Fe-bearing perovskite and post-perovskite: phase stability, spin transitions, and the consequences for the lower mantle

Using lattice dynamical calculations based on density functional perturbation theory we are able to disentangle a part of the complex phase diagram and spin behavior of the (Mg,Fe)SiO3 perovskite (pv). To do this we investigate the dynamic stability of Pbnm FeSiO3 pv and show the existence of unstable phonon modes. We track the eigen-displacements of the phonons modes to find low-spin and intermediate spin states. On solid-state physical basis we explore a set of hypothetical structures with various spin configurations and considerably lower enthalpy than the parent orthorhombic Pbnm structure. At megabar pressures we study various stable monoclinic and triclinic configurations with intermediate spin state and a triclinic low-spin structure. Our results demonstrate that pv with intermediate spin state can exist in the Earth as an independent phase or can coexist with domains in a high-spin configuration. Changes due to chemistry or temperature or both can easily shift the equilibrium from one spin configuration to the other; this would in turn affect the thermal and electrical conductivities, the seismic properties, the rheology and the chemical behavior of the mantle and thus could be at the origin of small-scale mantle heterogeneities and enhance chaotic convection. The elastic moduli and the bulk seismic wave velocities are weakly affected by the spin transition. However, the intrinsic differences in seismic anisotropy between the high-spin and low-spin phases of Fe-bearing pv coupled with lattice preferred orientation that can develop during mantle flow lead to distinct seismic signatures between the top and the bottom of the lower mantle. These signatures are detectable by seismic observations and they need to be taken into account in tomographic studies of the Earth's lower mantle. Then we extend the field of investigation of the pv to post-perovskite (ppv) phase transition with a study of the double substitution Mg+Si = Al+Fe. We distinguish two crystallographic cases: AlFeO3 and FeAlO3, corresponding respectively to two ordered cases: one with Fe and then one with Al in octahedral coordination. The pv-ppv phase transition occurs between antiferromagnetic configurations at 90GPa and is associated with a site exchange that triggers a partial collapse of the magnetic moment. The presence of Al+Fe3+ in perovskite/post-perovskite renders the phase transition sluggish, induces a large density jump at the transition and contributes into maintaining a residual magnetic spin down to the base of the mantle. Following this transition path the elastic moduli show positive jumps. The phase transition is marked by a positive jump of 0.04 km/s (0.33%) in Vp and by a negative jump of -0.15 km/s (-1.87%) in Vs. The effects of the Mg+Si = Al+Fe substitution on the seismic properties of MgSiO3 pv and ppv depend on the crystallography of the substitution, namely the position the exchanged cations take in the structure.