Atomic Scale Simulations of Hydrolytic Weakening in Olivine

Weakening of the upper mantle by the addition of small quantities of water to nominally anhydrous minerals is proposed to be critical for the emergence and ongoing viability of plate tectonics on Earth. However, the mechanism of hydrolytic weakening in olivine, the dominant phase that probably controls upper mantle rheology, is unclear. In diffusion creep a range of mechanisms have been proposed where the addition of hydrogen causes a change in the point-defect population and an increase in the number of slowly diffusing silicon vacancies. This leads to hydrolytic weakening for small grain-size at high temperature and low strain-rate but does not explain the weakening mechanism in the regime where dislocation motion is key. Here we present a series of atomic scale simulations designed to show how hydrogen in olivine can change the structure and mobility of dislocations in olivine. In the first simulations, involving fully atomistic descriptions of dislocation cores, we explore the energetics of interactions between hydrated point defects and dislocations. These calculations show that hydrated point defects are attracted to dislocation cores, but the strength of this attraction is different for different dislocations. The second group of simulations are designed to compare the mobility of dislocations with and without the addition of hydrogen. These make use of the Peierls-Nabarro model of dislocations where the effect of the atomic scale structure of the core is parameterised using the energy surface described by generalised stacking faults. These simulations suggest that the Peierls potential experienced by mobile dislocations is altered by the presence of hydrogen, and that this alters the relative mobility of different dislocations in olivine. Together, these simulations can begin to explain water weakening of the upper mantle and elucidate the relationship between rheology and the storage of volatile elements in planetary interiors.