Deformation of Wet Polycrystalline Olivine Aggregates at High Confining Pressure

Recent experiments have suggested that the dominant slip system in dry olivine changes from (010)[100] to (010)[001] at high confining pressures (>3 GPa) and low differential stresses (<300 MPa) due to the higher activation volume of the (010)[100] slip system (Raterron et al., 2009; Jung and Green, 2009). However, results from experiments performed at lower pressures (2 GPa) with water concentrations >200 H/106 Si and low differential stresses (<300 MPa) indicate that the dominant slip system is slip is either (001)[100] or (100)[001] (Jung et al., 2006). It is not clear which factor (high pressure or water content) has a larger effect on the dominant slip system in olivine aggregates in environments with high water contents and high confining pressures, such as subduction zones. In order to determine the dominant slip system at both high confining pressure and high water content, we have performed an experimental study deforming polycrystalline olivine aggregates in axial compression using the D-DIA apparatus (T = 1200oC, P = 3-8 GPa and strain rates ~1x10-5/s). Water was added to the olivine by dehydration of a talc liner surrounding the cylinder of polycrystalline olivine at the experimental conditions. Differential stresses varied from 250 MPa to 550 MPa at the lowest and highest strain rates, respectively. Water contents in samples deformed to ɛ = 15% contained between 3000-5000 H/106 Si, but samples deformed to ɛ = 30% contained much less water (COH ~1300 H/106 Si), indicating that water is being lost to the assembly during the experiment. Differential stresses measured in wet samples are considerably weaker than those measured in dry olivine deformed at similar conditions (>1 GPa, Durham et al., 2008; Nishihara et al., 2008) and generally agree with the stresses predicted by the Hirth and Kohlstedt (2003) flow law for “wet” olivine deforming by dislocation creep. The grain size in samples deformed to ɛ = 15% is highly variable 300-20 microns, but at ɛ = 30% the majority of grains are ~10 microns with few large grains remaining. No lattice preferred orientation was observed in samples only deformed to ɛ = 15%. However, in one sample deformed to ɛ = 30% (differential stress = 280 MPa, COH = 1300 H/106 Si), [100] and [001] form girdles perpendicular to the compression direction and [010] form a point maximum parallel to the compression direction. These results indicate that (010)[001] is most likey the dominant slip system in tectonic environments with high confining pressures and high water contents, such as subduction zones.