show several dissolution-precipitation features indicating that there was a pervasive fluid interaction with the host subsequent to cooling and crystallization. Within cavities and vugs, the precipitation is closely associated with monazite veins and formation of minerals like collinsite, synchysite and rare aragonite (Duraiswami and Shaikh, 2010). Carbon and oxygen isotope ratios were determined on 8 bulk carbonatite powders from Kangankunde using Thermo Fisher Scientific GasBench II, equipped with autosampler (CTC Analytics AG, Zwingen, Switzerland), and coupled to a Delta Plus XP Mass Spectrometer at NGRI, Hyderabad. International Standards NBS-19-1 and NBS-18-1 plus internal standards were used. The internal precision (1s) measured for raw d18O and d13C was 0.04–0.08‰ and 0.03 to 0.06 respectively. The ?13C VS V-PDB show restricted range (-0.31 to -2.76) but ?18O VS V-SMOW values vary widely (8.22 to 24.5). The samples analysed in the present study plot outside the Primary Igneous Carbonatite field (Demény et al., 2004) and are related either to carbo-hydrothermal fluids or alteration of dolomite by a later, cooling, deuteric fluid (Fendley et al., 2017). This study provides conclusive textural evidences (pseudomorphic replacements, atolls, iron-oxide exsolution fringes from dolomite/ankerite, coloform banding, secondary veinlets, etc.) to supports geochemical and stable isotopic inferences about the role of late stage hydrothermal fluids and subsequent alterations in the Kangankunde carbonatite complex (Wall and Mariano, 1996, Wall et al., 1994, Doroshkevich et al., 2009, Fendley et al., 2017).
Abstract: Natural-color blue diamonds are among the rarest and most valuable gemstones. Gray and violet diamonds are also included here, as these diamonds can coexist on a color continuum with blue diamonds. More so than most other fancy colors, many diamonds in this color range are sourced from specific locations-the Cullinan mine in South Africa and the Argyle mine in Australia. Although blue color is often associated with boron impurities, the color of diamonds in this range (including gray and violet) also originates from simple structural defects produced by radiation exposure or from more complex defects involving hydrogen. These different mechanisms can be characterized by absorption and luminescence spectroscopy. A fourth mechanism-micro-inclusions of grayish clouds or tiny graphite particles in gray diamonds-can be distinguished through microscopy. In this article, we summarize prior research as well as collected data such as color and carat weight on more than 15,000 naturally colored blue/gray/violet diamonds from the GIA database (along with an analysis of spectroscopic data on a subset of 500 randomly selected samples) to provide an unprecedented description of these beautiful gemstones.
Abstract: The Cenozoic East African rift (EAR), Cameroon Volcanic Line (CVL), and Atlas Mountains formed on the slow-moving African continent, which last experienced orogeny during the Pan-African. We synthesize primarily geophysical data to evaluate the role of magmatism in shaping Africa's crust. In young magmatic rift zones, melt and volatiles migrate from the asthenosphere to gas-rich magma reservoirs at the Moho, altering crustal composition and reducing strength. Within the southernmost Eastern rift, the crust comprises ~20% new magmatic material ponded in the lower crust and intruded as sills and dikes at shallower depths. In the Main Ethiopian Rift, intrusions comprise 30% of the crust below axial zones of dike-dominated extension. In the incipient rupture zones of the Afar rift, magma intrusions fed from crustal magma chambers beneath segment centers create new columns of mafic crust, as along slow-spreading ridges. Our comparisons suggest that transitional crust, including seaward dipping sequences, is created as progressively smaller screens of continental crust are heated and weakened by magma intrusion into 15-20 km thick crust. In the 30 Ma Recent CVL, which lacks a hot spot age progression, extensional forces are small, inhibiting the creation and rise of magma into the crust. In the Atlas orogen, localized magmatism follows the strike of the Atlas Mountains from the Canary Islands hot spot toward the Alboran Sea. CVL and Atlas magmatism has had minimal impact on crustal structure. Our syntheses show that magma and volatiles are migrating from the asthenosphere through the plates, modifying rheology, and contributing significantly to global carbon and water fluxes.
Abstract: Geochemical characterization including mineralogical measurements and kinetic testing was completed on samples from the Montviel carbonatite deposit, located in Quebec (Canada). Three main lithological units representing both waste and ore grades were sampled from drill core. A rare earth element (REE) concentrate was produced through a combination of gravity and magnetic separation. All samples were characterized using different mineralogical techniques (i.e., quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), X-ray diffraction (XRD), and scanning electron microscopy with X-ray microanalysis (SEM-EDS)) in order to quantify modal mineralogy, liberation, REE deportment and composition of REE-bearing phases. The REE concentrate was then submitted for kinetic testing (weathering cell) in order to investigate the REE leaching potential. The mineralogical results indicate that: (i) the main REE-bearing minerals in all samples are burbankite, kukharenkoite-Ce, monazite, and apatite; (ii) the samples are dominated by REE-free carbonates (i.e., calcite, ankerite, and siderite); and (iii) LREE is more abundant than HREE. Grades of REE minerals, sulfides and oxides are richer in the concentrate than in the host lithologies. The geochemical test results show that low concentrations of light REE are leached under kinetic testing conditions (8.8-139.6 ?g/L total light REE). These results are explained by a low reactivity of the REE-bearing carbonates in the kinetic testing conditions, low amounts of REE in solids, and by precipitation of secondary REE minerals.
Earth Planetary Science Letters, Vol. 491, pp. 148-159.
Africa, Australia, Canada, Europe
geothermometry
Abstract: The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100-150 km. The metasomatic component can consist of 0.5-1 wt% water bound in amphibole, antigorite and chlorite, ?0.2 wt% water plus potassium to form phlogopite, or ?5 wt% CO2 plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100-200?°C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.
Abstract: A model based on a thermodynamic framework for CO2 concentrations and speciation in natural silicate melts at graphite/diamond-saturated to fluid-saturated conditions is presented. The model is simultaneously calibrated with graphite-saturated and fluid-saturated conditions allowing for consistent model predictions across the CCO buffer. The model was calibrated using water-poor (?1?wt% H2O) silicate melts from graphite- to CO2-fluid-saturation over a range of pressure (P?=?0.05-3?GPa), temperature (T?=?950-1600?°C), composition (foidite-rhyolite; NBO?=?0.02-0.92; wt% SiO2?~?39-77, TiO2?~?0.1-5.8, Al2O3?~?7.5-18, FeO?~?0.2-24 MgO?~?0.1-24, CaO?~?0.3-14, Na2O~1-5, K2O?~?0-6), and fO2 (~QFM +1.5 to ~QFM ?6). The model can predict CO2 concentrations for a wide range of silicate melt compositions from ultramafic to rhyolitic compositions, i.e., melts that dissolve carbon only as carbonate anions CO32- and those that dissolve carbon both as CO32- and as molecular CO2mol as a function of pressure, temperature, and oxygen fugacity. The model also does a reasonable job in capturing CO2 solubility in hydrous silicate melts with ?2-3?wt% H2O. New CO2 solubility experiments at pressures >3?GPa suggest that the newly developed CO2 solubility model can be satisfactorily extrapolated to ~4-5?GPa. Above 5?GPa the model poorly reproduces experimental data, likely owing to structural change in silicate melt at pressures above 5?GPa. An Excel spreadsheet and a Matlab function are provided as online supplementary materials for implementing the new CO2 solubility model presented here.
Diamond & Related Materials, Vol. 89, pp. 101-107.
Mantle
boron
Abstract: Influence of growth medium composition on the efficiency of boron doping of carbonado-like diamond at 8-9 GPa was studied by diluting the C-B growth system with metallic solvents of carbon, Co and Ni. Addition of these metals to the original system leads to a decrease in the synthesis temperature, degree of doping with boron and suppression of superconductivity in diamond. According to XPS analysis, content of substitutional boron is equal to 0.07, 0.16 and 0.39 at.% in diamonds obtained in Co-C-B, Ni-C-B and C-B growth systems, respectively. Metallic behavior at normal temperatures and superconductivity below 5 K in diamond, synthesized in C-B system, change to semiconducting character of conductivity down to 2 K in diamonds obtained in the diluted systems; a faint hint of superconducting transition at 2 K was detected in the case of diamond grown in Ni-C-B system. By comparing phase composition of the inclusions and the doping efficiency of the diamonds, we are able to suggest that high chemical affinity of boron to boride-forming metals hinders the boron doping of diamond. The heavily boron-doped carbonado-like diamond compacts demonstrate high electrochemical activity in aqueous solutions and can be used as miniature electrodes in electrosynthesis and electroanalysis.
Abstract: We report preliminary results of a systematic search for fluid/melt microinclusions in mantle minerals. “Dusty” garnets from xenolith XRV6 [1], a heavily metasomatised Type I eclogite from Roberts Victor mine, SA, carry many microinclusions (<1 ?m). FTIR analyses of "dusty" zones indicate the presence of molecular water in the inclusions and hydroxyl groups in the garnet. EPMA analysis of 136 microinclusions constrains the bulk composition of the microinclusions. Compared to the host garnet, they are enriched in TiO2, FeO, CaO, Na2O and K2O and depleted in Al2O3 and MgO. The silica contents seem to be similar to that of the host garnet. Figure 1: a. Backscatter image of the microinclusions in XRV6 garnet. b. K2O vs. MgO of the clear garnet (red) and the microinclusions (+ their surrounding garnet, blue). Most of the elements form compositional mixing arrays of microinclusion+garnet (Fig. 1b). The arrays trend away from the compositions of large melt pools or secondary minerals found in the xenolith. They point towards the array of silicic to low-Mg carbonatitic high density fluids (HDFs) trapped in diamonds, indicating the role of such fluids in mantle metasomatism.
Diamond potential of the Dehcho region. Horn Plateau
2018 Yellowknife Geoscience Forum , p. 104. abstract
Canada, Northwest Territories
geochemistry
Abstract: Our knowledge of the diamond potential in the Dehcho region has progressed significantly in the past decade. We now recognize that the central Dehcho represents a world class diamond exploration district. Continued scientific and industry work in the area have clearly shown that the diamond potential of this area may be of the same magnitude as the Lac de Gras region, which hosts active diamond mines. The evidence for high diamond potential in the Dehcho, includes abundant Kimberlite Indicator Minerals (KIM) from stream sediment sampling work, a diamond found in a stream sediment sample, 39 drilled kimberlites, some of which are diamondiferous, and numerous untested kimberlite-like geophysical anomalies from both government and industry data. Recent work at the University of Alberta has shown that deep Earth conditions in the area of the Horn Plateau may be as favorable for diamond generation and preservation as the Lac de Gras region and that there that there may be multiple generations of kimberlites present in the region. Given the relative paucity of exploration work and geoscience data in the Dehcho region, the available evidence is strongly suggestive of the possibility of the presence of diamond deposits.
Abstract: Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization. Although many attempts have been made in the literature to categorize and name fenites, it is recommended that the IUGS metamorphic nomenclature be used to describe predominant mineralogy and textures. Complexing anions greatly enhance the solubility of REE and Nb in these fenitizing fluids, mobilizing them into the surrounding country rock, and precipitating REE- and Nb-enriched micro-mineral assemblages. As such, fenites have significant potential to be used as an exploration tool to find mineralized intrusions in a similar way alteration patterns are used in other ore systems, such as porphyry copper deposits. Strong trends have been identified between the presence of more complex veining textures, mineralogy and brecciation in fenites with intermediate stage Nb-enriched and later stage REE-enriched magmas. However, compiling this evidence has also highlighted large gaps in the literature relating to fenitization. These need to be addressed before fenite can be used as a comprehensive and effective exploration tool.
Geochemistry, Geophysics, Geosystems, Vol. 19, 3, pp. 865-881.
Mantle
subduction
Abstract: The buoyancy of continental crust opposes its subduction to mantle depths, except where mineral reactions substantially increase rock density. Sluggish kinetics limit such densification, especially in dry rocks, unless deformation and hydrous fluids intervene. Here we document how hydrous fluids in the subduction channel invaded lower crustal granulites at 50-60 km depth through a dense network of probably seismically induced fractures. We combine analyses of textures and mineral composition with thermodynamic modeling to reconstruct repeated stages of interaction, with pulses of high-pressure (HP) fluid at 650-6708C, rehydrating the initially dry rocks to micaschists. SIMS oxygen isotopic data of quartz indicate fluids of crustal composition. HP growth rims in allanite and zircon show uniform U-Th-Pb ages of 65 Ma and indicate that hydration occurred during subduction, at eclogite facies conditions. Based on this case study in the Sesia Zone (Western Italian Alps), we conclude that continental crust, and in particular deep basement fragments, during subduction can behave as substantial fluid sinks, not sources. Density modeling indicates a bifurcation in continental recycling: Chiefly mafic crust, once it is eclogitized to >60%, are prone to end up in a subduction graveyard, such as is tomographically evident beneath the Alps at 550 km depth. By contrast, dominantly felsic HP fragments and mafic granulites remain positively buoyant and tend be incorporated into an orogen and be exhumed with it. Felsic and intermediate lithotypes remain positively buoyant even where deformation and fluid percolation allowed them to equilibrate at HP.
Reviews in Mineralogy & Geochemistry, Vol. 83, Chap. 1, pp. 1-12.
Technology
geochronology
Abstract: Question: Why "Petrochronology"? Why add another term to an already cluttered scientific lexicon? Answer: Because petrologists and geochronologists need a term that describes the unique, distinctive way in which they apply geochronology to the study of igneous and metamorphic processes. Other terms just won’t do.
Abstract: For many years, Israel was considered to be an unlikely area for discovering commercially exploitable minerals. Studies only reported phosphorites, which are processed at the Dead Sea Works into plant fertilizer, building materials and chemicals for the cosmetic industry. However, a contemporary look at Israel’s economic mineral deposits suggests that this view needs substantial revision. This chapter provides an overview of all the economic deposits in Israel and then presents the second author's investigations in more detail. Aside from estimates of the industrial value of these deposits, methods for further probes are provided. Examples of the ways in which descriptions of geological formations, mineralogical analyses and geophysical methods can be applied to estimates of economic value and types of investigations are discussed along with the specific physical-geological profiles of the region. Since water reserves are crucially related to mineral rich deposits, new work around the Sea of Galilee is discussed.
IN: Horizons in Earth Science Research, Chapter 4, pp. 115-147. researchgate
Mantle
geophysics - gravity
Abstract: Satellite gravimetry is recognized now as a powerful and reliable tool for regional tectono-geodynamic zonation. Regular observation grid and comparatively high accuracy (1-1.5 mGal) of satellite gravity data retracked to the Earth’s surface (together with other kinds of gravity observations) makes these data indispensable instrument for examination of deep geological-geophysical features (mainly, Earth crust and upper mantle). Accomplished satellite derived gravity measurements (tens and hundreds of millions of observations) may be considered as ‘big data’ because of their volume and variety. For comprehensive analysis of these data, different methodologies can be applied: entropial and information transformations, computing of various derivatives, coherence features and multistatistical characteristics, inverse probability and advanced downward continuation, etc. Effectiveness of the satellite regional gravity analysis is demonstrated on numerous examples from the Eastern Mediterranean, South Caspian Sea and African-Arabian region. It was proposed that increasing satellite derived gravity data accuracy during nearest five years will allow to extend the circle of solvable physical-geological problems.
Global and Planetary Change, doi.org/j.gloplacha.2017.10.011 24p.
Africa
geodynamics
Abstract: Modern satellite gravimetry is now considered one of the most powerful and effective instrument for regional tectono-geodynamic zonation. Satellite gravity observations clearly fit the definition of 'big data' because of their volume and variety. The Arabian - NE African region discussed in this article has intricate geodynamic features including active rift zones, high seismic activity and collision processes, a rich structural pattern made up of the mosaic block system of continental and oceanic crusts of different ages, as well as several of the greatest gravity anomalies and complex magnetic anomaly mosaics. This region also has the world's main hydrocarbon resources and a vast number of other economic deposits. A comprehensive analysis of these satellite derived gravity data were used to construct a series of new maps that localize the key properties of the lithosphere of the region. A careful examination of numerous geological sources and their combined inspection with satellite derived gravity and other geophysical data resulted in this new integrated tectonic map of the Arabian-African region. An analysis of the series of gravity map transformations and certain geological indicators document the significant geodynamic features of the region.
Earth and Planetary Science Letters, Vol. 502, pp. 244-252.
Russia, Canada, Ontario, Attawapiskat
kimberlite, core boundary
Abstract: Mantle plumes are hot buoyant upwellings that rise from Earth's core-mantle-boundary to its surface where they can produce large igneous provinces (LIPs) and volcanic tracks, such as the Siberian Traps and the Hawaiian Emperor chain, respectively. We show that flattened mantle plume heads, which can have radii of >1200 km in the uppermost mantle, can heat the overlying lithospheric mantle to temperatures above the diamond stability field. As a consequence, they can destroy diamonds within the roots of Archean cratons, the principal source of diamonds in kimberlites. We quantitatively demonstrate that there is a ‘sour spot’ for this effect that occurs when lithospheric thicknesses are 165-185 km and the plume has a temperature of >150?°C above background mantle. Our model explains why the kimberlites associated with the 370 Ma Yakutsk-Vilyui plume in the Siberian craton are diamondiferous whilst those associated with the younger 250 Ma Siberian Traps plume are barren. We also show that the time required to restore the pre-plume thermal structure of the lithosphere is ca. 75-120 Myr, and that destroyed diamonds may regrow once the plume's thermal effect dissipates. The 1100 Ma Kyle Lake and adjacent 180-150 Ma Attawapiskat kimberlites in the southern Superior craton exemplify this, where the older kimberlites are associated with a narrower diamond window (<30 km) in comparison with the ca. 85 km diamond window of the younger Attawapiskat field.
Upper mantle structure underlying the diamondiferous Slave craton from teleseismic body-wave tomography.
2018 Yellowknife Geoscience Forum , p.104-105. abstract
Canada, Northwest Territories
tomography
Abstract: Cratons are, by definition, the most tectonically stable and oldest parts of the continental lithosphere on Earth. The Archean Slave craton is located in the northwestern part of the Canadian Shield. The propensity of diamondiferous kimberlite pipes in the central Slave craton raises many questions regarding their structural environment and source. Here, we provide the most robust teleseismic P and S body wave tomography models over the Slave craton region based on 20,547 P-wave delay times, 6,140 direct S-wave delay times and 3,381 SKS delay times. The P-wave model reveals an alternating pattern of relative positive and negative anomalies over a fine broad scale region within the central Slave craton. Furthermore, the P-wave model revealed two fine structures located in the lithosphere beneath the Lac de Gras kimberlite cluster, with relatively slow anomalies (B - C) that extend from 75 km to 350 km depths with an apparent dip to the north. These relatively slow P- and S-wave anomalies are associated with metasomatised regions within the lithosphere. The S-wave model displays a slow S-wave anomaly lying from 300 km depth to the transition zone beneath the central Slave craton. This anomaly is located beneath the Lac de Gras kimberlite cluster. We suggest that this anomaly is not the cause of the actual kimberlites at the surface since last eruption occurred 75-45 Ma ago but may be related to a potential kimberlite magma ascent in the asthenosphere.
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.
Mineralogy and Petrology, June 14, DOI:10.1007/ s00710-018- 0593-8, 14p.
Africa, Botswana
deposit - Nxau
Abstract: The Nxau Nxau kimberlites in northwest Botswana belong to the Xaudum kimberlite province that also includes the Sikereti, Kaudom and Gura kimberlite clusters in north-east Namibia. The Nxau Nxau kimberlites lie on the southernmost extension of the Congo Craton, which incorporates part of the Damara Orogenic Belt on its margin. The Xaudum kimberlite province is geographically isolated from other known clusters but occurs within the limits of the NW-SE oriented, Karoo-aged Okavango Dyke Swarm and near NE-SW faults interpreted as the early stages of the East African Rift System. Petrographic, geochronological and isotopic studies were undertaken to characterise the nature of these kimberlites and the timing of their emplacement. The Nxau Nxau kimberlites exhibit groundmass textures, mineral phases and Sr-isotope compositions (87Sr/86Sri of 0.7036?±?0.0002; 2?) that are characteristic of archetypal (Group I) kimberlites. U-Pb perovskite, 40Ar/39Ar phlogopite and Rb-Sr phlogopite ages indicate that the kimberlites were emplaced in the Cretaceous, with perovskite from four samples yielding a preferred weighted average U-Pb age of 84?±?4 Ma (2?). This age is typical of many kimberlites in southern Africa, indicating that the Xaudum occurrences form part of this widespread Late Cretaceous kimberlite magmatic province. This time marks a significant period of tectonic stress reorganisation that could have provided the trigger for kimberlite magmatism. In this regard, the Nxau Nxau kimberlites may form part of a NE-SW oriented trend such as the Lucapa corridor, with implications for further undiscovered kimberlites along this corridor.
Mineralogy and Petrology, Vol. 112, 5, pp. 603-616.
Europe, Austria
subduction
Abstract: Mafic layers displaying transition between clinopyroxenite and eclogite within peridotite from felsic granulite in the Bohemian Massif (Lower Austria) have been investigated. The mafic-ultramafic bodies shared a common granulite facies metamorphism with its hosting felsic rocks, but they still preserve evidence of eclogite facies metamorphism. The selected mafic layer for this study is represented by garnet with omphacite in the core of coarse-grained clinopyroxene, while fine-grained clinopyroxene in the matrix is diopside. In addition, garnet contains inclusions of omphacite, alkali feldspars, hydrous and other phases with halogens and/or CO2. Textural relations along with compositional zoning in garnet from the clinopyroxenite-eclogite layers favour solid-state recrystallization of the precursor minerals in the inclusions and formation of garnet and omphacite during subduction. Textures and major and trace element distribution in garnet indicate two stages of garnet growth that record eclogite facies and subsequent granulite facies overprint. The possible model explaining the textural and compositional changes of minerals is that the granulite facies overprint occurred after formation and exhumation of the eclogite facies rocks.
Dissolution features on diamond surfaces: what can they tell us?
Vancouver Kimberlite Cluster talk, 1p. Abstract
Mantle
diamond morphology
Abstract: Diamonds are valued for their brilliance achieved by faceting of diamond crystals into the cut shapes that most efficiently reflect the light. However, diamond cut removes the long history of diamond growth and dissolution recorded in the surface features on rough diamonds. Growth features on diamond surfaces reflect the conditions of diamond-forming events in the mantle but are rarely preserved on natural diamonds due to their partial dissolution. The majority of natural diamonds show surface features resulted from dissolution both in the mantle source due to metasomatism and in the kimberlite magma during the ascent to the Earth’s surfaces. Mantle-derived resorption features can be preserved on diamonds enclosed in mantle xenoliths and protected from the interaction with the kimberlite magma during the ascent. The diamonds exposed to the reaction with the kimberlite magma develop kimberlite-induced dissolution features. This talk will discuss the large diversity of surface dissolution features on diamonds recovered from kimberlites. It will examine how the shape, size, and orientation of certain features can be used to deduce the conditions of kimberlite emplacement using the results of diamond dissolution experiments and observations on natural diamonds. The talk will discuss what we can learn about the behavior of volatiles and exsolution of fluid in kimberlite magma and how this supports or contradicts to what we know about kimberlitic fluid. The results of experiments conducted at mantle conditions are used to examine the kind of dissolution surfaces developed by diamonds during mantle metasomatism in fluids, in aqueous silicate melts and in carbonatitic melts. Comparison to the features of natural diamonds will assess what media dissolves diamond in the mantle, the prevalent diamond-destructive metasomatic agent, and will try to explain the repetitive cycles of growth and dissolution events recorded in single diamond crystals.
Abstract: Roots of continental cratons keep a long record of multiple metasomatic events, but their trace is complicated due to the mixed signals left by these events in the composition of mantle silicate minerals. Simple composition helps diamonds to provide a more robust record of the latest metasomatic events which they witnessed. Growth and dissolution features on the diamond surface are sensitive to the composition of the reacting media. In this study we use mantle-derived resorption features on natural diamonds to examine the nature of metasomatic events in diamondiferous mantle lithologies. We use experiments at mantle conditions to examine how the composition of fluids and melts affect diamond resorption. We then compare these results to the features of natural diamonds to determine which of the tested compositions could have acted as metasomatic agents in Earth’s cratonic roots. Diamond dissolution experiments conducted at 6 GPa, 1200 - 1500oC using synthetic MgO-CaO-SiO2-CO2-H2O system examined the effect of CHO fluid, silica-saturated CHO fluid, aqueous and “dry” silica-carbonate and carbonate melts. Results show that the main control of diamond resorption morphology is the state of the reacting media: fluid vs. melt. We compared the experimental results to diamonds with mantle-derived resorption features from two kimberlites from the Orapa kimberlite cluster (Botswana). We identified twelve mantle-derived resorption types, none of which resembled the products of resorption in fluids. Most of the observed resorption types could be produced by dissolution in mantle melts with variable proportions of carbonate and silicate components and in the range of temperatures. The most abundant resorption type resembles the product of diamond dissolution in carbonate melts at temperatures above 1450oC. Our results suggest that fluid-metasomatism is not destructive for diamonds while melt-metasomatism is. The lower hydrous carbonated solidus of lherzolite compared to harzburgite can result in the shift the process from diamond growth in fluids to diamond dissolution in melts due to metasomatic transformation of harzburgite into lherzolite.
Journal of Geophysical Research Solid Earth, doi.org/10.1029/ 2017JB015349
Mantle
core mantle boundary
Abstract: The juxtaposition of a liquid iron?dominant alloy against a mixture of silicate and oxide minerals at Earth's core?mantle boundary is associated with a wide range of complex seismological features. One category of observed structures is ultralow?velocity zones, which are thought to correspond to either aggregates of partially molten material or solid, iron?enriched assemblages. We measured the phonon dispersion relations of (Mg,Fe) O magnesiowüstite containing 76 mol % FeO, a candidate ultralow?velocity zone phase, at high pressures using high?energy resolution inelastic X?ray scattering. From these measurements, we find that magnesiowüstite becomes strongly elastically anisotropic with increasing pressure, potentially contributing to a significant proportion of seismic anisotropy detected near the base of the mantle.
Earth and Planetary Science Letters, Vol. 458, 1, pp. 252-262.
Mantle
geochemistry
Abstract: The Earth and other terrestrial planets formed through the accretion of smaller bodies, with their core and mantle compositions primarily set by metal -silicate interactions during accretion. The conditions of these interactions are poorly understood, but could provide insight into the mechanisms of planetary core formation and the composition of Earth's core. Here we present modeling of Earth's core formation, combining results of 100 N-body accretion simulations with high pressure -temperature metal -silicate partitioning experiments. We explored how various aspects of accretion and core formation influence the resulting core and mantle chemistry: depth of equilibration, amounts of metal and silicate that equilibrate, initial distribution of oxidation states in the disk, temperature distribution in the planet, and target:impactor ratio of equilibrating silicate. Virtually all sets of model parameters that are able to reproduce the Earth's mantle composition result in at least several weight percent of both silicon and oxygen in the core, with more silicon than oxygen. This implies that the core's light element budget may be dominated by these elements, and is consistent with ?1 -2 wt% of other light elements. Reproducing geochemical and geophysical constraints requires that Earth formed from reduced materials that equilibrated at temperatures near or slightly above the mantle liquidus during accretion. The results indicate a strong tradeoff between the compositional effects of the depth of equilibration and the amounts of metal and silicate that equilibrate, so these aspects should be targeted in future studies aiming to better understand core formation conditions. Over the range of allowed parameter space, core and mantle compositions are most sensitive to these factors as well as stochastic variations in what the planet accreted as a function of time, so tighter constraints on these parameters will lead to an improved understanding of Earth's core composition.
Abstract: MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks are unusual mantle samples entrained by kimberlites and other alkaline volcanic rocks. The formation of MARID rocks remains hotly debated. Although the incompatible element (for example, large ion lithophile element) enrichment in these rocks suggests that they formed by mantle metasomatism, the layered textures of some MARID samples (and MARID veins in composite xenoliths) are more indicative of formation by magmatic processes. MARID lithologies have also been implicated as an important source component in the genesis of intraplate ultramafic potassic magmas (e.g., lamproites, orangeites, ultramafic lamprophyres), due to similarities in their geochemical and isotopic signatures. To determine the origins of MARID and PIC xenoliths and to understand how they relate to alkaline magmatism, this study presents new mineral major and trace element data and bulk-rock reconstructions for 26 MARID and PIC samples from the Kimberley-Barkly West area in South Africa. Similarities between compositions of PIC minerals and corresponding phases in metasomatised mantle peridotites are indicative of PIC formation by pervasive metasomatic alteration of peridotites. MARID genesis remains a complicated issue, with no definitive evidence precluding either the magmatic or metasomatic model. MARID minerals exhibit broad ranges in Mg# (e.g., clinopyroxene Mg# from 82 to 91), which may be indicative of fractionation processes occurring in the MARID-forming fluid/melt. Finally, two quantitative modelling approaches were used to determine the compositions of theoretical melts in equilibrium with MARID rocks. Both models indicate that MARID-derived melts have trace element patterns resembling mantle-derived potassic magma compositions (e.g., lamproites, orangeites, ultramafic lamprophyres), supporting inferences that these magmas may originate from MARID-rich mantle sources.
Mineralogy and Petrology, in press available, 14p.
Africa, South Africa
deposit - Bultfontein
Abstract: MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) xenoliths are thought to be formed by intense Bprimary^ mantle metasomatism. These rocks also display secondary features, such as cross-cutting veins and geochemical zonation of matrix minerals, which probably reflect latermetasomatic events. To investigate the nature and origin(s) of these secondary features, 28 MARID and PIC xenoliths from southern African kimberlites and orangeites have been studied. MARID-hosted veins contain both carbonate and Ti-rich phases (e.g., titanite, phlogopite), suggesting that they formed by the infiltration of a carbonated silicate melt. Elevated TiO2 contents in MARID matrix mineral rims are spatially associated with carbonate-dominated veins, suggesting a genetic relationship between vein formation and geochemical zonation. Spongy rims around primaryMARID and PIC clinopyroxene are depleted in Na2O andAl2O3 relative to their cores, possibly reflecting mineral dissolution in the xenoliths during ascent and emplacement of the entraining kimberlite. The preservation of compositional differences between primary and secondary phases in MARID and PIC xenoliths indicates that metasomatism occurred shortly before, or broadly coeval with, kimberlite/orangeite magmatism; otherwise, at typical mantle temperatures, such features would have quickly re-equilibrated. Increased Na2O in some mineral rims (e.g., K-richterite) may therefore reflect equilibration with a more Na-enriched primitive kimberlite melt composition than is commonly suggested. Vein-hosted clinopyroxene 87Sr/86Sri (0.70539 ± 0.00079) in one MARID sample is intermediate between primary clinopyroxene in the sample (0.70814 ± 0.00002) and the host Bultfontein kimberlite (0.70432 ± 0.00005), suggesting that vein minerals are derived from interactions between primary MARID phases and kimberlite-related melts/fluids. Sulfur isotope compositions of barite (?34SVCDT = +4.69 ‰) and sulfides (?34SVCDT = ?0.69 ‰) in carbonate veins reflect equilibration at temperatures of 850-900 °C, consistent with sulfurrich melt/fluid infiltration in the lithospheric mantle. In contrast, vein carbonate C-O isotope systematics (?13CVPDB = ?9.18 ‰ ?18OVSMOW = +17.22‰) are not typical of kimberlites or other mantle carbonates (?13CVPDB = ?3 to ?8‰ ?18OVSMOW = 6 to 9 ), and may represent post-emplacement hydrothermal interactions of the cooling kimberlite with crustal fluids. These constraints suggest protracted metasomatism of MARID rocks shortly before and during entrainment by the host kimberlite.
Abstract: Continental rifts most often nucleate within orogenic belts. However, some studies in the East African Rift System (EARS) have shown that continental rifts can also develop withincratons. This work investigated the ~1.5 Ma Eyasibasin,which propagates in a WSW direction into the Tanzanian craton. The basin is located where the Eastern Branch of the EARS transitions from a narrow rift (~70 km wide) thewider(~300 km wide) North Tanzanian Divergence. Unlike the rest of the Eastern Branch segments, the Eyasibasindoes not follow the Mozambique orogenic belt located on the eastern margin of the Tanzanian craton. This work generatedlithospheric?scale sections across the basinusing: (1) Digital Elevation Model to map surface rift?related brittle structures; (2) Aeromagnetic data to determine the depth to the Precambrian basement;and (3) World Gravity Model 2012 to estimatecrustal and lithospheric thickness by applying the two?dimensional(2D) radially?averaged power spectral analysis and 2D forward gravity modeling. These cross?sectionsshow that the Eyasibasinnucleates within a previously unidentified suture zone within the Tanzanian cratonand that this suture zone is characterized by thinner lithospherethat can be as thin as ~95 km. This zone ofthinner lithosphere is offset southeastwardfrom the surface expression of the Eyasibasinand might have facilitated the formation of other basins further south. Furthermore, the lithospheric thickness map indicates that the Tanzanian craton is heterogeneous and possibly composed of multiplesmaller cratonic fragments.
Abstract: The continental lithosphere is a vast store for carbon. The carbon has been added and reactivated by episodic freezing and re-melting throughout geological history. Carbon remobilization can lead to significant variations in CO2 outgassing and release in the form of magmas from the continental lithosphere over geological timescales. Here we use calculations of continental lithospheric carbon storage, enrichment and remobilization to demonstrate that the role for continental lithosphere and rifts in Earth’s deep carbon budget has been severely underestimated. We estimate that cratonic lithosphere, which formed 2 to 3 billion years ago, originally contained about 0.25 Mt C km -3. A further 14 to 28 Mt C km -3 is added over time from the convecting mantle and about 43 Mt C km -3 is added by plume activity. Re-melting focuses carbon beneath rifts, creating zones with about 150 to 240 Mt C km -3, explaining the well-known association of carbonate-rich magmatic rocks with rifts. Reactivation of these zones can release 28 to 34 Mt of carbon per year for the 40 million year lifetime of a continental rift. During past episodes of supercontinent breakup, the greater abundance of continental rifts could have led to short-term carbon release of at least 142 to 170 Mt of carbon per year, and may have contributed to the high atmospheric CO2 at several times in Earth's history.
Abstract: A comprehensive petrographic and in situ major and trace element study of rutile, ilmenite and Ti-magnetite was undertaken in six lower crustal xenoliths of metabasaltic (?underplate) and metasedimentary (subduction) origin from the Diavik kimberlites (central Slave Craton, Canada). The aims of the study were to improve our understanding of trace element incorporation into these Ti-minerals, and to use these systematics to obtain insights into lower continental crust formation and evolution. Abundant (oxy)exsolution of titanomagnetite lamellae, blocky rutile, as well as minor pleonaste and zircon in ilmenite from metabasaltic granulites are proposed to reflect cooling from magmatic or metamorphic temperatures and subsequent secular mantle cooling. This explains the large spread in Zr-in-rutile temperatures (>200°C) and may partly be responsible for the substantial heterogeneity of other trace element concentrations in rutile and ilmenite. Even after accounting for trace element heterogeneity and modal uncertainties, mass-balance calculations indicate that both Ti and Nb in lower crustal granulites are largely controlled by rutile and ilmenite. Rutile U-Pb data define discordia arrays that yield upper intercept ages broadly coincident with the 1•27 Ga giant Mackenzie dike swarm event, suggesting reheating of the lower crust above the rutile U-Pb closure temperature, whereas lower intercept ages roughly correspond to the age of Cretaceous to Eocene kimberlite magmatism. Subsequent cooling led to partial resetting and data spread along the concordia. Closer inspection reveals that inter-grain concentrations of elements that are compatible in rutile (Nb, Ta, W, U), but highly incompatible in the abundant silicate minerals (in equilibrium with melt), are heterogeneous and contrast with the more homogeneous concentrations of the transition metals (NiO, V). This may indicate that local reaction partners for diffusive homogenization of these element concentrations were absent. Nb/Ta is also highly variable at the sample scale. This may be explained by prograde growth from high-Nb/Ta mineral precursors (e.g. biotite) in the metasedimentary granulites and crystallization of the protoliths to the metabasaltic granulites from a mafic magma that had experienced fractionation of ilmenite with low Nb/Ta in a crustal magma chamber. Thus, (Fe)-Ti minerals represent high field strength element ‘islands’ in the granulite silicate matrix. The lack of homogenization and persistence of high-energy grain boundaries, such as exsolution lamellae, further indicate that the lower continental crust remained essentially dry and did not recrystallize, possibly since Neoarchaean metamorphism.
Abstract: In this study, we show how veined lithospheric mantle is involved in the genesis of ultrapotassic magmatism in cratonic settings. We conducted high pressure experiments to simulate vein + wall rock melting within the Earth's lithospheric mantle by reacting assemblages of harzburgite and phlogopite-rich hydrous mantle xenoliths. These comprised a mica-, amphibole-, rutile-, ilmenite-, diopside (MARID) assemblage at 3-5 GPa and 1325-1450 °C. Melting of the MARID assemblages results in infiltration of melt through the harzburgite, leading to its chemical alteration. At 3 and 4 GPa, melts are high in K2O (> 9 wt%) with K2O/Na2O > > 2 comparable to anorogenic lamproites. Higher pressures and temperatures (5 GPa/1450 °C) lead to increasing MgO contents of the melt and to some extent lower K2O contents (5-7 wt%) at equally high K2O/Na2O ratios. Our experiments provide insights into the role of alkalis in nickel-partitioning (DNi) between olivine and ultrapotassic melt. We observe that the high contents of Na, K, and Al are indicative of high DNi values, implying that the melt polymerization is the dominant factor influencing the olivine/melt nickel partitioning. The change of DNi as a function of melt composition results in a pressure independent, empirical geothermometer: Element oxides represent the composition of the glass (in wt%), and DNi is the liquid/olivine Ni-partitioning coefficient. We propose that this geothermometer is applicable to all natural silicate melts that crystallized olivine in a temperature interval between 1000 and 1600 °C. Application to glass-olivine pairs from calc-alkaline settings (Mexico), MORB (East Pacific Rise), and OIB (Hawaii) yielded reasonable values of 996-1199 °C, 1265 °C, and 1330 °C, respectively.
Society of Exploration Geophysicists, Interpretation, August T 313, 13p.
Canada, Northwest Territories
deposit - Tli Kwi Cho
Abstract: We focus on the task of finding a 3D conductivity structure for the DO-18 and DO-27 kimberlites, historically known as the Tli Kwi Cho (TKC) kimberlite complex in the Northwest Territories, Canada. Two airborne electromagnetic (EM) surveys are analyzed: a frequency-domain DIGHEM and a time-domain VTEM survey. Airborne time-domain data at TKC are particularly challenging because of the negative values that exist even at the earliest time channels. Heretofore, such data have not been inverted in three dimensions. In our analysis, we start by inverting frequency-domain data and positive VTEM data with a laterally constrained 1D inversion. This is important for assessing the noise levels associated with the data and for estimating the general conductivity structure. The analysis is then extended to a 3D inversion with our most recent optimized and parallelized inversion codes. We first address the issue about whether the conductivity anomaly is due to a shallow flat-lying conductor (associated with the lake bottom) or a vertical conductive pipe; we conclude that it is the latter. Both data sets are then cooperatively inverted to obtain a consistent 3D conductivity model for TKC that can be used for geologic interpretation. The conductivity model is then jointly interpreted with the density and magnetic susceptibility models from a previous paper. The addition of conductivity enriches the interpretation made with the potential fields in characterizing several distinct petrophysical kimberlite units. The final conductivity model also helps better define the lateral extent and upper boundary of the kimberlite pipes. This conductivity model is a crucial component of the follow-up paper in which our colleagues invert the airborne EM data to recover the time-dependent chargeability that further advances our geologic interpretation.
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.
Abstract: Dynamic topography is a well-established consequence of global geodynamic models of mantle convection with horizontal dimensions of >1000 km and amplitudes up to 2 km. Such physical models guide the interpretation of geological records on equal dimensions. Continent-scale geological maps therefore serve as reference frames of choice to visualize erosion/non-deposition as a proxy for long-wavelength, low-amplitude vertical surface motion. At a resolution of systems or series, such maps display conformable and unconformable time boundaries traceable over hundreds to thousands of kilometres. Unconformable contact surfaces define the shape and size of time gap (hiatus) in millions of years based on the duration of time represented by the missing systems or series. Hiatus for a single system or series base datum diminishes laterally to locations (anchor points) where it is conformable at the mapped resolution; it is highly dependent upon scale. A comparison of hiatus area between two successive system or series boundaries yields changes in location, shape, size and duration, indicative of the transient nature of vertical surface motion. As a single-step technique, it serves as a quantitative proxy for palaeotopography that can be calibrated using other geological data. The tool magnifies the need for geological mapping at the temporal resolution of stages, matching process rates. The method has no resolving power within conformable regions (basins) but connects around them. When applied to marine seismic sections that relate to rock record, not to time, biostratigraphic and radiometric data from deep wells are needed before hiatus areas - that relate to time - can be mapped.
Abstract: Mantle convection is a fundamental planetary process. Its plate mode is established and expressed by plate tectonics. Its plume mode also is established and expressed by interregional geological patterns. We developed both an event-based stratigraphic framework to illustrate the surface effects predicted by the plume model of Griffiths et al. (1989) and Griffiths and Campbell (1990) and a methodology to analyze continent-scale geological maps based on unconformities and hiatuses. The surface expression of ascending plumes lasts for tens-of-millions-of-years and rates vary over a few million years. As the plume ascends, its surface expression narrows, but increases in amplitude, leaving distinct geological and stratigraphic patterns in the geologic record, not only above the plume-head center, but also above its margins and in distal regions a few thousands-of-kilometers from the center. To visualize these patterns, we constructed sequential geological maps, chronostratigraphic sections, and hiatus diagrams. Dome-uplift with erosion (?engör, 2001) and the flood basalts (Duncan and Richards, 1991; Ernst and Buchan, 2001a) are diagnostic starting points for plume-stratigraphic analyses. Mechanical collapse of the dome results in narrow rifting (Burke and Dewey, 1973), drainage-network reorganization (Cox, 1989), and flood-basalt eruption. In the marginal region, patterns of vertical movement, deformation and surface response are transient and complex. At first, the plume margin is uplifted together with the central region, but then it subsides as the plume ascents farther; With plume-head flattening, the plume margin experiences renewed outward-migrating surface uplift, erosion, broad crustal faulting, and drainage reorganization. Knickpoint migration occurs first inward-directed at ½ the rate of plume ascent and later outward-directed at the rate of asthenospheric flow. Interregional-scale unconformity-bounded stratigraphic successions document the two inversions. The distal regions, which did not experience any plume-related uplift, yield complete sedimentary records of the event; Event-related time gaps (hiatuses) in the sedimentary record increase towards the center, but the event horizon is best preserved in the distal region; it may be recognized by tracing its contacts from the center outwards. We extracted system- and series-hiatuses from interregional geological maps and built hiatus maps as proxies for paleo-dynamic topography and as a basis for comparison with results from numerical models. Interregional-scale geological maps are well suited to visualize plume-related geological records of dynamic topography in continental regions. However, geological records and hiatus information at the resolution of stages will be needed at interregional scales. The plume-stratigraphic framework is event-based, interregional, but not global, with time-dependent amplitudes that are significantly larger than those of global eustatic sea-level fluctuations. Global stratigraphic syntheses require integration of plate- and plume-stratigraphic frameworks before eustatic contributions may be assessed.
Physics and Chemistry of Minerals, dor.org/10.1007/ d00269-018-0987-5 13p.
Russia, Kazakhstan, Alps
coesite, UHP
Abstract: The high-pressure silica polymorphs coesite and stishovite were synthesized under water-saturated conditions from a natural granitic composition doped with Li and B. Experiments were performed in a Multi-Anvil apparatus between 4 and 9.1 GPa and 900 and 950 °C, based on the conditions of a subducting continental crust as realistic for the ultrahigh-pressure metamorphic units Dora Maira and Kochetav massifs. Run products consisted of coesite/stishovite?+?kyanite?±?phengite?±?omphacite, and quench material. The synthesized silica polymorphs were successively analyzed by infrared spectroscopy, electron microprobe, and Secondary-Ion Mass Spectrometry (SIMS). No hydrous defects were observed in coesite synthesized at 4 GPa and 900 °C, whereas coesite grown at higher pressures revealed a triplet of infrared absorptions bands at 3575, 3523, and 3459 cm??1, two minor bands at 3535 and 3502 cm??1, and a small band at 3300 cm??1 that was only visible at 7.7 GPa. The total amount of Al was charge-balanced by H and the other monovalent cations. However, the band triplet could not be associated with AlOH defects, while the band doublet was inferred to BOH defects and the small band probably corresponded to interstitial H. Stishovite displayed one dominant band at 3116 cm??1 with a shoulder at 3170 cm??1, and a minor band at 2665 cm??1, probably all associated with AlOH defects. BOH defects were not observed in stishovite, and LiOH defects were neither observed in coesite nor stishovite, probably because of preferentially partition of Li in other phases such as omphacite. The total amount of defect protons increased with pressure and with metal impurity concentrations. The general increase in OH defects in silica polymorphs with increasing pressure (this study) contrasted the negative pressure trend of OH in quartz observed previously from the same starting material, and revealed an incorporation minimum of OH in silica polymorphs around the quartz/coesite phase transition.
Abstract: We document an exceptionally large, 25.85 ct diamond that shows a slight colour change but exhibits some atypical properties for chameleon diamonds, including white luminescence to long- and short-wave UV radiation, as well as a network-like pattern seen in most orientations with the DiamondView. In considering whether to call this a chameleon diamond, we undertook a review of available data to compile the properties that are commonly exhibited by these gems. We found that, in addition to their defining photochromic and thermochromic behaviour, nine characteristics all must be present: long-lasting yellow phosphorescence, a zoned DiamondView growth pattern showing yellow-green/blue/inert areas, the presence of dominant A aggregates and also some hydrogen in the infrared spectrum, a continuum of absorption in the visible range related to a very weak type Ib character, a 480 nm absorption band that is possibly related to trace amounts of oxygen, a 425 nm absorption band, a weaker absorption band in the red to near-infrared region consistent with hydrogen-related defects, and traces of nickel detected with photoluminescence spectroscopy.
Abstract: Eastern Hall Peninsula on southeastern Baffin Island, lies at the junction of a complex Paleoproterozoic collision between the Rae craton, Meta Incognita microcontinent and the North Atlantic craton from ca. 1.88 to 1.80?Ga. Several different interpretations of crustal correlations and the location of intervening sutures have been proposed based on reconnaissance-scale geologic investigation. Therefore, in this study, complex zircon grains from Archean orthogneiss units on eastern Hall Peninsula were analyzed in-situ to elucidate the detailed magmatic history of the region and assess crustal provenance. Magmatic zircons yielded U-Pb crystallization ages between ca. 2976 and 2720?Ma and metamorphic zircons record tectonothermal disturbances between ca. 2740 and 2700?Ma, a period coinciding with metamorphism documented in adjacent crustal blocks (e.g., west Greenland and northern Labrador). Magmatic rocks older than ca. 2740?Ma generally have positive ?Hf(t) signatures between 0 and 7 (±2) and depleted mantle model ages of ca. 3.1-3.0?Ga indicating the time that protolith melt was extracted from the mantle. The oldest, granodioritic crust crystallized at ca. 2976?Ma and was then reworked periodically at ca. 2.93, 2.84-2.81 and 2.77-2.69?Ma. Zircons from two orthogneiss samples, with U-Pb crystallization ages younger than ca. 2740?Ma, yielded negative ?Hf(t) values ranging from ?4 to ?12 and mean depleted mantle model ages of ca. 3.4 and 3.3?Ga respectively, indicating derivation from an older, potentially exotic, crustal source yet to be identified in outcrop on Hall Peninsula. Synthesizing regional U-Pb data we propose a new regional correlation model that retains the essentials of previous models and incorporates new data from eastern Hall Peninsula and other recent studies. This new tectonic correlation model groups eastern Hall Peninsula, southern Cumberland Peninsula and the Aasiaat domain into a “Core zone” that shared a geologic history prior to 1.90?Ga and potentially prior to 2.75?Ga.
Geophysical Research Letters, Vol. 45, 10, pp. 4725-4732.
Mantle
bridgmanite
Abstract: Seismic heterogeneities in the Earth's lower mantle have been attributed to thermal and/or chemical variations of constituent minerals. Bridgmanite is the most abundant lower?mantle mineral and contains Fe and Al in its structure. Knowing the effect of Fe on compressional and shear wave velocities (VP, VS) and density of bridgmanite at relevant pressure?temperature conditions can help to understand seismic heterogeneities in the region. However, experimental studies on both VP and VS of Fe?bearing bridgmanite have been limited to pressures below 40 GPa. In this study, VP and VS of Fe?bearing bridgmanite were measured up to 70 GPa in the diamond anvil cell. We observed drastic softening of VP by ~6(±1)% at 42.6-58 GPa and increased VS at pressures above 40 GPa. We interpret these observations as due to a spin transition of Fe3+. These observations are different to previous views on the effect of Fe on seismic velocities of bridgmanite. We propose that the abnormal sound velocities of Fe?bearing bridgmanite could help to explain the seismically observed low correlation between VP and VS in the mid?lower mantle. Our results challenge existing models of Fe enrichment to explain the origin of Large Low Shear Velocity provinces in the lowermost mantle.
Abstract: Nitrogen occupies about 80% of the Earth 's atmosphere and had an impact on the climate in the early Earth. However, the behavior of nitrogen especially in the deep Earth is still unclear. Nitrogen is depleted compared to other volatile elements in deep mantle (Marty et al., 2012). "Missing" nitrogen is an important subject in earth science. In this study, we compared nitrogen incorporation into lower-mantle minerals (bridgmanite, periclase and stishovite) from high-temperature high-pressure experiment using multianvil apparatus installed at Geodynamics Research Center, Ehime University under the conditions of 27 GPa and 1600°C-1900°C. In these experiments, we used Fe-FeO buffer in order to reproduce the redox state of the lower mantle. Two types of starting materials: a powder mixture of SiO2 and MgO and a powder mixture of SiO2, MgO, Al2O3 and Mg(OH)2 were used for starting materials. Nitrogen in recovered samples was analyzed using NanoSIMS installed at Atmosphere and Ocean Research Institute. A series of experimental results revealed that stishovite and periclase can incorporate more nitrogen than bridgmanite. This suggests that periclase, the major mineral in the lower mantle, may be a nitrogen reservoir. Furthermore, the results suggest that stishovite, which is formed by the transition of the SiO2-rich oceanic crustal sedimentary rocks transported to the lower mantle via subducting slabs, can incorporate more nitrogen than bridgmanite (20 ppm nitrogen solubility reported by Yoshioka et al. (2018)). Our study suggests that nitrogen would continue to be supplied to the lower mantle via subducting slabs since approximate 4 billion years ago when the plate tectonics had begun, forming a "Hidden" nitrogen reservoir in the lower mantle.
Abstract: This review paper focuses on joint modelling and interpretation of electromagnetic data and other geophysical and petrological observables. In particular, integrated geophysical-petrological modelling approaches, where the electrical conductivity and other physical properties of rocks are required to be linked by the common subsurface thermochemical conditions within a self-consistent thermodynamic framework, are reviewed. The paper gives an overview of the main geophysical electromagnetic techniques/data sets employed in lithospheric and mantle imaging including recent advances using satellite data, and an up-to-date summary of the most relevant laboratory experiments regarding the electrical conductivity of upper mantle minerals for various temperature-pressure-water conditions. The sensitivity of electrical conductivity and other geophysical parameters (density, seismic velocities) of mantle rocks to changes in temperature and composition are presented based on a Monte Carlo method parameter exploration. Finally, a case study in Central Tibet is presented where both seismological (long-period surface wave phase velocities) and electromagnetic (magnetotelluric) data—simultaneously including the constraints offered by topography, surface heat flow and mantle xenoliths—have been integrated. The modelling is based on a self-consistent petrological-geophysical thermodynamic framework where mantle properties are calculated as a function of temperature, pressure, and composition. The Tibetan case study offers an excellent opportunity to illustrate the different and complementary sensitivities of the various data sets used and to show how integrated thermochemical models of the lithosphere can help understand settings with a complex tectonic evolution.
Journal of Structual Geology, Vol. 11, pp. 329-335.
Mantle
magmatism
Abstract: The Earth's continental crust constitutes a major interface between the inner and outer envelops of the planet, controlling the differentiation of magmas produced in the mantle and their transfer to the surface. This close link facilitates the use of different chemical proxies to qualitatively unravel the crustal thickness related to fossil magmatic systems based on the message carried by magmas. This paper aims to bridge different results of statistical petrology, recently obtained at different scales of observation, in a global geodynamic model. Statistical analyses applied to a large multidimensional database of magmatic rocks show that crustal thickness could actually exert a first-order control on the composition of magmas, which become more calc-alkaline and comparatively less tholeiitic with increasing crustal thickness. Using this correlation, we document the progressive build-up of a thick (>40?km) Jurassic to Cretaceous accretionary belt along the Circum-Pacific Orogenic Belts (CPOB) that bounded the Panthalassa Ocean. The destruction of this thick belt started at ca. 125 Ma and was initially recorded by the thinnest magmatic systems hosting amphibole-bearing magma. Thinning of the CPOB became widespread in the northern regions of western America and in the western Pacific after ca. 75 Ma, possibly in response to oceanic plate segmentation, which triggered slab rollback and overriding plate extension. This chemical evolution is superimposed on a more global evolution of magma controlled by the temperature of the mantle that has gradually decreased since 200 Ma. Although the relative contribution of crust vs mantle cooling in the chemical signature of magmatic rocks should be further explored in the future, our results offer a new global perspective of the magmatic history of Pangea, the last supercontinent.
Geochemistry, Geophysics, Geosystems, Vol. 19, 7, pp. 2062-2086. doi.org/10/1029/ 2018GC007534
Mantle
geophysics - seismics
Abstract: Some seismic models derived from tomographic studies indicate elevated shear?wave velocities (?4.7 km/s) around 120-150 km depth in cratonic lithospheric mantle. These velocities are higher than those of cratonic peridotites, even assuming a cold cratonic geotherm (i.e., 35 mW/m2 surface heat flux) and accounting for compositional heterogeneity in cratonic peridotite xenoliths and the effects of anelasticity. We reviewed various geophysical and petrologic constraints on the nature of cratonic roots (seismic velocities, lithology/mineralogy, electrical conductivity, and gravity) and explored a range of permissible rock and mineral assemblages that can explain the high seismic velocities. These constraints suggest that diamond and eclogite are the most likely high?Vs candidates to explain the observed velocities, but matching the high shear?wave velocities requires either a large proportion of eclogite (>50 vol.%) or the presence of up to 3 vol.% diamond, with the exact values depending on peridotite and eclogite compositions and the geotherm. Both of these estimates are higher than predicted by observations made on natural samples from kimberlites. However, a combination of ?20 vol.% eclogite and ~2 vol.% diamond may account for high shear?wave velocities, in proportions consistent with multiple geophysical observables, data from natural samples, and within mass balance constraints for global carbon. Our results further show that cratonic thermal structure need not be significantly cooler than determined from xenolith thermobarometry.
Abstract: Some seismic models derived from tomographic studies indicate elevated shear?wave velocities (?4.7 km/s) around 120?150 km depth in cratonic lithospheric mantle. These velocities are higher than those of cratonic peridotites, even assuming a cold cratonic geotherm (i.e., 35 mW/m2 surface heat flux) and accounting for compositional heterogeneity in cratonic peridotite xenoliths and the effects of anelasticity. We reviewed various geophysical and petrologic constraints on the nature of cratonic roots (seismic velocities, lithology/mineralogy, electrical conductivity, and gravity) and explored a range of permissible rock and mineral assemblages that can explain the high seismic velocities. These constraints suggest that diamond and eclogite are the most likely high?Vs candidates to explain the observed velocities, but matching the high shear?wave velocities requires either a large proportion of eclogite (>50 vol.%) or the presence of up to 3 vol.% diamond, with the exact values depending on peridotite and eclogite compositions and the geotherm. Both of these estimates are higher than predicted by observations made on natural samples from kimberlites. However, a combination of ?20 vol.% eclogite and ~2 vol.% diamond may account for high shear?wave velocities, in proportions consistent with multiple geophysical observables, data from natural samples, and within mass balance constraints for global carbon. Our results further show that cratonic thermal structure need not be significantly cooler than determined from xenolith thermobarometry.
Abstract: The Tethys margin in central and eastern Asia is comprised of continental terranes separated by suture zones, some of which remain cryptic. Determining the crustal architecture, and therefore the geological history, of the Eastern Tethyan margin remains challenging. Sited in the heart of this region, Myanmar is a highly prospective but poorly explored minerals jurisdiction. A better understanding of Myanmar's mineralization can only be realized through a better understanding of its tectonic history, itself reflected in at least four major magmatic belts. The Eastern and the Main Range Provinces are associated with the Late Permian to Early Triassic closure of Palaeo-Tethys. The Mogok-Mandalay-Mergui Belt and Wuntho-Popa Arc are a response to the Eocene closure of Neo-Tethys. However, magmatic ages outside these two orogenic events are also recorded. We present new zircon U-Pb, Lu-Hf and O isotope data from magmatic rocks across Myanmar, which we append to the existing dataset to isotopically characterize Myanmar's magmatic belts. Eastern Province Permian I-type magmatism has evolved eHf (-10.9 to -6.4), whilst Main Range Province Triassic S-type magmatism also records evolved eHf (-13.5 to -8.8). The Mogok-Mandalay-Mergui Belt is here divided into the Tin Province and the Mogok Metamorphic Belt. The Tin Province hosts ca. 77-50 Ma magmatism with evolved eHf (-1.2 to -15.2), and d 18 O of 5.6-8.3‰. The Mogok Metamorphic Belt exhibits a more complex magmatic and metamorphic history, and granitoids record Jurassic, Late Cretaceous, and Eocene to Miocene phases of magmatism, all of which exhibit evolved eHf values between -4.6 and -17.6, and d 18 O between 6.3 and 9.2‰. From the Tagaung-Myitkyina Belt, we report a magmatic age of 172 Ma and eHf of 18.1 to 10.8. To accommodate the geological evidence, we propose a tectonic model for Myanmar involving a greater Sibumasu - where the documented zircon isotopic variations reflect compositional variations in magmatic source - and invoke the role of a Tengchong Block. The Baoshan Block and Greater Sibumasu were likely assembled on or before the Triassic, a former Andean margin and suture which may lie across the Northern Shan Plateau, and reflected in isotopic differences between the northern and southern parts of the Mogok Metamorphic Belt. This contiguous Sibumasu-Baoshan Block then sutured onto the Indochina margin in the Late Triassic. We propose that a Tengchong Block within Myanmar provides for a southerly termination of the Meso-Tethys suture immediately north of the Mogok area. A discrete Tengchong Block may explain a discontinuous arc of Late Triassic to Jurassic I-type magmatism in central Myanmar, representing an Andean-type margin sited above a subducting Meso-Tethys on the margin of Sibumasu. The Tengchong Block sutured onto Greater Sibumasu before the Late Cretaceous, after which subduction of Neo-Tethys drove the magmatism of the Wuntho-Popa Arc and ultimately that of the Tin Province. The metallogenic character of granite belts in Myanmar reflects the crustal architecture of the region, which is remarkable for its prolific endowment of granite-hosted Sn-W mineralization in two quite distinct granite belts related to sequential Indosinian and Himalayan orogenesis.
Abstract: This study focuses on the causes, modalities and obstacles of sediment transfer in the longest cell of littoral sand drift documented on Earth so far. Sand derived from the Orange River is dragged by swell waves and persistent southerly winds to accumulate in four successive dunefields in coastal Namibia to Angola. All four dunefields are terminated by river valleys, where aeolian sand is flushed back to the ocean; and yet sediment transport continues at sea, tracing an 1800 km long submarine sand highway. Sand drift would extend northward to beyond the Congo if the shelf did not become progressively narrower in southern Angola, where drifting sand is funnelled towards oceanic depths via canyon heads connected to river mouths. Garnet-magnetite placers are widespread along this coastal stretch, indicating systematic loss of the low-density feldspatho-quartzose fraction to the deep ocean. More than half of Moçamedes Desert sand is derived from the Orange River, and the rest in similar proportions from the Cunene River and from the Swakop and other rivers draining the Damara Orogen in Namibia. The Orange fingerprint, characterized by basaltic rock fragments, clinopyroxene grains and bimodal zircon-age spectra with peaks at ca 0•5 Ga and ca 1•0 Ga, is lost abruptly at Namibe, and beach sands further north have abundant feldspar, amphibole-epidote suites and unimodal zircon-age spectra with a peak at ca 2•0 Ga, documenting local provenance from Palaeoproterozoic basement. Along with this oblique-rifted continental margin, beach placers are dominated by Fe-Ti-Cr oxides with more monazite than garnet and thus have a geochemical signature sharply different from beach placers found all the way along the Orange littoral cell. High-resolution mineralogical studies allow us to trace sediment dispersal over distances of thousands of kilometres, providing essential information for the correct reconstruction of ‘source to sink’ relationships in hydrocarbon exploration and to predict the long-term impact of man-made infrastructures on coastal sediment budgets.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0633-4 13p.
Canada, Quebec
deposit - Renard
Abstract: Renard 65, a diamondiferous pipe in the Neoproterozoic Renard kimberlite cluster (Québec, Canada), is a steeply-dipping and downward-tapering diatreme comprised of three pipe-filling units: kimb65a, kimb65b, and kimb65d. The pipe is surrounded by a marginal and variably-brecciated country rock aureole and is crosscut by numerous hypabyssal dykes: kimb65c. Extensive petrographic and mineralogical characterization of over 700 m of drill core from four separate drill holes, suggests that Renard 65 is a Group I kimberlite, mineralogically classified as phlogopite kimberlite and serpentine-phlogopite kimberlite. Kimb65a is a massive volcaniclastic kimberlite dominated by lithic clasts, magmaclasts, and discrete olivine macrocrysts, hosted within a fine-grained diopside and serpentine-rich matrix. Kimb65b is massive, macrocrystic, coherent kimberlite with a groundmass assemblage of phlogopite, spinel, perovskite, apatite, calcite, serpentine and rare monticellite. Kimb65c is a massive, macrocrystic, hypabyssal kimberlite with a groundmass assemblage of phlogopite, serpentine, calcite, perovskite, spinel, and apatite. Kimb65d is massive volcaniclastic kimberlite with localized textures that are intermediate between volcaniclastic and coherent, with tightly packed magmaclasts separated by a diopside- and serpentine-rich matrix. Lithic clasts of granite-gneiss in kimb65a are weakly reacted, with partial melting of feldspars and crystallization of richterite and actinolite. Lithic clasts in kimb65b and kimb65d are entirely recrystallized to calcite + serpentine/chlorite + pectolite and display inner coronas of diopside-aegirine and an outer corona of phlogopite. Compositions are reported for all minerals in the groundmass of coherent kimberlites, magmaclasts, interclast matrices, and reacted lithic clasts. The Renard 65 rocks are texturally classified as Kimberley-type pyroclastic kimberlites and display transitional textures. The kimberlite units are interpreted to have formed in three melt batches based on their distinct spinel chemistry: kimb65a, kimb65b and kimb65d. We note a strong correlation between the modal abundances of lithic clasts and the textures of the kimberlites, where increasing modal abundances of granite/gneiss are observed in kimberlites with increasingly fragmental textures.
Gautam, I., Bhutani, R., Balakrishnan, S., Chatterjee, A., George, B.G., Ray, J.S.
142Nd/144Nd of alkaline magmas in Phenai Mat a complex, Chhota Udaipur, Deccan flood basalt province.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 14.
India
alkaline rocks
Abstract: The 65 million year old alkaline plug at Phenai Mata Complex, in Chota Udaipur sub province, is often linked to the last pulse of the Deccan volcanism. However, many believe that the Deccan-Reunion mantle plume that was responsible for the generation of flood basalts might not have been the source of Phenai Mata. It, however, could have acted as a heat source for these magmas derived from the subcontinentallithospheric- mantle (SCLM). Since the SCLM is generally considered to be a nonconvective mantle domain it has the potential to preserve some of the geochemical evidence of the early silicate Earth differentiation, e.g., 142Nd anomaly. In search of such signatures we analysed alkali basalts from the complex for their 142Nd/144Nd using high precision thermal ionization mass spectrometry. Whereas the geochemical characterization of these samples confirmed the lithospheric origin of their source magmas, their ? 142Nd compositions are found to be normal with respect to terrestrial standards. We infer that either the mantle source of Phenai Mata does not represent a true non-convective mantle or it is too young to retain any evidence of 146Sm decay.
gssaconferences.co.za /compliance -jse-reporting, June 28,29 th
Africa, South Africa
diamond resource and reserve reporting
Abstract: This two-day Workshop (28th and 29th June 2018) comprises an introduction to the SAMREC and SAMVAL Codes, and JSE Reporting. This course is aimed at geologists, mining engineers and other technical specialists, who include sign-off as Competent Persons ("CPs") or Competent Valuators ("CVs") in their job description. Day one focuses on the basics of the SAMREC and SAMVAL Codes, and concentrates on the requirements for CPs who compile documents, specifically Competent Persons Reports and Integrated Annual Reports, for companies listed on the Johannesburg Stock Exchange (applicable primarily to Solid Minerals). Day two focuses on the Section 12 Listing rules applicable to both the Main Board and the Alt-X as well as the requirements of SAMREC and SAMVAL, highlighting some of the issues experienced by CPs over the years as well as some of the updated requirements as a result of the implementation of the 2016 SAMREC/SAMVAL Codes. An introduction to the JSE Readers Panel and a discussion of some of the on-going compliance issues identified by the panel.
Geological Survey of Western Australia, Report 182, 297p.
Australia
deposit - Ellendale area
Abstract: In 2005, Frogtech Geoscience completed OZ SEEBASE - a continental-scale depth-to-basement grid which shows the distribution of Phanerozoic basins across Australia. OZ SEEBASE is an open-file study that has been downloaded 1000s of times by industry, government and academia. This was followed in 2006 by the Proterozoic OZ SEEBASE interpretation including the main Proterozoic basins of Australia. The 2005 and 2006 OZ SEEBASE incorporated results from the Canning Basin Project completed by SRK Consulting for Shell during 1998-99.
Geochemistry International, Vol. 56, 8, pp. 766-783.
Russia, Siberia
carbonatite
Abstract: In this study we present a fresh isotopic data, as well as U-Pb ages from different REE-minerals in carbonatites and phoscorites of Guli massif using in situ LA-ICPMS technique. The analyses were conducted on apatites and perovskites from calcio-carbonatite and phoscorite units, as well as on pyrochlores and baddeleyites from the carbonatites. The 87Sr/86Sr ratios obtained from apatites and perovskites from the phoscorites are 0.70308-0.70314 and 0.70306-0.70313, respectively; and 0.70310-0.70325 and 0.70314-0.70327, for the pyrochlores and apatites from the carbonatites, respectively. Furthermore, the in situ laser ablation analyses of apatites and perovskites from the phoscorite yield ?Nd from 3.6 (±1) to 5.1 (±0.5) and from 3.8 (±0.5) to 4.9 (±0.5), respectively; ?Nd of apatites, perovskites and pyrochlores from carbonatite ranges from 3.2 (±0.7) to 4.9 (±0.9), 3.9 (±0.6) to 4.5 (±0.8) and 3.2 (±0.4) to 4.4 (±0.8), respectively. Laser ablation analyses of baddeleyites yielded an eHf(t)d of +8.5 (± 0.18); prior to this study Hf isotopic characteristic of Guli massif was not known. Our new in situ ?Nd, 87Sr/86Sr and eHf data on minerals in the Guli carbonatites imply a depleted source with a long time integrated high Lu/Hf, Sm/Nd, Sr/Rb ratios. In situ U-Pb age determination was performed on perovskites from the carbonatites and phoscorites and also on pyrochlores and baddeleyites from carbonatites. The co-existing pyrochlores, perovskites and baddeleyites in carbonatites yielded ages of 252.3 ± 1.9, 252.5 ± 1.5 and 250.8 ± 1.4 Ma, respectively. The perovskites from the phoscorites yielded an age of 253.8 ± 1.9 Ma. The obtained age for Guli carbonatites and phoscorites lies within the range of ages previously reported for the Siberian Flood Basalts and suggest essentially synchronous emplacement with the Permian-Triassic boundary.
Dr. D. Twitchen ( CVD Element Six) discussion using magnetic resonance and optical techniques to study how defects in diamond affect colour and electrical conductivity. Keynote speaker
Earth and Planetary Science Letters, Vol. 499, pp. 205-218.
Ocean
plumes, hotspots
Abstract: The global mid-ocean ridge (MOR) system represents a major site for outgassing of volatiles from Earth's mantle. The amount of H2O released via eruption of mid-ocean ridge basalts varies along the global ridge system and greatest at sites of interaction with mantle plumes. These deep-sourced thermal anomalies affect approximately one-third of all MORs - as reflected in enrichment of incompatible trace elements, isotope signatures and elevated ridge topography (excess melting) - but the physical mechanisms involved are controversial. The “standard model” involves solid-state flow interaction, wherein an actively upwelling plume influences the divergent upwelling generated by a mid-ocean ridge so that melting occurs at higher pressures and in greater amounts than at a normal spreading ridge. This model does not explain, however, certain enigmatic features including linear volcanic ridges radiating from the active plume to the nearby MOR. Examples of these are the Wolf-Darwin lineament (Galápagos), Rodrigues Ridge (La Réunion), Discovery Ridge (Discovery), and numerous smaller ridge-like structures associated with the Azores and Easter-Salas y Gómez hot spots. An important observation from our study is that fractionation-corrected MORB with exceptionally-high H2O contents (up to 1.3 wt.%) are found in close proximity to intersections of long-lived plume-related volcanic lineaments with spreading centres. New algorithms in the rare-earth element inversion melting (INVMEL) program allow us to simulate plume-ridge interactions by mixing the compositions of volatile-bearing melts generated during both active upwelling and passively-driven corner-flow. Our findings from these empirical models suggest that at sites of plume-ridge interaction, moderately-enriched MORBs (with 0.2-0.4 wt.% H2O) result from mixing of melts formed by: (i) active upwelling of plume material to minimum depths of ?35 km; and (ii) those generated by passive melting at shallower depths beneath the ridge. The most volatile-rich MORB (0.4-1.3 wt.% H2O) may form by the further addition of up to 25% of “deep” small-fraction plume stem melts that contain >3 wt.% H2O. We propose that these volatile-rich melts are transported directly to nearby MOR segments via pressure-induced, highly-channelised flow embedded within a broader “puddle” of mostly solid-state plume material, spreading beneath the plate as a gravity flow. This accounts for the short wavelength variability (over 10s of km) in geochemistry and bathymetry that is superimposed on the much larger (many 100s of km) “waist width” of plume-influenced ridge. Melt channels may constitute a primary delivery mechanism for volatiles from plume stems to nearby MORs and, in some instances, be expressed at the surface as volcanic lineaments and ridges. The delivery of small-fraction hydrous melts from plume stems to ridges via a two-phase (melt-matrix) regime implies that a parallel, bimodal transport system is involved at sites of plume-ridge interaction. We estimate that the rate of emplacement of deep-sourced volatile-rich melts in channels beneath the volcanic lineaments is high and involves 10s of thousands of km3/Ma. Since mantle plumes account for more than half of the melt production at MORs our findings have important implications for our understanding of deep Earth volatile cycling.
Abstract: A detailed electron microprobe study has been carried out on the compositional variations of mica and apatite from carbonatites, phoscorites and associated pyroxenites (and fenites) of the Loolekop deposit, Palabora Carbonatite Complex (South Africa). Mica in pyroxenites and fenites is Mg-rich biotite, whilst micas in carbonatites and phoscorites are compositionally diverse including phlogopite, Ba-rich phlogopite (up to 30% kinoshitalite component), IVAl-rich phlogopite (up to 30% eastonite component) and tetraferriphlogopite. The various types of phlogopites are interpreted as orthomagmatic phases, whereas tetraferriphlogopite precipitation was a late-magmatic to hydrothermal process that additionally introduced REE into the system. Orthomagmatic apatite is generally REE- and Sr-poor fluorapatite and does not show large compositional differences between rock types. Apatite associated with the late-stage tetraferriphlogopite mineralization reaches higher levels of REE (up to 4.9?wt%), Si (up to 1.5?wt% SiO2), Sr (up to 2.6?wt% SrO) and Na (up to 1.0?wt% Na2O). The compositional variation of micas and apatites, which is affiliated with distinct rock types, reflects the multi-stage evolution of the Loolekop deposit and provides detailed insight into the relationships of the carbonatite-phoscorite assemblage. The obtained data support the separation of phoscorite and carbonatite by immiscibility from a common parental magma, which may happen due to a decrease of temperature and/or pressure during the ascent of the magma. This results in a density contrast between the carbonatitic and phoscoritic components that will lead to descending phoscorite accumulations at the outer zones of the magma channel and a jet-like ascent (further promoted by its extremely low viscosity) of the carbonatite magma. The genetic model deduced here explains the peculiar association of carbonatites, phoscorites and silicate rocks in many alkaline complexes worldwide.
Abstract: Two prominent features separate the Archean Wyoming and Hearne cratons: the Paleoproterozoic Great Falls tectonic zone (GFTZ) and the Medicine Hat block (MHB), neither of which is well defined spatially because of Phanerozoic sedimentary cover. Based on limited data, the MHB is thought to be a structurally complex mix of Archean (2.6-3.1?Ga) and Proterozoic (1.75?Ga) crust, but is recognized primarily by its geophysical signature, and its influence on the geochemistry of younger igneous rocks. Similarly, the GFTZ was recognized on the basis of broad differences in geophysical patterns, isopachs of Paleozoic sedimentary sections, and lineaments; however, juvenile arc rocks in the Little Belt Mountains (LBM) and strongly overprinted Archean rocks in southwestern Montana show it to be a dominantly Paleoproterozoic feature. The Little Rocky Mountains (LRM) of Montana provide access to exposures of the northeastern-most Precambrian crust in the MHB-GFTZ region. U/Pb ages of zircons from Precambrian rocks of the LRM range from 2.4 to 3.3?Ga, with most ages between 2.6 and 2.8?Ga. Whole-rock analyses yield Sm-Nd TDM from 3.1 to 4.0?Ga and initial ?Nd(T) values calculated at U-Pb zircon crystallization ages range from ?0.9 to ?10.5, indicating significant contributions from older Archean crust. The high proportion of 2.6-2.8?Ga U/Pb ages differentiates LRM crust from arc-related Paleoproterozoic magmatic rocks exposed in the LBM to the southwest. The age and isotopic composition of the LRM gneisses are similar to crust in the northern Wyoming Province (2.8-2.9?Ga), but Paleoproterozoic K-Ar cooling ages suggest crust in the LRM experienced the Paleoproterozoic metamorphism and deformation that characterizes the GFTZ. Consequently, its history differs markedly from the adjacent Beartooth-Bighorn magmatic zone of the northern Wyoming Province, which does not record Paleoproterozoic tectonism, but has a strong correlation with the Montana metasedimentary terrane that was strongly overprinted during the Paleoproterozoic Great Falls orogeny that defines the GFTZ. The LRM, therefore, likely provides a unique, and perhaps the only, opportunity to characterize Archean crust of the MHB.
Abstract: The subcontinental lithospheric mantle (SCLM) constitutes a significant portion of the upper mantle sourcing magmatic volatiles to the continents above, yet its geochemical signature and evolution remain poorly constrained. Here we present new interpretation of noble gas datasets from two magmatic CO2 fields in the SW US, namely Bravo Dome and Sheep Mountain, which provide a unique insight into the volatile character of the SCLM sourcing the Cenozoic volcanism in the region. We identify that reduction of 3He/4Hemantle ratio within the Sheep Mountain CO2 field can be attributed to radiogenic production within the SCLM. Using a Reduced Chi-Squared minimisation on the variation of derived 4He/21Necrust ratios within samples from the Sheep Mountain field, combined with a radiogenically raised 21Ne/22Nemantle end member, we resolve 3He/4Hemantle ratios of 2.59 ± 0.15 to 3.00 ± 0.18 Ra. These values correspond with a 21Ne/22Nemantle value of 0.136. Using these 3He/4Hemantle end member values with 21Nemantle resolved from Ne three component analysis, we derive the elemental 3He/22Nemantle of 2.80 ± 0.16 and radiogenic 4He/21Ne*mantle range of 1.11 ± 0.11 to 1.30 ± 0.14. A second Reduced Chi-Squared minimisation performed on the variation of 21Ne/40Arcrust ratios has allowed us to also determine both the 4He/40Armantle range of 0.78 to 1.21 and 21Ne/40Armantle of 7.66 ± 1.62 to 7.70 ± 1.54 within the field. Combining these ratios with the known mantle production ranges for 4He/21Ne and 4He/40Ar allows resolution of the radiogenic He/Ne and He/Ar ratios corresponding to the radiogenically lowered 3He/4Hemantle ratios. Comparing these values with those resolved from the Bravo Dome field allows identification of a clear and coherent depletion of He to Ne and He to Ar in both datasets. This depletion can only be explained by partial degassing of small melt fractions of asthenospheric melts that have been emplaced into the SCLM. This is the first time that it has been possible to resolve and account for both the mantle He/Ne and He/Ar ratios within a SCLM source. The data additionally rule out the involvement of a plume component in the mantle source of the two gas fields and hence any plume influence on the Colorado Plateau Uplift event.
Earth and Planetary Science Letters, Vol. 501, pp. 202-212.
Mantle
melting
Abstract: The viscosity of silicate melts is the most important physical property governing magma transport and eruption dynamics. This macroscopic property is controlled by composition and temperature but ultimately reflects the structural organization of the melt operating at the microscale. At present, there is no explicit relationship connecting viscosity to silicate melt structure and vice versa. Here, we use a single Raman spectroscopic parameter, indicative of melt structure, to accurately forecast the viscosity of natural, multicomponent silicate melts from spectroscopic measurements on glasses preserved on Earth and other planets. The Raman parameter is taken as the ratio of low and high frequency vibrational bands from the silicate glass by employing a green source laser wavelength of 514.5 nm (R514.5). Our model is based on an empirical linkage between R514.5 and coefficients in the Vogel-Fulcher-Tammann function for the temperature dependence of melt viscosity. The calibration of the Raman-based model for melt viscosity is based on 413 high-temperature measurements of viscosity on 23 melt compositions for which published Raman spectra are available. The empirical model obviates the need for chemical measurement of glass compositions, thereby, providing new opportunities for tracking physical and thermochemical properties of melts during igneous processes (e.g., differentiation, mixing, assimilation). Furthermore, our model serves as a milepost for the future use of Raman spectral data for predicting transport (and calorimetric) properties of natural melts at geological conditions (e.g., volatiles and pressure) and production.
Earth and Planetary Science Letters, Vol. 501, pp. 202-212.
Mantle
melting
Abstract: The viscosity of silicate melts is the most important physical property governing magma transport and eruption dynamics. This macroscopic property is controlled by composition and temperature but ultimately reflects the structural organization of the melt operating at the microscale. At present, there is no explicit relationship connecting viscosity to silicate melt structure and vice versa. Here, we use a single Raman spectroscopic parameter, indicative of melt structure, to accurately forecast the viscosity of natural, multicomponent silicate melts from spectroscopic measurements on glasses preserved on Earth and other planets. The Raman parameter is taken as the ratio of low and high frequency vibrational bands from the silicate glass by employing a green source laser wavelength of 514.5 nm (R514.5). Our model is based on an empirical linkage between R514.5 and coefficients in the Vogel-Fulcher-Tammann function for the temperature dependence of melt viscosity. The calibration of the Raman-based model for melt viscosity is based on 413 high-temperature measurements of viscosity on 23 melt compositions for which published Raman spectra are available. The empirical model obviates the need for chemical measurement of glass compositions, thereby, providing new opportunities for tracking physical and thermochemical properties of melts during igneous processes (e.g., differentiation, mixing, assimilation). Furthermore, our model serves as a milepost for the future use of Raman spectral data for predicting transport (and calorimetric) properties of natural melts at geological conditions (e.g., volatiles and pressure) and production.
Abstract: Diamond nucleation and growth were investigated experimentally at 7.5 and 10.5?GPa and temperatures up to 1500?°C. Samples consisted of two layers: i) H2O- and CO2-bearing model sediment and ii) graphite-bearing garnet harzburgite comprising natural minerals. Two experimental series were conducted, one under a controlled temperature gradient with the sedimentary layer usually in the cold zone and the other under isothermal conditions. In the latter case, diamond seeds were added to the sedimentary mixture. During the experiments, the sedimentary layer partially or completely melted, with the melt percolating and interacting with the adjacent harzburgite. The graphite-to-diamond transition in the peridotite was observed above 1300?°C at 7.5?GPa and 1200?°C at 10.5?GPa in the temperature-gradient experiments, and at temperatures ~100?°C lower in the isothermal experiments with diamond seeds. Newly formed diamond occurs mostly as individual grains up to 10??m in size and is separate from graphite aggregates. In some cases, an association of diamond with magnesite was observed. Diamond nucleation occurs in hydrous and CO2-bearing silicate melt following graphite dissolution and recrystallization. In the case of the diamond-magnesite association, diamond was probably formed through carbonate reduction coupled with graphite oxidation. The composition of the melts ranged from “carbonatitic” with ~10?wt% SiO2 and?>?50?wt% volatiles to hydrous silicate with ~40?wt% SiO2 and?
Abstract: The origin of intraplate carbonatitic to alkaline volcanism in Africa is controversial. A tectonic control, i.e., decompression melting associated with far-field stress, is suggested by correlation with lithospheric sutures, repeated magmatic cycles in the same areas over several million years, synchronicity across the plate, and lack of clear age progression patterns. Conversely, a dominant role for mantle convection is supported by the coincidence of Cenozoic volcanism with regions of lithospheric uplift, positive free-air gravity anomalies, and slow seismic velocities. To improve constraints on the genesis of African volcanism, here we report the first radiometric and isotopic results for the Catanda complex, which hosts the only extrusive carbonatites in Angola. Apatite (U-Th-Sm)/He and phlogopite 40Ar/39Ar ages of Catanda aillikite lavas indicate eruption at ca. 500-800 ka, more than 100 m.y. after emplacement of abundant kimberlites and carbonatites in this region. The lavas share similar high-? (HIMU)-like Sr-Nd-Pb-Hf isotope compositions with other young mantle-derived volcanics from Africa (e.g., Northern Kenya Rift; Cameroon Line). The position of the Catanda complex in the Lucapa corridor, a long-lived extensional structure, suggests a possible tectonic control for the volcanism. The complex is also located on the Bié Dome, a broad region of fast Pleistocene uplift attributed to mantle upwelling. Seismic tomography models indicate convection of deep hot material beneath regions of active volcanism in Africa, including a large area encompassing Angola and northern Namibia. This is strong evidence that intraplate late Cenozoic volcanism, including the Catanda complex, resulted from the interplay between mantle convection and preexisting lithospheric heterogeneities.
Earth and Planetary Science Letters, Vol. 401, pp. 132-147.
Africa, South Africa
metasomatism
Abstract: The Karoo igneous rocks (174-185 Ma) of southern Africa represent one of the largest continental flood basalt provinces on Earth. Available evidence indicates that Karoo magmas either originated in the asthenosphere and were extensively modified by interaction with the lithospheric mantle prior to emplacement in the upper crust; or were produced by partial melting of enriched mantle lithosphere. However, no direct evidence of interaction by Karoo melts (or their precursors) with lithospheric mantle rocks has yet been identified in the suites of mantle xenoliths sampled by post-Karoo kimberlites in southern Africa. Here we report U-Pb ages for lindsleyite-mathiasite (LIMA) titanate minerals (crichtonite series) from three metasomatised, phlogopite and clinopyroxene-rich peridotite xenoliths from the ?84 Ma Bultfontein kimberlite (Kimberley, South Africa), located in the southern part of the Karoo magmatic province. The LIMA minerals appear to have formed during metasomatism of the lithospheric mantle by fluids enriched in HFSE (Ti, Zr, Hf, Nb), LILE (K, Ba, Ca, Sr) and LREE. LIMA U-Pb elemental and isotopic compositions were measured in situ by LA-ICP-MS methods, and potential matrix effects were evaluated by solution-mode analysis of mineral separates. LIMA minerals from the three samples yielded apparent U-Pb ages of , and (). A single zircon grain extracted from the ?190 Ma LIMA-bearing sample produced a similar U-Pb age of , within uncertainty of the LIMA ages. These data provide the first robust evidence of fluid enrichment in the lithospheric mantle beneath the Kimberley region at ?180-190 Ma, and suggest causation of mantle metasomatism by Karoo melts or their precursor(s). The results further indicate that U-Pb dating of LIMA minerals provides a new, accurate tool for dating metasomatic events in the lithospheric mantle.
Earth and Planetary Science Letters, Vol. 482, pp. 253-264.
Africa, South Africa
metasomatism
Abstract: Radiogenic isotope variations unrelated to radiogenic ingrowth are common between minerals found in metasomatised mantle xenoliths entrained in kimberlite, basalts and related magmas. As the metasomatic minerals are assumed to have been in isotopic equilibrium originally, such variations are typically attributed to contamination by the magma host and/or interaction with mantle fluids during or before xenolith transport to surface. However, the increasing evidence of metasomatism by multiple, compositionally distinct fluids permeating the lithospheric mantle, coeval with specific magmatic events, suggests that isotopic disequilibrium might be a consequence of discrete, though complex, metasomatic events. Here we provide clear evidence of elemental and Sr isotope heterogeneity between coeval Ti-rich LIMA (lindsleyite–mathiasite) minerals at the time of their formation in the mantle. LIMA minerals occur in close textural association with clinopyroxene and phlogopite in low-temperature (?800–900?°C), strongly metasomatised mantle xenoliths from the ?84 Ma Bultfontein kimberlite (South Africa). Previous U/Pb dating of the LIMA phases was used to argue that each xenolith recorded a single event of LIMA crystallisation at ?180–190 Ma, coeval with the emplacement of Karoo magmas. SEM imaging reveals that up to four types of LIMA phases coexist in each xenolith, and occasionally in a single LIMA grain. Major element and in situ Sr isotope analyses of the different LIMA types show that each phase has a distinct elemental composition and initial 87Sr/86Sr ratio (e.g., 0.7068–0.7086 and 0.7115–0.7129 for two LIMA types in a single xenolith; 0.7053-0.7131 across the entire sample suite). These combined age and isotopic constraints require that multiple fluids metasomatised these rocks at broadly the same time (i.e. within a few thousands to millions of years), and produced similar mineralogical features. Elemental and isotopic variations between different LIMA types could be due to interaction between one (or more) Karoo-related Ti-rich silicate melts and previously metasomatised, phlogopite-rich lithospheric mantle. This study demonstrates that mantle metasomati
