Valence state and spin transitions of iron in Earth's mantle silicates

Abstract

Using ab initio simulations, we investigated the valence and spin states of iron impurities in the perovskite ( Pv) and post-perovskite ( PPv) polymorphs of MgSiO 3. In agreement with the previous experimental work, we find a valence disproportionation reaction: 3Fe 2+ → 2Fe 3+ Fe 0 metal. This exothermic reaction results in the predominance of Fe 3+ impurities in lower mantle silicates and produces free metallic iron. It occurs both in Pv and PPv, Al-free and Al-rich, at all lower mantle pressures. This reaction provides a possible mechanism for the growth of the Earth's core and core-mantle chemical equilibration. In the presence of Al 3+, iron forms Fe 3+–Al 3+ coupled substitutions in Pv, but separate Fe 3+-Fe 3+ and Al 3+-Al 3+ substitutions in PPv. Only the high-spin state is found for Fe 2+ impurities at all mantle pressures, while Fe 3+ impurities on the Si-site are low-spin at all pressures in both phases. Fe 3+ impurities on the Mg-site are in the high-spin state in PPv at all mantle pressures, but in Pv we predict a high-spin–low-spin transition. The pressure at which this transition occurs strongly depends on the Al 3+ content and according to our calculations increases from 76 GPa for Al-free to 134 GPa for aluminous Pv; this reconciles many of the previous experimental results. Our findings have implications for the chemical evolution of the Earth and for the radiative conductivity and dynamics of the D” layer.