The geological record of H2 production in the Archean

The oxidation of iron from rocks during subaqueous alteration is a key source of the molecular hydrogen (H₂) used as an energy source by chemosynthetic organisms, which may represent some of the earliest forms of life on Earth. In the Archean, a potential source of ultramafic material available for serpentinisation reactions that release H₂ are komatiites. Komatiites are highly magnesian lavas, which contain evidence of extensive serpentinisation and magnetite (Fe²⁺Fe³⁺₂O₄) production close to the Archean seafloor. H₂ production in komatiitic compositions has been modelled and experimentally investigated; however, the natural rock record has remained unexplored. Here, we examine the geological evidence of H₂ production from the basaltic to komatiitic rock record held in Archean cratons. From the petrological investigation of thirty-eight samples of komatiitic basalt to komatiite, we identify the unique serpentinisation reaction responsible for H₂ production from these lithologies. With support from over 1100 bulk rock geochemical analyses, we directly quantify Fe³⁺ and therefore H₂ production of komatiites in the Archean. The chemical (high Mg) and physical (low viscosity flow) characteristics of komatiite flows allowed for extensive hydration and serpentinisation in oceanic plateaus, and therefore high H₂ production available to chemosynthetic early life.