Geological evidence for high H2 production from komatiites in the Archaean

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 Archaean, a potential source of ultramafic material available for serpentinization reactions that release H2 are komatiites. Komatiites are highly magnesian lavas, which contain evidence of extensive serpentinization and magnetite (Fe2+Fe3+2O4) production close to the Archaean 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 Archaean cratons. From the petrological investigation of 38 samples of komatiitic basalt to komatiite, we identify the unique serpentinization reaction responsible for H2 production from these lithologies. With support from over 1,100 bulk rock geochemical analyses, we directly quantify Fe3+ and therefore H2 production of komatiites in the Archaean. The chemical (high Mg) and physical (low viscosity flow) characteristics of komatiite flows allowed for extensive hydration and serpentinization in oceanic plateaus and therefore high H2 production available to chemosynthetic early life.