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

Abstract

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.By considering magmatic differentiation at variable depths, we show that magma crystallization progressively changes the redox state of residual melts and coexisting gases due to differences in iron speciation between the melt and cumulate minerals. In thick crusts, high pressures of magma differentiation suppress magnetite saturation but promote garnet crystallization, increasing magmatic oxygen fugacity by ∼4–8 orders of magnitude and thereby resulting in oxidized gases. In thin crusts, the prevalence of magnetite prevents significant oxidation of the magma, yielding reduced gas emission. Hence, magmatic gases from thick crusts are much less effective O2 sinks, which does not appear to be affected by gas decompressional transport or polybaric degassing from the deep crustal magmatic zone to the atmosphere. Depending on how crustal thickness during active magmatism varies with time, magmatic gases may play an important role in modulating atmospheric O2 levels on million-year timescales. Rapid growth of continental crust in the late Archean appears to have been associated with the first evidence for significant crustal thickening during magmatism. If so, this would have led to a substantial decrease in O2 sink, possibly permitting atmospheric O2 to rise to a new state.