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Three‐dimensional structures and dynamics in the deep mantle: Effects of post‐perovskite phase change and deep mantle layering

Three‐dimensional numerical simulations of thermo‐chemical mantle convection with multiple phase transitions (olivine‐spinel‐perovskite‐post perovskite) are used to investigate the morphology of compositional and post‐perovskite (PPV) boundary structures above the core‐mantle boundary (CMB). Compositionally‐dense subducted MORB partially segregates above the CMB forming ridge‐like dense piles, morphologically similar to structures in previous spherical or Cartesian studies in which a dense layer was inserted a priori. An anti‐correlation is observed between regions where a thick post‐perovskite layer occurs and regions containing piles of dense material, as in our previous two‐dimensional study. Lateral variations in the thickness or occurrence of a PPV layer cause very strong seismic shear‐wave velocity anomalies which may dominate thermal or composition‐related seismic heterogeneity in the depth range where the variable PPV boundary occurs, and also generate sharp‐sided structures as observed seismologically. Both compositional and PPV effects may be necessary to simultaneously explain large‐scale heterogeneity and the global distribution of the discontinuity atop D″.