Hydrolytic weakening in olivine single crystals

Abstract

AbstractDeformation experiments on single crystals of San Carlos olivine under hydrous conditions were performed to investigate the microphysical processes responsible for hydrolytic weakening during dislocation creep. Hydrogen was supplied to the crystals using either talc or brucite sealed in nickel capsules with the crystal. Deformation experiments were carried out using a gas medium apparatus at temperatures of 1050° to 1250°C, a confining pressure of 300 MPa, differential stresses of 45 to 294 MPa, and resultant strain rates of 1.5 × 10−6 to 4.4 × 10−4 s−1. For talc‐buffered (i.e., water and orthopyroxene‐buffered) samples at high temperatures, the dependence of strain rate on stress follows a power law relationship with a stress exponent (n) of ∼2.5 and an activation energy of ∼490 kJ/mol. Brucite‐buffered samples deformed faster than talc‐buffered samples but contained similar hydrogen concentrations, demonstrating that strain rate is influenced by orthopyroxene activity under hydrous conditions. The values of n and dependence of strain rate on orthopyroxene activity are consistent with hydrolytic weakening occurring in the climb‐controlled dislocation creep regime that is associated with deformation controlled by lattice diffusion under hydrous conditions and by pipe diffusion under anhydrous conditions. Analyses of postdeformation electron‐backscatter diffraction data demonstrate that dislocations with [100] Burgers vectors are dominant in the climb‐controlled regime and dislocations with [001] are dominant in the glide‐controlled regime. Comparison of the experimentally determined constitutive equations demonstrates that under hydrous conditions crystals deform 1 to 2 orders of magnitude faster than under anhydrous conditions.