Deformation of δ‐AlOOH and Its Solid Solution With Phase H as a Potential Source of Intra‐Slab Seismic Anisotropy in the Mid‐Mantle

Abstract Seismic anisotropy is widely observed near subduction zones in the mantle transition zone and uppermost lower mantle, particularly along the western Pacific rim and tracks slabs geometries, implying an additional slab‐related source. Hydrous phases such as δ‐AlOOH and phase H (MgSiO 4 H 2 ), which form a solid solution (δ‐H) and are stable in cool, hydrated slabs, are potential contributors. We performed well‐controlled deformation experiments on δ‐AlOOH and δ‐H at 20.5–24.5 GPa and 800°C–1000°C. Deformed aggregates developed strong crystallographic preferred orientation (CPO), and simple‐shear experiments on δ‐AlOOH identified (010)[001] as the likely dominant slip system, with subsidiary systems inferred from intragranular misorientation. The resulting CPOs produce vertically polarized shear‐wave velocities exceeding horizontally polarized velocities ( V SV > V SH ) with strong azimuthal anisotropy under sub‐horizontal shearing. We suggest that these phases generate negative radial anisotropy under horizontal flow and can partly contribute to the anisotropy near flattened slab tops.