High pressure and temperature fabric transitions in olivine and variations in upper mantle seismic anisotropy

The effects of pressure on crystallographic preferred orientation (CPO) of olivine aggregates were investigated through simple-shear deformation experiments at pressures between 2.1 and 7.6 GPa and temperatures of 1493–1673 K under dry conditions using a deformation-DIA apparatus, and the variations in seismic anisotropy were evaluated under the Earthʹs upper mantle conditions. We found that the monotonic decrease in seismic anisotropy with depth is caused by the pressure-dependency of the seismic properties of A-type (developed by the (010) [ 100 ] slip system) olivine fabric, while the rapid decrease is caused by the fabric transition from A-type to B/C-type (by the (hk0) [ 001 ] slip systems) at 7.6 GPa and 1673 K. Moreover, an alternative transition, from A-type fabric to B-type-like fabric (by the (010) [ 001 ] slip system), occurs at 7.6 GPa and lower temperature. These two temperature-dependent fabric transitions occurring at 7.6 GPa result in low seismic anisotropy with VSH/VSV (the ratio of horizontally and vertically polarized shear waves) > 1 at low temperatures (i.e., old-continental mantle conditions) and VSH/VSV < 1 at high temperatures (i.e., oceanic mantle conditions) at greater depths, consistent with seismological observations. Thus, the variations of CPO with pressure and temperature in olivine under dry conditions can explain the seismic anisotropy signatures observed in the upper mantle, without invoking other mechanisms.