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

Using ab initio simulations, we investigated the valence and spin states of iron impurities in the perovskite (Pv) and post-perovskite (PPv) polymorphs of MgSiO3. In agreement with the previous experimental work, we find a valence disproportionation reaction: 3Fe(3+)-> 2Fe(3+) Fe-metal(0). This exothermic reaction results in the predominance of Fe3+ 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 Al3+, iron forms Fe3+-Al3+ coupled substitutions in Pv, but separate Fe3+-Fe3+ and Al3+-Al3+ substitutions in PPv. Only the high-spin state is found for Fe2+ impurities at all mantle pressures, while Fe3+ impurities on the Si-site are low-spin at all pressures in both phases. Fe3+ 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 Al3+ 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. (c) 2006 Elsevier B.V. All rights reserved.