In the present ocean hydrothermal systems, H2-rich alkaline fluids largely derive from lowtemperature (<~200 °C) water-rock interaction between ultramafic rocks (e.g., peridotite) and CO2depleted seawater . Beneath the primordial ocean floor with hotter mantle convection (Fig. ), an alternative mechanism of fluid basification was a high-temperature (>300 °C) hydrothermal alteration of basalt and komatiite under CO2-rich conditions . A few hundred mmol kg -1 of CO2 suffice to generate alkaline pH (>9) through anorthite decalcification and calcite precipitation . Additionally, hydrothermal fluids are expected to be basified in association with the phase separation between H2O and CO2 owing to the following reactions: