Contrasted Effect of Spinel and Pyroxene on Molecular Hydrogen (H2) Production during Serpentinization of Olivine

Serpentinization produces molecular hydrogen (H2) and hydrocarbons that can feed the colonies of microbes in hydrothermal vent fields, and therefore serpentinization may be important for the origins of life. However, the mechanisms that control molecular hydrogen (H2) production during serpentinization remain poorly understood. Here the effect of pyroxene minerals and spinel on molecular hydrogen (H2) generation during serpentinization is experimentally studied at 311–500 °C and 3.0 kbar, where olivine, individually and in combinations with pyroxene and/or spinel, is reacted with saline solutions (0.5 M NaCl). The results show a contrasting influence of spinel and pyroxeneon molecular hydrogen (H2) production. At 311 °C and 3.0 kbar, spinel promotes H2 generation by around two times, and pyroxene minerals decrease molecular hydrogen (H2) production by around one order of magnitude. Spinel leaches aluminum (Al) and chromium (Cr) during hydrothermal alteration, and Al and Cr enhance molecular hydrogen (H2) production. This is confirmed by performing experiments on the serpentinization of olivine with the addition of Al2O3 or Cr2O3 powders, and an increase in molecular hydrogen (H2) production was observed. Pyroxene minerals, however, not only leach Al and Cr, but they also release silica (SiO2) during serpentinization. The sharp decline in molecular hydrogen (H2) production in experiments with a combination of olivine and pyroxene minerals may be attributed to releases of silica from pyroxene minerals. With increasing temperatures (e.g., 400–500 °C), the effect of spinel and pyroxene minerals on molecular hydrogen (H2) production is much less significant, which is possibly related tothe sluggish kinetics of olivine serpentinization under these T-P conditions. In natural geological settings, olivine is commonly associated with spinel and pyroxene, and molecular hydrogen (H2) during serpentinization can be greatly affected.