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SDLRC - Scientific Articles all years by Author - O
The Sheahan Diamond Literature Reference Compilation
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcementscalled the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Resource Center
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
The SDLRC provides 3 types of references identified in the reference code. DS for scientific article, DM for a media article, and DC for a corporate announcement. Consider DS0512-0001. The DS stands for "diamond scientific". 05 stands for 2005, the year the reference was posted. 12 represents the month the reference was posted. For all years prior to 2015 the default month is 12. -0001 is the reference's identifier and it does not mean anything. The number below the refence code, ie 2015, is the year the article was published. Note that the posted year may sometimes be later than the published year.
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Abstract: Raman spectroscopy has been foreseen as a simple and non-destructive characterization method to determine the boron concentration in heavily boron-doped diamond with metallic conductivity. However, currently available empirical studies are not fully satisfactory for enabling accurate determination of the boron concentration in diamond. Here, we study Raman spectra of epitaxial boron-doped diamond as a function of the boron concentration and the excitation wavelength. The zone center phonon and the phonon density of state maximum (at ca. 1200?cm-1) lines are analyzed using a decoupled double Fano-function. This analysis method accurately describes the observed variation of the asymmetric parameters with atomic boron concentration and the photon excitation energy and enables the determination of the atomic boron concentration from the parameters of the examined Raman lines.
A new model for the Paleogene motion of Greenland relative to North America: plate reconstructions of the Davis Strait and Nares Strait regions between Canada and Greenland.
Journal of Geophysical Research, Vol. 117, B 10, B10401.
Abstract: Silicon carbide (SiC, moissanite) is a common industrial material that is rarely found in terrestrial rocks and meteorites. It has been found to adopt over 300 different crystal structures, most of which are polytypic: they consist of alternating layers of Si and C, with only small stacking faults or shears distinguishing them from one another. In nature, only a few polytypes of SiC have been found, primarily a cubic zincblende type (3C-SiC), several hexagonal wurtzite types (4H-SiC and 6H-SiC), and a rhombohedral type (15R-SiC). Our natural silicon carbide sample is from a Miocene tuff (Yizre’el Valley, Israel) related to interplate alkaline basalt volcanism. Three SiC grains with native silicon and metal silicide inclusions were analyzed using Raman spectroscopy and synchrotron Laue X-ray microdiffraction accompanied by mapping at a 5-8 um resolution. SiC is found to crystallize in only the 4H and 6H polytypes. Due to the crystal orientation of the grains, as well as the significant difference in the c-axis length (~10 vs. ~15 um in 4H and 6H respectively), we were able to unambiguously assign polytypes to each diffraction pattern. Each grain contains large areas where one polytype dominates as a single crystal. In some cases, multiple stacking faults and misoriented polycrystalline aggregates of SiC occur at the 4H/6H interface. In other cases we see intercalation of the 4H and 6H crystals throughout the diffracting volume without a significant change in their crystallographic axes orientation, pointing to a possibly incommensurate crystal structure. Stress and strain are also mapped for all three grains, showing a slight (< 2 ppt) compressive strain in the y direction of all three grains, and a tensile strain in the x and z directions. In the SiC-2 grain, a mostly single-crystalline Si inclusion was found, with an exposed surface diameter of ~30 um. We examine residual strain in Si by both Laue X-ray diffraction and Raman spectroscopy, and find results to generally agree between the two measurements.
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.
Diamonds & Related Materials, in press available, 31p. Pdf
Global
carbon
Abstract: Natural diamonds that have been partially replaced by graphite have been observed to occur in natural rocks. While the graphite-to-diamond phase transition has been extensively studied the opposite of this (diamond to graphite) remains poorly understood. We performed high-pressure and temperature hydrous and anhydrous experiments up to 1.0?GPa and 1300?°C using Amplex premium virgin synthetic diamonds (20-40?µm size) as the starting material mixed with Mg (OH)2 as a source of H2O for the hydrous experiments. The experiments revealed that the diamond-to-graphite transformation at P?=?1GPa and T?=?1300?°C was triggered by the presence of H2O and was accomplished through a three-stage process. Stage 1: diamond reacts with a supercritical H2O producing an intermediate 200-500?nm size “globular carbon” phase. This phase is a linear carbon chain; i.e. a polyyne or carbyne. Stage 2: the linear carbon chains are unstable and highly reactive, and they decompose by zigzagging and cross-linking to form sp2-bonded structures. Stage 3: normal, disordered, and onion-like graphite is produced by the decomposition of the sp-hybridized carbon chains which are re-organized into sp2 bonds. Our experiments show that there is no direct transformation from sp3 C-bonds into sp2 C-bonds. Our hydrous high-pressure and high-temperature experiments show that the diamond-to-graphite transformation requires an intermediate metastable phase of a linear hydrocarbon. This process provides a simple mechanism for the substitution of other elements into the graphite structure (e.g. H, S, O).
Earth and Planetary Science Letters, Vol. 498, pp. 387-396.
Europe, Bulgaria
moissanite
Abstract: Terrestrial moissanite (SiC) is widely reported as an ultra-high pressure mineral occurring in kimberlites, diamonds and ultramafic/mafic rocks of mantle origin. However, the conditions of crystallization remain largely unknown. Moreover, dozens of SiC occurrences have been reported from continental crust sources such as granitoids, andesite-dacite volcanic rocks and their breccia, metasomatic and metamorphic rocks, and even limestones. The validity of many of these reports is still debated primarily due to possible contaminations from the widespread use of synthetic SiC abrasives in samples preparation. Indeed, reports of well-documented in-situ occurrences of moissanite in association with co-existing minerals are still scarce. The only condition of moissanite formation that is agreed upon is that extremely reducing media are required (e.g. 4.5-6 log units below the iron-wustite buffer). Here, we report the new occurrence of moissanite that was found in-situ within the garnet-staurolite-mica schists of Topolovgrad metamorphic group of Triassic age in Southern Bulgaria. The 10-300 µm moissanite crystals are situated within 0.1-1.2 mm isolated clusters, filled with amorphous carbon and nanocrystalline graphite. Most of moissanite crystals are 15R (rhombohedral) and 6H (hexagonal) polytypes, and one prismatic crystal, found within them, exhibits unusual concentric polytypical zoning with core (15R), intermediate zone (6H) and rim (3C-cubic). Experimental data show that this type of polytypical zonation is likely due to a decrease in temperature (or/and pressure?) and changes in Si/C ratio. Indeed, amphibolite facies metamorphism (500-580?°C - garnet-staurolite zone) followed by a subsequent cooling during the retrograde stage of green schist facies metamorphism (~400-500?°C) could have provided a change in temperature. The SiC containing clusters exhibit evidence that they are pre-metamorphic, and we hypothesize that their protolith was a "lack shale" material likely rich in carbon, hydrocarbon and terrigenous silica. The latter served as a source of isolated chemically-reduced media, which is required for SiC formation. Other concepts to explain moissanite occurrences in metasedimentary rocks are also discussed. Importantly, our findings show that the formation conditions of moissanite are likely more variable than previously recognized.
Abstract: Moissanite, SiC, is an uncommon accessory mineral that forms under low oxygen fugacity. Here, we analyze natural SiC from a Miocene tuff-sandstone using synchrotron Laue microdiffraction and Raman spectroscopy, in order to better understand the SiC phases and formation physics. The studied crystals of SiC consist of 4H- and 6H-SiC domains, formed from either, continuous growth or, in one case, intergrown, together with native Si. The native Si is polycrystalline, with a large crystal size relative to the analytical beam dimensions (>1-2 µm). We find that the intergrown region shows low distortion or dislocation density in SiC, but these features are comparatively high in Si. The distortion/deformation observed in Si may have been caused by a mismatch in the coefficients of thermal expansion of the two materials. Raman spectroscopic measurements are discussed in combination with our Laue microdiffraction results. Our results suggest that these SiC grains likely grew from an igneous melt.
Topotaxic relationships between spinel and pyroxene in kelphite after garnet in mantle derived peridotites and their implications to reaction mechanism and kinetics.
Mineralogy and Petrology, Vol. 101, 3-4, pp. 217-224.
Wawrzenitz, N., Romer, R.L., Oberhansli, R., Dong, S.
Dating of subduction and differential exhumation of UHP rocks fromn the Central Dabie Complex ( E-China): constraints from microfabrics, Rb-Sr and U-Pb isotope systems.
Adam, J., Oberti, R., Camara, F., Green, T.H., Rushmer, T.
The effect of water on equilibrium relations between clinopyroxenes and basanitic magmas: tracing water and non- volatile incompatible elements in the Earth's mantle.
Obrebski, M., Allen, R.M., Pollitz, F., Hung, S-H.
Lithosphere asthenosphere interaction beneath the western United States from the joint inversion of body-wave traveltimes and surface wave phase veolocities.
Geophysical Journal International, March 25, In press available
Contributions to Mineralogy and Petrology, Vol. 171, pp. 1-12.
Europe
Bohemian
Abstract: A unique assemblage including kumdykolite and kokchetavite, polymorphs of albite and K-feldspar, respectively, together with cristobalite, micas, and calcite has been identified in high-pressure granulites of the Orlica-Snieznik dome (Bohemian Massif) as the product of partial melt crystallization in preserved nanogranites. Previous reports of both kumdykolite and kokchetavite in natural rocks are mainly from samples that passed through the diamond stability field. However, because the maximum pressure recorded in these host rocks is <3 GPa, our observations indicate that high pressure is not required for the formation of kumdykolite and kokchetavite, and their presence is not therefore an indicator of ultrahigh-pressure conditions. Detailed microstructural and microchemical investigation of these inclusions indicates that such phases should instead be regarded as (1) a direct mineralogical criteria to identify former melt inclusions with preserved original compositions, including H2O and CO2 contents and (2) indicators of rapid cooling of the host rocks. Thus, the present study provides novel criteria for the interpretation of melt inclusions in natural rocks and allows a more rigorous characterization of partial melts during deep subduction to mantle depth as well as their behavior on exhumation.
Lehtonen, M., O'Brien, H., Peltonen, P., Kukkonen, I., Ustinov, V., Verzhak, V.
Mantle xenocrysts from the Arkangelskaya kimberlite (Lomonosov); constraints on the composition and thermal state of the Diamondiferous lithospheric mantle.
Zozulya, D.R., O'Brien, H., Peltonen, P., Lehtonen, M.
Thermobarometry of mantle derived garnets and pyroxenes of Kola region ( NW Russia): lithosphere composition, thermal regime and diamond prospectivity.
Bulletin of the Geological Society of Finland, Vol. 81, pp. 143-158.
Zozulya, D.R., O'Brien, H., Peltonen, P., Lehtonen, M.
Thermobarometry of mantle derived garnets and pyroxenes of Kola region ( NW Russia): lithosphere composition, thermal regime and diamond prospectivity.
Bulletin of the Geological Society of Finland, Vol. 81, pp. 143-158.
Abstract: The levels of brilliance (brightness and contrast), fire (flashes of rainbow color), and scintillation (intense sparkles when moved) of diamonds are unmatched by any other gemstone. Also diamonds of gem size and quality are relatively rare. As a result, gem diamonds are extremely valuable, yet the supply of diamonds is ultimately limited. This reality has pushed diamond exploration and mining into extreme environments, from the far Arctic North to the deserts of southern Africa and onto the ocean bed off the coast of Namibia. About two-thirds of the annual production of diamonds by weight comes from ancient volcanoes that consist of the rock types kimberlite, orangeite, or lamproite. Tracking down the remnants of these small volcanoes requires sophisticated and efficient collection and processing of samples for kimberlite indicator minerals (i.e., peridotite constituent minerals) and evaluation of enormous amounts of mineral data to constrain the diamond prospectivity of a region, cluster of pipes, or particular diatreme. The exploration sampling stage is usually followed by aero- or ground-geophysical measurements, target evaluation, and, finally, drill testing. Diamond exploration is expensive, but the rewards can be great. Diamond exploration in Finland started in 1985, and has been continuous, albeit with varying levels of activity, since that time. As a result, diamondiferous rocks have been found in three regions—namely, the Kuhmo-Lentiira area hosting a group of 1200 Ma orangeites, the Kuusamo-Hossa area containing several 760 Ma kimberlites, and the Kaavi-Kuopio area with a cluster of ~600 Ma kimberlites. Driven by the needs of these exploration activities, our understanding of the makeup of the Karelian craton, and our understanding of the magmas that have transported diamonds to the surface in this part of the world have benefitted enormously.
Geochimica et Cosmochimica Acta, Vol. 206, pp. 312-342.
Europe, Finland
Deposit - Lahtojoki
Abstract: A collection of 61 xenocrystic and 12 eclogite xenolith-derived diamonds from the 600 Ma Lahtojoki kimberlite in central Finland has been investigated. Calculated pressure and temperature conditions for the diamondiferous eclogites are in excess of 5.5 GPa and 1300 °C, suggesting residence depths greater than 180 km, near the base of the Karelian cratonic mantle lithosphere. Geochemically, the eclogite xenoliths have gabbroic compositions showing positive Eu and Sr anomalies, relatively low SREE and elevated Al2O3 contents, yet garnets have ambiguous d18O values of 5.7‰ and 5.9‰. Gabbroic eclogite formation could therefore be linked to either subduction processes during the 1.9 Ga Svecofennian orogeny or to cumulate processes during 2.1 Ga rift-induced magmatism. Determination of the oxygen fugacity of Lahtojoki eclogite xenoliths from both this work and previous studies suggests that diamond-bearing eclogites may be more reduced (?FMQ-3.5) compared to barren eclogites (?FMQ-1.7). While recycled oceanic crust protoliths for the eclogites remain a possibility, the carbon isotopic compositions and nitrogen abundances of the Lahtojoki diamonds indicate mantle-derived volatile sources. All diamonds (i.e., loose and eclogite xenolith-derived) display a restricted range of d13C values from -7.8‰ to -3.7‰ that overlaps with the carbon isotopic composition of Earth’s mantle. The Lahtojoki diamond d13C values form a negatively skewed distribution, indicating diamond growth from reduced mantle-derived carbon sources such as methane- (CH4) bearing fluids. Nitrogen contents of the Lahtojoki diamonds range from 40 to 1830 atomic ppm with a mean of ~670 atomic ppm; these elevated nitrogen contents combined with the close association to eclogites suggest an eclogitic or crustal volatile source. However, the Karelian craton was periodically intruded by ultramafic alkaline magmas since at least 1.8 Ga, noting in particular the occurrence of phlogopite-rich kimberlites and olivine lamproites between 1200 and 700 Ma. We argue that this punctuated volatile-rich magmatism simultaneously metasomatised the cratonic mantle lithosphere, forming nitrogen enriched phlogopite-bearing metasomes. We propose that reduced, carbon-bearing and nitrogen-rich fluids were remobilized to form the Lahtojoki diamonds. The diamond-forming event(s) most probably occurred during or shortly prior to the entraining kimberlite magmatism as indicated by the diamond nitrogen aggregation systematics. Involvement of reduced diamond-forming fluids is supported by both the negative skewness of Lahtojoki diamond d13C values and the more reduced nature of the diamondiferous Lahtojoki eclogites compared with their more oxidized barren counterparts. Our results from the diamondiferous eclogites derived from the deepest parts of the Karelian cratonic mantle root are in support of methane being the stable carbon volatile species at the base of thick continental lithosphere.
Geochimica et Cosmochimica Acta, in press available, 14p.
Europe, Finland
deposit - Kaavi
Abstract: We present high-precision isotope dilution data for Os, Ir, Ru, Pt, Pd and Re in Group I and Group II kimberlites from the Karelian craton, as well as 2 samples of the Premier Group I kimberlite pipe from the Kaapvaal craton. The samples have, on average, 1.38 ppb Pt and 1.33 ppb Pd, with Pt/Pd around unity. These PGE levels are markedly lower, by as much as 80%, than those reported previously for kimberlites from South Africa, Brazil and India, but overlap with PGE results reported recently from Canadian kimberlites. Primitive-mantle-normalised chalcophile element patterns are relatively flat from Os to Pt, but Cu, Ni and, somewhat less so, Au are enriched relative to the PGE (e.g., Cu/Pd > 25.000). Pd/Ir ratios are 3,6 on average, lower than in most other mantle melts. The PGE systematics can be largely explained by two components, (i) harzburgite/lherzolite detritus of the SCLM with relatively high IPGE (Os-Ir-Ru)/PPGE (Rh-Pt-Pd) ratios, and (ii) a melt component that has high PPGE/IPGE ratios. By using the concentrations of iridium in the kimberlites as a proxy for the proportion of mantle detritus in the magma, we estimate that the analysed kimberlites contain 3–27% entrained and partially dissolved detritus from the sub-continental lithospheric mantle, consistent with previous estimates of kimberlites elsewhere (Tappe S. et al., 2016, Chem. Geol. http://dx.doi.org/10.1016/j.chemgeo.2016.08.019). The other major component in the samples is melt, modelled to contain an average of 0.85 ppb Pt and 1.09 ppb Pd. Assuming that Group II kimberlites are derived from relatively metasomatised SCLM, our data suggest that the metasomatised Karelian SCLM is relatively poor in Pt and Pd. If our data are representative of other Group II kimberlites elsewhere, this result could imply that the PGE enrichment in certain continental large igneous provinces, including Bushveld, is not derived from melting of metasomatised SCLM.
Geochimica et Cosmochimica Acta, Vol. 216, pp. 358-371.
Europe, Finland
deposit - Karelian
Abstract: We present high-precision isotope dilution data for Os, Ir, Ru, Pt, Pd and Re in Group I and Group II kimberlites from the Karelian craton, as well as 2 samples of the Premier Group I kimberlite pipe from the Kaapvaal craton. The samples have, on average, 1.38 ppb Pt and 1.33 ppb Pd, with Pt/Pd around unity. These PGE levels are markedly lower, by as much as 80%, than those reported previously for kimberlites from South Africa, Brazil and India, but overlap with PGE results reported recently from Canadian kimberlites. Primitive-mantle-normalised chalcophile element patterns are relatively flat from Os to Pt, but Cu, Ni and, somewhat less so, Au are enriched relative to the PGE (e.g., Cu/Pd > 25.000). Pd/Ir ratios are 3,6 on average, lower than in most other mantle melts. The PGE systematics can be largely explained by two components, (i) harzburgite/lherzolite detritus of the SCLM with relatively high IPGE (Os-Ir-Ru)/PPGE (Rh-Pt-Pd) ratios, and (ii) a melt component that has high PPGE/IPGE ratios. By using the concentrations of iridium in the kimberlites as a proxy for the proportion of mantle detritus in the magma, we estimate that the analysed kimberlites contain 3-27% entrained and partially dissolved detritus from the sub-continental lithospheric mantle, consistent with previous estimates of kimberlites elsewhere (Tappe S. et al., 2016, Chem. Geol. http://dx.doi.org/10.1016/j.chemgeo.2016.08.019). The other major component in the samples is melt, modelled to contain an average of 0.85 ppb Pt and 1.09 ppb Pd. Assuming that Group II kimberlites are derived from relatively metasomatised SCLM, our data suggest that the metasomatised Karelian SCLM is relatively poor in Pt and Pd. If our data are representative of other Group II kimberlites elsewhere, this result could imply that the PGE enrichment in certain continental large igneous provinces, including Bushveld, is not derived from melting of metasomatised SCLM.
Bulletin of the Geological Survey of Finland, Vol. 79, 2, pp. 203-215.
Europe, Finland
deposit - Lentiira Kuhmo
Abstract: The Lentiira-Kuhmo-Kostomuksha triangle, along the Finland - Russian border and within the central part of the Archean Karelian craton, contains numerous examples of phlogopite-rich, ultramafic, mantle-xenocryst-bearing and, in some cases, diamond-bearing dike rocks. Laser probe Ar-Ar data on phlogopite from 3 dike rocks on the Finnish side (Lentiira, Kuhmo) all gave ages within error of each other, 1202 ± 3 Ma (2s), 1199 ± 3 Ma (2s) and 1204 ± 4 Ma (2s) while a fourth sample produced mixed ages. Published Rb-Sr dates on mineralogically and chemically similar dikes from the Russian side (Kostomuksha) are 1232 ± 5 Ma. The question remains open whether these represent two distinct age populations or whether differences in isotopic system behavior are the reason for the 30 m.y. age difference.
Abstract: Diamond xenocrysts and eclogite-hosted diamonds from the Lahtojoki kimberlite (Karelian craton, Finland) indicate metasomatism of the deep lithosphere by N-rich, relatively reduced fluids. P-T-fO2 constraints show that all eclogites were derived from near the base of the lithospheric mantle (>5 GPa), but only the diamond-bearing samples are relatively reduced (?FMQ-3.5 vs. -1.7 for barren eclogites). The Lahtojoki diamonds show evidence of formation from reduced mantle-derived carbon, based on the restricted range of ?13C values (-3 and -7.8 ‰; n = 67) that form a negativelyskewed distribution. This reduced CHO fluid was also anomalously N-rich, based on the diamond N contents that range up to 1830 at. ppm. While N-rich sources for eclogiteassociated diamonds are often linked to recycled crustal materials, in this case we prefer derivation from K-rich cratonic mantle metasomes due to lack of firm crustal geochemical signatures in the eclogites (?18O = 5.7 - 5.9 ‰), in addition to the magmatic history of the Karelian craton. The Karelian craton has been periodically intruded by Krich alkaline lamprophyres, Group-2 kimberlites and olivine lamproites from 1800 to 700 Ma. Such K-rich ultramafic alkaline magmatism is likely linked to phlogopite-rich metasomes, which may represent significant repositories of N (NH4+ substitution for K+). Because the Lahtojoki eclogites resided near the base of the lithospheric mantle, they would have been susceptible to interaction with ascending asthenosphere-derived C-bearing fluids/melts, which were reducing. Following ingress into and interaction with the Krich metasomatised Karelian mantle lithosphere, the increasingly N-enriched, CH4-bearing fluids precipitated diamond during interaction with relatively oxidized eclogite wall rock. In contrast to the prevalent oxidizing effects of mantle metasomatism as identified within cratonic lithosphere-derived samples from worldwide locations, the eclogite-hosted diamonds at Lahtojoki represent a natural example of metasomatic overprinting that was highly reducing.
Abstract: Kimberlites are silica-poor, volatile-rich (CO2 ± H2O), volcanic rocks that are often described as ‘hybrid’, because their parental magmas include abundant xenocrystic (crustand mantle-derived) components. Unravelling the influence of mantle assimilation on kimberlite melt compositions represents an outstanding question of kimberlite petrology. To address this issue, we have carried out a comprehensive geochemical and petrographic investigation of nine kimberlites from the Kaavi-Kuopio field in Finland, that were emplaced on the southern margin of the Karelian Craton in the Neoproterozoic (~550-600 Ma). Olivine is the dominant mineral phase in kimberlites (~50 vol.%) with cores mainly derived from the disaggregation of mantle peridotite. In contrast, olivine rims crystallise directly from the kimberlitic melt and their Mg# (Mg/(Mg+Fe)) typically show remarkable homogeneity within and between kimberlites of a single cluster and field (e.g., Lac de Gras). The Kaavi-Kuopio kimberlites appear to represent a unique case where there is a (statistically) significant difference between the average Mg# of olivine rims in different pipes (89.9 ± 0.2 to 88.5 ± 0.3). Importantly, the Mg# of olivine rims exhibit a strong correlation with the Mg# of olivine cores. Furthermore, the compositions of olivine cores (and rims) exhibit a strong correlation with those of spinel (e.g., Mg#, TiO2 contents). These geochemical variations correlate with the modal mineralogy of the kimberlites: for example, higher abundances of monticellite and lower abundances of ilmenite are associated with higher Mg# olivine. The robust relationship between entrained and assimilated lithospheric mantle material (i.e. olivine cores) and magmatic components (i.e. olivine rims, spinel, and other groundmass minerals) suggests that assimilation of lithospheric mantle has impacted the compositions of kimberlitic melts to a greater extent than previously recognised. These new data also suggest significant variations in the composition of the mantle lithosphere beneath the Kaavi-Kuopio kimberlites, which are spaced less than 10 km apart.
Abstract: Hypabyssal kimberlites are subvolcanic intrusive rocks crystallised from mantle-derived magmas poor in SiO2 and rich in CO2 and H2O. They are complex, hybrid rocks containing significant amounts of mantle-derived fragments, primarily olivine with rare diamonds, set in a matrix of essentially magmatic origin. Unambiguous identification of kimberlites requires careful petrographic examination combined with mineral compositional analyses. Melt inclusion studies have shown that kimberlite melts contain higher alkali concentrations than previously thought but have not clarified the ultimate origin of these melts. Because of the hybrid nature of kimberlites and their common hydrothermal alteration by fluids of controversial origin (magmatic and/or crustal), the composition of primary kimberlite melts remains unknown.
Abstract: Kimberlites are complex, ‘hybrid’ igneous rocks because their parental magmas entrain abundant crust- and mantle-derived components that can be readily assimilated during ascent to surface. Recent studies of olivine zonation patterns have shown compositional relationships between xenocrystic cores and magmatic rims, suggesting that kimberlite melt compositions might be controlled by assimilation of mantle material during emplacement. However, the nature and extent to which this process, as well as assimilation of crustal material, influences melt compositions within single kimberlite fields remains unclear. To address this issue, we have conducted a comprehensive geochemical and petrographic investigation of kimberlites from eight pipes in the Kaavi-Kuopio field in Finland, which were emplaced on the southern margin of the Karelian craton during the Neoproterozoic (~550-600 Ma). While magmatic olivine rims are usually homogeneous in composition within and between kimberlites of a single cluster and field (e.g., Lac de Gras), the Kaavi-Kuopio kimberlites appear to represent a unique case where there are statistically significant differences between the average Mg# of olivine rims in different pipes (89.9 ± 0.2 to 88.5 ± 0.3). Importantly, the Mg# of magmatic olivine rims exhibit a strong correlation with the Mg# of their mantle-derived xenocrystic cores. Furthermore, the compositions of olivine cores and rims exhibit a robust relationship with those of magmatic spinel (e.g., Mg#, TiO2 contents). These geochemical variations also align with the mineralogy of the kimberlites: whereby abundances of phlogopite and oxides (e.g., spinel) are negatively correlated with olivine rim Mg#. The robust relationship between entrained and assimilated lithospheric mantle material (i.e. olivine cores) and magmatic components (i.e. olivine rims, spinel, and groundmass mineral abundance), combined with numerical modelling suggests that up to 10 wt% assimilation of lithospheric mantle material has modified the compositions of the Kaavi-Kuopio kimberlites. These new data are also consistent with significant variations in the lithospheric mantle composition of the Karelian craton beneath the closely spaced (<10 km) kimberlites. Finally, in addition to mantle assimilation, formation of Si-Fe-rich mica in some of the examined kimberlites might be linked to late-stage increases in oxygen fugacity potentially enhanced by crustal contamination. This study shows for the first time that variable assimilation of mantle and crustal material can generate significant variations in kimberlites derived from seemingly similar sources.
Journal of Petrology, in press available, 79p. Pdf
Europe, Finland
deposit - Kuusamo
Abstract: Kimberlites are often closely associated, both in time and space, with a wide variety of alkaline ultramafic rock types; yet the question of a genetic relationship between these rock types remains uncertain. One locality where these relationships can be studied within the same cluster is the Karelian craton in Finland. In this study we present the first petrographic, mineral and whole-rock geochemical results for the most recently discovered kimberlite cluster on this craton, which represents an example of the close spatial overlap of kimberlites with ultramafic lamprophyres. The Kuusamo cluster incorporates seven bodies (Kasma 45, Kasma 45 south, Kasma 47, Kalettomanpuro (KP), Kattaisenvaara (KV), Dike 15 and Lampi) distributed along a 60?km NE-SW corridor. Hypabyssal samples from KV, KP, Kasma 45 and Kasma 47 consist of altered olivine macrocrysts and microcrysts and phlogopite phenocrysts in a groundmass of perovskite, apatite, spinel, ilmenite, serpentine, and calcite. These petrographic features combined with mineral (e.g., Mg-rich ilmenite, Al-Ba-rich, Ti-Fe-poor mica) and whole-rock incompatible trace element compositions (La/Nb = 0.8 ± 0.1; Th/Nb = 0.07 ± 0.01; Nb/U = 66 ± 9) are consistent with these rocks being classified as archetypal kimberlites. These Kuusamo kimberlites are enriched in CaO and poor in MgO, which combined with the absence of chromite and paucity of olivine macrocrysts and mantle-derived xenocrysts (including diamonds), suggest derivation from differentiated magmas after crystal fractionation. Samples from Lampi share similar petrographic features, but contain mica with compositions ranging from kimberlitic (Ba-Al-rich cores) to those more typical of orangeites/lamproites (increasing Si-Fe, decreasing Al-Ti-Ba), and have higher bulk-rock SiO2 contents than the Kuusamo kimberlites. These features, combined with the occurrence of quartz and titanite in the groundmass, indicate derivation from a kimberlite magma that underwent considerable crustal contamination. This study shows that crustal contamination can modify kimberlites by introducing features typical of alkaline ultramafic rock types. Dike 15 represents a distinct carbonate-rich lithology dominated by phlogopite over olivine, with lesser amounts of titaniferous clinopyroxene and manganoan ilmenite. Phlogopite (Fe-Ti-rich) and spinel (high Fe2+/Fe2++Mg) compositions are also distinct from the other Kuusamo intrusions. The petrographic and geochemical features of Dike 15 are typical of ultramafic lamprophyres, specifically, aillikites. Rb-Sr dating of phlogopite in Dike 15 yields an age of 1178.8 ± 4.1?Ma (2s), which is considerably older than the ~750?Ma emplacement age of the Kuusamo kimberlites. This new age indicates significant temporal overlap with the Lentiira-Kuhmo-Kostomuksha olivine lamproites emplaced ~100?km to the southeast. It is suggested that asthenospheric aillikite magmas similar to Dike 15 evolved to compositions akin to the Karelian orangeites and olivine lamproites through interaction with and assimilation of MARID-like, enriched subcontinental lithospheric mantle. We conclude that the spatial coincidence of the Kuusamo kimberlites and Dike 15 is likely the result of exploitation of similar trans-lithospheric corridors.
Abstract: The Karelian Craton in Finland is host to (at least) two distinct pulses of kimberlite magmatism. Twenty kimberlite occurrences have so far been discovered on the southwest margin of the craton at Kaavi-Kuopio and seven kimberlites are located in the Kuusamo area within the core of the craton. Comprehensive radiometric age determinations (U-Pb, Ar- Ar and Rb-Sr) reveal that all kimberlite activity was restricted to the Proterozoic. The Kaavi-Kuopio field was emplaced over a protracted period from ~610 to 550 Ma and is predated by the Kuusamo cluster that represents a relatively short pulse of magmatism at ~750 to 730 Ma. The emplacement of kimberlites globally has recently been linked to supercontinent reorganisation and we propose a similar scenario for these Finnish occurrences which, at the time of kimberlite emplacement, were situated on the Baltica paleo-continent. This land mass was contiguous with Laurentia in the Proterozoic and together formed part of Rodinia. The breakup of Rodinia is considered to have commenced at ~750 Ma and initiation of the opening of the Iapetus ocean at ~615 Ma. Contemporaneous with Kaavi-Kuopio magmatism, this latter period of Neoproterozoic crustal extension also includes the emplacement of kimberlites and related rocks in areas that were linked with Baltica as part of Rodinia - West Greenland and eastern North America. Both the initial and final periods of Rodinia’s breakup have been linked to mantle upwellings from the core-mantle boundary. We suggest that kimberlite magmatism in Finland was promoted by the influx of heat from mantle upwellings and lithospheric extension associated with the demise of Rodinia. Although both magmatic episodes are potentially linked to the breakup of Rodinia, whole-rock and perovskite radiogenic isotope compositions for the Kuusamo kimberlites (eNd(i) +2.6 to +3.3, eHf(i) +3.1 to +5.6) are distinct from the Kaavi-Kuopio kimberlites (eNd(i) -0.7 to +1.8, eHf(i) -6.1 to +5.2). The spread in Hf isotope compositions for the Kaavi-Kuopio magmas may be linked to variable assimilation of diverse mantle lithologies.
Kara, J., Vaisanen, M., Heinonen, J.S., Lahaye, Y., O'Brien, H., Huhma, H.
Tracing arcologites in the Paleoproteroic era - a shift from 1.88 Ga calc-alkaline to 1.86 Ga high-Nb and adakite-like magmatism in central Fennoscandian shield.
Complex Zoning of Clinopyroxene in Shonkinites from Mafic Phonolites, Highwood Mountains, Montana: Evidence for Periodic Mixing with a K Rich Bananitic Magma.
Geological Society of America (GSA), Vol. 17, No. 3, P. 187. (abstract.).
Eclogite facies relics and a multistage breakdown in metabasites of the KTB pilot hole, northeast Bavaria: implications for the Variscan tectonometamorphic evol
Contributions to Mineralogy and Petrology, Vol. 112, pp. 261-278
Earth and Planetary Science Letters, Vol. 454, pp. 121-131.
Europe, Czech Republic
Bohemian Massif
Abstract: Carbonatites are peculiar magmatic rocks with mantle-related genesis, commonly interpreted as the products of melting of CO2-bearing peridotites, or resulting from the chemical evolution of mantle-derived magmas, either through extreme differentiation or secondary immiscibility. Here we report the first finding of anatectic carbonatites of crustal origin, preserved as calcite-rich polycrystalline inclusions in garnet from low-to-medium pressure migmatites of the Oberpfalz area, SW Bohemian Massif (Central Europe). These inclusions originally trapped a melt of calciocarbonatitic composition with a characteristic enrichment in Ba, Sr and LREE. This interpretation is supported by the results of a detailed microstructural and microchemical investigation, as well as re-melting experiments using a piston cylinder apparatus. Carbonatitic inclusions coexist in the same cluster with crystallized silicate melt inclusions (nanogranites) and COH fluid inclusions, suggesting conditions of primary immiscibility between two melts and a fluid during anatexis. The production of both carbonatitic and granitic melts during the same anatectic event requires a suitable heterogeneous protolith. This may be represented by a sedimentary sequence containing marble lenses of limited extension, similar to the one still visible in the adjacent central Moldanubian Zone. The presence of CO2-rich fluid inclusions suggests furthermore that high CO2 activity during anatexis may be required to stabilize a carbonate-rich melt in a silica-dominated system. This natural occurrence displays a remarkable similarity with experiments on carbonate-silicate melt immiscibility, where CO2 saturation is a condition commonly imposed.
Earth and Planetary Science Letters, Vol. 454, pp. 121-131.
Mantle
carbonatites
Abstract: Carbonatites are peculiar magmatic rocks with mantle-related genesis, commonly interpreted as the products of melting of CO2-bearing peridotites, or resulting from the chemical evolution of mantle-derived magmas, either through extreme differentiation or secondary immiscibility. Here we report the first finding of anatectic carbonatites of crustal origin, preserved as calcite-rich polycrystalline inclusions in garnet from low-to-medium pressure migmatites of the Oberpfalz area, SW Bohemian Massif (Central Europe). These inclusions originally trapped a melt of calciocarbonatitic composition with a characteristic enrichment in Ba, Sr and LREE. This interpretation is supported by the results of a detailed microstructural and microchemical investigation, as well as re-melting experiments using a piston cylinder apparatus. Carbonatitic inclusions coexist in the same cluster with crystallized silicate melt inclusions (nanogranites) and COH fluid inclusions, suggesting conditions of primary immiscibility between two melts and a fluid during anatexis. The production of both carbonatitic and granitic melts during the same anatectic event requires a suitable heterogeneous protolith. This may be represented by a sedimentary sequence containing marble lenses of limited extension, similar to the one still visible in the adjacent central Moldanubian Zone. The presence of CO2-rich fluid inclusions suggests furthermore that high CO2 activity during anatexis may be required to stabilize a carbonate-rich melt in a silica-dominated system. This natural occurrence displays a remarkable similarity with experiments on carbonate-silicate melt immiscibility, where CO2 saturation is a condition commonly imposed. In conclusion, this study shows how the investigation of partial melting through melt inclusion studies may unveil unexpected processes whose evidence, while preserved in stiff minerals such as garnet, is completely obliterated in the rest of the rock due to metamorphic re-equilibration. Our results thus provide invaluable new insights into the processes which shape the geochemical evolution of our planet, such as the redistribution of carbon and strategic metals during orogenesis.
Mineralogy and Petrology, in press available, 10p.
Russia, Aldan shield
carbonatites
Abstract: The Ust-Chulman apatite ore body is situated within the Nimnyrskaya apatite zone at the Aldan shield in Russia. The latest data confirm the carbonatitic origin of the Seligdar apatite deposit (Prokopyev et al. in Ore Geol Rev 81:296-308, 2017). The results of our investigations demonstrate that the magnetite-apatite-dolomitic rocks of the Ust-Chulman are highly similar to Seligdar-type dolomitic carbonatites in terms of the mineralogy and the fluid regime of formation. The ilmenite and spinel mineral phases occur as solid solutions with magnetite, and support the magmatic origin of the Ust-Chulman ores. The chemical composition of REE- and SO3-bearing apatite crystals and, specifically, monazite-(Ce) mineralisation and the formation of Nb-rutile, late hydrothermal sulphate minerals (barite, anhydrite) and haematite are typical for carbonatite complexes. The fluid inclusions study revealed similarities to the evolutionary trend of the Seligdar carbonatites that included changes of the hydrothermal solutions from highly concentrated chloride to medium-low concentrated chloride-sulphate and oxidized carbonate-ferrous.
Abstract: The timing of final assembly and initiation of subsequent rifting of Rodinia is disputed. New rutile ages (913?±?9?Ma, 900?±?8?Ma and 873?±?3?Ma) and published zircon, monazite, titanite, biotite, muscovite and xenotime geochronology from the Capricorn Orogen (West Australian Craton) reveal a significant early Neoproterozoic event characterized by very low to low metamorphic grade, abundant metasomatism, minor leucogranitic and pegmatitic magmatism and NW-SE fault reactivation episodes between ca. 955 and 830?Ma. Collectively, these are termed the ca. 955-830?Ma Kuparr Tectonic Event. An age range of ca. 955-830?Ma is concomitant with the final stages of Rodinia assembly and the initial stages of its attempted breakup. Very low- to low-grade metamorphic and structural geological evidence favor a distal north-south compressional regime as the driver for hydrothermal activity during ca. 955-830?Ma. Nearby continental collision or accretion from the west (e.g., South China and/or Tarim) are ruled out. The cessation of metasomatism and magmatism in the West Australian Craton after ca. 830?Ma is concomitant with the emplacement of the Gairdner-Amata dyke swarm and associated magmatic activity in South China and Laurentia, the inception of the Adelaide Rift Complex and the deposition of the Centralian Superbasin. We posit that the cessation of hydrothermal activity in the Capricorn Orogen was caused by a tectonic switch from compressional to extensional at ca. 830?Ma. Magmatic and hydrothermal fluids were transferred away from the Capricorn Orogen to the incipient Adelaide Rift Complex, terminating metasomatism in the West Australian Craton. Ultimately, the Kuparr Tectonic Event marked the final stages of Rodinia assembly and its cessation marks the initial stages of its attempted breakup.
A new occurrence of Rhoenite in a lherzolite xenolithof Victoria, Australia
Geological Association of Canada (GAC)/Mineralogical Association of Canada (MAC) Vancouver 90 Program with Abstracts, Held May 16-18, Vol. 15, p. A98. Abstract
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 304.
Global
diamond color
Abstract: Gem-quality laboratory-grown diamonds are manufactured in large quantities. With frequent reports of the mixing of meleesized synthetic diamonds with natural stones, demand for melee diamond screening is increasing. During melee diamond screening at GIA’s Tokyo lab, two notable types of samples with uncommon characteristics have been found. 1. Natural melee diamonds with silicon and nickel defects. Luminescence peaks derived from Si- and Ni-related defects are often observed in colorless melee grown by the HPHT method. The silicon-related defect, once considered proof of CVD-grown diamond, is now known to exist naturally as well (Breeding and Wang, 2008). Several colorless melee diamonds having both silicon- and nickel-related emissions have been found in GIA’s Tokyo lab; olivine inclusions were found in one of these samples. Spectroscopic and gemological features confirmed that the samples were grown in nature. 2. Irradiated laboratory-grown diamond melee found among irradiated natural melee diamonds. Several thousand greenish blue melee diamonds have been submitted by various clients to the Tokyo lab for testing. Each diamond’s color was attributed to a strong GR1 defect caused by irradiation treatment. Fourier-transform infrared (FTIR), photoluminescence (PL), and DiamondView analysis revealed that most of them were irradiated natural diamonds. Eight were irradiated CVD-grown diamonds, and one was an irradiated HPHT-grown specimen. The infrared spectrum of all the CVD samples showed a peak at 3123 cm-1, while their PL spectrum showed a doublet peak at 596/597 nm. Those peaks are specific to as-grown CVD diamonds, as annealing removes the peaks. From their spectra, these CVD specimens were considered irradiated without pre-annealing.
Abstract: The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth's mantle, hydrosphere, atmosphere, and biosphere for ~4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P-T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6-4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6-2.2 GPa and 900-950 °C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of ~30-45-km-thick Eoarchean oceanic crust, modified Earth's mantle and Eoarchean environments and ecosystems.
Geochemistry, Geophysics, Geosystems, Vol. 20, 7, pp. 3328-3346.
Mantle
melting
Abstract: Plate tectonics occurs because the strong tectonic plates sit on underlying weaker and softer mantle that flows over geological timescales. We do not fully understand why this deeper mantle is weak—the two main contenders are that a small part of it is molten or that it contains nominal amounts of the element hydrogen. The electrical conductivity of the mantle is increased both by the presence of molten rock and by hydrogen, so when we interpret conductivity data, it is difficult to distinguish between these two interpretations. We have written a new code to help this. It analyzes whether the conductivity of the mantle could only be explained by the presence of molten rock, whether it could only be explained by large hydrogen contents, or whether it could be explained by either. Our results show that the distribution of partially molten rock is very uneven: Most lies beneath hot spot volcanic islands, while there is no need for molten rock to be present beneath old continents or old parts of the ocean. Beneath young parts of the ocean, the electrical conductivities could be explained by either a small amount of molten rock or by large hydrogen contents.
Abstract: Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry, and electromagnetic data can identify contemporary melt zones, magma reservoirs, and, or, crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs), and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community.
Determination of sampling configuration for near shore Diamondiferous gravel occurrence using geostatistical methods. Mining area no. 1 - linear beach Namdeb
Journal of South African Institute of Mining and Metallurgy, Vol. 114, Jan. pp. 31-38.
Earth and Planetary Science Letters, Vol. 483, 1, pp. 90-104.
Europe, Iceland
olivines
Abstract: Trace element variability in oceanic basalts is commonly used to constrain the physics of mantle melting and the chemistry of Earth's deep interior. However, the geochemical properties of mantle melts are often overprinted by mixing and crystallisation processes during ascent and storage. Studying primitive melt inclusions offers one solution to this problem, but the fidelity of the melt-inclusion archive to bulk magma chemistry has been repeatedly questioned. To provide a novel check of the melt inclusion record, we present new major and trace element analyses from olivine macrocrysts in the products of two geographically proximal, yet compositionally distinct, primitive eruptions from the Reykjanes Peninsula of Iceland. By combining these macrocryst analyses with new and published melt inclusion analyses we demonstrate that olivines have similar patterns of incompatible trace element (ITE) variability to the inclusions they host, capturing chemical systematics on intra- and inter-eruption scales. ITE variability (element concentrations, ratios, variances and variance ratios) in olivines from the ITE-enriched Stapafell eruption is best accounted for by olivine-dominated fractional crystallisation. In contrast, ITE variability in olivines and inclusions from the ITE-depleted Háleyjabunga eruption cannot be explained by crystallisation alone, and must have originated in the mantle. Compatible trace element (CTE) variability is best described by crystallisation processes in both eruptions. Modest correlations between host and inclusion ITE contents in samples from Háleyjabunga suggest that melt inclusions can be faithful archives of melting and magmatic processes. It also indicates that degrees of ITE enrichment can be estimated from olivines directly when melt inclusion and matrix glass records of geochemical variability are poor or absent. Inter-eruption differences in olivine ITE systematics between Stapafell and Háleyjabunga mirror differences in melt inclusion suites, and confirm that the Stapafell eruption was fed by lower degree melts from greater depths within the melting region than the Háleyjabunga eruption. Although olivine macrocrysts from Stapafell are slightly richer in Ni than those from Háleyjabunga, their overall CTE systematics (e.g., Ni/(Mg/Fe), Fe/Mn and Zn/Fe) are inconsistent with being derived from olivine-free pyroxenites. However, the major element systematics of Icelandic basalts require lithological heterogeneity in their mantle source in the form of Fe-rich and hence fusible domains. We thus conclude that enriched heterogeneities in the Icelandic mantle are composed of modally enriched, yet nonetheless olivine-bearing, lithologies and that olivine CTE contents provide an incomplete record of lithological heterogeneity in the mantle. Modally enriched peridotites may therefore play a more important role in oceanic magma genesis than previously inferred.
Stability and compressibility of the high pressure phases of AL203 up to 200 GPa: implications for the electrical conductivity of the base of the lower mantle.
Earth and Planetary Science Letters, Vol. 246, 3-4, pp. 326-335.
Progress in Earth and Planetary Science, Vol. 3, 20, 17p.
United States, Colorado Plateau
Metasomatism
Abstract: This is the first report on amphibole exsolution in pyrope from the Colorado Plateau. Pyrope crystals delivered from mantle depths underneath the Colorado Plateau by kimberlitic volcanism at 30 Ma were collected at Garnet Ridge, northern Arizona. The garnet grains analyzed in this study occur as discrete crystals (without adjacent rock matrix) and are classified into two major groups, Cr-rich pyrope and Cr-poor pyrope. The Cr-poor pyrope group is divided into four subgroups based on exsolved phases: amphibole lamella type, ilmenite lamella type, dense lamellae type, and clinopyroxene/amphibole lamellae type. Exsolved amphibole occurs in amphibole lamella type, dense lamellae type, and clinopyroxene/amphibole lamellae type of Cr-poor pyrope. The amphibole crystals tend to have preferred orientations in their garnet hosts and occur as monomineralic hexagonal or rhombic prisms and tablets, and as multimineralic needles or blades with other exsolved phases. Exsolved amphibole has pargasitic compositions (Na2O up to 1.6 apfu based on 23 oxygen). Garnet host crystals that have undergone amphibole exsolution have low OH contents (2-42 ppmw H2O) compared to garnets that do not have amphibole lamellae (up to 115 ppmw H2O). The low OH contents of garnets hosting amphibole lamellae suggest loss of OH from garnet during amphibole exsolution. Amphibole exsolution from pyrope resulted from breakdown of a precursor “hydrous Na-garnet” composition (Mg,Na+ x)3(Al2?-?x, Mgx)2Si3O12?-?2x(OH)2x. Exsolution of amphibole and other phases probably occurred during exhumation to depths shallower than 100 km prior to volcanic eruption. Based on the abundance and composition of exsolved clinopyroxene and amphibole lamellae in one garnet, hydrous Na-garnet had excess silicon (Si3.017 apfu, 12 oxygen normalization, vs. X3Y2Si3O12 for typical garnet). Comparison with experimental data suggests crystallization at pressures near 6-8 GPa. Garnet crystals that host exsolved amphibole have compositions (Pyp49-76, 3-10 wt% CaO, and up to 0.6 wt% Cr2O3) similar to garnets reported from pyroxenites, and have pyrope-almandine-grossular compositional ranges that overlap with the Cr-rich pyrope (typical lherzolitic garnet). Hydrous Na-garnet was likely formed by metasomatic reactions between Cr-rich pyrope and Na-rich aqueous fluid in the deep upper mantle. The most likely source of metasomatic Na-rich fluid is ancient oceanic crust that was subducted before subduction of the Farallon Plate beneath the Colorado Plateau.
Geological Society of America Annual Meeting, Vol. 47, 7, p. 169. abstract
Russia
Metamorphic diamonds
Abstract: The metamorphic diamonds in the Kokchetav Massif show very diverse features in morphologies and grain sizes with other crystal characters, Raman spectra (FWHM, PL), cathodoluminescence spectra, C isotope, abundance, paragenesis with silicates and carbonates. The formations of these diamonds, however, seem to be related to H2O-fluid conditions. Dolomite marble has the highest abundance in diamond. The diamond grew at two stages and 2nd stage growth was from H2O fluid. Grt-Bt gneiss is 2nd highest and the diamond shows several morphologies; however, no 2nd stage growth. In dolomite marble, diamond at 2nd stage has light carbon isotope, -17 to -27 whereas 1st stage diamond has -8 to -15. The light carbon of 2nd stage could be organic carbon in gneisses carried by H2O-fluid; dissolution of diamond in gneisses could have occurred. No 2nd stage growth in gneisses supports this idea. Carbon-bearing H2O fluid infiltration into dolomite marble caused the change of carbon solubility in fluid to precipitate abundant fine diamonds (10-20 mm), quickly. Recently discovered sp2 graphitic carbon inclusions in 2nd stage diamond (AGU2014F V13B-4771), which is a relic of metastable intermediate phase for diamond formation, suggest the diamond participation from H2O fluid. A minor amount of diamond (large-grained, ca. 150 µm at average) occurs in Grt-Cpx rock. Recently, we found the overgrowth of large-grained cubic diamond on small-grained one by multi-layered 2D Raman mappings (JpGU2014 No.02541). This indicates slow-growth in H2O-fluid having low oversaturation degree of carbon. UHP calcite marble contains a trace amount of small-grained diamond (no 2nd stage growth) only in diopside; titanite with coesite exsolution does not contain diamond. These suggest that very high H2O activity for titanite stability makes diamond unstable, and dissolution of diamond was possible. This is a similar relation with UHP dolomitic marble, which Arg-Fo and Arg-Ti-Chum were stable but diamond was unstable. In Tur-Fel-Qtz rock, diamond is included in new mineral "maruyamaite" (K-rich Tur); recent experiments show high-P and fluid-bearing conditions for maruyamaite. Summarizing these diverse features of the Kokchetav diamonds, those formation and their possible dissolutions have strong relationships with H2O-fluid conditions.
Abstract: This richly illustrated history of diamonds illuminates myriad facets of the “king of gems,” including a cast of larger-than-life characters such as Alexander the Great, the Mughal emperor Jahangir, and East India Company adventurers. It’s an in-depth study tracing the story of diamonds from their early mining and trade more than two thousand years ago to the 1700s, when Brazil displaced India as the world’s primary diamond supplier. Jack Ogden, a historian and gemologist specializing in ancient gems and jewelry, describes the early history of diamond jewelry, the development of diamond cutting, and how diamonds were assessed and valued. The book includes more than one hundred captivating images, from close-up full-color photographs of historic diamond-set jewelry (some previously unpublished), to photomicrographs of individual gems and illustrations of medieval manuscripts, as well as diagrams depicting historical methods of cutting and polishing diamonds.
Rethinking laboratory reports for the geographical origin of gems.
Journal of Gemmology, Vol. 35, 5, pp. 416-423.
Technology
Reports - gem reports, tests, grades
Abstract: The proliferation of gemmological laboratory reports and the need for transparency to best protect against litigation suggest that some gem-testing laboratories should consider changes in the wording and content of their geographical origin reports. Based on the author's recent broader study of the legal aspects of the opinions provided by experts in the field of art and antiques, the main proposals presented here are that statements of opinion rather than fact should be clearly expressed as such where they are presented on a report, rather than relegating all mention of 'opinion' to the 'terms and conditions', and that the basic nature of the observational or analytical evidence on which any opinions are based should be noted. In addition, a laboratory might usefully provide some indication of the level of confidence in its opinion.
Abstract: In the present study, four samples of natural melilites were characterized using electron microprobe analysis, powder X-ray diffraction, FTIR, and Raman spectroscopy, and their thermodynamic properties were measured with a high-temperature heat-flux Tian-Calvet microcalorimeter. The enthalpies of formation from the elements were determined to be: -3796.3 ± 4.1 kJ/mol for Ca1.8Na0.2(Mg0.7Al0.2Fe2+0.1?)Si2O7, -3753.6 ± 5.2 kJ/mol for Ca1.6Na0.4(Mg0.5Al0.4Fe2+0.1?)Si2O7, -3736.4 ± 3.7 kJ/mol for Ca1.6Na0.4(Mg0.4Al0.4Fe2+0.2?)Si2O7, and -3929.2 ± 3.8 kJ/mol for Ca2(Mg0.4Al0.6)[Si1.4Al0.6O7]. Using the obtained formation enthalpies and estimated entropies, the standard Gibbs free energies of formation of these melilites were calculated. Finally, the enthalpies of the formation of the end-members of the isomorphic åkermanite-gehlenite and åkermanite-alumoåkermanite series were derived. The obtained thermodynamic properties of melilites of different compositions can be used for quantitative modeling of formation conditions of these minerals in related geological and industrial processes.
Abstract: Understanding the mechanisms and controlling factors of erosion rates is essential in order to sufficiently comprehend bigger processes such as landscape evolution. For decades, scientists have been researching erosion rates where one of the main objectives was to find the controlling factors. A variety of parameters have been suggested ranging from climate-related, basin morphometry and the tectonic setting of an area. This study focuses on the latter. We use previously published erosion rate data obtained mainly using 10Be and sediment yield and sediment yield data published by the United States Geological Survey. We correlate these data to tectonic-related factors, i.e., distance to tectonic plate boundary, peak ground acceleration (PGA), and fault distribution. We also examine the relationship between erosion rate and mean basin slope and find significant correlations of erosion rates with distance to tectonic plate boundary, PGA, and slope. The data are binned into high, medium, and low values of each of these parameters and grouped in all combinations. We find that groups with a combination of high PGA (>?0.2.86 g) and long distance (>?1118.69 km) or low PGA (
GSA Annual Meeting, Paper 131-14, 1p. Abstract only
Africa, South Africa
Carbonatite
Abstract: Petrologic and geochemical data for carbonatites and associated alkaline igneous rocks are presented for the Zandkopsdrift Carbonatite Complex, Namaqualand. The samples included in this study are relatively fresh, collected by coring at depths of >70 m below the weathered cap zone. The Zandkopsdrift complex is the only locality in the province known to contain significant carbonatite. The carbonatites studied are calico-, ferro- and silico- carbonatites, based on mineralogy, texture, and major element composition. They have low to moderate Mg-numbers (35-65), variable MgO contents (1.2-8.50 wt.%) and high atomic Ca/Ca+Mg (0.73-0.97), indicating that they are not likely simple mantle melts. The carbonatites contain significant apatite, magnetite, pyrochlore and phlogopite. Zandkopsdrift also contains significant amounts of aillikite and olivine melilitite. These rocks have relatively low SiO2 (25-31 wt.%) and Al2O3 (5.3- 6.1 wt.%), high K2O (6-6.3 wt.%) and TiO2 (5.6-9.5 wt.%) and moderate Mg numbers (51-58). d18O and d13C isotopes were measured for carbonatites and aillikites. d13CPDB values are close to those expected for mantle-derived carbonatites (-3.9 to -8.83), while the d18OSMOW values are significantly higher (+13. 25 to 21.84‰). The high d18O value observed in carbonatites and aillikites is most likely attributable to secondary alteration by hydrous/hydrothermal fluids. This supports the inference that the Zandkopsdrift carbonatite is magmatic in origin but was later affected by secondary alteration which resulted in the elevated O stable isotopes. The ‘mantle-like’ d13C is inconsistent with significant assimilation of C-bearing crustal rocks. Chondrite-normalised REE contents in the carbonatites are 2400 to 10,600 for La and 36 to 170 for Lu. The high REE contents of the carbonatites are most likely due to a combination of a source metasomatised by a highly LREE-enriched agent, as well as significant magmatic differentiation. The relatively fractionated composition of the Zandkopsdrift aillikites and melilitites is also consistent with this hypothesis. We propose that the Zandkopsdrift carbonatites were most likely formed by either immiscible liquid separation from or fractional crystallization of a moderately fractionated, carbonate-rich silicate parental magma.
Session No. 131--Booth# 338
Abstract: The 525 Ma Marinkas Quellen (MQ) Complex of southern Namibia, part of the Kuboos-Bremen Line (KBL) of alkaline igneous centers [1] consists of granites, nepheline syenites and carbonatites and is the only carbonatite locality in the KBL [1]. MQ carbonatite variteties include calciocarbonatites, magnesiocarbonatites and ferrocarbonatites. The enrichments in Ba, Nb and the REE vary widely in the carbonatites, with La ranging from 45 to 11154 ppm. All the carbonatites are characterised by large Zr, Hf, Ti depletions. Zr/Hf ratios ranges from 40 to 500, all greater than the chondritic value of 36. Such large Zr/Hf fractionations are often associated with carbonatite metasomatism. The values of carbon and oxygen isotope ratios of bulk carbonate in Marinkas Quellen carbonatites vary significantly (e.g., d13C = -3.95 to -6.02‰; d18 O = 8.84 to 22.22‰). The carbon isotope compositions are in the mantle range, while the oxygen isotope values extend to higher than typical mantle values, presumably due to interaction with hydrous fluids. All but two of the carbonatite samples have initial 87Sr/86Sr ratios falling in the range of 0.70236 to 0.70408. Of the remaining samples, one, a ferrocarbonatite, has a higher value of 0.70503 that is likely due to contamination by the surrounding rock or assimilation in the lower crust or Sr exchange with groundwater. The other, a magnesiocarbonatite, appears to have experienced an increase in its Rb/Sr ratio due to alteration, resulting in an over-corrected initial 87Sr/86Sr value. The relatively low Sr isotope ratios of most samples, plus their HNd(t) values (+3.9 to +4.8) values suggest that the carbonatite magma was generated from a long-lived low Rb/Sr, high Sm/Nd, relatively depleted mantle source. The radiogenic Pb isotope composition of the carbonatites (206Pb/204Pbi ratios from 18.06 to 22.38), suggests a high U/Pb source, akin to the HIMU mantle end member. This points to a sub-lithospheric (asthenospheric) source with only a relatively minor contribution from enriched lithospheric mantle
Low temperature eclogites and eclogitic schists in Mn-rich metabasites in Ward Creek, California: Mn and iron effects on the transition blueschist andeclogites
Journal of Petrology, Vol. 32, No. 2, April pp. 275-302
Glittering globe:could the earth be littered with diamonds in places no one has dreamed of looking? .. Hanlon ventures to an island paradise in search....
New Scientist, August 17, pp. 48-50.
Solomon Islands
Solsearch, Collerson, Zorba Mining, University of Queen
Feneyrol, J., Giuliani, G., Demaiffe, D., Ohenstetter, D., Fallick, A.E., Dubessy, J., Martelet, J-E., Rakotondrazafy, A.F.M., Omito, E., Ichangi, D., Nyamai, C., Wamunyu, W.
Abstract: The genetic model previously proposed for tsavorite- (and tanzanite-) bearing mineralization hosted in the Neoproterozoic Metamorphic Mozambique Belt (stretching from Kenya through Tanzania to Madagascar) is refined on the basis of new Sm-Nd age determinations and detailed Sr-O-S isotope and fluid-inclusion studies. The deposits are hosted within meta-sedimentary series composed of quartzites, graphitic gneisses, calc-silicate rocks intercalated with meta-evaporites, and marbles. Tsavorite occurs either in nodules (also called “boudins”) oriented parallel to the metamorphic foliation in all of the deposits in the metamorphic belt or in quartz veins and lenses located at the hinges of anticlinal folds (Lelatema fold belt and Ruangwa deposits, Tanzania). Gem tanzanite occurs in pockets and lenses in the Lelatema fold belt of northern Tanzania. The Sm-Nd isotopic data for tsavorites and tanzanites hosted in quartz veins and lenses from Merelani demonstrate that they formed at 600 Ma, during the retrograde metamorphic episode associated with the East African Orogeny. The tsavorites hosted in nodules do not provide reliable ages: their sedimentary protoliths had heterogeneous compositions and their Sm-Nd system was not completely rehomogenized, even at the local scale, by the fluid-absent metamorphic recrystallization. The initial 87Sr/86Sr isotopic ratios of calcite from marble and tanzanites from Merelani fit with the strontium isotopic composition of Neoproterozoic marine carbonates. Seawater sediment deposition in the Mozambique Ocean took place around 720 Ma. The quartz-zoisite O-isotopic thermometer indicates a temperature of formation for zoisite between 385 and 448 °C. The sulfur isotopic composition of pyrite (between –7.8 and –1.3‰ V-CDT) associated with tsavorite in the Lelatema fold belt deposits suggests the contribution of reduced marine sulfate. The sulfur in pyrite in the marbles was likely derived from bacterial sulfate reduction which produced H2S. Fluid inclusion data from tsavorite and tanzanite samples from the Merelani mine indicate the presence of a dominant H2S-S8±(CH4)±(N2)±(H2O)-bearing fluid. In the deposits in Kenya and Madagascar, the replacement of sulfate by tsavorite in the nodules and the boron isotopic composition of tourmaline associated with tsavorite are strong arguments in favor of the participation of evaporites in garnet formation.
Geochemistry International, Vol. 54, 10, pp. 834-838.
South America, Brazil
Deposit - Sao Luiz
Abstract: We report cloudy micro- and nano-inclusions in a superdeep diamond from São-Luiz, Brazil which contains inclusions of ferropericlase (Mg, Fe)O and former bridgmanite (Mg, Fe)SiO3 and ringwoodite (Mg, Fe)2SiO4. Field emission-SEM and TEM observations showed that the cloudy inclusions were composed of euhedral micro-inclusions with grain sizes ranging from tens nanometers to submicrometers. Infrared absorption spectra of the cloudy inclusions showed that water, carbonate, and silicates were not major components of these micro- and nano-inclusions and suggested that the main constituent of the inclusions was infrared-inactive. Some inclusions were suggested to contain material with lower atomic numbers than that of carbon. Mineral phase of nano- and micro-inclusions is unclear at present. Microbeam X-ray fluorescence analysis clarified that the micro-inclusions contained transition metals (Cr, Mn, Fe, Co, Ni, Cu, Zn) possibly as metallic or sulfide phases. The cloudy inclusions provide an important information on the growth environment of superdeep diamonds in the transition zone or the lower mantle.
Abstract: An open question for solid-earth scientists is the amount of water in Earth’s interior. The uppermost mantle and lower mantle contain little water because their dominant minerals, olivine and bridgmanite, have limited water storage capacity. In contrast, the mantle transition zone (MTZ) at a depth of 410 to 660 km is considered to be a potential water reservoir because its dominant minerals, wadsleyite and ringwoodite, can contain large amounts of water [up to 3 weight % (wt %)]. However, the actual amount of water in the MTZ is unknown. Given that water incorporated into mantle minerals can lower their viscosity, we evaluate the water content of the MTZ by measuring dislocation mobility, a property that is inversely proportional to viscosity, as a function of temperature and water content in ringwoodite and bridgmanite. We find that dislocation mobility in bridgmanite is faster by two orders of magnitude than in anhydrous ringwoodite but 1.5 orders of magnitude slower than in water-saturated ringwoodite. To fit the observed mantle viscosity profiles, ringwoodite in the MTZ should contain 1 to 2 wt % water. The MTZ should thus be nearly water-saturated globally.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 306-7.
Mantle
diamond inclusions
Abstract: Nitrogen is one of the most common impurities in diamond, and its aggregation styles have been used as criteria for diamond classification. Pure type IaB diamonds (with 100% nitrogen in B aggregation) are rather rare among natural diamonds. The occurrence of the B center is generally associated with high temperature and a long residence time of the host diamond, which would potentially provide information on the earth’s deep interior. Seawater circulation is the unique process that shapes the surface of our planet and potentially has a profound effect on its interior due to slab subduction. In about 50 type IaB diamonds with detectable micro-inclusions submitted to GIA for screening, we found that more than 70% of them contained a typical mineral assemblage from the sublithosphere. Jeffbenite (TAPP), majorite garnet, enstatite, and ferropericlase have been observed, which could be retrograde products of former bridgmanite. CaSiO3-walstromite with larnite and titanite is the dominant phase present in approximately 40% of all diamond samples. Direct evidence from oxygen isotope ratios measured by secondary ion mass spectrometry, or SIMS, (d18OVSMOWin the range +10.7 to +12.5‰) of CaSiO3-walstromite with coexisting larnite and titanite that retrograde from CaSiO3-perovskite suggest that hydrothermally altered oceanic basalt can subduct to depths of >410 km in the transition zone. Incorporation of materials from subducted altered oceanic crust into the deep mantle produced diamond inclusions that have both lower mantle and subduction signatures. Ca(Si,Al)O3-perovskite was observed with a high concentration of rare earth elements (>5 wt.%) that could be enriched under P-Tconditions in the lower mantle. Evidence from ringwoodite with a hydroxide bond, coexisting tuite and apatite, precipitates of an NH3phase, and cohenite with trace amounts of Cl imply that the subducted brines can potentially introduce hydrous fluid to the bottom of the transition zone. In the diamonds with subducted materials, the increasing carbon isotope ratio from the core to the rim region detected by SIMS (d13C from -5.5‰ to -4‰) suggests that an oxidized carbonate-dominated fluid was associated with recycling of the subducted hydrous material. The deep subduction played an important role in balancing redox exchange with the reduced lower mantle indicated by precipitated iron nanoparticles and coexisting hydrocarbons and carbonate phases.
Abstract: The enigmatic appearance of cuboctahedral diamonds in ophiolitic and arc volcanic rocks with morphology and infrared characteristics similar to synthetic diamonds that were grown from metal solvent requires a critical reappraisal. We have studied 15 diamond crystals and fragments from Tolbachik volcano lava flows, using Fourier transform infrared spectrometry (FTIR), transmission electron microscopy (TEM), synchrotron X-ray fluorescence (SRXRF) and laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS). FTIR spectra of Tolbachik diamonds correspond to typical type Ib patterns of synthetic diamonds. In TEM films prepared using focused ion beam technique, we find Mn-Ni and Mn-Si inclusions in Tolbachik diamonds. SRXRF spectra indicate the presence of Fe-Ni and Fe-Ni-Mn inclusions with Cr, Ti, Cu, and Zn impurities. LA-ICP-MS data show variable but significantly elevated concentrations of Mn, Fe, Ni, and Cu reaching up to 70?ppm. These transition metal concentration levels are comparable with those determined by LA-ICP-MS for similar diamonds from Tibetan ophiolites. Mn-Ni (+Fe) solvent was widely used to produce industrial synthetic diamonds in the former USSR and Russia with very similar proportions of these metals. Hence, it appears highly probable that the cuboctahedral diamonds recovered from Kamchatka arc volcanic rocks represent contamination and are likely derived from drilling tools or other hard instruments. Kinetic data on diamond dissolution in basaltic magma or in fluid phase demonstrate that diamond does not form under the pressures and temperature conditions prevalent within the magmatic system beneath the modern-day Klyuchevskoy group of arc volcanoes. We also considered reference data for inclusions in ophiolitic diamonds and compared them with the composition of solvent used in industrial diamond synthesis in China. The similar inclusion chemistry close to Ni70Mn25Co5 for ophiolitic and synthetic Chinese diamonds scrutinized here suggests that most diamonds recovered from Tibetan and other ophiolites are not natural but instead have a synthetic origin. In order to mitigate further dubious reports of diamonds from unconventional tectonic settings and source rocks, we propose a set of discrimination criteria to better distinguish natural cuboctahedral diamonds from those produced synthetically in industrial environments and found as contaminants in mantle- and crust-derived rocks.
Abstract: The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO3-Fe0 system at 70-150 GPa and 800-2600 K in a laser-heated diamond anvil cell. Using in situ synchrotron X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by: 6MgCO3 + 19Fe = 8FeO +10(Mg0.6Fe0.4)O + Fe7C3 + 3C. The presented results suggest that the interaction of carbonates with Fe0 or Fe0-bearing rocks can produce Fe-carbide and diamond, which can accumulate in the D’’ region, depending on its carbon to Fe ratio. Due to the sluggish kinetics of the transformation, diamond can remain metastable at the core-mantle boundary (CMB) unless it is in a direct contact with Fe-metal. In addition, it can be remobilized by redox melting accompanying the generation of mantle plumes.
Nishiyama, T., Ohfuji, H., Fukuba, K., Terauchi, M., Nishi, U., Harada, K., Unoki, K., Moribe, Y., Yoshiasa, A., Ishimaru, S., Mori, Y., Shigeno, M., Arai, S.
Nature Scientific Reports, Vol. 10, 11645 11p. Pdf
Asia, Japan
microdiamond
Abstract: Microdiamonds in metamorphic rocks are a signature of ultrahigh-pressure (UHP) metamorphism that occurs mostly at continental collision zones. Most UHP minerals, except coesite and microdiamond, have been partially or completely retrogressed during exhumation; therefore, the discovery of coesite and microdiamond is crucial to identify UHP metamorphism and to understand the tectonic history of metamorphic rocks. Microdiamonds typically occur as inclusions in minerals such as garnet. Here we report the discovery of microdiamond aggregates in the matrix of a metapelite from the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan. The Nishisonogi unit represents a Cretaceous subduction complex which has been considered as an epidote-blueschist subfacies metamorphic unit, and the metapelite is a member of a serpentinite mélange in the Nishisonogi unit. The temperature condition for the Nishisonogi unit is 450 °C, based on the Raman micro-spectroscopy of graphite. The coexistence of microdiamond and Mg-carbonates suggests the precipitation of microdiamond from C-O-H fluid under pressures higher than 2.8 GPa. This is the first report of metamorphic microdiamond from Japan, which reveals the hidden UHP history of the Nishisonogi unit. The tectonic evolution of Kyushu in the Japanese Archipelago should be reconsidered based on this finding.
In situ x-ray observations of phase assemblages in peridotite and basalt compositions at lower mantle conditions: implications for density of subducted...
Maeda, F., Ohtani, E., Kamada, S., Sakamaki, T., Ohishi, Y., Hirao, N.
The reactions in the MgCO3-SiO2 system in the slabs subducted into the lower mantle and formation of deep diamond.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 1p. Abstract
Abstract: Diamond is an evidence for carbon existing in the deep Earth. Some diamonds are considered to have originated at various depth ranges from the mantle transition zone to the lower mantle. These diamonds are expected to carry significant information about the deep Earth. Here, we determined the phase relations in the MgCO3-SiO2 system up to 152?GPa and 3,100?K using a double sided laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction. MgCO3 transforms from magnesite to the high-pressure polymorph of MgCO3, phase II, above 80?GPa. A reaction between MgCO3 phase II and SiO2 (CaCl2-type SiO2 or seifertite) to form diamond and MgSiO3 (bridgmanite or post-perovsktite) was identified in the deep lower mantle conditions. These observations suggested that the reaction of the MgCO3 phase II with SiO2 causes formation of super-deep diamond in cold slabs descending into the deep lower mantle.
Earth and Planetary Science Letters, Vol. 465, pp. 29-37.
Mantle
Geothermometry
Abstract: (Mg,?Fe)O ferropericlase (Fp) is one of the important minerals comprising Earth's lower mantle, and its thermal conductivity could be strongly influenced by the iron content and its spin state. We examined the lattice thermal conductivity of (Mg,?Fe)O Fp containing 19 mol% iron up to 111 GPa and 300 K by means of the pulsed light heating thermoreflectance technique in a diamond anvil cell. We confirmed a strong reduction in the lattice thermal conductivity of Fp due to iron substitution as reported in previous studies. Our results also show that iron spin crossover in Fp reduces its lattice thermal conductivity as well as its radiative conduction. We also measured the electrical conductivity of an identical Fp sample up to 140 GPa and 2730 K, and found that Fp remained an insulator throughout the experimental conditions, indicating the electronic thermal conduction in Fp is negligible. Because of the effects of strong iron impurity scattering and spin crossover, the total thermal conductivity of Fp at the core-mantle boundary conditions is much smaller than that of bridgmanite (Bdg). Our findings indicate that Bdg (and post-perovskite) is the best heat conductor in the Earth's lower mantle, and distribution of iron and its valence state among the lower mantle minerals are key factors to control the lower mantle thermal conductivity.
Earth and Planetary Science Letters, Vol. 547, 9p. Pdf
Mantle
perovskite
Abstract: The thermal conductivity of post-perovskite (ppv), the highest-pressure polymorph of MgSiO3 in the Earth's mantle, is one of the most important transport properties for providing better constraints on the temperature profile and dynamics at the core-mantle boundary (CMB). Incorporation of Fe into ppv can affect its conductivity, which has never been experimentally investigated. Here we determined the lattice thermal conductivities of ppv containing 3 mol% and 10 mol% of Fe at high P-T conditions - of pressures up to 149 GPa and 177 GPa, respectively, and temperatures up to 1560 K - by means of the recently developed pulsed light heating thermoreflectance technique combining continuous wave heating lasers. We found that the incorporation of Fe into ppv moderately reduces its lattice thermal conductivity as it increases the Fe content. The bulk conductivity of ppv dominant pyrolite is estimated as 1.5 times higher than that of pyrolite consisting of bridgmanite and ferropericlase in the lower mantle, which agrees with the traditional view that ppv acts as a better heat conductor than bridgmanite in the Earth's lowermost mantle.
International symposium on mineralization related to mafic and ultramafic rocks with a special session on alkaline and carbonatitic magmatism and associated min
Crscm-cnrs, To Be Held September 1-3, Orleans France,
Giuliani, G.,Pivin, M., Fallick, A.E., Ohnenstetter, D., Song, Y., Demaiffe, D.
Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite, Democratic Republic of Congo and the Changle alkali basalt, China.
Comptes Rendus Geoscience, Vol. 347, 1, pp. 24-34.
Symposium on critical and strategic materials, British Columbia Geological Survey Paper 2015-3, held Nov. 13-14, pp. 69-74.
Canada, Quebec
Alkalic
Abstract: The Crevier alkaline intrusion is in the Grenville Province, north of the Lac Saint-Jean region of Québec (Fig. 1). It covers ~25 km2 (Bergeron, 1980) and intrudes charnockitic suites in the allochthon belt defi ned by Rivers et al. (1989). This intrusion has a U-Pb zircon age of 957.5 ± 2.9 Ma (Groulier et al., 2014) and is oriented N320°, along the axis of crustal weakness known as the Waswanipi-Saguenay corridor (Bernier and Moorhead, 2000). This corridor is related to the Saguenay graben, which hosts the Saint-Honoré (Niobec) Nb-Ta-REE deposit and Montviel REE deposit. The age of the Saint-Honoré carbonatite was estimated at 584 to 650 Ma (K-Ar whole rock; Vallée and Dubuc, 1970; Boily and Gosselin, 2004). The Montviel intrusion has a U-Pb zircon age of 1894 ± 3.5 Ma (David et al., 2006; Goutier, 2006). These crystallization ages are very different and cannot be related to a single event for the injection of alkaline intrusions. As mapped by Bergeron (1980), the Crevier alkaline intrusion is broadly composed of syenite and carbonatite rocks (Fig. 2). The Nb- Ta mineralization consists of pyrochlore hosted by a nepheline syenite dike swarm in the centre of the intrusion. The highest REE concentrations, up to 729 ppm La and 1465 ppm Ce, are at the edge of the Crevier alkaline intrusion (Niotaz sud showing; Fig. 2).
Earth and Planetary Science Letters, Vol. 465, pp. 29-37.
Mantle
Geothermometry
Abstract: (Mg,?Fe)O ferropericlase (Fp) is one of the important minerals comprising Earth's lower mantle, and its thermal conductivity could be strongly influenced by the iron content and its spin state. We examined the lattice thermal conductivity of (Mg,?Fe)O Fp containing 19 mol% iron up to 111 GPa and 300 K by means of the pulsed light heating thermoreflectance technique in a diamond anvil cell. We confirmed a strong reduction in the lattice thermal conductivity of Fp due to iron substitution as reported in previous studies. Our results also show that iron spin crossover in Fp reduces its lattice thermal conductivity as well as its radiative conduction. We also measured the electrical conductivity of an identical Fp sample up to 140 GPa and 2730 K, and found that Fp remained an insulator throughout the experimental conditions, indicating the electronic thermal conduction in Fp is negligible. Because of the effects of strong iron impurity scattering and spin crossover, the total thermal conductivity of Fp at the core-mantle boundary conditions is much smaller than that of bridgmanite (Bdg). Our findings indicate that Bdg (and post-perovskite) is the best heat conductor in the Earth's lower mantle, and distribution of iron and its valence state among the lower mantle minerals are key factors to control the lower mantle thermal conductivity.
Earth and Planetary Science Letters, Vol. 547, 9p. Pdf
Mantle
perovskite
Abstract: The thermal conductivity of post-perovskite (ppv), the highest-pressure polymorph of MgSiO3 in the Earth's mantle, is one of the most important transport properties for providing better constraints on the temperature profile and dynamics at the core-mantle boundary (CMB). Incorporation of Fe into ppv can affect its conductivity, which has never been experimentally investigated. Here we determined the lattice thermal conductivities of ppv containing 3 mol% and 10 mol% of Fe at high P-T conditions - of pressures up to 149 GPa and 177 GPa, respectively, and temperatures up to 1560 K - by means of the recently developed pulsed light heating thermoreflectance technique combining continuous wave heating lasers. We found that the incorporation of Fe into ppv moderately reduces its lattice thermal conductivity as it increases the Fe content. The bulk conductivity of ppv dominant pyrolite is estimated as 1.5 times higher than that of pyrolite consisting of bridgmanite and ferropericlase in the lower mantle, which agrees with the traditional view that ppv acts as a better heat conductor than bridgmanite in the Earth's lowermost mantle.
Shatskiy, A., Litasov, K., Palyanov, Y.N., Ohtaini, E.
Phase relationships on the K2CO3 MgCOs join at 6 Gpa and 900-1400C: implications for incipient melting in carbonated mantle domains.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Africa, South Africa, Guinea, Australia,South America, Brazil, Canada, Northwest Territories
deposit - Koffiefontein, Kankan, Lac de Gras, Juina, Machado, Orroroo
Abstract: (Mg,Fe)SiO3 bridgmanite is the dominant phase in the lower mantle; however no naturally occurring samples had ever been found in terrestrial samples as it undergoes retrograde transformation to a pyroxene-type structure. To identify retrograde phases of former bridgmanite single-phase and composite inclusions of (Mg,Fe)SiO3 in a series of superdeep diamonds have been examined with electron microscopy, electron microprobe, Raman spectroscopy and X-ray diffraction techniques. Our study revealed that (Mg,Fe)SiO3 inclusions are represented by orthopyroxene. Orthopyroxenes in single-phase and composite inclusions inherit initial chemical composition of bridgmanites, including a high Al and low Ni contents. In composite inclusions they coexist with jeffbenite (ex-TAPP) and olivine. The bulk compositions of these composite inclusions are rich in Al, Ti, and Fe, which are similar but not fully resembling Al-rich bridgmanite produced in experiments on the MORB composition. The retrograde origin of composite inclusions due to decomposition of Al-rich bridgmanite may be doubtful because each of observed minerals may represent coexisting HP phases, i.e. bridgmanite or ringwoodite.
Stability Field of Knorringite Mg3 Chromium 2 Si3 012 at High Pressure and its implication to the Occurrence of Chromium Rich Pyrope in the Upper Mantle.
Physics of The Earth And Plan. Interiors, Vol. 27, PP. 263-272.
Phase and melting relations of peridotite H2O and basalt H2O systems to the top of the lower mantle and implications for slab subduction and hot mantle plume.
18th. International Mineralogical Association Sept. 1-6, Edinburgh, abstract p.75.
Sano, A., Ohtani, E., Litasov, K., Kubo, T., Hosoya, T., Funakoshi, K., Kikegawa, T.
In situ x-ray diffraction study of the effect of water on the garnet perovksite transformation in MORB and implications for the penetration of oceanic crust...
Physics of the Earth and Planetary Interiors, Vol. 159, 1-2, pp. 118-126.
Majoritic garnet: a new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamonds.
Geochimica et Cosmochimica Acta, Vol. 74, 20, pp. 5939-5937.
Dymshits, A., Litasov, K., Sharygin, I., Shatskiy, A., Ohtani, E.
Mineral physics of high pressure garnets.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Maeda, F., Ohtani, E., Kamada, S., Sakamaki, T., Ohishi, Y., Hirao, N.
The reactions in the MgCO3-SiO2 system in the slabs subducted into the lower mantle and formation of deep diamond.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 1p. Abstract
Speciation of and D/H partioning between fluids and melts in silicate D-O-H-C-N systems determined in-situ at upper mantle temperatures, pressures, and redox conditions.
Sharygin, I., Litasov, K., Shatskiy, A., Golovin, A., Ohtani, E., Pokhilenko, N.
Is kimberlite magma ascent fuelled by CO2 degassing via orthopyroxene assimilation?
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Sharygin, I., Litasov, K., Shatskiy, A., Golovin, A., Ohtani, E., Pokhilenko, N.
Melting phase relations of the Udachnaya East Group 1 kimberlite at 3.0-6.5 GPA: experimental evidence for alkali-carbonatite composition of primary kimberlite melt.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Sharygin, I.S., Litasov, K.D., Shatskiy, A., Golovin, A.V., Ohtani, E., Pokhilenko, N.P.
Melting phase relations of the Udachnaya-East Group 1 kimberlite at 3.0-6.5GPa: experimental evidence for alkali-carbonatite composition of primary kimberlite melts and implications for mantle plumes.
Abstract: Water is transported into the deep mantle via hydrous minerals in subducting slabs. During subduction, a series of minerals in these slabs such as serpentine or chlorite, Mg-sursassite and/or the 10 Å phase, and phase A can be stable at different pressures within the slab geotherms, and may transport significant amount of water into the Earth's interior. The transition zone has a large water storage capacity because of the high solubility of water in wadsleyite and ringwoodite. The recent discovery of hydrous ringwoodite and phase Egg as inclusions in ultra deep diamonds from Juina, Brazil suggests that the transition zone may indeed contain water. Seismic tomographic studies and electrical conductivity observations suggest that the transition zone may contain large amount of water, at least locally, beneath the subduction zones. The discovery of a new hydrous phase H, MgSiO2(OH)2, and its solid solution with isostructural phase d-AlOOH, suggests that a significant amount of water could be stored in this hydrous magnesium silicate phase which is stable down to the lower mantle. Water may be transported into the bottom of the lower mantle via phase H–d solid solution in descending slabs. This new high pressure hydrous phase solid solution has a high bulk modulus and sound velocity owing to strong O-H bonding due to hydrogen bond symmetrization in the lower mantle. Therefore, water stored in this hydrous phase would not reduce the seismic wave velocity in the lower mantle, and is seismically invisible. Dehydration melting could then occur at the base of the lower mantle, providing a potential explanation for the ultralow-velocity zone at the core-mantle boundary. When this hydrous magnesium silicate phase or hydrous melt makes contact with the metallic outer core at the core-mantle boundary, then hydrogen is likely to dissolve into the core.
Abstract: Experiments on the origin of the Udachnaya-East kimberlite (UEK) have been performed using a Kawai-type multianvil apparatus at 3-6.5GPa and 900-1500°C. The studied composition represents exceptionally fresh Group-I kimberlite containing (wt.%): SiO2=25.9, TiO2=1.8, Al2O3=2.8, FeO=9.0, MgO=30.1, CaO=12.7, Na2O=3.4, K2O=1.3, P2O5=1.0, Cl=0.9, CO2=9.9, and H2O=0.5. The super-solidus assemblage consists of melt, olivine (Ol), Ca-rich (26.0-30.2wt.% CaO) garnet (Gt), Al-spinel (Sp), perovskite (Pv), a CaCO3 phase (calcite or aragonite), and apatite. The low pressure assemblage (3-4GPa) also includes clinopyroxene. The apparent solidus was established between 900 and 1000°C at 6.5GPa. At 6.5GPa and 900°C Na-Ca carbonate with molar ratio of (Na+K)/Ca˜0.44 was observed. The UEK did not achieve complete melting even at 1500°C and 6.5GPa, due to excess xenogenic Ol in the starting material. In the studied P-T range, the melt has a Ca-carbonatite composition (Ca#=molar Ca/(Ca+Mg) ratio=0.62-0.84) with high alkali and Cl contents (7.3-11.4wt.% Na2O, 2.8-6.7wt.% K2O, 1.6-3.4wt.% Cl). The K, Na and Cl contents and Ca# decrease with temperature. It is argued that the primary kimberlite melt at depths>200km was an essentially carbonatitic (<5wt.% SiO2), but evolved toward a carbonate-silicate composition (up to 15-20wt.% SiO2) during ascent. The absence of orthopyroxene among the run products indicates that xenogenic orthopyroxene was preferentially dissolved into the kimberlite melt. The obtained subliquidus phase assemblage (Ol+Sp+Pv+Ca-rich Gt) at P-T conditions of the UEK source region, i.e. where melt was in the last equilibrium with source rock before magma ascent, differs from the Opx-bearing peridotitic mineral assemblage of the UEK source region. This difference can be ascribed to the loss of substantial amounts of CO2 from the kimberlite magma at shallow depths, as indicated by both petrological and experimental data. Our study implies that alkali-carbonatite melt would be a liquid phase within mantle plumes generated at the core-mantle boundary or shallower levels of the mantle, enhancing the ascent velocity of the plumes. We conclude that the long-term activity of a rising hot mantle plume and associated carbonatite melt (i.e. kimberlite melt) causes thermo-mechanical erosion of the subcontinental lithosphere mantle (SCLM) roots and creates hot and deformed metasomatic regions in the lower parts of the SCLM, which corresponds to depths constrained by P-T estimates of sheared Gt-peridotite xenoliths. The sheared Gt-peridotites undoubtedly represent samples of these regions.
Abstract: Although kimberlite magma carries large amounts of mantle-derived xenocrysts and xenoliths (with sizes up to meters), this magma ascends from the Earth's mantle (> 150-250 km) to the surface in a matter of hours or days, which enables diamonds to survive. The recently proposed assimilation-fuelled buoyancy model for kimberlite magma ascent emphasizes the importance of fluid CO2 that is produced via the reactive dissolution of mantle-derived orthopyroxene xenocrysts into kimberlite melt, which initially has carbonatitic composition. Here, we use a series of high-pressure experiments to test this model by studying the interaction of orthopyroxene (Opx) with an alkali-dolomitic melt (simplified to 0.7Na2CO3 + 0.3K2CO3 + 2CaMg(CO3)2), which is close to the melt that is produced by the partial melting of a kimberlite source, at P = 3.1-6.5 GPa and T = 1200-1600 °C, i.e., up to pressures that correspond to depths (~ 200 km) from where the ascent of kimberlite magma would start. During the first set of experiments, we study the reaction between powdered Opx and model carbonate melt in a homogeneous mixture. During the second set of experiments, we investigate the mechanism and kinetics of the dissolution of Opx crystals in alkali-dolomitic melt. Depending on the P-T conditions, Opx dissolves in the alkali-dolomitic melt (CL) either congruently or incongruently via the following reactions: Mg2Si2O6 (Opx) + CaMg(CO3)2 (CL) = CaMgSi2O6 (clinopyroxene) + 2MgCO3 (CL) and Mg2Si2O6 (Opx) = Mg2SiO4 (olivine) + SiO2 (CL). The experiments confirm that the dissolution of Opx causes gradual SiO2 enrichment in the initial carbonate melt, as previously suggested. However, the assimilation of Opx by carbonate melt does not produce fluid CO2 in the experiments because the CO2 is totally dissolved in the evolved melt. Thus, our results clearly demonstrate the absence of exsolved CO2 fluid at 3.1-6.5 GPa in ascending kimberlite magma and disprove the assimilation-fuelled buoyancy model for kimberlite magma ascent in the lithospheric mantle. We alternatively suggest that the extreme buoyancy of kimberlite magma at depths of 100-250 km is an exclusive consequence of the unique physical properties (i.e., low density, ultra-low viscosity and, thus, high mobility) of the kimberlite melt, which are dictated by its carbonatitic composition.
Abstract: Diamond is an evidence for carbon existing in the deep Earth. Some diamonds are considered to have originated at various depth ranges from the mantle transition zone to the lower mantle. These diamonds are expected to carry significant information about the deep Earth. Here, we determined the phase relations in the MgCO3-SiO2 system up to 152?GPa and 3,100?K using a double sided laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction. MgCO3 transforms from magnesite to the high-pressure polymorph of MgCO3, phase II, above 80?GPa. A reaction between MgCO3 phase II and SiO2 (CaCl2-type SiO2 or seifertite) to form diamond and MgSiO3 (bridgmanite or post-perovsktite) was identified in the deep lower mantle conditions. These observations suggested that the reaction of the MgCO3 phase II with SiO2 causes formation of super-deep diamond in cold slabs descending into the deep lower mantle.
Earth and Planetary Science Letters, Vol. 465, pp. 208-227.
Russia
Deposit - Udachnaya-East
Abstract: The critical issue in the study of kimberlites, known as principal host rocks of diamonds, is the reconstruction of their primary melt composition, which is poorly constrained due to contamination by xenogenic materials, significant loss of volatiles during eruption, and post-magmatic alteration. It is generally accepted that the last equilibration of primary kimberlite melt with surrounding mantle (garnet lherzolite) occurred beneath cratons at 5-7 GPa (150-230 km depths). However, the subliquidus mineral assemblages obtained in kimberlite melting experiments at mantle pressures differ from lherzolite, probably owing to unaccounted loss of CO2. Here we present experiments at 6.5 GPa and 1200-1600 °C on unaltered kimberlite with an addition of 2-22 mol% CO2 over its natural abundance in the rock (13 mol%), but keeping proportions of other components identical to those in an exceptionally fresh anhydrous kimberlite from Udachnaya-East pipe in Siberia. We found that the partial melt achieves equilibrium with garnet lherzolite at 1500 °C and 19-23 mol% CO2 in the system. Under these conditions this melt contains (mol%): SiO2 = 9, FeO = 6-7, MgO = 23-26, CaO = 16, Na2O = 4, K2O = 1, and CO2 = 30-35. We propose, therefore, the alkali-rich carbonatitic composition of primary kimberlite melt and loss of 34-45 mol% (34-46 wt%) CO2 during ascent of the kimberlite magma to the surface.
Abstract: Although kimberlite magma carries large amounts of mantle-derived xenocrysts and xenoliths (with sizes up to meters), this magma ascends from the Earth's mantle (> 150–250 km) to the surface in a matter of hours or days, which enables diamonds to survive. The recently proposed assimilation-fuelled buoyancy model for kimberlite magma ascent emphasizes the importance of fluid CO2 that is produced via the reactive dissolution of mantle-derived orthopyroxene xenocrysts into kimberlite melt, which initially has carbonatitic composition. Here, we use a series of high-pressure experiments to test this model by studying the interaction of orthopyroxene (Opx) with an alkali-dolomitic melt (simplified to 0.7Na2CO3 + 0.3K2CO3 + 2CaMg(CO3)2), which is close to the melt that is produced by the partial melting of a kimberlite source, at P = 3.1–6.5 GPa and T = 1200–1600 °C, i.e., up to pressures that correspond to depths (~ 200 km) from where the ascent of kimberlite magma would start. During the first set of experiments, we study the reaction between powdered Opx and model carbonate melt in a homogeneous mixture. During the second set of experiments, we investigate the mechanism and kinetics of the dissolution of Opx crystals in alkali-dolomitic melt. Depending on the P-T conditions, Opx dissolves in the alkali-dolomitic melt (CL) either congruently or incongruently via the following reactions: Mg2Si2O6 (Opx) + CaMg(CO3)2 (CL) = CaMgSi2O6 (clinopyroxene) + 2MgCO3 (CL) and Mg2Si2O6 (Opx) = Mg2SiO4 (olivine) + SiO2 (CL). The experiments confirm that the dissolution of Opx causes gradual SiO2 enrichment in the initial carbonate melt, as previously suggested. However, the assimilation of Opx by carbonate melt does not produce fluid CO2 in the experiments because the CO2 is totally dissolved in the evolved melt. Thus, our results clearly demonstrate the absence of exsolved CO2 fluid at 3.1–6.5 GPa in ascending kimberlite magma and disprove the assimilation-fuelled buoyancy model for kimberlite magma ascent in the lithospheric mantle. We alternatively suggest that the extreme buoyancy of kimberlite magma at depths of 100–250 km is an exclusive consequence of the unique physical properties (i.e., low density, ultra-low viscosity and, thus, high mobility) of the kimberlite melt, which are dictated by its carbonatitic composition.
Contribution to Mineralogy and Petrology, Vol. 173, 22p.
Mantle
carbonatite
Abstract: We performed an experimental study, designed to reproduce the formation of an unusual merwinite?+?olivine-bearing mantle assemblage recently described as a part of a Ca-rich suite of inclusions in sublithospheric diamonds, through the interaction of peridotite with an alkali-rich Ca-carbonatite melt, derived from deeply subducted oceanic crust. In the first set of experiments, we studied the reaction between powdered Mg-silicates, olivine and orthopyroxene, and a model Ca-carbonate melt (molar Na:K:Ca?=?1:1:2), in a homogeneous mixture, at 3.1 and 6.5 GPa. In these equilibration experiments, we observed the formation of a merwinite?+?olivine-bearing assemblage at 3.1 GPa and 1200 °C and at 6.5 GPa and 1300-1400 °C. The melts coexisting with this assemblage have a low Si and high Ca content (Ca#?=?molar 100?×?Ca/(Ca?+?Mg)?>?0.57). In the second set of experiments, we investigated reaction rims produced by interaction of the same Ca-carbonate melt (molar Na:K:Ca?=?1:1:2) with Mg-silicate, olivine and orthopyroxene, single crystals at 3.1 GPa and 1300 °C and at 6.5 GPa and 1400 °C. The interaction of the Ca-carbonate melt with olivine leads to merwinite formation through the expected reaction: 2Mg2SiO4 (olivine)?+?6CaCO3 (liquid)?=?Ca3MgSi2O8 (merwinite)?+?3CaMg(CO3)2 (liquid). Thus, our experiments confirm the idea that merwinite in the upper mantle may originate via interaction of peridotite with Ca-rich carbonatite melt, and that diamonds hosting merwinite may have a metasomatic origin. It is remarkable that the interaction of the Ca-carbonate melt with orthopyroxene crystals does not produce merwinite both at 3.1 and 6.5 GPa. This indicates that olivine grain boundaries are preferable for merwinite formation in the upper mantle.
Abstract: Bridgmanite (Mg,Fe)SiO3, a high pressure silicate with a perovskite structure, is dominant material in the Lower Mantle and therefore is probably the most abundant mineral in the Earth. One single-phase and two composite inclusions of (Mg,Fe)SiO3 coexisting with jeffbenite ((Mg,Fe)3Al2Si3O12), and with jeffbenite and olivine ((Mg,Fe)2SiO4) have been analyzed to identify retrograde phases of former bridgmanite in diamonds from Juina (Brazil). XRD and Raman spectroscopy have revealed that (Mg,Fe)SiO3 inclusions are orthopyroxene at ambient conditions. XRD patterns of these inclusions indicate that they consist of polycrystals. This polycrystalline textures together with high lattice strain of host diamond around these inclusions observed from EBSD may be an evidence for the retrograde phase transition of former bridgmanite. Single-phase inclusions of (Mg,Fe)SiO3 in superdeep diamonds are suggested to represent a retrograde phase of bridgmanite and fully inherit its initial chemical composition, including a high Al and low Ni contents [1,2]. The composite inclusions of (Mg,Fe)SiO3 with jeffbenite and other silicate and oxide phases may be interpreted as exsolution products from originally homogeneous bridgmanite [3]. The bulk compositions of these inclusions are rich in Al, Ti, and Fe which are similar to bridgmanite produced in experiments on the MORB composition. However, the retrograde origin of composite inclusions due to decomposition of Al-rich bridgmanite may be doubtful because each of observed phases may represent single-phase inclusions, i.e. bridgmanite and high pressure garnet (majoritic garnet), with similar compositional features.
Progress in Earth and Planetary Science, Vol. 7, 23, 7p. Pdf
Mantle
wustite
Abstract: The longitudinal sound velocity (VP) and the density (?) of wüstite, FeO, were measured at pressures of up to 112.3?GPa and temperatures of up to 1700?K using both inelastic X-ray scattering and X-ray diffraction combined with a laser-heated diamond-anvil cell. The linear relationship between VP and ?, Birch’s law, for wüstite can be expressed as VP = 1.55 (1) × ? [g/cm3] - 2.03 (8) [km/s] at 300?K and VP = 1.61 (1) × ? [kg/m3] - 2.82 (10) [km/s] at 1700?K. The sound velocity of wüstite is significantly lower than that of bridgmanite and ferropericlase under lower mantle conditions. In other words, the existence of wüstite in the lower mantle can efficiently decrease the seismic velocity. Considering its slow velocity and several mechanisms for the formation of FeO-rich regions at the core-mantle boundary, we confirm earlier suggestions indicating that wüstite enrichment at the bottom of the Earth’s mantle may contribute to the formation of denser ultra-low velocity zones.
South America, Brazil, Africa, South Africa, Guinea, Canada, Northwest Territories
deposit - Sao Luis, Juina
Abstract: Bridgmanite (Mg,Fe)SiO3, a high pressure silicate with a perovskite structure, is dominant material in the lower mantle at the depths from 660 to 2700 km and therefore is probably the most abundant mineral in the Earth. Although synthetic analogues of this mineral have been well studied, no naturally occurring samples had ever been found in a rock on the planet’s surface except in some shocked meteorites. Due to its unstable nature under ambient conditions, this phase undergoes retrograde transformation to a pyroxene-type structure. The identification of the retrograde phase as ‘bridgmanite’ in so-called superdeep diamonds was based on the association with ferropericlase (Mg,Fe)O and other high-pressure (supposedly lower-mantle) minerals predicted from theoretical models and HP-HT experiments. In this study pyroxene inclusions in diamond grains from Juina (Brazil), one single-phase (Sample SL-14) and two composite inclusions of (Mg,Fe)SiO3 coexisting with (Mg,Fe)3Al2Si3O12 (Sample SL-13), and with (Mg,Fe)3Al2Si3O12 and (Mg,Fe)2SiO4 (Sample SL-80) have been analyzed to identify retrograde phases of former bridgmanite. XRD and Raman spectroscopy have revealed that these are orthopyroxene (Opx). (Mg,Fe)2SiO4 and (Mg,Fe)3Al2Si3O12 in these inclusions are identified as olivine and jeffbenite (TAPP). These inclusions are associated with inclusions of (Mg,Fe)O (SL-14), CaSiO3 (SL-80) and composite inclusion of CaSiO3+CaTiO3 (SL-13). XRD patterns of (Mg,Fe)SiO3 inclusions indicate that they consist of polycrystals. This polycrystalline textures together with high lattice strain of host diamond around these inclusions observed from EBSD may be an evidence for the retrograde phase transition of former bridgmanite. Single-phase inclusions of (Mg,Fe)SiO3 in superdeep diamonds are suggested to represent a retrograde phase of bridgmanite and fully inherit its initial chemical composition, including a high Al and low Ni contents [Harte, Hudson, 2013; Kaminsky, 2017]. The composite inclusions of (Mg,Fe)SiO3 with jeffbenite and other silicate and oxide phases may be interpreted as exolusion products from originally homogeneous bridgmanite [Walter et al., 2011]. The bulk compositions of these composite inclusions are rich in Al, Ti, and Fe which are similar to Al-rich bridgmanite produced in experiments on the MORB composition. However, the retrograde origin of composite inclusions due to decomposition of Al-rich bridgmanite may be doubtful because each of observed phases may represent single-phase inclusions, i.e. bridgmanite and high pressure garnet (majoritic garnet), with similar compositional features.
Crystallographic preferred orientation of wadsleyite and ringwoodite: effects of phase transformation and water on seismic anisotropy in the mantle transition zone.
Earth and Planetary Science Letters, Vol. 397, pp. 133-144.
Abstract: Seismic waves that propagate through a layer of Earth’s upper mantle are highly attenuated. Contrary to general thinking, this attenuation seems to be strongly affected by oxidation conditions, rather than by water content.
Contributions to Mineralogy and Petrology, Vol. 173, 21p. Doi.org/10.1007/s00410-018-1515-9
Mantle
subduction
Abstract: The role of aqueous fluid in fracturing in subducting slabs was investigated through a series of deformation experiments on dunite that was undersaturated (i.e., fluid-free) or saturated with water (i.e., aqueous-fluid bearing) at pressures of 1.0-1.8 GPa and temperatures of 670-1250 K, corresponding to the conditions of the shallower regions of the double seismic zone in slabs. In situ X-ray diffraction, radiography, and acoustic emissions (AEs) monitoring demonstrated that semi-brittle flow associated with AEs was dominant and the creep/failure strength of dunite was insensitive to the dissolved water content in olivine. In contrast, aqueous fluid drastically decreased the creep/failure strength of dunite (up to ~ 1 GPa of weakening) over a wide range of temperatures in the semi-brittle regime. Weakening of the dunite by the aqueous fluid resulted in the reduction of the number of AE events (i.e., suppression of microcracking) and shortening of time to failure. The AE hypocenters were located at the margin of the deforming sample while the interior of the faulted sample was aseismic (i.e., aseismic semi-brittle flow) under water-saturated conditions. A faulting (slip rate of ~ 10?³ to 10?4 s?¹) associated with a large drop of stress (?s ~ 0.5 to 1 GPa) and/or pressure (?P ~ 0.5 GPa) was dominant in fluid-free dunite, while a slow faulting (slip rate < 8 × 10?5 s?¹) without any stress/pressure drop was common in water-saturated dunite. Aseismic semi-brittle flow may mimic silent ductile flow under water-saturated conditions in subducting slabs.
Abstract: We performed melting experiments on Fe-O alloys up to 204 GPa and 3500 K in a diamond-anvil cell (DAC) and determined the liquidus phase relations in the Fe-FeO system based on textural and chemical characterizations of recovered samples. Liquid-liquid immiscibility was observed up to 29 GPa. Oxygen concentration in eutectic liquid increased from >8 wt% O at 44 GPa to 13 wt% at 204 GPa and is extrapolated to be about 15 wt% at the inner core boundary (ICB) conditions. These results support O-rich liquid core, although oxygen cannot be a single core light element. We estimated the range of possible liquid core compositions in Fe-O-Si-C-S and found that the upper bounds for silicon and carbon concentrations are constrained by the crystallization of dense inner core at the ICB.
Yamaguchi, H.,Kudo, Y., Masuzawa, T., Kudo, M., Yamada, Takakuwa, Okano
Combine x-ray photoelectron spectroscopy/ultraviolet photoelectron spectroscopy/field emission spectroscopy for characterization of electron emmision of diamond.
Journal of Vacuum Science and Technology B Microelectronics and Nanometer Structures, Vol. 26, 2, pp. 730-734. American Vacuum Society
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 267.
Global
synthetics
Abstract: Pink diamond is extremely popular among fancy-color diamonds, which has prompted numerous attempts to produce pink diamond artificially. Pink CVD synthetic diamonds appeared on the gem market around 2010. Their color was produced by a multistep process combining post-growth HPHT treatment to remove the brown hue and subsequent electron irradiation, followed by low-temperature annealing. Pink CVD synthetic diamonds treated only with low pressure and high temperature (LPHT), without additional post-growth irradiation, have also been reported but are rarely seen on the market. Recently, a loose pink stone (figure 1) was submitted to the Central Gem Laboratory in Tokyo for grading purposes. Our examination revealed that this 0.192 ct brilliant-cut marquise was a CVD synthetic diamond that had been LPHT treated. Visually, this diamond could not be distinguished from natural diamonds with similar color. However, three characteristics of CVD origin were detected: 1. C-H related absorption peaks between 3200 and 2800 cm-1, located with infrared spectroscopy 2. A luminescence peak at 737 nm, detected with photoluminescence (PL) spectroscopy 3. A trace of lamellar pattern seen in the DiamondView However, irradiation-related peaks such as at 1450 cm-1 (H1a), 741.1 nm (GR1), 594.3 nm, or 393.5 nm (ND1) that are seen in the pink CVD diamonds treated with common multi-step processes were not detected. The presence of four peaks at 3123, 2901, 2870, and 2812 cm-1 between 3200 and 2800 cm-1 suggests this stone was LPHT treated; the following observations indicate that it was not HPHT treated: 1) The 3123 cm-1 peak presumably derived from NVH0 disappears after a normal HPHT treatment. 2) The 2901, 2870, and 2812 cm-1 peaks are known to shift toward higher wavenumbers as the annealing temperature rises. Our own HPHT treatment experiments on CVDgrown diamonds proved that the 2902 and 2871 cm-1 peaks detected after 1600°C annealing shifted to 2907 and 2873 cm-1 after 2300°C annealing. The peak shift of 2901, 2870, and 2812 cm-1 is also related to the pressure during the annealing, as these peaks shifted to 2902, 2871, and 2819 cm-1 at the higher pressure of 7 GPa compared to 2900, 2868, and 2813 cm-1 at the ambient pressure under the same annealing temperature of 1600°C. 3) Absorption peaks at 7917 and 7804 cm-1 in the infrared region and at 667 and 684 nm in the visible range were also detected, which coincide with the features seen in LPHTtreated stones. From the combination of the intensity ratios of optical centers such as H3 and NV centers that were detected with PL measurement, this sample is presumed to have been treated with LPHT annealing at about 1500- 1700°C as a post-growth process. In recent years, CVD synthetic diamonds have been produced in a wider range of colors due to progress in the crystal growth techniques and post-growth treatments. Although HPHT treatment has been employed mainly to improve the color in a diamond, LPHT annealing may become widespread as the technique is further developed. Gemologists need to have deep knowledge about the optical defects in such LPHT-treated specimens.
Science Adavances, Vol. 6, eaba7118 May 27, 9p. Pdf
Asia, Java
geophysics -seismic
Abstract: New passive- and active-source seismic experiments reveal unusually high mantle P-wave speeds that extend beneath the remnants of the world’s largest known large igneous province, making up the 120-million-year-old Ontong-Java-Manihiki-Hikurangi Plateau. Sub-Moho Pn phases of ~8.8 ± 0.2 km/s are resolved with negligible azimuthal seismic anisotropy, but with strong radial anisotropy (~10%), characteristic of aggregates of olivine with an AG crystallographic fabric. These seismic results are the first in situ evidence for this fabric in the upper mantle. We show that its presence can be explained by isotropic horizontal dilation and vertical flattening due to late-stage gravitational collapse and spreading in the top 10 to 20 km of a depleted, mushroom-shaped, superplume head on a horizontal length scale of 1000 km or more. This way, it provides a seismic tool to track plumes long after the thermal effects have ceased.
Seismic identification of basement reflectors: the Bagdad reflection sequence in the Basin and Range Province- Colorado Plateau transition zone, Arizona
Geophysical Research Letters, Vol. 45, 24, pp. 13,298-13,305.
Mantle
subduction
Abstract: A subduction zone's mantle wedge can have a complex pattern of seismic anisotropy where the fast direction often rotates from trench-parallel close to the trench to trench-normal in the backarc. This pattern can be interpreted as induced by either 3-D trench-parallel flow or by the presence of water close to the trench. Almost all models so far favored the trench-parallel flow hypothesis, usually based on indirect or complementary indicators such as the evolution of geochemical signatures of volcanoes along the arc. Here we examine a seismic anisotropy observational signature that can be used to discriminate between the two explanations. The concept is defined using an interdisciplinary approach linking a direct modeling of the flow in the subduction wedge and a computation of seismic wave propagation in anisotropic media. We define a unique water-induced signature that is the presence of a “morph zone” characterized by a weak anisotropy and a decrease of seismic velocities. We apply the model to the Lau Basin where we find this predicted signature, demonstrating for the first time that water rather than trench-parallel flow is responsible for the observed anisotropy pattern there.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 24.
Canada, Northwest Territories
Deposit - Ekati
Abstract: How would a large open pit mine on caribou range (e.g., the Ekati Diamond Mine in the Bathurst caribou’s summer range) have influenced caribou? A traditional knowledge study on the cumulative impacts on the Bathurst caribou herd qualitatively described how mining activities might have influenced the herd (Mackenzie et al. 2013): caribou migration routes deflected away from the mines probably due to seeing mining activities or hearing the noises; and skinny caribou or abnormal smells and materials in caribou meat, liver, or the hide linings probably related to changes in caribou forage and quality of water and air. In other words, the potential influences of mining operations on caribou were most likely through altering what caribou can see, hear, smell (e.g., dusts and fine particle matter < 2.5 ?m (PM2.5) in the air, and from acidity in the soil), and taste (e.g., dust on foliage, vegetation composition change). Boulanger et al. (2012) estimated the size of a zone of influence (ZOI) of the Ekati-Diavik mining complex in the Bathurst caribou summer range, using caribou presence dataset. They also explored the mechanisms of ZOI using the spatial distribution of the total suspended particles, which was simulated with an atmospheric transport and dispersion model (Rescan, 2006). While these studies have added to our understanding of the possible impacts of mining operations on caribou, knowledge gaps remain. One outstanding gap is the lack of direct measurements about the caribou relevant environmental changes caused by mining operations. For example, exactly from how far away can caribou clearly see the vehicles driving on a mining road, or the buildings and the elevated waste piles in a camp? From how far away might caribou hear the noise caused by mining operations? To what spatial extent had the dusts and PM2.5 from mining operations influenced the tundra ecosystems? And how the dusts and PM2.5 from mining operations might have influenced caribou forage quality? Potentially these questions can be answered by in-situ measurements and satellite remote sensing. For example, studies have showed that it is possible to remotely sense PM2.5 distribution using twice-daily MODIS data at a spatial resolution of 1 km (Lyapustin et al., 2011; Chudnovsky et al., 2013; Hu et al., 2014). The objective of this study is thus to quantitatively measure these changes around the Ekati Diamond Mine, by means of in-situ surveys and satellite remote sensing. We conducted field surveys at more than 100 sites around the Ekati Diamond Mine during August 14-23, 2015, a collaborative effort of the NWT CIMP project entitled “Satellite Monitoring for Assessing Resource Development’s Impact on Bathurst Caribou (SMART)”, and the Dominion Diamond Ekati Corporation. In this presentation, we will report preliminary results and lessons learned from our first year’s study.
Experimental study of partition of rare elements between minerals and melts of diamond forming eclogite carbonatite and peridotite carbonatites systems.
Abstract: Precambrian supercontinents Nuna-Columbia (1.7 to 1.3 billion years ago) and Rodinia (1.1 to 0.7 billion years ago) have been proposed. However, the arrangements of crustal blocks within these supercontinents are poorly known. Huge, dominantly basaltic magmatic outpourings and intrusions, covering up to millions of square kilometres, termed Large Igneous Provinces, typically accompany (super) continent breakup, or attempted breakup and offer an important tool for reconstructing supercontinents. Here we focus on the Large Igneous Province record for Siberia and Laurentia, whose relative position in Nuna-Columbia and Rodinia reconstructions is highly controversial. We present precise geochronology—nine U -Pb and six Ar -Ar ages—on dolerite dykes and sills, along with existing dates from the literature, that constrain the timing of emplacement of Large Igneous Province magmatism in southern Siberia and northern Laurentia between 1,900 and 720 million years ago. We identify four robust age matches between the continents 1,870, 1,750, 1,350 and 720 million years ago, as well as several additional approximate age correlations that indicate southern Siberia and northern Laurentia were probably near neighbours for this 1.2-billion-year interval. Our reconstructions provide a framework for evaluating the shared geological, tectonic and metallogenic histories of these continental blocks.
Eclogite facies relics and a multistage breakdown in metabasites of the KTB pilot hole, northeast Bavaria: implications for the Variscan tectonometamorphic evol
Contributions to Mineralogy and Petrology, Vol. 112, pp. 261-278
Schmadicke, E., Okrusch, M., Rupprecht-Gutowski, P., Will, T.M.
Garnet pyroxenite, eclogite and alkremite xenoliths from the off-craton Gibeon kimberlite field, Namibia: a window into the upper mantle of the Rehoboth Terrane.
Schmadicke, E., Okrusch, M., Rupprecht-Gutpwski, P., Will, T.M.
Garnet pyroxenite, eclogite and alkremite xenoliths from the off-craton Gibeon kimberlite field, Namibia: a window into the upper mantle of Rehoboth Terrane.
Physics of the Earth and Planetary Interiors, Vol. 247, pp. 11-16.
Mantle
Experimental Petrology
Abstract: Lattice thermal conductivity of ferropericlase and radiative thermal conductivity of iron bearing magnesium silicate perovskite (bridgmanite) - the major mineral of Earth’s lower mantle- have been measured at room temperature up to 30 and 46 GPa, respectively, using time-domain thermoreflectance and optical spectroscopy techniques in diamond anvil cells. The results provide new constraints for the pressure dependencies of the thermal conductivities of Fe bearing minerals. The lattice thermal conductivity of ferropericlase Mg0.9Fe0.1O is 5.7(6) W/(m * K) at ambient conditions, which is almost 10 times smaller than that of pure MgO; however, it increases with pressure much faster (6.1(7)%/GPa vs 3.6(1)%/GPa). The radiative conductivity of a Mg0.94Fe0.06SiO3 bridgmanite single crystal agrees with previously determined values for powder samples at ambient pressure; it is almost pressure-independent in the investigated pressure range. Our results confirm the reduced radiative conductivity scenario for the Earth’s lower mantle, while the assessment of the heat flow through the core-mantle boundary still requires in situ measurements at the relevant pressure-temperature conditions.
Earth and Planetary Science Letters, Vol. 547, 9p. Pdf
Mantle
perovskite
Abstract: The thermal conductivity of post-perovskite (ppv), the highest-pressure polymorph of MgSiO3 in the Earth's mantle, is one of the most important transport properties for providing better constraints on the temperature profile and dynamics at the core-mantle boundary (CMB). Incorporation of Fe into ppv can affect its conductivity, which has never been experimentally investigated. Here we determined the lattice thermal conductivities of ppv containing 3 mol% and 10 mol% of Fe at high P-T conditions - of pressures up to 149 GPa and 177 GPa, respectively, and temperatures up to 1560 K - by means of the recently developed pulsed light heating thermoreflectance technique combining continuous wave heating lasers. We found that the incorporation of Fe into ppv moderately reduces its lattice thermal conductivity as it increases the Fe content. The bulk conductivity of ppv dominant pyrolite is estimated as 1.5 times higher than that of pyrolite consisting of bridgmanite and ferropericlase in the lower mantle, which agrees with the traditional view that ppv acts as a better heat conductor than bridgmanite in the Earth's lowermost mantle.
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: In the last two decades, airborne laser scanning (ALS) has found widespread application and driven fundamental advances in the Earth sciences. With increasing availability and accessibility, multi-temporal ALS data have been used to advance key research topics related to dynamic Earth surface processes. This review presents a comprehensive compilation of existing applications of ALS change detection to the Earth sciences. We cover a wide scope of material pertinent to the broad field of Earth sciences to encourage the cross-pollination between sub-disciplines and discuss the outlook of ALS change detection for advancing scientific discovery. While significant progress has been made in applying repeat ALS data to change detection, numerous approaches make fundamental assumptions that limit the full potential of repeat ALS data. The use of such data for 3D change detection is, therefore, in need of novel, scalable, and computationally efficient processing algorithms that transcend the ever-increasing data density and spatial coverage. Quantification of uncertainty in change detection results also requires further attention, as it is vitally important to understand what 3D differences detected between epochs represent actual change as opposed to limitations in data or methodology. Although ALS has become increasingly integral to change detection across the Earth sciences, the existence of pre- and post-event ALS data is still uncommon for many isolated hazard events, such as earthquakes, volcanic eruptions, wildfires, and landslides. Consequently, data availability is still a major limitation for many ALS change detection applications.
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Magnetic inversion of three airborne dat a sets over the Tli Kwi Cho kimberlite complex.
SEG Annual Meeting Denver, pp. 1790-1794 extended abstract
Fournier, D., Heagy, L., Corcoran, N., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Multi-EM systems inversion - towards a common conductivity model for Tli Kwi Cho complex.
SEG Annual Meeting Denver, pp. 1795-1798. Extended abstract
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO-18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO- 18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Marchant, M., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
ASEG-PESA-AIG 2016 25th Geophysical Conference, 4p. Pdf
Canada, Northwest Territories
Deposit - Tli Kwi Cho
Abstract: In this study, we revisit three airborne EM surveys over Tli Kwi Cho (TKC). These consist of a frequency domain DIGHEM data set, and two time domain surveys, VTEM and AeroTEM. Negative transients have been recorded in both of the time domain surveys and we interpret these as arising from chargeable bodies. The kimberlite pipes are referred to as DO-27 and DO-18. We look in more detail at the transient data and apply the ATEM-IP inversion procedure to recover a 3D pseudo-chargeability distribution. Important components of the analysis involve estimating a background conductivity for the region. For DO-27 we have used a 3D parametric inversion to recover the conductivity from TEM data. The IP signal for the inversion is obtained by subtracting the time domain responses estimated by EM inversion from the observed background signal. This process also removes EM coupling noise that might be contaminating the data. The resultant IP data are inverted with a linear inverse approach using the sensitivity from the background conductivity. This yields a 3D model of pseudo-chargeability.
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.
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.
Society of Exploration Geophysicists, Interpretation, August T 327, 14p.
Canada, Northwest Territories
deposit -Tli Kwi Cho
Abstract: The geologically distinct DO-27 and DO-18 kimberlites, often called the Tli Kwi Cho (TKC) kimberlites, have been used as a testbed for airborne geophysical methods applied to kimberlite exploration. This paper, which is the last of a three-part series, focuses on extracting chargeability information from time-domain electromagnetic (TEM) data. Three different TEM surveys, having similar coincident-loop geometry, have been carried out over TKC. Each records negative transients over the main kimberlite units and this is a signature of induced polarization (IP) effects. By applying a TEM-IP inversion workflow to a VTEM data set we decouple the EM and IP responses in the observations and then recover 3D pseudo-chargeability models at multiple times. A subsequent analysis is used to recover Cole-Cole parameters. Our models demonstrate that both DO-18 and DO-27 pipes are chargeable, but they have different Cole-Cole time constants: 110 and 1160 µs, respectively. At DO-27, we also distinguish between two adjacent kimberlite units based on their respective Cole-Cole time constants. Our chargeability models are combined with the den-sity, magnetic susceptibility and conductivity models from Papers I and II and allow us to build a 3D petrophysical model of TKC using only information obtained from airborne geophysics. Comparison of this final petrophysical model to a 3D geological model derived from the extensive drilling program demonstrates that we can characterize the three main kimberlite units at TKC: HK, VK, and PK in 3D by using airborne geophysics.
Earth and Planetary Science Letters, Vol. 428, pp. 108-119.
Russia, Siberia
Deposit - Obnazhennaya
Abstract: The formation age of the lithospheric mantle of the Siberian craton (one of the largest on Earth) is not well established; nearly all published whole-rock Re–Os data are for mantle xenoliths from a single kimberlite in the center of the craton (Udachnaya). We report Re–Os isotope and PGE concentration data for 19 spinel and garnet peridotite xenoliths from the Obnazhennaya kimberlite in the northeastern portion of the craton. Most samples in this study, and many Obnazhennaya peridotites in general, show a combination of relatively low Al2O3 (0.1–2%) with high CaO (1.4–4%) concentrations. Only four dunites and harzburgites in our sample suite have low contents of both Al2O3 and CaO (0.1–0.8%), but their relatively low Mg# (0.888–0.919) and highly variable Os concentrations (0.6–35 ppb) suggest they may have formed in melt migration channels rather than as residues of partial melt extraction. A group of six Ca-rich (2.0–3.2% CaO) peridotites yields the highest Re–Os model ages (mean TRD = 2.8 Ga, mean TMA = 3.5 Ga). Eight peridotites with low to moderate Al2O3 (<2%) and Mg# =0.91, including three low-Ca harzburgites, yield lower Re–Os model ages (mean TRD = 1.9 Ga, mean TMA = 2.2 Ga). The remainder of the samples may not yield meaningful TRD ages because they are not refractory (Al2O3 >2.6% and/or Mg# =0.90). We interpret these results as evidence for a two-stage formation of the lithospheric mantle. The peridotites formed at the two stages show very similar chemical compositions. The enrichment in Ca, which we attribute to widespread post-melting metasomatism by carbonate-rich melts, may have taken place either at the end of the Archean melting event, when at least one Ca–Al-rich peridotite was formed, or later. The combined Re–Os age data on xenoliths from Obnazhennaya and Udachnaya suggest that the lithospheric mantle beneath the Siberian craton was not formed in a single event, but grew in at least two events, one in the late Archean and the other in the Paleoproterozoic. This study further indicates that the formation of highly melt-depleted lithospheric mantle was not limited to the Archean, but continued well into the Paleoproterozoic when the Siberian craton was stabilized.
Abstract: Coalingite, Mg10Fe2(CO3)(OH)24 • 2H2O, rare Mg -Fe hydrous carbonate, has been found in the course of the mineralogical study of a disintegrated kimberlite breccia from the Manchary pipe of the Khompu -May field located in the Tamma Basin, Central Yakutia, 100 km south of Yakutsk. Coalingite occurs as small reddish brown platelets, up to 0.2 mm in size. It is associated with lizardite, chrysotile and brucite, which are typical kimberlitic assemblage. Coalingite is a supergene mineral, but in this case, it is produced by the interaction of brucite-bearing kimberlite and underground water circulating through a vertical or oblique fault zone.
Formation and evolution of hypabyssal kimberlites from the Siberian craton: part 1 - new insights from cathodluminescence of the carbonates. Anabar and Olenek area
Journal of Asian Earth Sciences, Vol. 145, pt. B, pp. 670-678.
Geochimica et Cosmochimica Acta, in press available, 46p.
Russia, Siberia
deposit - Obnazhennaya
Abstract: The Obnazhennaya kimberlite in the NE Siberian craton hosts a most unusual cratonic xenolith suite, with common rocks rich in pyroxenes and garnet, and no sheared peridotites. We report petrographic and chemical data for whole rocks (WR) and minerals of 20 spinel and garnet peridotites from Obnazhennaya with Re-depletion Os isotope ages of 1.8-2.9 Ga (Ionov et al., 2015a) as well as 2 pyroxenites. The garnet-bearing rocks equilibrated at 1.6-2.8 GPa and 710-1050°C. Some xenoliths contain vermicular spinel-pyroxene aggregates with REE patterns in clinopyroxene mimicking those of garnet. The peridotites show significant scatter of Mg# (0.888-0.924), Cr2O3 (0.2-1.4 wt.%) and high NiO (0.3-0.4 wt.%). None are pristine melting residues. Low-CaO-Al2O3 (=0.9 wt.%) dunites and harzburgites are melt-channel materials. Peridotites with low to moderate Al2O3 (0.4-1.8 wt.%) usually have CaO > Al2O3, and some have pockets of calcite texturally equilibrated with olivine and garnet. Such carbonates, exceptional in mantle xenoliths and reported here for the first time for the Siberian mantle, provide direct evidence for modal makeover and Ca and LREE enrichments by ephemeral carbonate-rich melts. Peridotites rich in CaO and Al2O3 (2.7-8.0 wt.%) formed by reaction with silicate melts. We infer that the mantle lithosphere beneath Obnazhennaya, initially formed in the Mesoarchean, has been profoundly modified. Pervasive inter-granular percolation of highly mobile and reactive carbonate-rich liquids may have reduced the strength of the mantle lithosphere leading the way for reworking by silicate melts. The latest events before the kimberlite eruption were the formation of the carbonate-phlogopite pockets, fine-grained pyroxenite veins and spinel-pyroxene symplectites. The reworked lithospheric sections are preserved at Obnazhennaya, but similar processes could erode lithospheric roots in the SE Siberian craton (Tok) and the North China craton, where ancient melting residues and reworked garnet-bearing peridotites are absent.The modal, chemical and Os-isotope compositions of the Obnazhennaya xenoliths produced by reaction of refractory peridotites with melts are very particular (high Ca/Al, no Mg#-Al correlations, highly variable Cr, low 187Os/188Os, continuous modal range from olivine-rich to low-olivine peridotites, wehrlites and websterites) and distinct from those of fertile lherzolites in off-craton xenoliths and peridotite massifs. These features argue against the concept of ‘refertilization’ of cratonic and other refractory peridotites by mantle-derived melts as a major mechanism to form fertile to moderately depleted lherzolites in continental lithosphere. The Obnazhennaya xenoliths represent a natural rock series produced by ‘refertilization’, but include no rocks equivalent in modal, major and trace element to the fertile lherzolites. This study shows that ‘refertilization’ yields broad, continuous ranges of modal and chemical compositions with common wehrlites and websterites that are rare among off-craton xenoliths.
Geochimica et Cosmochimica Acta, Vol. 224, pp. 132-153.
Russia, Siberia
deposit - Obnazhennaya
Abstract: The Obnazhennaya kimberlite in the NE Siberian craton hosts a most unusual cratonic xenolith suite, with common rocks rich in pyroxenes and garnet, and no sheared peridotites. We report petrographic and chemical data for whole rocks (WR) and minerals of 20 spinel and garnet peridotites from Obnazhennaya with Re-depletion Os isotope ages of 1.8-2.9?Ga (Ionov et al., 2015a) as well as 2 pyroxenites. The garnet-bearing rocks equilibrated at 1.6-2.8?GPa and 710-1050?°C. Some xenoliths contain vermicular spinel-pyroxene aggregates with REE patterns in clinopyroxene mimicking those of garnet. The peridotites show significant scatter of Mg# (0.888-0.924), Cr2O3 (0.2-1.4?wt.%) and high NiO (0.3-0.4?wt.%). None are pristine melting residues. Low-CaO-Al2O3 (=0.9?wt.%) dunites and harzburgites are melt-channel materials. Peridotites with low to moderate Al2O3 (0.4-1.8?wt.%) usually have CaO?>?Al2O3, and some have pockets of calcite texturally equilibrated with olivine and garnet. Such carbonates, exceptional in mantle xenoliths and reported here for the first time for the Siberian mantle, provide direct evidence for modal makeover and Ca and LREE enrichments by ephemeral carbonate-rich melts. Peridotites rich in CaO and Al2O3 (2.7-8.0?wt.%) formed by reaction with silicate melts. We infer that the mantle lithosphere beneath Obnazhennaya, initially formed in the Mesoarchean, has been profoundly modified. Pervasive inter-granular percolation of highly mobile and reactive carbonate-rich liquids may have reduced the strength of the mantle lithosphere leading the way for reworking by silicate melts. The latest events before the kimberlite eruption were the formation of the carbonate-phlogopite pockets, fine-grained pyroxenite veins and spinel-pyroxene symplectites. The reworked lithospheric sections are preserved at Obnazhennaya, but similar processes could erode lithospheric roots in the SE Siberian craton (Tok) and the North China craton, where ancient melting residues and reworked garnet-bearing peridotites are absent. The modal, chemical and Os-isotope compositions of the Obnazhennaya xenoliths produced by reaction of refractory peridotites with melts are very particular (high Ca/Al, no Mg#-Al correlations, highly variable Cr, low 187Os/188Os, continuous modal range from olivine-rich to low-olivine peridotites, wehrlites and websterites) and distinct from those of fertile lherzolites in off-craton xenoliths and peridotite massifs. These features argue against the concept of ‘refertilization’ of cratonic and other refractory peridotites by mantle-derived melts as a major mechanism to form fertile to moderately depleted lherzolites in continental lithosphere. The Obnazhennaya xenoliths represent a natural rock series produced by ‘refertilization’, but include no rocks equivalent in modal, major and trace element to the fertile lherzolites. This study shows that ‘refertilization’ yields broad, continuous ranges of modal and chemical compositions with common wehrlites and websterites that are rare among off-craton xenoliths.
Geochimica et Cosmochimica Acta, Vol. 248, pp. 1-13.
Russia, Siberia
carbonatite
Abstract: Ca isotopes can be strongly fractionated at the Earth’s surface and thus may be tracers of subducted carbonates and other Ca-rich surface materials in mantle rocks, magmas and fluids. However, the d44/40Ca range in the mantle and the scope of intra-mantle isotope fractionation are poorly constrained. We report Ca isotope analyses for 22 mantle xenoliths: four basalt-hosted refractory peridotites from Tariat in Mongolia and 18 samples from the Obnazhennaya (Obn) kimberlite on the NE Siberian craton. Obn peridotites are Paleoproterozoic to Archean melting residues metasomatised by carbonate-rich and/or silicate melts including unique xenoliths that contain texturally equilibrated carbonates. d44/40Ca in 15 Obn xenoliths shows limited variation (0.74-0.97‰) that overlaps the value (0.94?±?0.05‰) inferred for the bulk silicate Earth from data on fertile lherzolites, but is lower than d44/40Ca for non-metasomatised refractory peridotites from Mongolia (1.10?±?0.03‰). Bulk d44/40Ca in four Obn peridotites containing metasomatic carbonates ranges from 0.81?±?0.08‰ to 0.83?±?0.06‰, with similar values in acid-leachates and leaching residues, indicating isotopic equilibration of the carbonates with host rocks. We infer that (a) metasomatism tends to decrease d44/40Ca values of the mantle, but its effects are usually limited (=0.3‰); (b) Ca isotopes cannot distinguish "carbonatite" and "silicate" types of mantle metasomatism. The lowest d44/40Ca value (0.56‰) was obtained for a phlogopite-bearing Obn peridotite with a very high Ca/Al of 8 suggesting that the greatest metasomatism-induced Ca isotope shifts may be seen in rocks initially low in Ca that experienced significant Ca input leading to high Ca/Al. Two Obn peridotites, a dunite (melt channel material) and a veined spinel wehrlite, have high d44/40Ca values (1.22‰ and 1.38‰), which may be due to isotope fractionation by diffusion during silicate melt intrusion and percolation in the host mantle. Overall, we find no evidence that recycling of crustal carbonates may greatly affect Ca isotope values in the global mantle or on a regional scale.
Geochimica et Cosmochimica Acta, Vol. 248, pp. 1-13.
Mantle
carbonatite
Abstract: Ca isotopes can be strongly fractionated at the Earth’s surface and thus may be tracers of subducted carbonates and other Ca-rich surface materials in mantle rocks, magmas and fluids. However, the d44/40Ca range in the mantle and the scope of intra-mantle isotope fractionation are poorly constrained. We report Ca isotope analyses for 22 mantle xenoliths: four basalt-hosted refractory peridotites from Tariat in Mongolia and 18 samples from the Obnazhennaya (Obn) kimberlite on the NE Siberian craton. Obn peridotites are Paleoproterozoic to Archean melting residues metasomatised by carbonate-rich and/or silicate melts including unique xenoliths that contain texturally equilibrated carbonates. d44/40Ca in 15 Obn xenoliths shows limited variation (0.74-0.97‰) that overlaps the value (0.94?±?0.05‰) inferred for the bulk silicate Earth from data on fertile lherzolites, but is lower than d44/40Ca for non-metasomatised refractory peridotites from Mongolia (1.10?±?0.03‰). Bulk d44/40Ca in four Obn peridotites containing metasomatic carbonates ranges from 0.81?±?0.08‰ to 0.83?±?0.06‰, with similar values in acid-leachates and leaching residues, indicating isotopic equilibration of the carbonates with host rocks. We infer that (a) metasomatism tends to decrease d44/40Ca values of the mantle, but its effects are usually limited (=0.3‰); (b) Ca isotopes cannot distinguish "carbonatite" and "silicate" types of mantle metasomatism. The lowest d44/40Ca value (0.56‰) was obtained for a phlogopite-bearing Obn peridotite with a very high Ca/Al of 8 suggesting that the greatest metasomatism-induced Ca isotope shifts may be seen in rocks initially low in Ca that experienced significant Ca input leading to high Ca/Al. Two Obn peridotites, a dunite (melt channel material) and a veined spinel wehrlite, have high d44/40Ca values (1.22‰ and 1.38‰), which may be due to isotope fractionation by diffusion during silicate melt intrusion and percolation in the host mantle. Overall, we find no evidence that recycling of crustal carbonates may greatly affect Ca isotope values in the global mantle or on a regional scale.
Abstract: We have analyzed 141 grains of pyrope from Neogene sediments in a quarry of construction materials, in the Kenkeme River catchment, along its left-side tributary (Chakiya River), about 60 km northwest of Yakutsk city. The mineral chemistry patterns of pyropes are typical of Jurassic-Cretaceous barren kimberlites, like the pipes of Obnazhennaya or Muza, but are uncommon to diamondiferous Middle Paleozoic kimberlites. The results allow identifying the magmatic event and placing time constraints on kimberlite magmatism in the southeastern flank of the Vilui basin, which was part of the Late Jurassic-Early Cretaceous tectonic-magmatic event in northeastern Asia.
Nature Communications, doi:.org/10.1038/ s41467-020-17442 -8 11p. Pdf
Africa, South Africa, Russia, Siberia
water
Abstract: Trace amounts of water dissolved in minerals affect density, viscosity and melting behaviour of the Earth’s mantle and play an important role in global tectonics, magmatism and volatile cycle. Water concentrations and the ratios of hydrogen isotopes in the mantle give insight into these processes, as well as into the origin of terrestrial water. Here we show the presence of molecular H2 in minerals (omphacites) from eclogites from the Kaapvaal and Siberian cratons. These omphacites contain both high amounts of H2 (70 to 460 wt. ppm) and OH. Furthermore, their ?D values increase with dehydration, suggesting a positive H isotope fractionation factor between minerals and H2-bearing fluid, contrary to what is expected in case of isotopic exchange between minerals and H2O-fluids. The possibility of incorporation of large quantities of H as H2 in nominally anhydrous minerals implies that the storage capacity of H in the mantle may have been underestimated, and sheds new light on H isotope variations in mantle magmas and minerals.
Geochimica et Cosmochimica Acta, Vol. 248, pp. 1-13.
Mantle, Asia, Mongolia, Russia, Siberia
metasomatism
Abstract: Ca isotopes can be strongly fractionated at the Earth’s surface and thus may be tracers of subducted carbonates and other Ca-rich surface materials in mantle rocks, magmas and fluids. However, the d44/40Ca range in the mantle and the scope of intra-mantle isotope fractionation are poorly constrained. We report Ca isotope analyses for 22 mantle xenoliths: four basalt-hosted refractory peridotites from Tariat in Mongolia and 18 samples from the Obnazhennaya (Obn) kimberlite on the NE Siberian craton. Obn peridotites are Paleoproterozoic to Archean melting residues metasomatised by carbonate-rich and/or silicate melts including unique xenoliths that contain texturally equilibrated carbonates. d44/40Ca in 15 Obn xenoliths shows limited variation (0.74-0.97‰) that overlaps the value (0.94?±?0.05‰) inferred for the bulk silicate Earth from data on fertile lherzolites, but is lower than d44/40Ca for non-metasomatised refractory peridotites from Mongolia (1.10?±?0.03‰). Bulk d44/40Ca in four Obn peridotites containing metasomatic carbonates ranges from 0.81?±?0.08‰ to 0.83?±?0.06‰, with similar values in acid-leachates and leaching residues, indicating isotopic equilibration of the carbonates with host rocks. We infer that (a) metasomatism tends to decrease d44/40Ca values of the mantle, but its effects are usually limited (=0.3‰); (b) Ca isotopes cannot distinguish “carbonatite” and “silicate” types of mantle metasomatism. The lowest d44/40Ca value (0.56‰) was obtained for a phlogopite-bearing Obn peridotite with a very high Ca/Al of 8 suggesting that the greatest metasomatism-induced Ca isotope shifts may be seen in rocks initially low in Ca that experienced significant Ca input leading to high Ca/Al. Two Obn peridotites, a dunite (melt channel material) and a veined spinel wehrlite, have high d44/40Ca values (1.22‰ and 1.38‰), which may be due to isotope fractionation by diffusion during silicate melt intrusion and percolation in the host mantle. Overall, we find no evidence that recycling of crustal carbonates may greatly affect Ca isotope values in the global mantle or on a regional scale.
Abstract: Pyrope-almandine garnets (Mg# = 28.3-44.9, Ca# = 15.5-21.3) from a heavy mineral concentrate of diamondiferous kimberlites of the largest diamond deposit, the Yubileinaya pipe, along with kimberlite- like rocks and diamondiferous volcano-sediments of the Laptev Sea coast, have been found to contain polymineral, predominantly acicular inclusions, composed of aggregates of shrilankite (Ti2ZrO6), rutile, ilmenite, clinopyroxene, and apatite. The presence of shrilankite as an inclusion in garnets from assumed garnet-pyroxene rocks of the lower crust, lifted up by diamond-bearing kimberlite, allows it to be considered as an indicator mineral of kimberlite, which expands the possibilities when searching for kimberlite in the Arctic.
Abstract: Pyrope-almandine garnets (Mg# = 28.3-44.9, Ca# = 15.5-21.3) from a heavy mineral concentrate of diamondiferous kimberlites of the largest diamond deposit, the Yubileinaya pipe, along with kimberlite- like rocks and diamondiferous volcano-sediments of the Laptev Sea coast, have been found to contain polymineral, predominantly acicular inclusions, composed of aggregates of shrilankite (Ti2ZrO6), rutile, ilmenite, clinopyroxene, and apatite. The presence of shrilankite as an inclusion in garnets from assumed garnet-pyroxene rocks of the lower crust, lifted up by diamond-bearing kimberlite, allows it to be considered as an indicator mineral of kimberlite, which expands the possibilities when searching for kimberlite in the Arctic.
Abstract: The extraction of geological lineaments from digital satellite data is a fundamental application in remote sensing. The location of geological lineaments such as faults and dykes are of interest for a range of applications, particularly because of their association with hydrothermal mineralization. Although a wide range of applications have utilized computer vision techniques, a standard workflow for application of these techniques to mineral exploration is lacking. We present a framework for extracting geological lineaments using computer vision techniques which is a combination of edge detection and line extraction algorithms for extracting geological lineaments using optical remote sensing data. It features ancillary computer vision techniques for reducing data dimensionality, removing noise and enhancing the expression of lineaments. We test the proposed framework on Landsat 8 data of a mineral-rich portion of the Gascoyne Province in Western Australia using different dimension reduction techniques and convolutional filters. To validate the results, the extracted lineaments are compared to our manual photointerpretation and geologically mapped structures by the Geological Survey of Western Australia (GSWA). The results show that the best correlation between our extracted geological lineaments and the GSWA geological lineament map is achieved by applying a minimum noise fraction transformation and a Laplacian filter. Application of a directional filter instead shows a stronger correlation with the output of our manual photointerpretation and known sites of hydrothermal mineralization. Hence, our framework using either filter can be used for mineral prospectivity mapping in other regions where faults are exposed and observable in optical remote sensing data.
Abstract: The timing of final assembly and initiation of subsequent rifting of Rodinia is disputed. New rutile ages (913?±?9?Ma, 900?±?8?Ma and 873?±?3?Ma) and published zircon, monazite, titanite, biotite, muscovite and xenotime geochronology from the Capricorn Orogen (West Australian Craton) reveal a significant early Neoproterozoic event characterized by very low to low metamorphic grade, abundant metasomatism, minor leucogranitic and pegmatitic magmatism and NW-SE fault reactivation episodes between ca. 955 and 830?Ma. Collectively, these are termed the ca. 955-830?Ma Kuparr Tectonic Event. An age range of ca. 955-830?Ma is concomitant with the final stages of Rodinia assembly and the initial stages of its attempted breakup. Very low- to low-grade metamorphic and structural geological evidence favor a distal north-south compressional regime as the driver for hydrothermal activity during ca. 955-830?Ma. Nearby continental collision or accretion from the west (e.g., South China and/or Tarim) are ruled out. The cessation of metasomatism and magmatism in the West Australian Craton after ca. 830?Ma is concomitant with the emplacement of the Gairdner-Amata dyke swarm and associated magmatic activity in South China and Laurentia, the inception of the Adelaide Rift Complex and the deposition of the Centralian Superbasin. We posit that the cessation of hydrothermal activity in the Capricorn Orogen was caused by a tectonic switch from compressional to extensional at ca. 830?Ma. Magmatic and hydrothermal fluids were transferred away from the Capricorn Orogen to the incipient Adelaide Rift Complex, terminating metasomatism in the West Australian Craton. Ultimately, the Kuparr Tectonic Event marked the final stages of Rodinia assembly and its cessation marks the initial stages of its attempted breakup.
Abstract: The Earth’s mantle is currently divided into the African and Pacific domains, separated by the circum-Pacific subduction girdle, and each domain features a large low shear-wave velocity province (LLSVP) in the lower mantle. However, it remains controversial as to whether the LLSVPs have been stationary through time or dynamic, changing in response to changes in global subduction geometry. Here we compile radiogenic isotope data on plume-induced basalts from ocean islands and oceanic plateaus above the two LLSVPs that show distinct lead, neodymium and strontium isotopic compositions for the two mantle domains. The African domain shows enrichment by subducted continental material during the assembly and breakup of the supercontinent Pangaea, whereas no such feature is found in the Pacific domain. This deep-mantle geochemical dichotomy reflects the different evolutionary histories of the two domains during the Rodinia and Pangaea supercontinent cycles and thus supports a dynamic relationship between plate tectonics and deep-mantle structures.
Abstract: The Earth’s mantle is currently divided into the African and Pacific domains, separated by the circum-Pacific subduction girdle, and each domain features a large low shear-wave velocity province (LLSVP) in the lower mantle. However, it remains controversial as to whether the LLSVPs have been stationary through time or dynamic, changing in response to changes in global subduction geometry. Here we compile radiogenic isotope data on plume-induced basalts from ocean islands and oceanic plateaus above the two LLSVPs that show distinct lead, neodymium and strontium isotopic compositions for the two mantle domains. The African domain shows enrichment by subducted continental material during the assembly and breakup of the supercontinent Pangaea, whereas no such feature is found in the Pacific domain. This deep-mantle geochemical dichotomy reflects the different evolutionary histories of the two domains during the Rodinia and Pangaea supercontinent cycles and thus supports a dynamic relationship between plate tectonics and deep-mantle structures.
Billion dollar babies... the rush is on, as junior miners like Kensington race for riches in Canada's burgeoning diamond industry. Jellicoe - interview.
Abstract: Unusually deep earthquakes occur beneath rift segments with and without surface expressions of magmatism in the East African Rift system. The Tanganyika rift is part of the Western rift and has no surface evidence of magmatism. The TANG14 array was deployed in the southern Tanganyika rift, where earthquakes of magnitude up to 7.4 have occurred, to probe crust and upper mantle structure and evaluate fault kinematics. Four hundred seventy-four earthquakes detected between June 2014 and September 2015 are located using a new regional velocity model. The precise locations, magnitudes, and source mechanisms of local and teleseismic earthquakes are used to determine seismogenic layer thickness, delineate active faults, evaluate regional extension direction, and evaluate kinematics of border faults. The active faults span more than 350 km with deep normal faults transecting the thick Bangweulu craton, indicating a wide plate boundary zone. The seismogenic layer thickness is 42 km, spanning the entire crust beneath the rift basins and their uplifted flanks. Earthquakes in the upper mantle are also detected. Deep earthquakes with steep nodal planes occur along subsurface projections of Tanganyika and Rukwa border faults, indicating that large offset (=5 km) faults penetrate to the base of the crust, and are the current locus of strain. The focal mechanisms, continuous depth distribution, and correlation with mapped structures indicate that steep, deep border faults maintain a half-graben morphology over at least 12 Myr of basin evolution. Fault scaling based on our results suggests that M > 7 earthquakes along Tanganyika border faults are possible.
Abstract: Diamonds are the deepest accessible “fragments” of Earth, providing records of deep geological processes. Absolute ages for diamond formation are crucial to place these records in the correct time context. Diamond ages are typically determined by dating inclusions, assuming that they were formed simultaneously with their hosts. One of the most widely used mineral inclusions for dating diamond is garnet, which is amenable to Sm-Nd geochronology and is common in lithospheric diamonds. By investigating worldwide garnet-bearing diamonds, we provide crystallographic evidence that garnet inclusions that were previously considered to be syngenetic may instead be protogenetic, i.e., they were formed before the host diamond, raising doubts about the real significance of many reported diamond “ages.” Diffusion modeling at relevant pressures and temperatures, however, demonstrates that isotopic resetting would generally occur over geologically short time scales. Therefore, despite protogenicity, the majority of garnet-based ages should effectively correspond to the time of diamond formation. On the other hand, our results indicate that use of large garnet inclusions (e.g., >100 µm) and diamond hosts formed at temperatures lower than ~1000 °C is not recommended for diamond age determinations.
Abstract: Mantle lithologies in orogenic massifs and xenoliths commonly display strikingly different Hf- and Nd-isotope compositions compared to oceanic basalts. While the presence of pyroxenites has long been suggested in the source region of mantle-derived magmas, very few studies have reported their combined HfNd isotope compositions. We here report the first LuHf data along with ReOs data and S concentrations on the Cabo Ortegal Complex, where the pyroxenite-rich Herbeira massif has been interpreted as remnants of a delaminated arc root. The pyroxenites, chromitites and their host harzburgites show a wide range of whole-rock 187Re/188Os and 187Os/188Os (0.16-1.44), indicating that Re was strongly mobilized, partly during hydrous retrograde metamorphism but mostly during supergene alteration that preferentially affected low-Mg#, low Cu/S pyroxenites. Samples that escaped this disturbance yield an isochron age of 838 ± 42 Ma, interpreted as the formation of Cabo Ortegal pyroxenites. Corresponding values of initial 187Os/188Os (0.111-0.117) are relatively unradiogenic, suggesting limited contributions of slab-derived Os to primitive arc melts such as those parental to these pyroxenites. This interpretation is consistent with radiogenic Os in arc lavas being mostly related to crustal assimilation. Paleoproterozoic to Archean Os model ages confirm that Cabo Ortegal pyroxenites record incipient volcanic arc magmatism on the continental margin of the Western African Craton, as notably documented by zircon UPb ages of 2.1 and 2.7 Ga. LuHf data collected on clinopyroxene and amphibole separates and whole-rock samples are characterized by uncorrelated 176Lu/177Hf and 176Hf/177Hf (0.2822-0.2855), decoupled from Nd-isotope compositions. This decoupling is ascribed to diffusional disequilibrium during melt-peridotite interaction, in good agreement with the results of percolation-diffusion models simulating the interaction of an arc melt with an ancient melt-depleted residue. These models notably show that HfNd isotopic decoupling such as recorded by Cabo Ortegal pyroxenites and peridotites (??Hf(i) up to +97) is enhanced during melt-peridotite interaction by slow diffusional re-equilibration and can be relatively insensitive to chromatographic fractionation. Finally, we discuss the hypothesis that arc-continent interaction may provide preferential conditions for such isotopic decoupling and propose that its ubiquitous recognition in peridotites reflects the recycling of sub-arc mantle domains derived from ancient, reworked SCLM.
Earth and Planetary Science Letters, Vol. 544, 116365 14p. Pdf
Australia
carbonatite
Abstract: Petrological and geochemical studies have revealed the contribution of garnet pyroxenites in basalt petrogenesis. However, whether primary mantle melts are produced with such signature or acquired it subsequently remains somewhat controversial. We here integrate new major-, trace-element and Sr-Nd-Hf isotopic compositions of garnet pyroxenite xenoliths in Holocene alkali basalts from Lakes Bullenmerri and Gnotuk, Southeastern Australia, to relate their petrogenesis to mantle-wedge melt circulation and subsequent lithospheric evolution. Results show that the clinopyroxenites have lower MgO and Cr2O3 contents than the associated websterites, and range in compositions from depleted LREE patterns and highly radiogenic Nd and Hf isotopic signatures in relatively low-MgO samples (Type 1), to enriched REE patterns with negative HFSE anomalies, unradiogenic Nd and Hf isotopes, and extremely radiogenic Sr-isotopic ratios in samples with higher MgO (Type 2). Such compositional variabilities suggest that these pyroxenites represent segregates from melts derived from a recycled oceanic lithosphere with a potential contribution from pelagic sediments. Variable LREE contents and isotopic compositions between those of Type 1 and 2 clinopyroxenites are observed in amphibole-bearing samples (Type 3), which are interpreted as Type 1-like protoliths metasomatized by the basaltic and carbonatitic melts, possibly parental to Type 2 clinopyroxenites. The lithosphere beneath Southeastern Australia thus has received variable melt contributions from a heterogeneous mantle-wedge source, which notably includes a subducted oceanic slab package that has retained its integrity during subduction. On this basis, we suggest that the compositional heterogeneity and temporal evolution of the subsequent Southeastern Australian basaltic magmatism were probably affected by the presence of pyroxenite fragments in the basalt source and formed by the tectonic reactivation of this lithosphere during Cenozoic rifting. This interpretation is notably consistent with a trend of Nd-Pb isotopes towards EMII in Older Volcanic Provinces (OVP basalts) and limited Sr-Nd-Pb isotopic variations towards HIMU in the Newer Volcanic Provinces (NVP basalts, including the host lavas), which also exhibit low SiO2, high FeO and high CaO/Al2O3 commonly interpreted as due to pyroxenite contributions. Therefore, the identification of a subduction signature in these rift-related lavas attests to a "lithospheric memory" of earlier subduction episodes (as documented by the xenoliths), rather than a reflection of contemporaneous subduction tectonics.
Dyke Swarms of the World: a modern perspective Ed. Srivastava et al. Springer , Chapter pp. 111-154. available
South America, Guiana, Brazil
craton
Abstract: We review geochronological data including U-Pb baddelyite ages of Proterozoic mafic dyke swarms and sills of the Amazonian Craton, as well as their geochemical character and geological settings, in order to arrive at an integrated tectonic interpretation. The information together with the characteristics of coeval volcanic-plutonic suites indicates a cyclicity of the mafic-felsic activity through time and space. At least four LIP/SLIP events are apparent, and each one appears to accompany the stepwise accretionary crustal growth of Amazonia. The oldest two, the Orocaima (1.98-1.96 Ga) and Uatumã (c. 1.89-1.87 Ga) SLIPs, comprise calc-alkaline I-type and subordinate A-type plutonic and volcanic rocks. Synchronous mafic intraplate activity occurs across the Guiana and Central-Brazil Shields. These two events may be caused by interaction between subduction-related processes and mantle plumes with synchronous lithosphere extension during the two time periods. The Avanavero (1.79 Ga) LIP event mostly consists of mafic dykes and sills which are intrusive into the Roraima platform cover, in the Guiana Shield. They show tholeiitic chemistry and similarities with E-MORB and subcontinental lithospheric mantle-derived basalts, whereas the REE pattern suggests affinity with intraplate settings. The age of the Avanavero rocks is identical to the Crepori Diabase, located ca. 1800 km away to the south (Central-Brazil Shield). The youngest LIP event (1.11 Ga), the Rincón del Tigre-Huanchaca, has the Rio Perdido Suite as a component in the Rio Apa Terrane, which is ca. 300 km away from the Rincón del Tigre Complex, located in the SW portion of the Amazonian Craton. Furthermore, the Central-Brazil and Guiana Shields boasts widespread intraplate mafic activity, highlighted by the Mata-Matá (1.57 Ga), Salto do Céu (1.44 Ga) and Nova Floresta (1.22 Ga) mafic sills and the Cachoeira Seca Troctolite (1.19 Ga). Contemporaneous A-type, rapakivi granites with roughly similar ages also occur elsewhere. These particular episodes are extension specific steps of the Mesoproterozoic Amazonia, and the quite large distribution is consistent with LIP events. In a broader perspective, the intermittent Proterozoic intracratonic activity has a barcode that matches LIP/SLIP events in Columbia and Rodinia.
Dyke swarms of the world: a modern perspective., 10.1007/978-981-13-1666-1_4
South America, Guiana, Brazil
magmatism
Abstract: We review geochronological data including U-Pb baddelyite ages of Proterozoic mafic dyke swarms and sills of the Amazonian Craton, as well as their geochemical character and geological settings, in order to arrive at an integrated tectonic interpretation. The information together with the characteristics of coeval volcanic-plutonic suites indicates a cyclicity of the mafic-felsic activity through time and space. At least four LIP/SLIP events are apparent, and each one appears to accompany the stepwise accretionary crustal growth of Amazonia. The oldest two, the Orocaima (1.98-1.96 Ga) and Uatumã (c. 1.89-1.87 Ga) SLIPs, comprise calc-alkaline I-type and subordinate A-type plutonic and volcanic rocks. Synchronous mafic intraplate activity occurs across the Guiana and Ce
