Yu. V. Bataleva

High-pressure synthesis and characterization of diamond from an Mg–Si–C system

Diamond crystallization in the Mg–Si–C system has been studied at high-pressure high-temperature conditions of 7 GPa and 1500–1900 °C. The features of nucleation and growth of diamond from the carbon solution in the Mg–Si melt are established. The degree of the graphite-to-diamond transformation is found to depend significantly on the crystallization temperature. As opposed to the pure Mg–C system where the cubic morphology dominates, the octahedron with the antiskeletal structure of faces is the dominant form of growth in the Mg–Si–C system over the entire temperature range. The possibility of epitaxial growth of silicon carbide tetrahedral crystals on diamond upon their co-crystallization was noted. Synthesized diamonds are found to contain optically active silicon-vacancy (Si-V) centers and inactive substitutional silicon defects, giving rise to the 1.68 eV system in the photoluminescence spectra and an absorption peak at 1338 cm−1 in the infrared absorption spectra, respectively.

Interaction of iron carbide and sulfur under P–T conditions of the lithospheric mantle

Experimental studies were performed in the Fe3C–S system at P = 6.3 GPa, T = 900–1600°C, and t = 18–20 h. The study aimed to characterize the conditions of iron carbide stability in a reduced lithospheric mantle and to reveal the possibility of the formation of elemental carbon by the interaction of iron carbide and sulfur. It was found that the reaction at T < 1200°C proceeds with the formation of a pyrrhotite–graphite assemblage by the following scheme: 2Fe3C + 3S2 → 6FeS + 2C0. The crystallization of graphite at T < 1200°C is accompanied by the generation of sulfide and metal–sulfide melts and via 2Fe3C + 3S2 → 6[Fe–S(melt) + Fe–S–C(melt)] + 2C(graphite)0 reaction. Resulting from the carbon-generating reactions, not only graphite crystallized in sulfide or metal–sulfide melts, but the growth of diamond also takes place. The obtained data allow one to consider cohenite as a potential source of carbon in the processes of diamond and graphite crystallization under the conditions of a reduced lithospheric mantle. The interaction of iron carbide and sulfur under which carbon extraction proceeds may be one of possible processes of the global carbon cycle.

Graphite and diamond formation via the interaction of iron carbide and Fe,Ni-sulfide under mantle P–T parameters

Experimental research in the Fe3C–(Fe,Ni)S system was carried out. The objective of the investigation was to model the reactions of carbide–sulfide interaction related to graphite (diamond) formation in reduced lithosphere mantle domains. T ≤ 1200°C is the formation temperature of the Ni-cohenite + graphite assemblage coexisting with two immiscible melts such as sulfide (Fe60–Ni3–S37)L and metal–sulfide (Fe71–Ni7–S21–C1)L containing dissolved carbon. Т ≥ 1300°C is the generation temperature of a unified melt such as (Fe80–Ni6–S10–C4)L characterized by graphite crystallization and diamond growth. The extraction of carbide carbon during the interaction with the sulfide melt can be considered as one of the potential mechanisms of graphite and diamond formation in the reduced mantle.

Formation of the Fe,Mg-Silicates, Fe 0 , and Graphite (Diamond) Assemblage as a Result of Cohenite Oxidation under Lithospheric Mantle Conditions

Experimental studies in the Fe3C–SiO2–MgO system (P = 6.3 GPa, T = 1100–1500°C, t = 20–40 h) have been carried out. It has been established that carbide-oxide interaction resulted in the formation of Fe-orthopyroxene, graphite, wustite, and cohenite (1100 and 1200°C), as well as a Fe–C–O melt (1300–1500°C). The main processes occurring in the system at 1100 and 1200°C are the oxidation of cohenite, the extraction of carbon from carbide, and the crystallization of metastable graphite, as well as the formation of ferrosilicates. At T ≥ 1300°C, graphite crystallization and diamond growth occur as a result of the redox interaction of a predominantly metallic melt (Fe–C–O) with oxides and silicates. The carbide–oxide interaction studied can be considered as the basis for modeling a number of carbon-producing processes in the lithospheric mantle at fO2 values near the iron–wustite buffer.

The formation of graphite upon the interaction of subducted carbonates and sulfur with metal-bearing rocks of the lithospheric mantle

Experimental studies of the Fe0–(Mg, Ca)CO3–S system were carried out during 18–20 h at 6.3 GPa, 900–1400°C. It is shown that the major processes resulting in the formation of free carbon include reduction of carbonates upon redox interaction with Fe0 (or Fe3C), extraction of carbon from iron carbide upon interaction with a sulfur melt/fluid, and reduction of the carbonate melt by Fe–S and Fe⎯S–C melts. Reconstruction of the processes of graphite formation indicates that carbonates and iron carbide may be potential sources of carbon under the conditions of subduction, and participation of the sulfur melt/fluid may result in the formation of mantle sulfides.

Experimental modeling of the interaction of subducted carbonates and sulfur with mantle silicates

Experimental studies in the system Fe,Ni–olivine–carbonate–S (P = 6.3 GPa, T = 1050–1550°C, t = 40–60 h) aimed at modeling of the interaction of subducted carbonates and sulfur with rocks of the silicate mantle and at investigation of the likely mechanism of the formation of mantle sulfides were performed. It is shown that an association of olivine + orthopyroxene + magnesite + pyrite coexisting with a sulfur melt/fluid with dissolved Fe, Ni, and O is formed at T ≤ 1250°C. An association of low-Fe olivine, orthopyroxene, and magnesite and two immiscible melts of the carbonate and S–Fe–Ni–O compositions are formed at T ≥ 1350°C. It is shown that the reduced S-bearing fluids may transform silicates and carbonates, extract metals from the solid-phase matrix, and provide conditions for generation of sulfide melts.