H2 is a common reductive component in fluids from submarine hydrothermal systems. A wellknown geochemical mechanism of H2 production is serpentinization, a subseafloor water-rock interaction causing H2O reduction by coupling with Fe 2+ oxidation. Although a high concentration of CO2 suppresses H2 generation through low-temperature serpentinization (<~300 °C) owing to the precipitation of Fe 2+ -bearing carbonates, the high-temperature serpentinization (>~300 °C) generates extraordinarily high concentrations of H2 even in CO2-saturated seawater .