Iron carbide as a source of carbon for graphite and diamond formation under lithospheric mantle P-T parameters

Experimental modeling of natural carbide-involving reactions, implicated in the graphite and diamond formation and estimation of the iron carbide stability in the presence of S-bearing fluids, sulfide melts as well as mantle silicates and oxides, was performed using a multi-anvil high-pressure split-sphere apparatus. Experiments were carried out in the carbide-sulfur (Fe₃C-S), carbide-sulfur-oxide (Fe₃C-S-SiO₂-MgO) and carbide-sulfide (Fe₃C-FeS₂) systems, at pressure of 6.3 GPa, temperatures in the range of 900–1600 °C and run time of 18–40 h. During the interaction of cohenite with S-rich reduced fluid or pyrite at 900–1100 °C, extraction of carbon from carbide was realized, resulting in the formation of graphite in assemblage with pyrrhotite and cohenite. At higher temperatures complete reaction of cohenite with newly-formed sulfide melt was found to produce metal-sulfide melt with dissolved carbon (Fe₆₄S₂₇C₉ (1200 °C)–Fe₅₄S₄₀C₆ (1500 °C), at.%), which acted as a crystallization medium for graphite (1200–1600 °C) and diamond growth on seeds (1300–1600 °C). Reactions of cohenite and oxides with S-rich reduced fluid resulted in the formation of graphite in assemblage with highly ferrous orthopyroxene and pyrrhotite (900–1100 °C) or in hypersthene formation, as well as graphite crystallization and diamond growth on seeds in the Fe-S-C melt (1200–1600 °C). We show that the main processes of carbide interaction with S-rich fluid or sulfide melt are recrystallization of cohenite (900–1100 °C), extraction of carbon and iron in the sulfide melt, and graphite formation and diamond growth in the metal-sulfide melt with dissolved carbon. Our results evidence that iron carbide can act as carbon source in the processes of natural graphite and diamond formation under reduced mantle conditions. We experimentally demonstrate that cohenite in natural environments can be partially consumed in the reactions with mantle silicates and oxides, and is absolutely unstable in the presence of S-bearing reduced fluid or sulfide melt at temperatures higher than 1100 °C, under lithospheric mantle pressures.