Hydrolytic weakening of olivine at mantle pressure: Evidence of [1 0 0](0 1 0) slip system softening from single-crystal deformation experiments

To date, most of the experiments performed to quantify the hydrolytic weakening of olivine single crystals have been performed at low confining pressure (and low water fugacities). In order to determine if the degree of hydrolytic weakening changes at higher water fugacities (higher water contents), we have performed a series of high pressure (P) and high temperature (T) deformation experiments in hydrous condition on San Carlos olivine (Fo92) and forsterite (Fo100) single crystals, in a Deformation-DIA apparatus (D-DIA). Deformation was carried out in axial compression along the [1 1 0]c crystallographic reference frame direction in order to activate [1 0 0](0 1 0) dislocation slip system alone, at P ranging from 4 to 7 GPa and T = 1200 °C. Water was supplied during the experiments through dehydration of a talc sleeve (talc → enstatite + coesite + H2O) around the sample, while the differential stress and strain rate of the specimen were calculated from in situ X-ray diffraction and time-resolved imaging, respectively. Run products were investigated using Fourier transform infrared (FTIR) spectroscopy, and optical/electron microscopy (SEM and TEM). Structurally bound hydroxyl content (COH) derived from absorbance in FTIR spectra, based on the Paterson (1982) relation, were typically between 290 and 720 ppm H/Si in the run products and the majority of these hydroxyls were observed as distinct peaks in the range of wavenumber from 3620 to 3500 cm−1. TEM reveals that the [1 0 0](0 1 0) slip system was effectively activated and responsible for crystal deformation, and that dislocation glide was significantly assisted by recovering processes such as dislocation climb and dipole annihilation. Combining the experimental data from this study and Mackwell et al. (1985), we estimate the activation volume V∗[110]c for San Carlos olivine in order to produce a reasonable water-fugacity exponent r in the flow law, and obtain V∗[110]c = 12.1 cm3/mol for r = 0 and V∗[110]c = 17.3 cm3/mol for r = 1.2. Comparisons of the strengths of the wet single crystals to the strengths of dry single crystals calculated from the rheological laws ( Bai et al., 1991, Darot and Gueguen, 1981 and Raterron et al., 2009) indicate that the wet single crystal cylinders were between 1.3 and 1.6 times weaker than their dry counterpart at same conditions, which is consistent with previous studies of [1 0 0](0 1 0) olivine rheology performed at low confining pressure (300 MPa).

► We report experimental results on the effect of water on the [1 0 0] slip of olivine. ► The weakening factor (σdry/σwet at same strain rate) ranges from 1.3 to 1.6. ► The water weakening is insensitive to hydroxyl content above a certain threshold. ► The presence of dislocation dipoles annihilating might be responsible for weakening. ► For a stress exponent of 3.5, and r = 0, (V∗[110]c) = 12.1 cm3/mol.