Home – Home – Jornals – Russian Geology and Geophysics 2016 number 1

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.