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Ryabchikov I.D.,RAS Institute of Geology and Mineralogy | Kaminsky F.V.,KM Diamond Exploration Ltd.
Geochemistry International

Thermodynamic analysis of equilibria involving minerals of the lower mantle of pyrolite composition and crystalline carbon-bearing compounds indicates that the range of oxygen fugacity values at which diamond can be formed is separated from the region in which Fe-rich metallic alloy is generated by a field in which Fe carbides are stable. This implies that diamond can be formed in the lower mantle under more oxidizing conditions than those thought to be dominant in this geosphere. The absence of a metallic phase from the lower-mantle diamond-bearing mineral assemblage is consistent with the high (approximately 1%) Ni concentration in the ferropericlase found as inclusions in diamonds (Fe-rich metallic alloy is able to intensely extract Ni). An elevated redox potential also follows from the occurrence of carbonate phases found among mineral inclusions in lower-mantle diamonds. The main reason for a local increase in oxygen fugacity in the lower mantle may be shifts of redox equilibria toward a decrease in the amount, and then the disappearance of the Fe-Ni alloy with increasing temperature. An important role in the formation of diamond may be played by the generation of carbonate-phosphate and silicate melts in high-temperature zones and the migration of these melts and their interaction with wall rocks. © 2014, Pleiades Publishing, Ltd. Source

Kaminsky F.V.,KM Diamond Exploration Ltd. | WiRTh R.,Helmholtz Center Potsdam | MORAleS L.,Helmholtz Center Potsdam
Canadian Mineralogist

Samples of carbonado from Brazil and the Central African Republic were studied with the use of electron backscatter diffraction (EBSD) for quantitative textural analysis (QTA) and transmission electron microscopy (TEM). The grain size distribution in carbonado is either random or may be approximated by a log-normal distribution model with a mode at 6-8 ìm. No bimodal distribution, as suggested previously for some carbonado samples, was observed. The crystallographic orientations of diamond grains in carbonado are quasi-random. The following minerals were identified among syngenetic mineral inclusions, enclosed in diamond grains of carbonado: garnet, apatite (including fluorapatite), phlogopite (or high-silica mica), SiO2, Ca-Mg-Sr-and Ca-Ba-carbonates, halides (sylvite and bismocolite BiOCl), native Ni and metal alloys (Fe-Ni, Cr-Fe-Mn, and Pb-As-Mo), oxides (FeO, Fe-Sn-O, TiO2, SnO2, and PbO2), and Fe-sulfides, as well as fluid inclusions. Most of these occur over a very wide range of stability conditions. Only bismocolite, which is characteristic of the weathered crust, can be considered an entirely crustal mineral phase, which implies a possible crustal origin of the entire mineral association. Among syngenetic liquid inclusions in diamond grains comprising carbonado, silicate-carbonate ones overwhelmingly predominate. In addition to the usual silicate components, such as Si, Ti, Al, and Fe, they have Ca, K, and Cl; the latter three comprise 11.9 at.% of the only analyzed fluid inclusion. Source

Kaminsky F.V.,KM Diamond Exploration Ltd. | Ryabchikov I.D.,RAS Institute of Geology and Mineralogy | McCammon C.A.,University of Bayreuth | Longo M.,University of Bayreuth | And 3 more authors.
Earth and Planetary Science Letters

Ferropericlase (fPer) inclusions from kimberlitic lower-mantle diamonds recovered in the Juina area, Mato Grosso State, Brazil were analyzed with transmission electron microscopy, electron energy-loss spectroscopy and the flank method. The presence of exsolved non-stoichiometric Fe3+-enriched clusters, varying in size from 1-2 nm to 10-15 nm and comprising ~3.64 vol.% of fPer was established. The oxidation conditions necessary for fPer formation within the uppermost lower mantle (P=25 GPa, T=1960 K) vary over a wide range: δlogfO2 (IW) from 1.58 to 7.76 (δ=6.2), reaching the fayalite-magnetite-quartz (FMQ) oxygen buffer position. This agrees with the identification of carbonates and free silica among inclusions within lower-mantle Juina diamonds. On the other hand, at the base of the lower mantle δlogfO2 values may lie at and below the iron-wüstite (IW) oxygen buffer. Hence, the variations of δlogfO2 values within the entire sequence of the lower mantle may reach ten logarithmic units, varying from the IW buffer to the FMQ buffer values. The similarity between lower- and upper-mantle redox conditions supports whole mantle convection, as already suggested on the basis of nitrogen and carbon isotopic compositions in lower- and upper-mantle diamonds. The mechanisms responsible for redox differentiation in the lower mantle may include subduction of oxidized crustal material, mechanical separation of metallic phase(s) and silicate-oxide mineral assemblages enriched in ferric iron, as well as transfer of fused silicate-oxide material presumably also enriched in ferric iron through the mantle. © 2015 Elsevier B.V. Source

Palot M.,University Paris Diderot | Cartigny P.,University Paris Diderot | Harris J.W.,University of Glasgow | Kaminsky F.V.,KM Diamond Exploration Ltd. | Stachel T.,University of Alberta
Earth and Planetary Science Letters

Diamond, as the deepest sample available for study, provides a unique opportunity to sample and examine parts of the Earth's mantle not directly accessible. In order to provide further constraints on mantle convection and deep volatile cycles, we analysed nitrogen and carbon isotopes and nitrogen abundances in 133 diamonds from Juina (Brazil) and Kankan (Guinea). Host syngenetic inclusions within these diamonds indicate origins from the lithosphere, the asthenosphere-transition zone and the lower mantle.Juina and Kankan diamonds both display overall carbon isotopic compositions within the current upper mantle range but the δ13C signatures of diamonds from the asthenosphere-transition zone extend toward very negative and positive values, respectively. Two Kankan diamonds with both lower mantle and asthenosphere-transition zone inclusions (KK-45 and KK-83) are zoned in δ13C, and have signatures consistent with multiple growth steps likely within both the lower mantle and the asthenosphere-transition zone illustrating the transfer of material through the 670km seismic discontinuity.At a given locality, diamonds from the upper and the lower mantle show similar δ15N distributions with coinciding modes within the range defined by typical upper mantle samples, as one might expect for a well stirred reservoir resulting from whole mantle convection.Kankan diamonds KK-11 (lower mantle), KK-21 and KK-92 (both lithospheric) display the lowest δ15N values (-24.9%, -39.4% and -30.4%) ever measured in terrestrial samples, which we interpret as reflecting primordial heterogeneity preserved in an imperfectly mixed convective mantle.Our diamond data thus provide support for deeply rooted convection cells, together with the preservation of primordial volatiles in an imperfectly mixed convecting mantle, thereby reconciling the conflicting interpretations regarding mantle homogeneity derived from geochemical and geophysical studies. © 2012 Elsevier B.V. Source

Kaminsky F.,KM Diamond Exploration Ltd.
Earth-Science Reviews

Starting from the late 1980s, several groups of lower-mantle mineral inclusions in diamond have been found. Three associations were established among them: juvenile ultramafic, analogous to eclogitic, and carbonatitic. The juvenile ultramafic association strongly predominates, and it is composed of ferropericlase, MgSi-, CaSi- and CaTi-perovskites, stishovite, tetragonal almandine-pyrope phase (TAPP), and some others. The association analogous to the upper-mantle eclogitic association, formed from subducting lithosphere, comprises: majorite, CaSi-perovskite bearing compositional Eu anomalies, phase 'Egg' with a tetragonal structure, and stishovite. The carbonatitic association is represented by various carbonates, halides, and associated minerals. Some mineral associations (wüstite. +. periclase and native iron + iron carbides) are, possibly, related to the D″ layer at the core/mantle boundary. The mineralogical composition of the lower mantle is now understood to be more complex than had been suggested in theoretic and experimental works. The proportion of ferropericlase in the lower mantle is higher than it was suggested before, and its composition is more iron-rich (mg=0.36-0.90) as compared to experimental and theoretical data. Free silica (stishovite) is always present in lower-mantle associations, and a separate aluminous phase (TAPP) has been identified in several areas. These discrepancies suggest that the composition of the lower mantle differs to that of the upper-mantle, and experiments based solely on 'pyrolitic' compositions are not, therefore, applicable to the lower mantle. These data indicate a probability of an alternative to the CI-chondrite model of the Earth's formation, for example, an enstatite-chondrite model. © 2011 Elsevier B.V. Source

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