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Earth’s lower mantle is believed to contain ∼75 vol.% (Mg,Fe)SiO3 perovskite, 20 vol.% (Mg,Fe)O ferropericlase, 5 vol.% CaSiO3 perovskite. In the lowermost ∼200 km of the mantle, (Mg,Fe)SiO3 adopts the post-perovskite structure [1,2]. While the properties and behaviour of pure MgSiO3 phases are well understood (see e.g. ref. [2-5]), the effects of impurities most importantly iron and aluminium, are poorly known and the existing experimental studies are contradictory. Here, we perform ab initio simulations that address the valence and spin states of iron impurities in MgSiO3 polymorphs. In agreement with [6] we find a valence disproportionation reaction: 3Fe→2Fe+Femetal. This highly exothermic (and somewhat counterintuitive) reaction results in the predominance of Fe impurities in lower mantle silicates and produces free metallic iron. It occurs both in perovskite and post-perovskite, Al-free and Al-rich, at all lower mantle pressures. In presence of Al, iron forms Fe -Al coupled substitutions in perovskite, but separate Fe-Fe and Al-Al substitutions in post-perovskite. Only the high-spin state is found for Fe impurities at all mantle pressures, while Fe impurities on the Si-site are low-spin at all pressures in both phases. Fe impurities on the Mg-site are in the high-spin state in post-perovskite at all mantle pressures, but in perovskite we predict a high-spin low-spin transition. The pressure at which this transition occurs strongly depends on the Al content and according to our calculations increases from 76 GPa for Al-free to 134 GPa for Al-bearing perovskites. This explains previous experimental results [e.g., 7,8], which were thought to contradict each other. These findings have important geophysical implications [9].

Valence and spin states of iron in (Mg,Fe)SiO3perovskite and post-perovskite at pressures of the Earth's lower mantle

Valence and spin states of iron in (Mg,Fe)SiO₃perovskite and post-perovskite at pressures of the Earth's lower mantle