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Silversmit G.,Ghent University | Vekemans B.,Ghent University | Appel K.,German Electron Synchrotron | Schmitz S.,Goethe University Frankfurt | And 4 more authors.
Analytical Chemistry | Year: 2011

A stream of 1-20 μm sized mineral inclusions having the negative crystal shape of its host within an "ultra-deep" diamond from Rio Soriso (Juina area, Mato Grosso State, Brazil) has been studied with confocal μ-X-ray absorption near edge structure (μXANES) at the Fe K and Mn K edges. This technique allows the three-dimensional nondestructive speciation of the Fe and Mn containing minerals within the inclusion cloud. The observed Fe-rich inclusions were identified to be ferropericlase (Fe,Mg)O, hematite and a mixture of these two minerals. Confocal μ-X-ray fluorescence (μXRF) further showed that Ca-rich inclusions were present as well, which are spatially separated from or in close contact with the Fe-rich inclusions. The inclusions are aligned along a plane, which most likely represents a primary growth zone. In the close vicinity of the inclusions, carbon coated planar features are visible. The three-dimensional distribution indicates a likely fluid overprint along an open crack. Our results imply that an imposed negative diamond shape of an inclusion alone does not exclude epigenetic formation or intense late stage overprint. © 2011 American Chemical Society.

Ryabchikov I.D.,RAS Institute of Geology and Mineralogy | Kaminsky F.V.,KM Diamond Exploration Ltd.
Geochemistry International | Year: 2014

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.

Kaminsky F.V.,KM Diamond Exploration Ltd. | Wirth R.,Helmholtz Center Potsdam
Canadian Mineralogist | Year: 2011

Iron carbides in association with native iron, graphite, and magnetite were identified in a crystal of diamond from the Juina area, Brazil, that contains a series of other, deep-mantle mineral inclusions. Among the iron carbides, Fe3C, Fe2C ("chalypite"), and Fe 23C6 (haxonite) are present; the two latter phases are identified in the terrestrial environment for the first time. Some of the analyzed iron carbide grains contain 7.3-9.1 at.% N and are, in fact, nitrocarbide. We suggest, on the basis of the high-pressure mineral parageneses previously observed in the diamond and experimental data on the system Fe-C, that "chalypite" crystallized within a pressure interval of 50-130 GPa from an iron-carbon melt rich in nitrogen. Following crystallization, iron carbides and native iron were partially oxidized to magnetite, and encapsulated in diamond along with other high-pressure minerals. The finds of various iron carbides, some of which are rich in nitrogen, in lower-mantle diamond confirm a significant role of carbides and nitrogen in the Earth's interior.

Kaminsky F.V.,KM Diamond Exploration Ltd. | Kaminsky F.V.,Macquarie University | Sablukov S.M.,KM Diamond Exploration Ltd. | Belousova E.A.,Macquarie University | And 3 more authors.
Lithos | Year: 2010

The Juina diamond field, in the 1970-80s, was producing up to 5-6 million carats per year from rich placer deposits, but no economic primary deposits had been found in the area. In 2006-2007, Diagem Inc. discovered a group of diamondiferous kimberlitic pipes within the Chapadão Plateau (Chapadão, or Pandrea cluster), at the head of a drainage system which has produced most of the alluvial diamonds mined in the Juina area. Diamonds from placer deposits and newly discovered kimberlites are identical; they have super-deep origins from the upper-mantle and transition zone. Field observations and petrographic studies have identified crater-facies kimberlitic material at seven separate localities. Kimberlitic material is represented by tuffs, tuffisites and various epiclastic sediments containing chrome spinel, picroilmenite, manganoan ilmenite, zircon and diamond. The diamond grade varies from 0.2-1.8 ct/m 3. Chrome spinel has 30-61 wt.% Cr 2O 3. Picroilmenite contains 6-14 wt.% MgO and 0.2-4 wt.% Cr 2O 3. Manganoan ilmenite has less than 3 wt.% MgO and 0.38-1.41 wt.% MnO. The 176Hf/ 177Hf ratio in kimberlitic zircons is 0.028288-0.28295 with ε Hf = 5.9-8.3, and lies on the average kimberlite trend between depleted mantle and CHUR. The previously known barren and weakly diamondiferous kimberlites in the Juina area have ages of 79-80 Ma. In contrast, zircons from the newly discovered Chapadão kimberlites have a mean 206Pb/ 238U age of 93.6 ± 0.4 Ma, corresponding to a time of magmatic activity related to the opening of the southern part of the Atlantic Ocean. The most likely mechanism of the origin of kimberlitic magma is super-deep subduction process that initiated partial melting of zones in lower mantle with subsequent ascent of proto-kimberlitic magma. © 2009 Elsevier B.V. All rights reserved.

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 | Year: 2015

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.

Ryabchikov I.D.,RAS Institute of Geology and Mineralogy | Kaminsky F.V.,KM Diamond Exploration Ltd
Geology of Ore Deposits | Year: 2013

Thermodynamic calculations have shown that when a metallic phase arising due to ferroan ion disproportionation is contained in lower-mantle rocks, carbon occurs as iron carbide and the oxygen fugacity corresponds to the equilibrium of ferropericlase with Fe-Ni alloy. The typical values of oxygen fugacity in zones of diamond formation in the lower mantle lie between the iron-wüstite buffer and six logarithmic units above this level. The processes that proceed in the lower mantle give rise to variation of fO_2 within several orders of magnitude above the elevated fO_2 values, which are necessary for the formation of diamond, as compared with a common level typical of the lower mantle. The mechanisms responsible for redox differentiation in the lower mantle comprise the subduction of oxidized crustal material, mechanical separation of metallic phase and silicate-oxide mineral assemblage enriched in ferric ions, as well as transfer of fused silicate material presumably enriched in Fe3+ through the mantle. © 2013 Pleiades Publishing, Ltd.

Kaminsky F.,KM Diamond Exploration Ltd.
Earth-Science Reviews | Year: 2012

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.

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 | Year: 2012

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.

Kaminsky F.V.,KM Diamond Exploration Ltd. | WiRTh R.,Helmholtz Center Potsdam | MORAleS L.,Helmholtz Center Potsdam
Canadian Mineralogist | Year: 2013

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.

Pechnikov V.A.,Central Research Institute of Geological Prospecting for Base and Precious Metals TsNIGRI | Kaminsky F.V.,KM Diamond Exploration Ltd.
Canadian Mineralogist | Year: 2011

The diamond content of metamorphic rocks of the Kumdy-Kol and Barchi-Kol deposits in the Kokchetav Massif, northern Kazakhstan, varies widely from less than one carat to several hundred carats per tonne (cpt). Diamond mineralization is neither controlled by a special rock type(s), nor is its spatial distribution random, as could be expected from the UHP nature of the parent process. Diamond mineralization occurs in linear zones coinciding with tectonic discontinuities and associated metasomatic rocks. The morphology of diamondiferous zones follows branching linear patterns of metasomatic zones developed along the fault systems. Results of a factor analysis of whole-rock chemical compositions including the abundance of diamond confirm a close relationship of diamond content with metasomatic rocks, and demonstrate that factors characterizing metasomatic processes also reflect the diamond content in rocks from both the Kumdy-Kol and the Barchi-Kol deposits. However, the fluids that produced metasomatic alterations were different at these two localities. In the case of the former, predominantly hydrous fluid carried a moderate amount of CO2, whereas at Barchi-Kol, the fluid was rich in CO2. Crystals of diamond occur in intergranular interstices and within the grains of the rock-forming minerals. Their distribution is not random but tends to be associated with fractures in rocks and rock-forming minerals, including secondary minerals. Diamond grains form chains and clusters along the fractures. Diamondiferous clusters are morphologically variable: five to ten microcrystals may form 2D patterns; 3D botryoidal aggregates are observed as well. These data confirm the hypothesis favoring a crustal fluid - metasomatic origin of diamond in the metamorphic sequence of the Kokchetav Massif.

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