Hydrogen-rich hydrothermal environments in the Hadean ocean inferred from serpentinization of komatiites at 300 °C and 500 bar

Serpentinization of Al-depleted and Al-undepleted komatiites (and olivine for comparison) was experimentally characterized under high-temperature and high-pressure conditions of 300 degrees C and 500 bar to evaluate the H-2 generation potential in komatiite-hosted hydrothermal systems in the early Earth. From the results, the steady-state H-2 concentrations of fluids were estimated to be approximately 20 and 0.05 mmol/kg during the serpentinization reactions for Al-depleted and Al-undepleted komatiites, respectively (60 mmol/kg in the case of olivine). The H-2 concentration of hydrothermal fluid generated from the serpentinization of Al-depleted komatiite is lower than that from olivine but is comparable to that of typical modern peridotite-hosted hydrothermal systems (similar to 16 mmol/kg). The relatively low H-2 concentration from Al-undepleted komatiite is similar to the levels in modern basalt-hosted hydrothermal fluids. Considering that the generation of Al-depleted komatiite melt requires a hotter mantle upwelling (plume) than the generation of Al-undepleted komatiite melt and that the temperature of the mantle has gradually decreased throughout Earth's history, Al-depleted komatiite may have constituted ultramafic volcanism in Hadean oceanic islands/plateaus. Furthermore, it seems unlikely that seafloor exposure of mantle peridotites occurred frequently in the Hadean because the oceanic crust of that time was presumably much thicker than the modern equivalent. Therefore, the serpentinization of Al-depleted komatiites may have been the main process that provided abundant H-2-rich seafloor hydrothermal environments in the Hadean ocean, which potentially acted as a nursery for the prebiotic chemical evolution and the emergence and early evolution of life on Earth.