Townsend, J. P., S. D. Jacobsen C. R. Bina, and J. Tsuchiya, Water partitioning between bridgmanite, postperovskite, and periclase in the lowermost mantle, Eos, Transactions of the American Geophysical Union, 97, Fall Meeting Supplement, MR34A-01, 2016.
We used ab-initio lattice dynamics to calculate the water partition coefficient between bridgmanite (brg), postperovskite (ppv), and periclase (per) 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 brg and ppv 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 postperovksite 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 feeding deep-sourced ocean island basalts in regions of upwelling at the base of the mantle.