The post-perovskite transition in Fe- and Al-bearing bridgmanite: effects on seismic observables*

The major mineral phase of the Earth’s lower mantle, (Al,Fe)-bearing bridgmanite, transitions to a seemingly layered structure known as post-perovskite at Earth’s deep lower mantle conditions. Despite extensive investigations, there are still important aspects of this transformation that need clarification. Here, we address this transition in (Al³⁺, Fe³⁺)-, (Al³⁺)-, (Fe²⁺)- and (Fe³⁺)-bearing bridgmanite using ab initio calculations, particularly the phase boundary dependence on the chemistry and acoustic velocity changes across this transformation. These results help us to constrain possible seismic signatures of this phase transition, which is necessary for a better understanding of the nature of the D” region. While our results are consistent with previous mineral physics studies, we find that the seismic features produced by the post-perovskite transition depend on the chemical composition of bridgmanite. Therefore, the absence of a D“ seismic discontinuity or signature of a double-crossing of the post-perovskite phase boundary has clear implications for the local aggregate chemistry and/or temperature.