Elastic anisotropy of experimental analogues of perovskite and post-perovskite help to interpret D′′ diversity

Recent studies show that the D′′ layer, just above the Earth's core–mantle boundary, is composed of MgSiO(3) post-perovskite and has significant lateral inhomogeneity. Here we consider the D′′ diversity as related to the single-crystal elasticity of the post-perovskite phase. We measure the single-crystal elasticity of the perovskite Pbnm-CaIrO(3) and post-perovskite Cmcm-CaIrO(3) using inelastic X-ray scattering. These materials are structural analogues to same phases of MgSiO(3). Our results show that Cmcm-CaIrO(3) is much more elastically anisotropic than Pbnm-CaIrO(3), which offers an explanation for the enigmatic seismic wave velocity jump at the D′′ discontinuity. Considering the relation between lattice preferred orientation and seismic anisotropy in the D′′ layer, we suggest that the c axis of post-perovskite MgSiO(3) aligns vertically beneath the Circum-Pacific rim, and the b axis vertically beneath the Central Pacific.