Lubrication of dislocation glide in forsterite by Mg vacancies: insights from Peierls-Nabarro modeling

Dislocation glide is an important contributor to the rheology of olivine under conditions of high stress and low to moderate temperature, such as occur in mantle wedges. Interactions between point defects and dislocation core may alter the Peierls stress, σp, and has been suggested that vacancy-related defects may selectively enhance glide on certain slip systems, changing the olivine deformation fabric. In this study, the Peierls-Nabarro model, parameterized by generalized stacking fault (GSF) energies calculated atomistically using empirical interatomic potentials, is used to determine the effect of bare Mg vacancies on the Peierls stresses of [100](010) and [001](010) dislocations in forsterite. Mg vacancies considerably reduce GSF energies and, consequently, σp for dislocations gliding on (010) in olivine. The magnitude of this decrease depends strongly on dislocation and the type of the lattice site, with vacant M2 sites producing the largest reduction of σp. The [001](010) slip system is found to be more sensitive than the [100](010) slip system to the presence of vacancies. Although, at ambient pressure, σp is lower for [100](010) than [001](010) edge dislocations, dσp/dP is greater for [100](010) dislocations, resulting in a change in the preferred slip system at 1.5 GPa. By preferentially lubricating [001](010) glide, Mg vacancies reduce the pressure at which this cross-over occurs. An M2 vacancy concentration at the glide plane of 0.125 defects/site is sufficient to reduce cross-over to 0.7 GPa. This may account for the existence of the B-type olivine deformation fabric in the corners of mantle wedges.