REDOX EVOLUTION OF MAGMAS WITH C-H-O-S VOLATILES DURING DEEP AND SHALLOW CRUSTAL CRYSTALLIZATION AND ITS IMPACT ON ATMOSPHERIC OXYGENATION

Through most of Earth’s history, atmospheric O₂ has been generated by oxygenic photosynthesis. Yet the rise of atmospheric O₂ to an appreciable level did not occur until the Great Oxidation Event, ~500 Myr after the emergence of O₂-producing organisms at ~3 Ga (1). The primary cause of this event remains unclear. A popular view attributes this event to a decrease in the global flux of reduced magmatic gases. Proposed hypotheses include changes in mantle oxygen fugacity (2), volcanic emission depths (3), or volcanic gas temperature (4). However, these hypotheses remain debated and are likely over-simplified without considering the complexity of dynamically evolving multiple-phase magmatic systems.

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.

[1] Lyons et al. (2014) Nature 506: 307–315; [2] Kadoya et al. (2020) Nat. Commun. 11: 2774; [3] Gaillard et al. (2011) Nature 478: 229–232; [4] Moussallam et al. (2019) EPSL 520: 260–267.