Pyrrhotite was also identified on komatiite after its 7-day exposure to H2S under scCO2 (SI Appendix, Fig. ; again, H2 was excluded from the starting material). The resultant H2 from 500 mg of komatiite was 22 μmol, whereas the amount of H2 generated in the absence of H2S was only 2.5 μmol. Thus, komatiite and mackinawite in H2S-rich environments can serve as the source of H2