Change of olivine a-axis alignment induced by water: Origin of seismic anisotropy in subduction zones

The effects of water on the crystallographic preferred orientation (CPO) of olivine aggregates were investigated through simple-shear deformation experiments under asthenospheric upper mantle conditions (pressure = 2.1–5.2 GPa, temperature = 1490–1830 K) using a deformation-DIA apparatus. Formation of the A-type olivine fabric (developed by the (010)[100] slip system) was observed under water-depleted conditions (COH < 650 ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) or a B-type-like fabric (by the (010)[h0l] slip system) were predominantly formed under water-rich conditions (> 1000 ppm H/Si). In comparison with fabrics of anhydrous olivine (≤ 111 ppm H/Si), those of olivine having higher water contents (≥ 625 ppm H/Si in olivine) showed stronger anisotropic properties (e.g., P-wave anisotropy, S-wave polarization anisotropy, and the ratio of horizontally and vertically polarized shear waves). The water-induced olivine CPO transition from A-type to B-type(-like) fabric accompanies a change in the alignment of the seismic fast a-axes, resulting in flow-parallel and flow-perpendicular shear wave splitting under water-depleted and water-rich conditions, respectively. The rotation of the fast direction of shear-wave splitting across an arc, which is observed in many subduction zones, is well explained by the likely bimodal water distribution along the mantle wedge (i.e., water-rich in fore-arc and water-depleted in back-arc regions) and the developments of two different types of olivine fabrics (i.e., B-type(-like) fabric in fore-arc and A-type fabric in back-arc regions).