Redox evolution of crystallizing magmas with C-H-O-S volatiles and its implications for atmospheric oxygenation

The evolution of Earth’s atmospheric O2 levels on long timescales is modulated in part by the composition of magmatic gases, specifically their oxidation state and speciation. Here, we examine how magma differentiation affects the oxygen fugacity and composition of magmatic gases. Variations in temperature, pressure, composition, and redox conditions of evolving magmatic systems result in complexities, which have not been fully treated previously. Taking all these variables into account, we develop a multipronged approach to a comprehensive understanding on the role of magmatic gases on atmospheric oxygenation. This includes a compilation of petrological observations on the redox systematics of mid-ocean ridge basalts, a geochemical model for magma crystallization in oxygen-closed systems with variable initial redox conditions, and a thermodynamic framework for gas–melt systems with multi-component C-H-O-S volatiles.