Elasticity of Hydrated Al‐Bearing Stishovite and Post‐Stishovite: Implications for Understanding Regional Seismic <i>V</i> _<i>S</i> Anomalies Along Subducting Slabs in the Lower Mantle

Seismic studies have found seismic scatterers with -2 to -12% shear velocity anomalies along some subducting slabs at 700-1900 km depth. The ferroelastic post-stishovite transition in subducted mid-ocean ridge basalt (MORB) has been linked these features, but compressional and wave velocities (VP VS) full elastic moduli (Cij) of Al,H-bearing stishovite high pressure remain uncertain. Here we determined Raman shifts optic modes equation state parameters two hydrated Al-bearing crystals, Al1.3-SiO2 (1.34 mol% Al 0.55 H) Al2.1-SiO2 (2.10 0.59 H), up ∼70 GPa diamond anvil cells coupled spectroscopy X-ray diffraction. experimental data are modeled using a pseudoproper Landau theory derive Cij sound across the pressure. H dissolution significantly reduces 21.1 16.1 Al2.1-SiO2, where is manifested by approximately 29% VS reduction. Considering that 1.3 0.6 could account for 20 vol% MORB top-lower mantle, Clapeyron slope 65 K/GPa would occur about 1060 depth -7(4)% anomaly. can help explain depth-dependent seismically-observed top- mid-lower-mantle depths including Tonga slabs.