Natural basalt and komatiite contain Mo as a trace but ubiquitous component at the reported concentrations up to 18.7 ppm . Importantly, komatiite was substantially carbonatized through our scCO2 experiment (SI Appendix, Fig. ). Because Mo does not form a stable carbonate mineral, hydrothermal processes of komatiite in subseafloor scCO2 environments likely extract the rockcontaining Mo into the fluids. Once the released Mo is sulfurized to form MoS2, the surface can initiate the synthetic steps to methanethiol, as demonstrated in the present study (Fig. ).