Mantle oxidation state and oxygen fugacity: Constraints on mantle chemistry, structure, and dynamics

Abstract

This review of mantle oxidation state, based on Mossbauer measurements of mantle xenoliths and synthetic high-pressure phases, suggests relatively low Fe 3+ /ΣFe in upper mantle and transition zone phases but high Fe 3+ /ΣFe in lower mantle (Mg,Fe)(Si,Al)O 3 perovskite, even under reducing conditions, with significant implications for physical and chemical properties. Whole-rock Fe 2 O 3 concentrations for pyrolite mantle are calculated to be low in the upper mantle and transition zone (ca. 0.3 wt% Fe 2 O 3 ), but high in the lower mantle (ca. 4 wt% Fe 2 O 3 ). High Fe 2 O 3 concentrations are probably balanced according to the iron disproportionation reaction 3 Fe 2+ = Fe° + 2 Fe 3+ . Oxygen fugacity is relatively high at the top of the upper mantle (near the fayalite-magnetite-quartz buffer) due to the concentration of Fe 3+ in modally minor phases, but probably low in the transition zone and lower mantle (close to metal equilibrium). Minerals in subducting slabs may be more oxidised, however, with higher Fe 2 O 3 concentrations. Coupled redox reactions between iron oxidation.and the reduction of oxidised species such as carbonate may contribute to the genesis of diamonds in the lower mantle. Disproportionation of iron in the lower mantle to produce ca. 1 wt% Fe-rich metal may have occurred in the early Earth, with important consequences for mantle geochemistry. If the metal phase does not segregate appreciably, material can move across the transition zone-lower mantle boundary without a net enrichment or depletion in oxygen.