Ab initio study of water speciation in forsterite

Water or hydrogen in the Earth’s mantle can be stored as hydrous defects in nominally anhydrous minerals (NAMs). Although in modest concentrations, these defects change the physical properties of their hosts, including electrical conductivity and viscosity, properties that affect mantle processes such as convection. To understand the influence of water on mantle properties, the mechanisms of water incorporation in olivine, the most voluminous mineral in the upper mantle, must be determined. The most likely incorporation mechanism of hydrogen in olivine is substituting for Mg and Si cations. However, a long-standing debate remains concerning the relative thermodynamic stability of these defects.

In this ab initio study, we reinvestigate the energetics of these defects – (4H)^x_Si and (2H)^x_Mg – in forsterite, the Mg end-member of olivine. Several configurations of (2H)^x_Mg and (4H)^x_Si defects, lattice and internal configurational entropy, and vibrational effects are included in the calculation of relative stability of these defects. We conclude that entropic effects are key to the stabilization of the hydrous Mg defect, which should predominate over the hydrous Si defect at typical upper mantle conditions.