High-Pressure Basalt Experiments Show Magma Holds Over 5% Excess Oxygen at 7 GPa

Researchers at HPSTAR and collaborators used piston-cylinder and cubic presses to subject synthetic basalt plus PtO2 to mantle-like conditions, then quenched melts to glass for electron microprobe and volatile analysis. Excess oxygen was calculated after subtracting oxygen bound in major elements, water, and CO2, yielding a clear pressure-dependent increase that reached >5 wt% at 7 GPa. The design isolates the effect of oxygen fugacity on melt structure without relying on natural samples whose redox history is ambiguous.

Prior models treated oxygen in magma as fixed by cation charge balance, missing an independent volatile reservoir. This finding tightens links between deep-mantle oxidation state, arc volcanism, and the Great Oxidation Event by implying that episodic degassing from oxidized melts could have supplied an unrecognized O2 flux billions of years ago. It also reframes ore-genesis models, because excess oxygen in ascending magmas would alter metal speciation and sulfide stability in ways that affect porphyry copper and iron-oxide deposits.

The principal limitation is that experiments used synthetic compositions and added PtO2 rather than natural mantle fluids; natural confirmation requires in-situ measurements or xenolith studies. Next steps include coupling these solubility data with global mantle convection models to quantify cumulative oxygen release over geologic time and testing whether similar behavior occurs in more silica-rich or hydrous melts.