The post-perovskite transition in Fe- and Al-bearing bridgmanite*

The major phase of the Earth's mantle, (Al,Fe)-bearing bridgmanite, transitions to the post-perovskite (PPv) phase at Earth's deep lower mantle conditions. Despite extensive experimental and ab initio 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. We find that the seismic features produced by the PPv transition depend distinctly on the chemical composition. For instance, Fe³⁺-, Al³⁺-, or (Al³⁺, Fe³⁺)-alloying increase the transition pressure, while Fe²⁺-alloying has the opposite effect. Consequently, the absence of a D" seismic discontinuity or signature of a double-crossing of the PPv phase boundary point to a Fe²⁺-poor and Fe³⁺-rich bridgmanite composition with profound implications for the redox state of the deep lower mantle. These chemistry-specific seismic features together, along with thermochemical equilibrium calculations will be fundamental for resolving the chemical composition of the D" region by direct inspection of tomographic images.