title: Redox Evolution of the Early Earth’s Mantle
Creators: name: Armstrong, Katherine
Publisher: name: University of Bayreuth
Year: 2018
Subjects: subject: Mantle redox
subject: Earth evolution
subject: Core formation
subject: Magma ocean
subject: Oxygen fugacity sensor
subject: 500

description: 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 Mössbauer 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 would have approached ~0.2, which means that the melt-metal equilibrium, 3FeO = Fe + 2FeO1.5, must shift to the right with pressure. A magma ocean initially poor in FeO1.5 would, therefore, precipitate metallic iron at depth. Separation of this iron to the core would raise the redox state of the mantle. Such a magma ocean would develop a gradient in oxygen fugacity with depth, from conditions of metallic iron saturation at lower mantle pressures to that of the present day mantle at the surface. This magma ocean redox stratification ensured that H2O and CO2 were the dominant atmosphere forming gases throughout the later stages of accretion. There are a number of further important implications for the distribution of volatiles within the Earth and differences in the redox states between planetary interiors.

Another frequently proposed scenario is mantle oxidation by H2O. One mechanism for this to operate is if the resulting hydrogen is sequestered into the core rather than reducing the surrounding mantle back to the initial level. At upper mantle conditions the oxygen fugacity of the mantle could only be raised above iron metal equilibrium if this core-forming phase were a sulfide liquid. A slightly hydrous mid ocean ridge basaltic melt was equilibrated with an FeS melt at 3 GPa, to test the effect of an exsolving sulfide melt on the redox state of the remaining silicate. Hydrogen contents of quenched sulphide liquids were measured for the first time using Elastic Recoil Detection Analysis and found to be the equivalent of up to 1300 weight ppm H2O. A lack of consistency in H partitioning between silicate melt and sulfide, however, suggests that the entire H content of the initial sulphide may not be preserved in the crystallised liquid. The partitioning of H and O into a separating sulfide melt is examined based on the available data and found to have insignificant effects on the oxidation state of the remaining mantle.