Effect of water on high pressure and high temperature deformation of olivine single crystal [110]c and [011]c and quantification of activation volumes

Seismic velocity anisotropies observed in the upper mantle are interpreted from lattice-preferred orientations (LPO) produced experimentally in olivine, which depends on the dominant dislocation slip systems. At low pressure P<3GPa, mantle temperature (T) and in dry conditions, olivine [100] dislocation slip dominates the less active [001] slip. This tends to align crystal fast velocity [100] axis with the principal shear direction. Yet recent high-pressure deformation experiments [1] show that [001](010) slip system dominates over [100](010) system in the (P,T) range of the deep upper mantle. This may promote a shear-parallel slow-velocity [001] axis and may explain the seismic-velocity attenuation observed at depth >200 km [2]. According to those studies, the slip systems activities depend on pressure. However previous studies revealed that at high temperature and pressure the water could affect the olivine plastic deformation and induce the transition between the [100] slip to the [001] slip. [3] The influence of water on the lattice prefer orientation (LPO) of olivine single crystal [110]c and [011]c was investigated at high pressure (4-7GPa) and high temperature (1000OC-1200oC) using deformation-DIA press coupled with synchrotron X-ray beam to measure in situ stress, strain and strain rate of the samples. Water was supplied to the sample using a talc sleeve surrounding the single crystals. The water diffused in the crystal before we started the deformation. Water contents were measured on the recovered samples, using FTIR at U2 beamline of the NSLS. Deformations were carried on samples with 300, 600 and 800ppm H/Si. Deformation microstructures were observed and characterized using TEM. The data analyses revealed a transition of slip systems from [100] slip to [001] slip occurring between 5-6GPa. The quantification of the effect of pressure on the slip system activity was obtained by the calculation of activation volume for each single crystal orientation. Activation volume for [100](010) slip system is determined to be about 4cm3/mol, and [001](010) slip system seems to be less affected by pressure with an activation volume around 0cm3/mol.