The incorporation of water into lower-mantle perovskites: A first-principles study

We have used first principles methods to calculate the partitioning of water between perovskite and ringwoodite under lower mantle and Fe-free conditions. We find that incorporation of water into ringwoodite is more favourable than into perovskite by about 0.25 eV per formula unit, or about 24 kJ/mol. This translates to a ringwoodite to perovskite partition coefficient of between 10 and 13, depending on temperature. These values are in good agreement with the partitioning experiments of Inoue et al. (2010) on Fe-bearing samples, where they find a partition coefficient of about 15. We also find that water incorporates into perovskite more readily than into periclase (also under Fe-free conditions), and we predict a perovskite to periclase partition coefficient of 90 at 24 GPa and 1500 K. We conclude, therefore, that the lower-mantle is able to contain substantial amounts of water, perhaps as much as 1000 ppm.