Transmission electron microscopy characterization of dislocations and slip systems in K-lingunite: Implications for the seismic anisotropy of subducted crust

Abstract

In order to estimate the seismic anisotropy of subducted crust, polycrystalline samples of KAlSi 3O 8 K-lingunite (25% of the total subducted transformed sediments), have been synthesized and deformed under the temperature and pressure conditions of the subducted slabs. Transmission electron microscopy (TEM) characterizations of the recovered samples reveal that the microstructures are clearly dominated by [0 0 1] glide involving screw dislocations. For this reason, only {1 0 0} could be identified as glide planes, the question of [0 0 1] slip on {1 1 0} remains open. Few 1/2〈1 1 1〉 dislocations were observed gliding on {1 1 0} planes, which implies that 1/2〈1 1 1〉{1 1 0} is a harder slip system than those involving [0 0 1] slip. The occurrence of sub-grain boundaries suggests that diffusion and climb might be active under these conditions. To assess the texture of polycrystalline K-lingunite, the crystal preferred orientations (CPOs) were calculated using visco-plastic self-consistent (VPSC) polycrystalline plasticity model in simple shear using the slip systems identified by TEM. Finally, the seismic properties of K-lingunite aggregates were calculated from the CPO and single crystal elasticity tensor. K-lingunite is predicted to have a high seismic anisotropy, which could combine constructively with one of the stishovite (same proportion as K-lingunite at the transition zone depth ranges).