Redox-State Dependent Water Partitioning and the Sequestration of Earth’s Deep Water in a Basal Magma Ocean

The distribution of water in Earth’s deep interior critically influences planetary differentiation and long-term geodynamics. However, the water content of the lower mantle is poorly constrained, as its estimation depends on complex, redox-sensitive partitioning processes under extreme pressure–temperature conditions during magma ocean crystallization.

To address this, we perform large-scale simulations of magma ocean crystallization using a machine learning interatomic potential—trained on first-principles data and specifically optimized for bridgmanite and silicate melt. This approach enables efficient sampling of a vast parameter space, including pressures, temperatures, melt water contents, and oxygen fugacities relevant to the early lower mantle. We use these simulations to quantify the water partition coefficient between bridgmanite and melt and to assess redox controls on iron partitioning between the mantle and core.

Our results reveal that water is highly incompatible in bridgmanite, with its partitioning strongly modulated by redox state. Numerical models based on our partition data indicate that upon lower mantle crystallization, a substantial portion of Earth’s deep water was sequestered into a long-lived basal magma ocean, leaving the overlying solid mantle relatively dry. Furthermore, we find that oxygen fugacity profiles remained largely stable throughout this process. Our analysis suggests Earth’s water was predominantly accreted during its early formation stages, with only a limited addition post mantle differentiation—a budget that could be supplied by a small mass fraction of a late veneer with CI chondrite–like composition.

These findings provide novel quantitative constraints on deep-Earth water storage and redox evolution, offering pivotal insights into the coupled chemical and thermal history of the early Earth and the dynamics of magma ocean crystallization.