Water partitioning between bridgmanite and postperovskite in the lowermost mantle

The lowermost mantle appears to contain geochemically primitive reservoirs of volatile components including water, as evidenced by certain ocean island basalts (Hallis et al., 2015). We used ab initio lattice dynamics to calculate the water partition coefficient between bridgmanite and postperovskite using quasi-harmonic free energies to determine how water is distributed between nominally anhydrous minerals in the D″ region. In the absence of aluminum, hydrogen was incorporated into both phases by a simple substitution of Mg²⁺⇔2H⁺, and we found that water favors bridgmanite over postperovskite by a factor of about 5:1 at conditions where an average mantle geotherm intersects the phase boundary. In the Al-bearing system, hydrogen and aluminum were coupled as Si⁴⁺⇔Al³⁺+H⁺ defects into both phases, and we found that water favors postperovskite over bridgmanite in the Al-bearing system by a factor of about 3:1 at ambient mantle conditions, and by about 8:1 at colder slab conditions. Our results indicate that aluminum controls the partitioning of water between bridgmanite and postperovskite, and that aluminous postperovskite may be a potential host for primordial water in the lowermost region of the mantle. The strong partitioning of water into aluminous postperovskite over bridgmanite provides a potential mechanism for dehydration melting in the lowermost mantle that could be a source for ocean island basalts in regions of upwelling.