Water partitioning between bridgmanite and postperovskite in the lowermost mantle

Joshua P. Townsend*, Jun Tsuchiya, Craig R. Bina, Steven D. Jacobsen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

17 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)20-27
Number of pages8
JournalEarth and Planetary Science Letters
Volume454
DOIs
StatePublished - Nov 15 2016

Keywords

  • density functional theory
  • nominally anhydrous minerals
  • water partitioning

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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