A. V. Bobrov

Parental media of natural diamonds and primary mineral inclusions in them: Evidence from physicochemical experiment

A generalized diagram was constructed for the compositions of multicomponent heterogeneous parental media for diamonds of kimberlite deposits on the basis of the mantle carbonatite concept of diamond genesis. The boundary compositions on the diagram of the parental medium are defined by the components of minerals of the peridotite and eclogite parageneses, mantle carbonatites, carbon, and the components of volatile compounds of the C-O-H system and accessory phases, both soluble (chlorides, phosphates, and others) and insoluble (sulfides and others) in carbonate-silicate melts. This corresponds to the compositions of minerals, melts, and volatile components from primary inclusions in natural diamonds, as well as experimental estimations of their phase relations. Growth media for most natural diamonds are dominated by completely miscible carbonate-silicate melts with dissolved elemental carbon. The boundary compositions for diamond formation (concentration barriers of diamond nucleation) in the cases of peridotite-carbonate and eclogite-carbonate melts correspond to 30 wt % peridotite and 35 wt % eclogite; i.e., they lie in the carbonatite concentration range. Phase relations were experimentally investigated at 7 GPa for the melting of the multicomponent heterogeneous system eclogite-carbonatite-sulfide-diamond with a composition close to the parental medium under the conditions of the eclogite paragenesis. As a result, “the diagram of syngenesis” was constructed for diamond, as well as paragenetic and xenogenic mineral phases. Curves of diamond solubility in completely miscible carbonate-silicate and sulfide melts and their relationships with the boundaries of the fields of carbonate-silicate and sulfide phases were determined. This allowed us to establish the physicochemical mechanism of natural diamond formation and the P-T conditions of formation of paragenetic silicate and carbonate minerals and coexistence of xenogenic sulfide minerals and melts. Physicochemical conditions of the capture of paragenetic and xenogenic phases by growing diamonds were revealed. Based on the mantle carbonatite concept of diamond genesis and experimental data, a genetic classification of primary inclusions in natural diamond was proposed. The phase diagrams of syngenesis of diamond, paragenetic, and xenogenic phases provide a basis for the analysis of the physicochemical history of diamond formation in carbonatite magma chambers and allow us to approach the formation of such chambers in the mantle material of the Earth.

Crystal chemistry of sodium in the Earth’s interior: The structure of Na2MgSi5O12 synthesized at 17.5 GPa and 1700 °C

Abstract The crystal structure and chemical composition of a crystal of Na2MgSi5O12 garnet synthesized in the model system Mg3Al2Si3O12-Na2MgSi5O12 at 17.5 GPa and 1700 °C have been investigated. Quantitative analysis leads to the following formula: Na1.98Mg1.00Si5.01O12. Na2MgSi5O12 garnet was found to be tetragonal, space group I41/acd, with lattice parameters a = 11.3966(6), c = 11.3369(5) Å, V = 1472.5(1) Å3. The structure was refined to R = 5.13% using 771 independent reflections. Sodium and Mg are disordered at the X sites (with a mean bond distance of 2.308 Å for both the sites), whereas Si is ordered at both the Y (mean: 1.793 Å) and Z sites (means: 1.630 and 1.624 Å). Na-bearing majoritic garnet may be an important potential sodium concentrator in the lower parts of the upper mantle and transition zone. The successful synthesis of the Na2MgSi5O12 end-member and its structural characterization is of key importance because the study of its thermodynamic constants combined with the data of computer modeling provides new constraints on thermobarometry of majorite garnet assemblages

Conditions of magmatic crystallization of Na-bearing majoritic garnets in the earth mantle: Evidence from experimental and natural data

Results of experimental study at 7.0–8.5 GPa and 1300–1900°C of the systems pyrope Mg3Al2Si3O12 (Prp)-Na2MgSi5O12 (NaGrt) modeling solid solutions of Na-bearing garnets, Prp-jadeite NaAlSi2O6 (Jd) in a simplified mode demonstrating melting relations of Na-rich eclogite, and Prp-Na2CO3 are presented. Prp-Na2MgSi5O12 join is a pseudobinary that results from the decomposition of NaGrt on to coesite and Na-pyroxene. Synthesized garnets are characterized by Na admixture (>0.32 wt % Na2O) and excess Si (3.05–3.15 f.u.). Maximal Na2O concentrations (1.5 wt % Na2O) are reached on the solidus of the system at 8.5 GPa. Clear correlation between Na and Si was established in synthesized garnets; this provides evidence for heterovalent isomorphism of the Mg + Al → Na + Si type with the appearance of Na2MgSi5O12 component as a mechanism of such garnet formation. The Prp-Jd join is also pseudobinary that results from the formation of two series of solid solutions: (1) garnet (Prp-NaGrt-majorite) and (2) pyroxene (Jd-clinoenstatite-Eskola molecule), and the appearance of kyanite at the solidus of the system, where garnets with the highest Na2O contents (>0.8 wt %) are formed. In spite of quite a wide field of garnet crystallization (20–100 mol % Prp), garnets with significant sodium concentration (>0.3 wt % Na2O) are formed in a Jd-rich part of the system (20–50 mol % Prp). In the Prp-Na2CO3 system at 8.5 GPa garnet crystallizes in a wide range of starting compositions as a liquidus mineral containing up to 0.9 wt % Na2O. Our experiments demonstrate that melt alkalinity, as well as PT-parameters control the crystallization of Na-bearing majoritic garnets. The results obtained provide evidence for the fact that the majority of natural diamonds with inclusions of Na-bearing majoritic garnets containing <0.4 wt % Na2O were formed in alkaline silicate (carbonate-silicate) melts at a pressure of <7 GPa. Only a small portion of garnets with higher sodium concentrations (>1 wt % Na2O) could be formed at a pressure of >8.5 GPa. 1 This article was translated by the authors.

Incorporation of Fe3+ in phase-X, A2−xM2Si2O7Hx, a potential high-pressure K-rich hydrous silicate in the mantle

Abstract Phase-X, a potential sink for K in the mantle, is a synthetic hydrous K-rich silicate formed by the breakdown of K-amphibole at high pressure. It has the general formula A2-xM2Si2O7Hx where A = K, Na, Ca, □ (vacancy), and M = Mg, Al, or Cr. No other isomorphic substitutions, either for the A or the M site, were reported for such a synthetic compound. Here we report the crystal structure and chemical composition of a crystal of phase-X containing large amounts of trivalent Fe. This crystal was synthesized in the model system garnet lherzolite-K2CO3 at lower pressure (P = 7 GPa) and higher temperature (T = 1450-1650°C) with respect to the stability range reported in the literature (i.e. P = 9-17 GPa and T = 1150-1400°C). Quantitative analysis led to the following formula: (K1.307Na0.015Ca0.007)Σ = 1.329(Mg1.504Fe3+0.373Al0.053Ti4+0.004Mn2+0.001)Σ = 1.935Si2O7.00H0.360. The lattice parameters (hexagonal setting) are: a = 5.005(1), c = 13.148(2) Å, V = 285.23(9) Å3, and Z = 2. The structure was refined in space group P63cm to R = 5.06% using 199 independent reflections and consists of MO6 octahedra layers stacked along the c axis and linked together by Si2O7 groups. The Si2O7 groups form pillars in the layer that contains A atoms in the cavities between the pillars. The Raman spectrum in the OH-stretching region indicates the existence of two different OH environments. However, the position of H could not be determined. The substitution of Fe3+ for Mg shortens octahedral bond distances. In addition, the entry of Fe3+ in M induces geometrical changes to the adjacent A site. The crystal-chemical characteristics are compared with published data on synthetic Fe-free phase-X.

Structural and chemical characterization of Mg[(Cr,Mg)(Si,Mg)]O4, a new post-spinel phase with sixfold-coordinated silicon

Abstract The crystal structure and chemical composition of a crystal of Mg(Mg,Cr,Si)2O4 post-spinel phase synthesized in the model system MgCr2O4-Mg2SiO4 at 16 GPa and 1600 °C have been investigated. The compound was found to crystallize with a distorted orthorhombic calcium-titanate (CaTi2O4) structure type, space group Cmc21, with lattice parameters a = 2.8482(1), b = 9.4592(5), c = 9.6353(5) Å, V = 259.59(1) Å3, and Z = 4. The structure was refined to R1 = 0.018 using 345 independent reflections. The loss of the inversion center is due to the ordering of cations at the octahedral sites: Cr is mainly hosted at the M1 site, whereas Si at the M2 site. Such an ordered distribution induces a distortion thus provoking a change in coordination of Mg, which becomes sevenfold-coordinated instead of the usual eightfold coordination observed in post-spinel phases. Electron microprobe analysis gave the Mg[(Cr0.792Mg0.208) (Si0.603Mg0.397)]O4 stoichiometry for the studied phase. The successful synthesis of this phase can provide new constraints on thermobarometry of wadsleyite/ringwoodite-bearing assemblages.

Chromium solubility in perovskite at high pressure: The structure of (Mg1–xCrx)(Si1–xCrx)O3 (with x = 0.07) synthesized at 23 GPa and 1600 ºC

Abstract The crystal structure and chemical composition of a crystal of (Mg1-xCrx)(Si1-xCrx)O3 perovskite (with x = 0.07) synthesized in the model system Mg3Cr2Si3O12-Mg4Si4O12 at 23 GPa and 1600 °C have been investigated. The compound was found to be orthorhombic, space group Pbnm, with lattice parameters a = 4.8213(5), b = 4.9368(6), c = 6.9132(8) Å, V = 164.55(3) Å3. The structure was refined to R = 0.046 using 473 independent reflections. Chromium was found to substitute for both Mg at the dodecahedral X site (with a mean bond distance of 2.187 Å) and Si at the octahedral Y site (mean: 1.814 Å), according to the reaction Mg2+ + Si4+ = 2Cr3+. Such substitutions cause a shortening of the and a lengthening of the distances with respect to the values typically observed for pure MgSiO3 perovskite. Although high Cr-contents are not considered in the pyrolite model, Cr-bearing perovskite may be an important host for chromium in the lower mantle. The successful synthesis of perovskite with high-Cr content and its structural characterization are of key importance because the study of its thermodynamic constants combined with the data on phase relations in the lower-mantle systems can provide new constraints on thermobarometry of perovskite-bearing assemblages.

Mineral equilibria of diamond-forming carbonate-silicate systems

Based on experimental and mineralogical data, the model of mantle carbonate-silicate (carbonatite) melts as dominating parental media for natural diamonds was substantiated. It was demonstrated that the compositions of silicate constituents of parental melts were variable and saturated with respect to mantle rocks, namely pyrope peridotite, garnet pyroxenite, and eclogite. Based on concentration contributions and role in diamond genesis, major (carbonate and silicate) and minor (admixture) components were distinguished. The latter components may be both soluble (oxides, phosphates, chlorides, carbon dioxide, and water) and insoluble (sulfides, metals, and carbides) in silicate-carbonate melts. This paper presents the results of a study of diamond crystallization in multicomponent melts of variable composition with carbonate components (K2CO3, CaCO3 · MgCO3, and K-Na-Ca-Mg-Fe carbonatite) and silicate components represented by model peridotite (60 wt % olivine, 16 wt % orthopyroxene, 12 wt % clinopyroxene, and 12 wt % garnet) and eclogite (50 wt % garnet and 50 wt % clinopyroxene). Carbonate-silicate melts behave like completely miscible liquid phases in experiments performed under the P-T conditions of diamond stability. The concentration barriers of diamond nucleation (CBDN) in melts with variable proportions of silicates and carbonates were determined at 8.5 GPa. In the peridotite system with K2CO3, CaCO3 · MgCO3, and carbonatite, they correspond to 30, 25, and 30 wt % silicates, respectively, and in the eclogite system, the CBDN is shifted to 45, 30, and 35 wt % silicates. In the silicate-carbonate melts with higher silicate contents, diamond grows on seeds, which is accompanied by the crystallization of thermodynamically unstable graphite. At P = 7.0 GPa and T = 1200−1800°C, we studied and constructed phase diagrams for the multicomponent peridotite-carbonate and eclogite-carbonate systems as a physicochemical basis for revealing the syngenetic relationships between diamond and its silicate (olivine, ortho- and clinopyroxene, and garnet) and carbonate (aragonite and magnesite) inclusions depending on the physicochemical conditions of growth media. The results obtained allowed us to reconstruct the evolution of diamond-forming systems. The experiments revealed similarity between the compositions of synthetic silicate minerals and inclusions in natural diamonds (high concentrations of Na in garnets and K in clinopyroxenes). It was experimentally demonstrated that the formation of Na-bearing majoritic garnets is controlled by the P-T parameters and melt alkalinity. Diamonds with inclusions of such garnets can be formed in alkalic carbonate-silicate (aluminosilicate) melts. A mechanism was suggested for sodic end-member dissolution in majoritic garnets, and garnet with the composition Na2MgSi5O12 and tetragonal symmetry was synthesized for the first time.

High-pressure synthesis of skiagite-majorite garnet and investigation of its crystal structure

Abstract Skiagite-rich garnet was synthesized as single crystals at 9.5 GPa and 1100 °C using a multi-anvil apparatus. The crystal structure [cubic, space group Ia3̅d, a = 11.7511(2) Å, V = 1622.69(5) Å3, Dcalc = 4.4931 g/cm3] was investigated using single-crystal synchrotron X‑ray diffraction. Synchrotron Mössbauer source spectroscopy revealed that Fe2+ and Fe3+ predominantly occupy dodecahedral (X) and octahedral (Y) sites, respectively, as expected for the garnet structure, and confirmed independently using nuclear forward scattering. Single-crystal X‑ray diffraction suggests the structural formula of the skiagite-rich garnet to be Fe32+(Fe2+0.234(2)Fe3+1.532(1)Si4+0.234(2))(SiO4)3, in agreement with electron microprobe chemical analysis. The formula is consistent with X‑ray absorption near-edge structure spectra. The occurrence of Si and Fe2+ in the octahedral Y-site indicates the synthesized garnet to be a solid solution of end-member skiagite with ~23 mol% of the Fe-majorite end-member Fe32+(Fe2+Si4+)(SiO4)3.

Garnet-pyroxene barometry for the assemblages with Na-bearing majoritic garnet

The majority of diamonds from kimberlite and lamproite pipes were formed within a quite narrow range of 140‐220 km, as evident from calculations of pressures and temperatures for the formation of their silicate inclusions [1]. The upper boundary of this range corresponds to the graphite‐diamond transition in the Earth’s mantle. The lower boundary is limited by the maximal lithosphere thickness that is typical only for the old parts of continents (cratons). Mineral inclusions in ultradeep diamonds comprising majoritic garnet, magnesiowustite, and phases of CaSiO 3 and MgSiO 3 compositions (presumably, with a perovskite structure), as well as tetragonal phase of pyrope-almandine composition (TAPP) [2] are direct samples from mantle depths, which allow us to test models of the Earth’s deep structure based on geophysical and experimental investigations. Of special interest among all the ultrahigh-pressure phases is majoritic garnet, because its composition and parageneses are most indicative for estimation of physicochemical conditions of the formation of deep mineral assemblages. According to the experimental data, pyroxene component starts to dissolve in garnet at a depth of ~250 km, resulting in the increase of Si and Na and the decrease of Al concentrations in this mineral according to the scheme Na + + Si 4+ = Mg 2+ + Al 3+ [3]. Experiments performed for mantle eclogite systems in a wide PT range using a high-pressure multianvil apparatus [4‐8] confirmed the influence of pressure on the composition of garnets (mainly Na admixture and excess Si contents) obtained in experiments (Fig. 1). In our study, we have analyzed the experimental data in the multicomponent Na 2 O–CaO–MgO–FeO–Al 2 O 3 – SiO 2 system and suggested a model of the garnet‐clinopyroxene barometer based on the change of Na concentration in garnet. In the opinion of Haggerty and Sauter [9], jadeiterich pyroxene lamellae in garnet are related to the decomposition of high-pressure Na-bearing garnets. The increase of pressure may lead to the opposite process of pyroxene microlite dissolution in garnet. This fact provides an explanation for the increase in the Si content up to 3.3 f.u. and a decrease in the Al content down to 1.3 f.u., as observed in garnets from the Monastery kimberlite pipe. According to the experimental data, such garnets were formed at a pressure of >10 GPa. Therefore, Na incorporation in garnet may be considered as a result of jadeite dissolution accompanied by Na and Si ingress according to the reaction [1]

Chromium-bearing phases in the Earth’s mantle: Evidence from experiments in the Mg2SiO4–MgCr2O4 system at 10–24 GPa and 1600 °C

Abstract Phase relations in the system Mg2SiO4–MgCr2O4 were studied at 10–24 GPa and 1600 °C using a high-pressure Kawai-type multi-anvil apparatus. We investigated the full range of starting compositions for the forsterite-magnesiochromite system to derive a P–X phase diagram and synthesize chromium-bearing phases, such as garnet, wadsleyite, ringwoodite, and bridgmanite of a wide compositional range. Samples synthesized at 10 GPa contain olivine with small chromium content and magnesiochromite. Mg2SiO4 wadsleyite is characterized by the pressure-dependent higher chromium solubility (up to 7.4 wt% Cr2O3). The maximal solubility of chromium in ringwoodite in the studied system (~ 18.5 wt% Cr2O3) was detected at P = 23 GPa, which is close to the upper boundary of the ringwoodite stability. Addition of chromium to the system moves the boundaries of olivine/wadsleyite and wadsleyite/ringwoodite phase transformations to lower pressures. Our experiments simulate Cr-rich phase assemblages found as inclusions in diamonds, mantle xenoliths, and UHP podiform chromitites.