Seismic anisotropy in the mantle transition zone induced by shear deformation of wadsleyite

Mantle flow in the Earth’s mantle transition zone (between 410 and 660 km depth) plays a key role to understand the nature of mantle convection, which can be mapped by observed seismic anisotropy combined with crystallographic preferred orientations of mantle minerals. Although wadsleyite is the most important mineral to cause seismic anisotropy observed in the mantle transition zone, there have been limited experimental data on its crystallographic preferred orientation because of experimental limitations. We experimentally evaluated the preferred orientation of wadsleyite developed by shear deformation at pressure–temperature conditions of the mantle transition zone (17.6 GPa and 1800–1900 K) using a deformation-DIA apparatus. The deformation experiments reveal that the [0 0 1] axis and the (0 1 0) plane of wadsleyite tend to be subparallel to the shear direction and the shear plane during deformation, respectively. These results demonstrate that polarization seismic anisotropy (velocity contrast between horizontally-polarized and vertically-polarized S-waves, VSH/VSV) observed in the mantle transition zone might be attributed to the preferred orientation of wadsleyite caused by horizontal mantle flow.