Magma oceans, iron and chromium redox, and the origin of comparatively oxidized planetary mantles

Abstract

The mantles of both Earth and Mars are more oxidized than would be expected based on low pressure equilibration of molten silicate and alloy during their magma ocean stages. High pressure silicate-alloy equilibration in a magma ocean can produce appreciable ferric iron in the silicate, leading to comparatively oxidized near surface conditions and overlying atmospheres. Upon crystallization, this may feasibly be sufficient to account for oxygen fugacities prevailing in basalt source regions of Earth and Mars. Experiments and first principles studies affirm that Fe3+ is stabilized at high pressure, but to date there has been no model that accounts accurately for the combined effects of melt composition, temperature, pressure, and oxygen fugacity on magma ocean Fe3+/FeT. We calibrate a new model for Fe3+/FeT as a function of temperature, pressure, melt composition, and fO2 which reproduces Fe3+/FeT for experimental peridotite liquids and which incorporates differences in FeO and Fe2O3 liquid heat capacities into a potentially realistic temperature function. For the effects of pressure, two versions of the model are implemented based on recent equations of state (EOS), though only the EOS of Deng et al. (2020) is applicable to pressures relevant to metal-silicate equilibration in a deep terrestrial magma ocean. For Earth, metal-silicate equilibration at 28–53 GPa, 2300–4100 K, and fO2 set by plausible mantle and core compositions produces Fe3+/FeT between 0.034 and 0.10, with variation mostly owing to differences in assumed temperatures. For Mars, different proposed mantle compositions produce Fe3+/FeT ratios that range from 0.026 for FeO* of 13.5 wt.% up to 0.038 for FeO* of 18.1 wt.%.Although significant Fe3+ may be present in magma oceans owing to high pressure equilibration with alloy, the budget of Fe2O3 in crystallized mantles is expected to be modified from that in the molten state. An important additional factor is the influence of Cr, which is Cr2+ in molten silicate equilibrated with alloy and Cr3+ in terrestrial upper mantles. Production of Cr3+ and Fe2+ by reaction with Cr2+ and Fe3+ during crystallization can destroy much of the Fe2O3 present during the magma ocean stage. Considering the stability of Cr2+ in olivine and the temperature-dependent partitioning of Cr3+ between mantle silicates, we construct an empirical model for the fraction of Cr that is Cr2O3 in solid spinel peridotite as a function of temperature and fO2. For Earth, at least 0.35 wt.% Fe2O3 is destroyed by oxidation of magma ocean CrO and for Mars, more than 0.55 wt.% Fe2O3 should be destroyed. Consequently, either the terrestrial and martian magma oceans were significantly more enriched in Fe2O3 than their present-day upper mantles or other processes contributed to oxidation of the latter. Over-enrichment of Fe2O3 in the magma oceans is plausible only if terrestrial metal-silicate equilibration occurred above 3300 K and if the martian mantle contains >17 wt.% FeO*. Subsolidus disproportionation of ferrous iron may have contributed to the present-day redox state of the Earth’s mantle, and late accretion of chondrite-like material and hydrogen degassing also likely affected the solidified mantles of both Earth and Mars.