Plasticity of the dense hydrous magnesium silicate phase A at subduction zones conditions

Abstract

The plasticity of the dense hydrous magnesium silicate (DHMS) phase A, a key hydrous mineral within cold subduction zones, was investigated by two complementary approaches: high-pressure deformation experiments and computational methods. The deformation experiments were carried out at 11GPa, 400 and 580°C, with in situ measurements of stress, strain and lattice preferred orientations (LPO). Based on viscoplastic self-consistent modeling (VPSC) of the observed LPO, the deformation mechanisms at 580°C are consistent with glide on the (0001) basal and (011¯0) prismatic planes. At 400°C the deformation mechanisms involve glide on (2¯110) prismatic, (0001) basal and {112¯1} pyramidal planes. Both give flow stresses of 2.5–3GPa at strain rates of 2–4×10−5s−1. We use the Peierls–Nabarro–Galerkin (PNG) approach, relying on first-principles calculations of generalized stacking fault (γ-surface), and model the core structure of potential dislocations in basal and prismatic planes. The computations show multiple dissociations of the 13[21¯1¯0] and [011¯0] dislocations (〈a〉 and 〈b〉 dislocations) in the basal plane, which is compatible with the ubiquity of basal slip in the experiments. The γ-surface calculations also suggest 13[21¯1¯3] and [01¯11] dislocations (〈a+c〉 or 〈c−b〉 directions) in prismatic and pyramidal planes, which is also consistent with the experimental data. Phase A has a higher flow strength than olivine. When forming at depths from the dehydration of weak and highly anisotropic hydrated ultramafic rocks, phase A may not maintain the mechanical softening antigorite can provide. The seismic properties calculated for moderately deformed aggregates suggest that S-wave seismic anisotropy of phase A-bearing rocks is lower than hydrous subduction zone lithologies such as serpentinites and blueschists.