Yuri M. Borzdov

Diamond formation through carbonate-silicate interaction

Abstract Crystallization of diamond and graphite from the carbon component of magnesite, upon its decarbonation in reactions with coesite and enstatite at pressures of 6-7 GPa and temperatures of 1350-1800°C has been accomplished experimentally. In a series of experiments, diamond was obtained in association with enstatite, coesite, and magnesite, as well as with forsterite, enstatite, and magnesite. Octahedral diamond crystals with sizes up to 450 mm were studied by FTIR spectroscopy and were found to contain nitrogen and hydrogen, which are known as the most abundant impurities in natural type Ia diamonds. We found that growth of diamond on the cubic faces of seed crystals proceeds with formation of a cellular surface structure, which is similar to natural fibrous diamonds. The isotopic composition of synthesized diamonds (δ13C = -1.27‰) was determined to be close to that of the initial magnesite (δ13C = -0.2‰)

Mantle-slab interaction and redox mechanism of diamond formation.

Significance The primary question that we address in this study is what happens when a carbonate-bearing crust is subducted to depths where the Earth’s mantle is metal saturated. Subduction plays an important role in the evolution of the Earth’s interiors, but the mechanism of the interaction between the oxidized slab and reduced mantle remains unclear. Here we report the results of high-pressure redox-gradient experiments on the interaction between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle–slab boundary, and present mechanisms of diamond formation ahead of and behind the redox front. We demonstrate that the redox mechanism revealed in this study can explain the contrasting heterogeneity of natural diamonds on the composition of inclusions, carbon isotopic composition, and nitrogen impurity content. Subduction tectonics imposes an important role in the evolution of the interior of the Earth and its global carbon cycle; however, the mechanism of the mantle–slab interaction remains unclear. Here, we demonstrate the results of high-pressure redox-gradient experiments on the interactions between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle–slab boundary; thereby, we present mechanisms of diamond formation both ahead of and behind the redox front. It is determined that, at oxidized conditions, a low-temperature Ca-rich carbonate melt is generated. This melt acts as both the carbon source and crystallization medium for diamond, whereas at reduced conditions, diamond crystallizes only from the Fe-C melt. The redox mechanism revealed in this study is used to explain the contrasting heterogeneity of natural diamonds, as seen in the composition of inclusions, carbon isotopic composition, and nitrogen impurity content.

The system K2CO3-MgCO3 at 6 GPa and 900–1450 °C

Abstract Phase relations in the K2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900-1450 °C temperatures. Subsolidus assemblies comprise the fields K2CO3+K2Mg(CO3)2 and K2Mg(CO3)2+MgCO3 with the transition boundary near 50 mol% MgCO3 in the system. The K2CO3-K2Mg(CO3)2 eutectic is established at 1200 °C and 25 mol% MgCO3. Melting of K2CO3 occurs between 1400 and 1450 °C. We propose that K2Mg(CO3)2 disappears between 1200 and 1300 °C via congruent melting. Magnesite is observed as a subliquidus phase to temperatures in excess of 1300 °C. At 6 GPa, melting of the K2Mg(CO3)2+MgCO3 assemblage can be initiated either by heating to 1300 °C under “dry” conditions or by adding a certain amount of water at 900-1000 °C. Thus, the K2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the K- and Mg-rich carbonatite melts similar to those found as microinclusions in “fibrous” diamonds. The K2Mg(CO3)2 compound was studied using in situ X‑ray coupled with a DIA-type multi-anvil apparatus. At 6.5 GPa and 1000 °C, the structure of K2Mg(CO3)2 was found to be orthorhombic with lattice parameters a = 8.8898(7), b = 7.8673(7), and c = 5.0528(5), V = 353.39(4). No structure change was observed during pressure decrease down to 1 GPa. However, recovered K2Mg(CO3)2 exhibited a trigonal R3̅m structure previously established at ambient conditions.

Germanium: a new catalyst for diamond synthesis and a new optically active impurity in diamond.

Diamond attracts considerable attention as a versatile and technologically useful material. For many demanding applications, such as recently emerged quantum optics and sensing, it is important to develop new routes for fabrication of diamond containing defects with specific optical, electronic and magnetic properties. Here we report on successful synthesis of diamond from a germanium-carbon system at conditions of 7 GPa and 1,500–1,800 °C. Both spontaneously nucleated diamond crystals and diamond growth layers on seeds were produced in experiments with reaction time up to 60 h. We found that diamonds synthesized in the Ge-C system contain a new optical centre with a ZPL system at 2.059 eV, which is assigned to germanium impurities. Photoluminescence from this centre is dominated by zero-phonon optical transitions even at room temperature. Our results have widened the family of non-metallic elemental catalysts for diamond synthesis and demonstrated the creation of germanium-related optical centres in diamond.

Monitoring diamond crystal growth, a combined experimental and SIMS study

Detailed ion microprobe measurements have been made on two synthetic diamond crystals in order to investigate how variations in chemical and isotopic compositions are related to growth sectors and overall growth history. The crystals were grown by the metal catalyst technique under identical T - P conditions of 1450 °C and 5.5 GPa, but with different source nitrogen abundances. Measurements for carbon and nitrogen isotope compositions and nitrogen abundance were made in traverses across the crystal sectors, which included cubic sectors and octahedral sectors of both relatively rapid and relatively slow growth. In both crystals an early phase of growth dominated by falling δ 13 C and rising of N ppm is followed by an extensive phase of growth with moderately constant δ 13 C and gradually descending N ppm . The change from falling to stable δ 13 C ratios has been numerically modelled on the basis of the carbon isotope fractionation between the carbon solution in metal melt and the growing diamond in closed system; the stabilization of the diamond carbon isotope composition is achieved once a steady state is attained and diamond grows with the same carbon isotope composition as the graphite source. The decreasing N ppm values appear to be a product of Rayleigh fractionation. Carbon isotope compositions show little difference between different growth sectors, and δ 15 N values in any given sector show no change with time of growth. However, the nitrogen isotope compositions show major differences of ca. 30 ‰ between octahedral and cubic sectors. These differences are not related to growth rate or time of growth and appear to represent a consistent difference in N isotope adhesion between the two faces.

Melting and subsolidus phase relations in the system Na2CO3-MgCO3±H2O at 6 GPa and the stability of Na2Mg(CO3)2 in the upper mantle

Abstract Phase relations in the Na2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900-1400 °C temperatures. Sub-solidus assemblages are represented by Na2CO3+Na2Mg(CO3)2 and Na2Mg(CO3)2+MgCO3, with the transition boundary near 50 mol% MgCO3 in the system. The Na2CO3-Na2Mg(CO3)2 eutectic is established at 1200 °C and 29 mol% MgCO3. Melting of Na2CO3 occurs between 1350 and 1400 °C. We propose that Na2Mg(CO3)2 disappears between 1200 and 1250 °C via congruent melting. Magnesite remains as a liquidus phase above 1300 °C. Measurable amounts of Mg in Na2CO3 suggest an existence of MgCO3 solid-solutions in Na2CO3 at given experimental conditions. The maximum MgCO3solubility in Na-carbonate of about 9 mol% was established at 1100 and 1200 °C. The Na2CO3 and Na2Mg(CO3)2 compounds have been studied using in situ X‑ray coupled with a DIA-type multi-anvil apparatus. The studies showed that eitelite is a stable polymorph of Na2Mg(CO3)2 at least up to 6.6 GPa and 1000 °C. In contrast, natrite, γ-Na2CO3, is not stable at high pressure and is replaced by β-Na2CO3. The latter was found to be stable at pressures up to 11.7 GPa at 27 °C and up to 15.2 GPa at 1200 °C and temperatures at least up to 800 °C at 2.5 GPa and up to 1000 °C at 6.4 GPa. The X‑ray and Raman study of recovered samples showed that, under ambient conditions, β-Na2CO3 transforms back to γ-Na2CO3. Eitelite [Na2Mg(CO3)2] would be an important mineral controlling insipient melting in subducting slab and upwelling mantle. At 6 GPa, melting of the Na2Mg(CO3)2+MgCO3 assemblage can be initiated, either by heating to 1300 °C under “dry” conditions or at 900-1100 °C under hydrous conditions. Thus, the Na2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the Na- and Mg-rich carbonatite melts similar to those found as inclusions in olivines from kimberlites and the deepest known mantle rock samples-sheared peridotite xenoliths (190-230 km depth).

Phase relations in the system FeCO3-CaCO3 at 6 GPa and 900–1700 °C and its relation to the system CaCO3-FeCO3-MgCO3

Abstract The subsolidus and melting phase relations in the CaCO3-siderite system have been studied in multianvil experiments using graphite capsules at pressure of 6 GPa and temperatures of 900-1700 °C. At low temperatures, the presence of ankerite splits the system into two partial binaries: siderite + ankerite at 900 °C and ankerite + aragonite up to 1000 °C. Extrapolated solvus curves intersect near 50 mol% just below 900 °C. At 1100 and 1200 °C, the components appear to form single-phase solid solutions with space group symmetry R3c, while CaCO3 maintains aragonite structure up to 1600 °C and 6 GPa. The FeCO3 solubility in aragonite does not exceed 1.0 and 3.5 mol% at 900-1000 and 1600 °C, respectively. An increase of FeCO3 content above the solubility limit at T > 1000 °C, leads to composition-induced phase transition in CaCO3 from aragonite, Pmcn, to calcite, R3c, structure, i.e., the presence of FeCO3 widens the calcite stability field down to the P-T conditions of sub-cratonic mantle. The siderite-CaCO3 diagram resembles a minimum type of solid solutions. The melting loop for the FeCO3-CaCO3 join extends from 1580 °C (FeCO3) to 1670 °C (CaCO3) through a liquidus minimum near 1280 ± 20 °C and 56 ± 3 mol% CaCO3. At X(Ca) = 0-30 mol%, 6 GPa and 1500-1700 °C, siderite melts and dissolves incongruently according to the reaction: siderite = liquid + fluid. The apparent temperature and X(Ca) range of siderite incongruent dissolution would be determined by the solubility of molecular CO2 in (Fe,Ca)CO3 melt. The compositions of carbonate crystals and melts from the experiments in the low-alkali carbonated eclogite (Hammouda 2003; Yaxley and Brey 2004) and peridotite (Dasgupta and Hirschmann 2007; Brey et al. 2008) systems are broadly consistent with the topology of the melting loop in the CaCO3- MgCO3-FeCO3 system at 6 GPa pressure: a Ca-rich dolomite-ankerite melt coexists with Mg-Fe-calcite in eclogites at CaO/MgO > 1 and Mg-dolomite melt coexists with magnesite in peridotites at CaO/MgO < 1. However, in fact, the compositions of near solidus peridotite-derived melts and carbonates are more magnesian than predicted from the (Ca,Mg,Fe)CO3 phase relations.

Phase relationships in the system K2CO3-CaCO3 at 6 GPa and 900-1450 °C

Abstract Phase relations in the system K2CO3-CaCO3 have been studied in the compositional range, X(K2CO3), from 100 to 10 mol%, at 6.0 GPa and 900-1450 °C. At 900-950 °C, the system has three intermediate compounds: K6Ca2(CO3)5, K2Ca(CO3)2, and K2Ca3(CO3)4. The K2Ca(CO3)2 compound decomposes to the K6Ca2(CO3)5 + K2Ca3(CO3)4 assembly above 950 °C. The K6Ca2(CO3)5 and K2Ca3(CO3)4 compounds melt congruently slightly above 1200 and 1300 °C, respectively. The eutectics were established at 64 and 44 mol% near 1200 °C and at 23 mol% near 1300 °C. K2CO3 remains as a liquidus phase at 1300 °C and 75 mol% and melts at 1425 ± 20 °C. Aragonite remains as a liquidus phase at 1300 °C and 20 mol% and at 1400 °C and 10 mol%. CaCO3 solubility in K2CO3 and K2CO3 solubility in aragonite are below the detection limit (<0.5 mol%). Infiltration of subduction-derived K-rich Ca-Mg-Fe-carbonatite into the Fe0-saturated mantle causes the extraction of (Mg,Fe)CO3 components from the melt, which shifts its composition toward K-Ca-carbonatite. According to our data this melt can be stable at the P-T conditions of subcratonic lithosphere with geothermal gradient of 40 mW/m2 corresponding to temperature of 1200 °C at 6 GPa.

Sulfidation of silicate mantle by reduced S-bearing metasomatic fluids and melts

Sulfur, along with hydrogen, oxygen, and carbon, is one of the most common volatiles in magmatic mantle processes. As a redox-sensitive element, sulfur can have a direct influence on the redox evolution of mantle rocks, melts, and fluids, and participate in processes of mantle metasomatism. Modern concepts suggest that subduction processes play a key role in the global sulfur cycle. We report the results of the first high-pressure–high-temperature experiments in olivine-sulfur and olivine-pyrite systems aimed at modeling sulfidation processes in a silicate mantle with involvement of S-bearing fluids or melts and determining a potential mechanism of sulfide formation under deep subduction conditions. It was found that at the lower temperature stage of sulfidation, the partial recrystallization of olivine was accompanied by extraction of Fe and Ni into an S-bearing fluid and, finally, an olivine, orthopyroxene, and pyrite assemblage was formed; i.e., sulfide mineralization of an ultramafic rock occurred. At higher temperatures, the complete sulfidation and recrystallization of olivine resulted in the formation of forsterite and enstatite, containing inclusions of Ni-rich sulfide melt. Strong enrichment of S-bearing fluids in Fe and Ni led to a further sulfide melt generation. It is thus experimentally demonstrated that the influence of ephemeral S-bearing fluids on ultramafic mantle rocks results in an extraction of base metals from the solid-phase silicates, modifying their mineral and chemical compositions, and providing conditions for mobilization of an ore material in the form of sulfides at pressure-temperature conditions of the lithospheric mantle.

The system Na2CO3-FeCO3 at 6 GPa and its relation to the system Na2CO3-FeCO3-MgCO3

Abstract The phase relations in the Na2CO3-(Fe0.87Mn0.06Mg0.07)CO3 system have been studied in Kawai-type multi-anvil experiments using graphite capsules at 6.0 GPa and 900-1400 °C. Subsolidus assemblages comprise the stability fields of Na2CO3 + Na2Fe(CO3)2 and Na2Fe(CO3)2 + siderite with the transition boundary at X(Na2CO3) = 50 mol%. Intermediate Na2Fe(CO3)2 compound has rhombohedral R3̅ eitelite structure with cell parameters a = 4.9712(16), c = 16.569(4) Å, V = 354.61(22). The Na2CO3- Na2Fe(CO3)2 eutectic is established at 1000 °C and 66 mol% Na2CO3. Na2Fe(CO3)2 disappears between 1000 and 1100 °C via incongruent melting to siderite and a liquid containing about 55 mol% Na2CO3. Siderite remains a subliquidus phase at 1400 °C at X(Na2CO3) ≤ 30 mol%. The ternary Na2CO3-FeCO3-MgCO3 system can be built up from the corresponding binary systems: two systems with intermediate Na2(Mg,Fe)(CO3)2 phase, which melts congruently at the Mg-rich side and incongruently at the Fe-rich side, and the (Mg,Fe)CO3 system with complete solid solution. The phase relations suggest that the maximum contribution of FeCO3 component into the lowering solidus temperatures of Na-bearing carbonated mantle domains could not exceed several tens of degrees Celsius.