2016 AGU Abstract

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.

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