Magma as a carrier of non-stoichiometric oxygen linking Earth's interior and atmosphere.

Magma generation and transport govern planetary evolution. Although oxygen is the most abundant anion in Earth's silicate interior, its role in magma is conventionally ignored, with magma oxygen contents assumed to be simply fixed by cation stoichiometry. Here we show, using high-pressure and high-temperature experiments supported by molecular dynamics simulations, that silicate magma can incorporate substantially more oxygen than permitted by traditional valence-state constraints. Excess oxygen acts as a previously unrecognized volatile component, increasing in concentration from near zero at 1 atm to > ∼5 wt.% above 5 GPa, more than five times the maximum oxygen increase attainable in Earth's solid mantle through iron redox changes. As oxygen-rich magmas ascend and decompress, their capacity to retain excess oxygen decreases, driving the release of O2. This behaviour provides a direct mechanism with the potential to link mantle melting to oxidation of Earth's shallow interior and atmosphere, and identifies magma as a dynamic reservoir in planetary oxygen cycles.