Links between the Archean hydrosphere and Earth's interior

Earth's long-term habitability depends on cycling of surface volatiles into and out of the mantle, yet when this cycling began and how the early hydrosphere became connected to Earth's interior remain debated. Komatiites are ultramafic magmas produced mostly during the Archean by deep, high-degree mantle melting, providing a sensitive archive of early mantle composition. Here we present high-precision K isotope data for komatiites from ten localities. Our data show that Archean komatiites from three localities record heavy potassium isotope compositions, well above values typical of the mantle and most surface reservoirs. This signal is best explained by metasomatism of mantle domains by potassium-rich fluids released from subducted, water-altered oceanic lithosphere, later sampled by komatiite magmas. The data indicate that surface-derived water was transferred into mantle reservoirs by at least 3.6 billion years ago. Komatiites thus link the Archean hydrosphere to Earth's interior and record early operation of subduction-style volatile recycling.