Influence of the post-perovskite transition on thermal and thermo-chemical mantle convection
Résumé
Several studies have focused on the post-perovskite (PPV) transition's possible dynamical effect, as well as the complex seismological structures that may arise through the interplay of variations in temperature, composition and the PPV phase transition. Here these issues are explored using numerical models of thermal and thermo-chemical convection in various geometries including a three-dimensional spherical shell. A zero-, single- or double- crossing of the PPV phase boundary is observed depending on the temperatures of the CMB and deep mantle; this evolves with time as the core and mantle cool. The PPV transition has a minor effect on the dynamics and mantle temperature, mildly destabilizing the lower boundary layer and slightly increasing mantle temperature, depending on its depth relative to the thermal boundary layer. If piles of dense subducted MORB accumulate above the CMB then there is an anticorrelation between regions with a thick PPV layer and hot dense piles, but with a composition-dependent PPV transition this can change. Lateral variations in the occurrence of PPV are likely the dominant contributor to long-wavelength lateral shear-wave velocity heterogeneity in the deepest mantle, depending on some uncertain scaling parameters. The different contributions to seismic heterogeneity have different spectral slopes: temperature is "red", composition is "white" and PPV is intermediate. Theoretical considerations suggest that when compositional effects on the stability of PPV are taken into account, a large potential variety of complex behavior could occur, generating structures such as discontinuities, gaps or holes, and multiple (i.e., >2) crossings.