RELATIVE VISCOSITIES OF WET OLIVINE AGGREGATES WITH DIFFERENT CRYSTALLOGRAPHIC PREFERRED ORIENTATIONS; IMPLICATIONS FOR SEISMIC ANISOTROPY IN THE MANTLE WEDGE

Within the mantle wedge of subduction zones, seismic anisotropies are observed nearing the subducting slab as the fast seismic direction changes from trench perpendicular to trench parallel. Possible causes for this seismic anisotropy are trench parallel flow or a change in the dominant slip system in olivine crystals due to the presence of water. However, the exact cause is unclear. Results of Jung and Karato (2001) indicate the olivine Type C and E crystallographic preferred orientations (CPO) should form at conditions of the mantle wedge, but these CPOs will not produce a fast seismic direction in the mantle wedge parallel to the trench. However, higher pressure experiments by Ohuchi et. al. (2011) indicate Type B CPOs are critical to solving this problem. In order to bridge the gap between these differing experimental rock deformation results, we performed experiments on a naturally deformed olivine aggregate (Anita Bay dunite) with a pre-existing CPO. Samples were cored in three orientations relative to the CPO that put the Type A, B or C/E CPOs in easy slip orientations. The cores were deformed in the deformation-DIA multi-anvil press with a talc sleeve inside the assembly to hydrate the olivine. Preliminary results indicate that Type A-orientated core is slightly weaker than Type C/E and much weaker than Type B at 1.5 GPa with modest water contents. At 3 GPa, the Type C/E-orientated core is weaker than Type B and much weaker than Type A. These results are consistent with those of Jung and Karato (2001), but at higher pressures (and higher water contents) the Type B olivine CPO may become dominant, as observed by Ohuchi et. al. (2011). These results indicate that the dominant slip system may change multiple times during hydration and subduction.