Single-crystal elasticity of (Al,Fe)-bearing bridgmanite and seismic shear wave radial anisotropy at the topmost lower mantle

In this study, we investigated the single-crystal elasticity of (Al,Fe)-bearing bridgmanite (Bgm)with chemical compositions of Mg_0.95Fe_0.033 ²⁺Fe_0.027 ³⁺Al_0.04Si_0.96O₃ (Fe6-Al4-Bgm)and Mg_0.89Fe_0.024 ²⁺Fe_0.096 ³⁺Al_0.11Si_0.89O₃ (Fe12-Al11-Bgm)using combined experimental results from Brillouin light scattering (BLS), impulsive stimulated light scattering (ISLS), and X-ray diffraction (XRD)measurements in diamond anvil cells at 25 and 35 GPa. Based on experimentally measured compressional and shear wave velocities (V_P, V_S)as a function of azimuthal angles within selected crystal platelets that are sensitive to derivation of nine elastic constants for each composition, we reliably derived the full elastic constants of Fe6-Al4-Bgm and Fe12-Al11-Bgm at the two experimental pressures. Our results show that the combined Fe and Al substitution results in a reduction of both V_S and V_P in Fe12-Al11-Bgm up to 2.6(±0.5)% and 1.5(±0.3)%, respectively, compared with those in Fe6-Al4-Bgm at the experimental pressures. In particular, we observed strong combined Fe and Al effects on V_S splitting anisotropy of (Al,Fe)-bearing Bgm at the two experimental pressures: Fe6-Al4-Bgm exhibits the highest V_S splitting anisotropy of ∼8.23-9.0% along the [001]direction, while the direction shifts to the midway between [100]and [001]directions for Fe12-Al11-Bgm with V_S splitting anisotropy of ∼7.68-11.06%. Combining the single-crystal elasticity data of Fe6-Al4-Bgm and Fe12-Al11-Bgm with the crystallographic preferred orientation (CPO)results of deformed Bgm at relevant lower-mantle pressure-temperature (P-T)conditions from literature, we modeled the seismic V_S radial anisotropy of deformed (Al,Fe)-bearing Bgm near a subducting slab at conditions relevant to the topmost lower mantle. Taking into account the Fe and Al contents in (Al,Fe)-bearing Bgm with depth in the Earth's topmost lower mantle, the results of our model show that the deformation of Fe6-Al4-Bgm and Fe12-Al11-Bgm crystals would produce ∼0.9% and ∼0.8% V_S radial anisotropy at depths of ∼670 and ∼920 km, respectively. These findings provide mineral physics explanations to the distinct seismically-detected V_S radial anisotropies at the topmost lower mantle near subducted slabs, especially in the Tonga-Kermadec subduction region.