Brillouin Scattering Observations of Elastic Behavior Across the Post‐Stishovite Transition Under Deviatoric Stress in Polycrystalline Al‐Bearing Stishovite

Abstract Stishovite (SiO 2 ), a key phase in subducted mid‐ocean ridge basalt (MORB), undergoes a ferroelastic transition to a CaCl 2 ‐type structure, potentially explaining localized shear wave velocity ( V S ) anomalies in the shallow to mid‐lower mantle. However, the phase boundary determining this transition remains debated. The transition is strongly influenced by stress and chemical impurities. We examined the influence of strong deviatoric stress on the transition pressure and elastic anomalies of polycrystalline Al‐bearing stishovite using Brillouin scattering spectroscopy up to 37 GPa. In this study, Brillouin spectroscopy provides the primary constraint, with synchrotron XRD and Raman measurements used only to confirm the structural state of the sample. A 13% drop in V S was observed at 8 GPa under deviatoric stress, with the uniaxial stress component of | t | = 5.3 GPa. This transition pressure is lower than previous studies, reporting transitions above 16 GPa. The ∼5.3 GPa stress inferred here is comparable to ∼4 GPa in subducting MORB slabs at 100–300 km depth. Our findings suggest that this transition could explain small‐scale seismic scatterers near ∼700 km depth in subduction zones of northwestern Pacific (e.g., northwest Japan), or Fiji‐Tonga, potentially extending to ∼1,825 km. The reconstructed MORB phase boundary indicates that deeper scatterers are favored under modest deviatoric stress (∼1 GPa) and elevated temperatures relative to cold slabs. These conditions correspond to pressure‐temperature regimes where previous experimental studies report limited Al partitioning into stishovite, resulting in Al 2 O 3 contents of approximately ∼0.4 wt.%.