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- Title
Elasticity of Hydrated Al‐Bearing Stishovite and Post‐Stishovite: Implications for Understanding Regional Seismic V<sub>S</sub> Anomalies Along Subducting Slabs in the Lower Mantle.
- Authors
Zhang, Yanyao; Fu, Suyu; Karato, Shun‐ichiro; Okuchi, Takuo; Chariton, Stella; Prakapenka, Vitali B.; Lin, Jung‐Fu
- 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 (VP and VS) and full elastic moduli (Cij) 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‐SiO2 (1.34 mol% Al and 0.55 mol% H) and Al2.1‐SiO2 (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 Cij 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‐SiO2 and to 16.1 GPa in Al2.1‐SiO2, where the transition is manifested by approximately 29% VS 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)% VS anomaly. The VS anomalies across the Al,H‐bearing post‐stishovite transition can help explain the seismically‐observed depth‐dependent VS anomalies along some subducting slabs in the top‐ to mid‐lower‐mantle depths including the Tonga subducting slab. Plain Language Summary: Seismologists have found that shear wave travels 2%–12% slower along some regions of subducting slabs at 700–1,900 km depths than the surrounding lower mantle. This observation cannot be explained by the presence of cold subducting oceanic crusts alone, but the transition from stishovite to post‐stishovite could be a possible cause. Stishovite is a high‐pressure dense silica polymorph that makes up about one fifth volume of subducting mid‐ocean ridge basalt in the lower mantle. We designed high‐pressure Raman spectroscopy and X‐ray diffraction experiments to probe lattice vibration modes and lattice parameters, respectively, of Al,H‐bearing stishovite and post‐stishovite. These results are used to evaluate the speed of sound across the post‐stishovite transition. Our study shows that the shear wave velocity of stishovite with 1.3–2.1 mol% Al and 0.5–0.6 mol% H significantly slows down by −29% at 16–21 GPa. If one fifth volume of the subducting oceanic crust is made of stishovite with 1.3 mol% Al and 0.6 mol% H, the velocity reduction across the transition could be ∼7% at ∼1,060 km depth. Regional seismic observations of VS anomalies along some subducting slabs in the top‐ to mid‐lower mantle can be explained by the presence of the Al,H‐bearing post‐stishovite transition. Key Points: Elastic moduli across the Al,H‐bearing post‐stishovite transition are derived from high‐pressure Raman and X‐ray diffraction dataStishovite with 1.3 mol% Al and 0.6 mol% H undergoes the post‐stishovite transition at ∼1,060 km depth with −29% VS anomalyVS reductions across the Al,H‐bearing post‐stishovite transition can explain regional seismic VS anomalies along some subducting slabs
- Subjects
TONGA; SLABS (Structural geology); FRICTION velocity; SPEED of sound; LANDAU theory; SHEAR waves; SEISMIC waves
- Publication
Journal of Geophysical Research. Solid Earth, 2022, Vol 127, Issue 4, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2021JB023170