Elasticity of Hydrated Al‐Bearing Stishovite and Post‐Stishovite: Implications for Understanding Regional Seismic <i>V</i><sub><i>S</i></sub> Anomalies Along Subducting Slabs in the Lower Mantle
Yanyao Zhang, Suyu Fu, Shun‐ichiro Karato, Takuo Okuchi, Stella Chariton, Vitali B. Prakapenka, Jung‐Fu Lin
Abstract
Abstract 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 to these seismic features, but compressional and shear wave velocities ( V P and V S ) and full elastic moduli ( C ij ) of Al,H‐bearing stishovite and post‐stishovite at high pressure remain uncertain. Here we have determined Raman shifts of optic modes and equation of state parameters of two hydrated Al‐bearing stishovite crystals, Al1.3‐SiO 2 (1.34 mol% Al and 0.55 mol% H) and Al2.1‐SiO 2 (2.10 mol% Al and 0.59 mol% H), up to ∼70 GPa in diamond anvil cells coupled with Raman spectroscopy and X‐ray diffraction. The experimental data are modeled using a pseudoproper Landau theory to derive full C ij and sound velocities across the post‐stishovite transition at high pressure. The Al and H dissolution in stishovite significantly reduces the transition pressure to 21.1 GPa in Al1.3‐SiO 2 and to 16.1 GPa in Al2.1‐SiO 2 , where the transition is manifested by approximately 29% V S reduction. Considering that stishovite with approximately 1.3 mol% Al and 0.6 mol% H could account for 20 vol% in subducted MORB at the top‐lower mantle, the Al,H‐bearing post‐stishovite transition with a Clapeyron slope of 65 K/GPa would occur at about 1060 km depth with −7(4)% V S anomaly. The V S anomalies across the Al,H‐bearing post‐stishovite transition can help explain the seismically‐observed depth‐dependent V S anomalies along some subducting slabs in the top‐ to mid‐lower‐mantle depths including the Tonga subducting slab.