Reconciling the Post-Perovskite Phase with Seismological Observations of Lowermost Mantle Structure

The discovery of post-perovskite (pPv), the high pressure polymorph of (Mgx,Fe1-x)SiO₃ perovskite (Pv), may have profound implications for the thermal, chemical and dynamical structure of the deep Earth, if it can be demonstrated that pPv currently exists in the lower mantle. The requisite basis for such a demonstration is seismological observation of elastic velocity and density structures diagnostic of or at least compatible with the expected properties of pPv occurrence in a realistic lower mantle chemical and thermal environment. A critical assessment of seismological observations versus predictions for a lower mantle model with pPv is undertaken, with the limitations and robust attributes of the seismological data being summarized. The existence of a seismic velocity discontinuity several hundred kilometers above the core-mantle boundary is a primary line of evidence for the presence of a Pv-pPv phase change. However, some attributes of the discontinuity, such as localized P wave reflections from a large P velocity increase and the sharpness of observed P and S velocity increases, reveal inconsistencies with expected properties of a Pv-pPv phase transition. Lateral variations in temperature can produce complex phase boundary structure that explains variable S wave observations, but such elastic heterogeneity intrinsically complicates testing the pPv hypothesis. The combination of rapidly expanding seismological data sets and new high resolution data analysis procedures reveal multiple seismic discontinuities near the base of the mantle; in some cases these may be consistent with forward and reverse Pv-pPv transformations, bounding a pPv layer or lens in D" above the CMB. Other phase changes or compositional contrasts could also be involved. At present, existence of pPv in the deep Earth is quite plausible, but not yet conclusively demonstrated. Future seismological and mineral physics research directions for further testing the hypothesis that pPv is present in the lower mantle are suggested.