Ab initio study of water speciation in forsterite

The Earth’s interior contains 0.5 to 100 times of water on the Earth’s surface based on different studies. Water or hydrogen (in hydroxyl form) can be stored as hydrous defects in nominally anhydrous minerals (NAMs) in the Earth’s mantle. 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. The most likely incorporation mechanism of hydrogen in mantle minerals is the substitution of Mg and Si cations by hydrogens. However, a long-standing debate remains concerning the relative thermodynamic stability of these defects. Using ab initio calculations we investigate the energetics of these defects, (4H)^x _Si and (2H)^x _Mg, in forsterite, the Mg end-member of olivine, the most abundant upper mantle phase. We address the role of vibrational free energy and lattice and internal defect configurational entropy in the 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.