The Mantle-Atmosphere Connection: Oxidation of the Atmosphere through Mantle Convection

Abstract:

Earth’s mantle connects the surface with the deep interior through convection, and the evolution of its redox state will affect the distribution of siderophile elements¹, recycling of refractory isotopes² and the oxidation state of the atmosphere through volcanic outgassing³. The rise of oxygen in atmosphere, i.e., the Great Oxidation Event (G.O.E.) occurred ~2.4 billion years ago (Ga)⁴. However, multiple lines of evidence point to biological oxygen production well before 2.4 Ga⁵; while chromium isotopes in iron formations indicates a decline of atmospheric oxygen about 1.88 Ga⁶. In contrast to the fluctuation of atmospheric oxygen, vanadium in Archean mantle lithosphere suggests that the mantle redox state has been constant for ~3.5 Ga⁷. Indeed, the redox state of the deep Earth’s interior is not well constrained⁸ and its effect on mantle dynamics is unknown. Here we show a redox-induced density difference affects mantle convection and may potentially cause the oxidation of the upper mantle. From two synthetic enstatite chondritic samples with identical bulk compositions but formed under different oxygen fugacities (fO₂) compressed to lower mantle pressures and temperatures, we find Al₂O₃ forms its own phase separate from the dominant Mg-silicate perovskite phase (i.e., bridgmanite⁹) in the more reduced composition, in contrast to a more Al-rich, bridgmanite-dominated assemblage for a more oxidized starting composition. As a result, the reduced material is ~1-1.5% denser than the oxidized material. Geodynamical numerical simulations show that the redox-induced density difference could lead to an increased oxidation of Earth’s upper mantle but is buffered by slow mixing with more reduced material through hot upwellings, which will potentially affect mantle redox and rise of oxygen in atmosphere.