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 MgSiO3. In agreement with the previous experimental work, we find a valence disproportionation reaction: 3Fe2+ → 2Fe3+ Fe0metal. 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. © 2006 Elsevier B.V. All rights reserved.
