Deformation 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.

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Grain size sensitive (GSS) power law creep of San Carlos olivine aggregates was investigated by comparing strain rates measured in laboratory deformation experiments to strain rates determined from a micromechanical model of intragranular dislocation processes. The plastic flow behavior of olivine aggregates due solely to intragranular slip was determined using flow laws for olivine single crystals in combination with grain orientations measured by electron backscatter diffraction. Measured strain rates were compared to results from the micromechanical model for samples deformed in compression to an axial strain of <0.2 and in torsion to a shear strain of up to 7.4. Olivine aggregates deform up to a factor of 4.6 times faster than the maximum possible rates determined from the micromechanical model of intragranular slip. Comparison of our data to published flow laws indicates that diffusion creep cannot account for this difference. The ratio of experimentally determined strain rates to those from the micromechanical model is strongly dependent upon grain size but is independent of stress and strength of lattice‐preferred orientation. These observations indicate that GSS power law creep, consistent with dislocation‐accommodated grain boundary sliding, occurs in both weakly and strongly textured olivine aggregates at the studied conditions.

The nature of lattice-preferred orientation (LPO) in olivine-rich rocks strongly influences many important physical properties of Earth's upper mantle. Different LPO types have been observed to develop in deformation experiments on olivine-rich rocks carried out at different water fugacity conditions. The development of the different LPO types has been attributed to dislocation slip systems in olivine having different sensitivities to water fugacity, but this hypothesis has not been directly tested. To measure the influence of water fugacity on the relative strengths of olivine dislocation slip systems, we carried out a series of deformation experiments on olivine single crystals under either anhydrous or hydrous conditions. The crystals were oriented to activate either the (010)[100], (001)[100], or (100)[001] dislocation slip systems using a direct shear geometry, which allows for isolation of single slip systems, in contrast to the multiple systems activated in experiments carried out in compression. Post-deformation electron backscatter diffraction analyses reveal orientation gradients consistent with deformation occurring via the motion of dislocations on the activated slip systems. Crystals in all of the investigated orientations exhibit hydrolytic weakening, but crystals oriented to activate the (001)[100] slip system exhibit the largest degree of weakening. These results are consistent with a water-induced change in LPO in olivine-rich rocks deforming by dislocation creep. The rheological data obtained from the experiments can be used to improve models of LPO evolution in Earth's mantle, which is critical for imaging the structure of Earth's interior and predicting the movement of Earth's tectonic plates.