Experimental studies were performed in the Fe3C–S system at P = 6.3 GPa, T = 900–1600°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 < 1200°C proceeds with the formation of a pyrrhotite–graphite assemblage by the following scheme: 2Fe3C + 3S2 → 6FeS + 2C0. The crystallization of graphite at T < 1200°C is accompanied by the generation of sulfide and metal–sulfide melts and via 2Fe3C + 3S2 → 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.