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 = 900-1600A degrees 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 < 1200A degrees C proceeds with the formation of a pyrrhotite-graphite assemblage by the following scheme: 2Fe(3)C + 3S(2) -> 6FeS + 2C(0). The crystallization of graphite at T < 1200A degrees C is accompanied by the generation of sulfide and metal-sulfide melts and via 2Fe(3)C + 3S(2) -> 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.