Redox Evolution of the Early Earth’s Mantle

Abstract

The redox state of the Earth’s upper mantle controls the nature of volatile phases degassing from the interior and has, therefore, influenced the development of habitable surface conditions. An important event in the creation of these conditions was the rapid oxidation of the upper mantle after core formation. During accretion, mantle silicates equilibrated with core-forming metallic iron. This would have imposed a low mantle oxygen fugacity (fO2), where H2O, CH4 and H2 would be the dominant degassing species. Throughout the geological record, however, fO2 of the upper mantle has been 4-5 orders of magnitude higher, such that H2O and CO2 are the dominant volcanic gasses. The mechanism by which the mantle oxidised has implications for volatile delivery and fractionation in the early Earth and for the evolution of an oxygen-rich atmosphere. In this experimental study, mantle oxidation mechanisms have been investigated. Pressure has been found to stabilise ferric iron components in some mantle minerals, such that they contain a significant fraction of Fe3+ even in equilibrium with iron metal. If a ferric component in silicate magmas undergoes similar stabilisation, melt at the base of a deep magma ocean could have precipitated iron metal via the reaction 3FeO = Fe0 + 2FeO1.5. Separation of this iron metal to the core could then have raised the redox state of the mantle. In order to test this scenario, the proportions of Fe3+ and Fe2+ silicate melt components have been measured as a function of pressure at buffered oxygen fugacities. First, an oxygen buffering assemblage for use at pressures at the top of Earth’s lower mantle was calibrated. Phase relations, compressibility and thermal expansivity of Ru and RuO2 were investigated in a multianvil device using in-situ X-ray diffraction at the Advanced Photon Source in Chicago. Rutile-structured RuO2 was found to undergo two phase transformations, first at 7 GPa to an orthorhombic structure and then above 12 GPa to a cubic structure. The phase boundary of the cubic phase was constrained for the first time at high pressure and temperature. A thermodynamic description of the phase transformations along with equation of state data allows the oxygen fugacity buffered by the Ru + O2 = RuO2 equilibrium to be accurately determined to lower mantle conditions. Secondly, an andesitic melt was equilibrated with the Ru-RuO2 buffer in a multianvil press between 5 and 24 GPa, and further experiments were performed on the same melt in equilibrium with iron metal. The recovered sample were analysed using Mossbauer spectroscopy to determine the Fe3+/∑Fe ratio. This ratio was found to decrease with pressure up to 8 GPa, but above 15 GPa this trend reverses. Based on the equation of state properties of the iron melt components, we develop a model that describes this behaviour. The predictions of this model at conditions of iron-metal saturation are in good agreement with further experiments. This indicates that the Fe3+/∑Fe ratio of a magma ocean extending into the lower mantle…