Modeling dislocation glide in Mg2SiO4 ringwoodite: Towards rheology under transition zone conditions

Deformation resulting from thermally activated plastic slip is modeled in Mg2SiO4 ringwoodite at 20 GPa for a wide range of temperatures. The model relies on the structures of the rate controlling 1/2〈110〉 screw dislocations which have been modeled using the Peierls-Nabarro-Galerkin method. These calculations are parametrized by density functional theory calculations of γ-surfaces of the {001},{110} and {111} planes. At finite temperatures, dislocation mobility is controlled by kink-pair nucleation on the thermally activated 1/4〈110〉 partial screw dislocations as they occur in ringwoodite. Single slip critical resolved shear stresses (CRSS) corresponding to this mechanism are deduced from Orowan's equation. The results are found to be in reasonably good agreement with experimental data at 20 GPa which show high effective flow stresses under laboratory conditions. Finally, the CRSS's are calculated for typical mantle strain rates of ∊̇=10-16 s−1 at appropriate temperatures expected in the lower transition zone. Results show that dislocation glide remains difficult and that lattice friction is not yet negligible in ringwoodite under natural conditions.