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Russian Geology and Geophysics

2016 year, number 1


Yu.V. Bataleva1,2, Yu.N. Palyanov1,2, Yu.M. Borzdov1,2, O.A. Bayukov3, N.V. Sobolev1,2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
3L.V. Kirensky Institute of Physics, Siberian Branch of the Russian Academy of Sciences, Akademgorodok, Krasnoyarsk, 660036, Russia
Keywords: Iron carbide, wГјstite, graphite, diamond, oxides, olivine, subduction, redox interaction, lithospheric mantle, high-pressure experiment


To estimate conditions for the stability of iron carbide under oxidation and to assess the possibility of formation of elemental carbon by interaction between iron carbide and oxides, experimental modeling of redox interaction in the systems Fe 3C-Fe 2O 3 and Fe 3C-Fe 2O 3-MgO-SiO 2 was carried out on a “split-sphere” high-pressure multianvil apparatus at 6.3 GPa and 900-1600 °C for 18-20 h. During carbide-oxide interaction in the system Fe 3C-Fe 2O 3, graphite crystallizes in assemblage with Fe 3+-containing wüstite. Graphite forms from carbide carbon mainly by cohenite oxidation: Fe 3C + 3Fe 2O 3 → 9FeO + C 0 and FeO + Fe 3C → (Fe 2+, Fe 3+)O + + C 0. At above-solidus temperatures (≥1400 °C), when metal-carbon melt is oxidized by wüstite, graphite and diamond crystallize by the redox mechanism and form the Fe 3+-containing wüstite + graphite/diamond assemblage. Interaction in the system Fe 3C-Fe 2O 3-MgO-SiO 2 results in the Fe 3+-containing magnesiowüstite-olivine-graphite assemblage. At ≥1500 °C, two melts with contrasting f O2 values are generated: metal-carbon and silicate-oxide; their redox interaction leads to graphite crystallization and diamond growth. Under oxidation conditions, iron carbide is unstable in the presence of iron, silicon, and magnesium oxides, even at low temperatures. Iron carbide-oxide interaction at the mantle temperature and pressure leads to the formation of elemental carbon; graphite is produced from carbide carbon mainly by redox reactions of cohenite (or metal-carbon melt) with Fe 2O 3 and FeO as well as by interaction between metal-carbon and silicate-oxide melts. The results obtained suggest that cohenite is a potential source of carbon during graphite (diamond) formation in the lithospheric mantle and the interaction of iron carbide with iron, silicon, and magnesium oxides, during which carbon is extracted, is a process of the global carbon cycle.