Identifying the mechanism for the early oxidation of the Earths interior

• Applicant: Professor Dr. Daniel J. Frost
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 277057085

One of the most important events in the Earths development towards a habitable planet was the rapid increase in the oxidation state of the mantle during or just after core formation. During core formation the presence of iron metal would have buffered the oxygen fugacity of the mantle at a level below the iron-wüstite oxygen buffer. However, the earliest rock record reveals a mantle oxygen fugacity up to 5 log units higher. The cause and timing of this oxidation event are not only relevant to the evolution of the atmosphere but have implications for the early differentiation of the mantle and the proportions of volatile elements accreted and retained within the Earth. In this project three mechanisms for the early oxidation of the mantle will be tested through high pressure and temperature experiments and modelling. The first is that disproportionation of FeO occurred during the crystallisation of a magma ocean to form Fe2O3 bearing minerals and iron metal. The oxygen content of the mantle could have been raised as some of the iron metal that formed separated to the core. This mechanism will be tested by measuring the Fe2O3 content of minerals forming on the peridotite solidus in equilibrium with iron metal at conditions equivalent to the entire depth of the mantle. The second possibility proposes that FeO disproportionation occurred within silicate melts, rather than minerals, at lower mantle pressures as a result of melt Fe2O3 components becoming increasingly stable at lower oxygen fugacities as pressure increases. This will be tested by measuring melt Fe2O3 contents equilibrated at controlled oxygen fugacities as a function of pressure to deep mantle conditions. The third possibility is that the accretion of either Fe2O3 or H2O rich material led to the gradual oxidation of the mantle. This was only possible, however, if core formation continued from these oxidised regions through the separation of a sulphide melt. Otherwise highly siderophile elements would have become over abundant in the mantle. This will be tested by constraining the composition of core forming sulphide melts as a function of oxygen fugacity at pressures reflecting much of the Earths mantle. Using these results in models for the final stages of core formation and magma ocean crystallisation the dominant oxidation mechanism will be determined. The implications for volatile accretion and speciation in the earth will be examined and the initial redox profile through the Hadean mantle determined.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1833: Building a Habitable Earth
• International Connection: USA
• Cooperation Partner: Professor Dr. Andrew Campbell