REDOX EVOLUTION OF MAGMAS WITH C-H-O-S VOLATILES DURING DEEP AND SHALLOW CRUSTAL CRYSTALLIZATION AND ITS IMPACT ON ATMOSPHERIC OXYGENATION
Here we explore the redox effects of crystallization and degassing in dynamically evolving magmatic systems with C-H-O-S volatiles by combining petrological/geochemical observations and thermodynamic models. We show that polybaric crystallization of magmas as they transit from the mantle to the surface progressively changes the redox state of magmas and coexisting gases due to differences in iron speciation between crystallizing phases and melts. In thick crusts, high pressures of magmatic differentiation suppress magnetite saturation and enhance garnet crystallization, increasing magmatic oxygen fugacity by ~3–5 orders of magnitude, thereby resulting in oxidized magmatic gases. In thin crusts, the prevalence of magnetite prevents significant oxidation of the magma, resulting in reduced magmatic gases. The redox state of magmatic gases may play a key role in modulating atmospheric O2 levels because reduced gases serve as efficient O2 sinks. Rapid growth of continental crust in the late Archean appears to have been associated with the first evidence for significant crustal thickening. If so, this would have led to a decrease in the O2 sink, permitting atmospheric O2 to rise.
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