Conditions for diamond and graphite formation from iron carbide at the P-T parameters of lithospheric mantle

To estimate conditions for the stability of iron carbide under oxidation conditions 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 Fe3C–Fe2O3 and Fe3C–Fe2O3–MgO–SiO2 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 Fe3C–Fe2O3, graphite crystallizes in assemblage with Fe3 +-containing wüstite. Graphite forms from carbide carbon mainly by cohenite oxidation: Fe3C + 3Fe2O3 → 9FeO + C0 and FeO + Fe3C → (Fe2 +,Fe3 +)O + C0. 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 Fe3 +-containing wüstite + graphite/diamond assemblage. Interaction in the system Fe3C–Fe2O3–MgO–SiO2 results in the formation of Fe3 +− containing magnesiowüstite–olivine–graphite assemblage. At ≥ 1500 °C, two melts with contrasting fO2 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 temperatures and pressures leads to the formation of elemental carbon; graphite is produced from carbide carbon mainly by redox reactions of cohenite (or metal–carbon melt) with Fe2O3 and FeO as well as by interaction between metal–carbon and silicate–oxide melts. The results obtained suggest that cohenite can be 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 can be regarded as a process of the global carbon cycle.