Yuriy A. Litvin

Experimental investigation of the effect of metasomatism by carbonatic melt on the composition and structure of the deep mantle

Abstract The compositions of various transition-zone and lower-mantle phases and coexisting carbonatic melts were determined by exploratory melting experiments in chemically complex CO 2 -bearing systems at 20–24.5 GPa and 1600–2000 °C. The melts are highly ultramafic, enriched in K, Na, Ca, Fe, and Mg, and depleted in Al and Si. Melting experiments were also carried out with the compositions on the join Mg 2 SiO 4 –Na 2 CO 3 at 3.7 GPa and 1200–1600 °C. The solidus assemblage of MgCO 3 and Na 2 MgSiO 4 melts incongruently to produce forsterite and Na-rich melt. The new results and other recent studies in CO 2 -bearing systems suggest that carbonatic melt could be present, either transiently or permanently, in the whole Earths upper mantle and at least the uppermost lower mantle. Carbonate-melt metasomatism is recognized as a process that could have a major effect on the composition and structure of the deep mantle, and thus play an important role in its evolution. Due to the unique properties of the carbonatic melt, its circulation in an otherwise static mantle could be a more efficient process than the solid-state convection for maintaining equilibrium in most of the mantle not involved directly in plate tectonics.

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

Diamond formation in carbonate-silicate-sulfide-carbon melts : Raman-and IR-microspectroscopy

An experimental study on diamond nucleation and growth in a carbon solution in multicomponent carbonate, carbonate-silicate, silicate and sulfide melts was performed using a high-pressure toroidal anvil-with-hole cell with graphite resistive furnace. The boundary conditions of diamond spontaneous crystallization and seeded growth are determined for all the diamond growth media with the use of the PT diagram of diamond crystal growth. A density of diamond nucleation in the studied carbonate, carbonate-silicate, silicate, and sulfide melts with dissolved carbon exceeds (3.0–5.0) × 10 2 nuclei/mm 3 with a maximum around 1.0 × 10 5 nuclei/mm 3 at formation of polycrystalline “diamondite”. Raman spectra of quenched carbonate-carbon, carbonate-silicate-carbon, silicate-carbon and sulfide-carbon melts contain bands relating to the region of C-C stretching modes in diamond and graphite microphases. FTIR spectra show that nitrogen defects in the carbonate-synthetic diamond are characterized with the mixed Ib-IaA type and reveal a high nitrogen concentration (up to 850 ppm).

Ultrahigh potassium content in the clinopyroxene structure: an X-ray single-crystal study

Thecrystalstructuresandchemicalcompositionsoftwocrystalsofclinopyroxene(labelledK1andK2)synthesizedinthe model system CaMgSi2O6 - KAlSi2O6 at 7 GPa, have been investigated. Quantitative analysis leads to the following formulae: (Ca0.75K0.23Mg0.02)(Mg0.73Al0.27)(Si1.97Al0.03)O6.00and (Ca0.90K0.07Mg0.03)(Mg0.86Al0.14)(Si1.94Al0.06)O6.00, for K1 and K2 respectively. Latticeparametersare:a =9.803(2),b=8.985(2),c =5.263(1)A, =105.69(1)° forK1anda =9.744(1),b=8.904(2),c =5.273(1) A, b = 106.14(1)° for K2. The structures were refined to Rall = 1.16 and 2.02 %, respectively, using 982 independent reflections. Substitution of K for Ca causes strong modifications to the average structure, mainly a lengthening of the M2-O3C1 bond distance and a shortening of the T-O3A1 bond distance. The crystal K1 shows the largest volume for a M2 polyhedron ever reported for the clinopyroxene structure withdivalentcations inthissite. No evidence was found fora smallcation such as Mg coexistingwithCa in the M2 site that might act as a structural stabilizer for the accommodation of K in the clinopyroxene structure.

Synthetic hypersilicic Cl-bearing mica in the phlogopite-celadonite join : A multimethodical characterization of the missing link between di-and tri-octahedral micas at high pressures

Abstract A hypersilicic Cl-bearing mica was synthesized at 4 GPa and 1200-1250 °C, close to the solidus of the join diopside-jadeite-KCl, in association with diopside-jadeite pyroxene, K-rich aluminosilicate glass and/or sanidine and (K,Na)Cl. The mica shows a negative correlation between tetrahedral Si and octahedral (Al + Mg), suggesting an Al-celadonitic substitution (Si + VIAl + VI □ = IVAl + VIMg) and a chemical formula: K1.01(Mg2.45Al0.19□ 0.35)Σ=3(Si3.52Al0.48)Σ=4O10[(OH,O)1.66Cl0.34)]Σ=2. The presence of hydroxyl was confirmed by OH stretching modes at 3734 and 3606 cm-1 in the Raman spectra. Singlecrystal X-ray diffraction data provide the unit-cell parameters (space group C2/m, 1M polytype): a = 5.299(4), b = 9.167(3), c = 10.226(3) Å, β =100.06(4)°, V = 489.1(4) Å3. The structure refinement shows the presence of vacancies on the octahedral sites (15% for M1 and 6.5% for M2). Chlorine occupies a position about 0.5 Å from O4 with partial occupancy (0.39 apfu). Crystal-chemical mechanisms seem to govern chlorine incorporation in mica, since a large A site is necessary to locate the anion in the structure. A large A site results when the six-tetrahedra ring is hexagonal and the tetrahedral rotation angle α is 0°. Such a geometry is achieved either by increasing the annite component in biotite or by increasing the hypersilicic character of phlogopite through the Al-celadonite substitution. The present Si-rich mica shows a partial dioctahedral character due to the Al-celadonite substitution, which lowers the α angle and expands its stability field at high pressure. High aK₂O conditions, like in potassium-rich brine or potassic carbonatitic melts, increase the Alceladonite component in the phlogopite solid solution, explaining the association of Si-rich micas with inclusions of potassic liquids in kimberlitic diamonds.

MELTING OF JADEITE TO 16.5 GPA AND MELTING RELATIONS ON THE ENSTATITE-JADEITE JOIN

Abstract A split-sphere anvil apparatus (USSA-2000) was used to determine the melting curve of jadeite (NaAlSi2O6) from 2.4 to 16.5 GPa, and the melting relations on the enstatite (Mg2Si2O6)-jadeite join at 2.8, 4, and 6 GPa. The melting temperatures of jadeite increase more rapidly with pressure than the known melting temperatures of any other mantle material and exceed the melting temperatures of enstatite around 13 GPa. The melting curve of jadeite was fitted with the Simon equation, as follows: P (GPa) = 2.5 + 2.01[T(K)/1543)3.7− 1]. Melting on the enstatite-jadeite join is peritectic in the investigated pressure range, with the peritectic composition becoming more magnesium-rich with increasing pressure. The melting changes to azeotropic at pressures higher than 8 GPa. Thus, while the fractionation at lower pressures can proceed to the most sodium-rich compositions, the fractionation at pressures greater than 8 GPa is controlled by the composition of the minimum on the azeotrope. This could have important implications for the fractionation of the upper mantle, applicable to fractional crystallization in a magma ocean.

Compressibility of synthetic potassium-rich clinopyroxene: In-situ high-pressure single-crystal X-ray study

Abstract The crystal structure of a synthetic potassium-rich clinopyroxene, (Ca0.88K0.12)(Mg0.83Al0.17)(Si1.98 Al0.02)O6, was studied using high-pressure single-crystal X-ray diffraction methods. A four-pin diamond anvil cell with 4:1 methanol:ethanol pressure medium was used to achieve pressures to 9.72 GPa. Unit-cell data were measured at 17 pressures, and intensity data were collected at 6 pressures. Fitting the P-V data to the third-order Birch-Murnaghan equation of state yields V0 = 435.49(3) Å3, K0= 129(1) GPa, K’ = 2.7(3). Anisotropic compression was observed with unit strain axial ratios of 1:1.94:1.90. Unit-cell parameters decrease gradually as a function of pressure with axial compressibilities βb > βc ~ βa. They match those found for kosmochlor but are stiffer than those observed for synthetic diopside and hedenbergite. Compressibilities of the bond distances within the M2, M1, and T polyhedra show significant anisotropy. The incorporation of K into the clinopyroxene structure has little effect on its compressibility, although the concomitant substitution of Al in M1 from the K-Jd component reduces its compressibility. The K atom is softer than the M2 polyhedron and thus shrinks enough at high pressure to fit into the pyroxene structure.

Diamond nucleation and growth in sulfide-carbon melts: an experimental study at 6.0–7.1 GPa

Experimental study of diamond nucleation and growth in pyrrhotite-carbon melts was carried out at 6.0–7.1 GPa. The PT -diagram of diamond crystallization was constructed, and we found that the PT boundary conditions of diamond formation are limited by the diamond-graphite equilibrium line and the PT curve of the pyrrhotite-carbon eutectics. The fields of diamond spontaneous nucleation and mass crystallization as well as of diamond-seeded growth were determined. Diamond nucleation density up to 1.8 × 10 5 grains in 1 mm 3 and growth rate within the range of several mm/min to several μm/min were estimated. A possibility for formation of both monocrystalline and polycrystalline diamonds in the molten sulfide solvents was also demonstrated. Experimental data are in agreement with the melt-solution model of diamond formation in sulfide solvents of carbon. The experimental and literature data applied to the model of the mantle-derived diamond genesis demonstrate that sulfide melts with dissolved carbon are capable to form a limited mass of diamonds with specific mineralogical and physical properties.

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.

Mantle-Carbonatite Conception of Diamond and Associated Phases Genesis

The mantle-carbonatite conception of diamond genesis has been worked out as the result of consistent concordance of mineralogical and experimental evidence. This makes possible to have arrived to the construction of the generalized diagrams for compositions of diamond-producing melts under the upper mantle, transition zone and lower mantle conditions.