The deep Earth oxygen cycle: Mass balance considerations on the origin and evolution of mantle and surface oxidative reservoirs

Abstract

The deep Earth oxygen cycle addresses the origin and long-term exchange of oxidized species between the mantle and exosphere and creates the geochemical context in which the interior and surface environment have evolved. The redox power of the bulk silicate Earth (BSE) can be measured in terms of its redox budget, RB, relative to a reference state of the predominant valences of redox-sensitive elements in the mantle. 82±4% (1.9±0.5 × 1023 moles) of the redox power resides in the mantle and 18±4% (4.2±0.5 × 1022 moles) in the exosphere. Vigorous outfluxes of oxidized species from the mantle (3.8±0.7 × 1013 mol/yr) can replenish the exosphere reservoir in 1.1±0.2 Ga, which requires efficient long-term recycling of redox power via subduction. Within uncertainties, recent (last 200 Ma) mantle RB outfluxes and subduction influxes (4.9±1.3 × 1013 mol/yr) are balanced, but outfluxes likely exceeded influxes earlier in Earth history. CO2 is 66±20% of the RB outfluxes but only 24±13% of the influxes. This is largely because of conjugate intervalence reactions; one in the shallow mantle creates carbonate at the expense of Fe2O3 and the other creates Fe2O3 from CO2 on the surface by a combination of organic carbon fixation and oxidative weathering.Scaling of mantle oxygen fugacity, fO2, to redox mass balance is approximately Δlog⁡(Fe3+Fe⊤)peridotite ≈14Δlog⁡(fO2)mantle . Consequently, inferences of secular evolution of mantle oxygen fugacity from the Archaean to the Proterozoic, amounting to about 1.5 log units in fO2, imply that the Archaean mantle had an Fe3+/FeT ratio <0.02, rather than the modern value of 0.04±0.01. Oceanic basalts derive from sources with greater redox budgets than mid-ocean ridges, and this is partly expressed as higher source Fe3+/FeT ratios, but more importantly, as greater source CO2 concentrations. In combination, these require that plumes in the deep upper mantle have Fe3+/FeT ratios significantly greater than the depleted mantle.The oxidative inventory of the BSE originated by a combination of H2O disproportionation in the atmosphere, leading to H2 escape, and FeO disproportionation in the deep mantle, with loss of Fe to the core. FeO disproportionation in a deep magma ocean inevitably produces a significant fraction of the BSE RB, but additional contributions are likely required. Further Fe loss could be from bridgemanite crystallization followed by Fe escape through a basal magma ocean. Gradual mixing of the resulting deep oxidized layer may account for secular oxidation of the mantle source regions of igneous rocks from 3 to 2 Ga. H2O disproportionation assisted in accumulation of the oxidized surface species, but was not a significant source of mantle oxidative power, as mechanisms of oxidative influx are quantitatively insufficient.