Two Transitions in the D″: A Pyrolitic Deep Lower Mantle and a Ca‐Rich Bridgmanite Altering Chemistry at the CMB as Determined by Ab Initio Calculations

Abstract The D″ is a layer lying below a seismic discontinuity that occurs ∼250 km above the Core‐Mantle boundary (CMB). A commonly proposed origin of this discontinuity is the transformation of bridgmanite (Bdm) to a post‐perovskite structure (Bdm‐ppv) but this generally produces results that do not match all the seismic observations. In this work we build a thermodynamic model of this transition using ab initio calculations incorporating the effects of Fe, Al, and Ca and predict that it is not one but two overlapping transitions. The second transition is the high temperature (>∼2,700 K) dissolution of Davemaoite (Dvm) phases into the Bdm phase to make a chemically mixed Bdm‐Dvm phase. This reaction is controlled by Dvm and ferropericlase (fp) phases which means different mineralogical compositions will exhibit different phase behaviors and thus different seismic and dynamical behaviors near the CMB. We predict that assemblages with pyrolitic compositions match closest to the seismic observations of the D″ and that the presence of velocity increases near the top of the D″ followed by linked velocity increases near the CMB are both indicators of pyrolitic assemblages. Harzburgitic and basaltic mixtures are predicted to lack some of these features. In a pyrolitic mantle we then predict the existence of Bdm‐Dvm at the CMB which will have different chemical, thermal and dynamical properties to Bdm + Dvm and could alter interactions between the mantle and the core.