The constant oxidation state of Earth's mantle since the Hadean.

Determining the evolutionary history of mantle oxygen fugacity (fo 2 ) is crucial, as it controls the fo 2 of mantle-derived melts and regulates atmospheric composition through volcanic outgassing. However, the evolution of mantle fo 2 remains controversial. Here, we present a comprehensive dataset of plume-derived komatiites, picrites, and ambient mantle-derived (meta)basalts, spanning from ~3.8 Ga to the present, to investigate mantle thermal and redox states evolution. Our results indicate that fo 2 of both mantle plume-derived and ambient mantle-derived melts was lower during the Archean compared to the post-Archean period. This increase in the fo 2 of mantle-derived melts over time correlates with decreases in mantle potential temperature and melting depth. By normalizing fo 2 to a constant reference pressure (potential oxygen fugacity), we show that the fo 2 of both the mantle plume and ambient upper mantle has remained constant since the Hadean. These findings suggest that secular mantle cooling reduced melting depth, increasing the fo 2 of mantle-derived melts and contributing to atmospheric oxygenation.