The distribution of ferric iron within the Earth's interior and evolution in the mantle's redox state. (Invited)

The Earth's redox state has influenced the nature of volatile species within both the Earth and mantle derived magmas. During core formation equilibrium with metallic iron would have imposed a low oxygen fugacity, but some of the oldest magmas on Earth indicate a mantle oxygen fugacity more oxidised than this. One mechanism for early mantle oxidation is through disproportionation of ferrous iron in the lower mantle to form ferric iron in magnesium silicate perovskite and iron metal. This is supported by evidence showing that Al-bearing perovskite synthesised in equilibrium with iron metal contains at least 50 % of iron in the ferric state. Loss of some of the resulting iron metal to the core may have rendered the lower mantle more oxidised, which may then have mixed with the upper mantle. The questions remain, however, to what extent might the lower mantle have been oxidised by loss of metal and whether mixing with the upper mantle was complete? To address these issues requires interpretation of evidence for the oxidation state of both normal sublithospheric mantle and plume source mantle, which may bring material from the lower mantle. We have performed experiments to understand redox processes that may potentially mask higher ferric iron contents in the deeper upper and lower mantle. Carbon, for example, should exist in reduced form as diamond or dissolved in metallic alloy in the deep mantle but will oxidise during decompression of the sublithospheric mantle causing reduction of ferric iron closer to the surface. We consider this and other effects that may imply the existence of a more oxidised deeper mantle source than previously considered.