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

Abstract

In this study, we investigated the single-crystal elasticity of (Al,Fe)-bearing bridgmanite (Bgm) with chemical compositions of Mg0.95Fe0.0332+Fe0.0273+Al0.04Si0.96O3 (Fe6-Al4-Bgm) and Mg0.89Fe0.0242+Fe0.0963+Al0.11Si0.89O3 (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 (VP, VS) 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 VS and VP 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 VS splitting anisotropy of (Al,Fe)-bearing Bgm at the two experimental pressures: Fe6-Al4-Bgm exhibits the highest VS 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 VS 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 VS 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% VS radial anisotropy at depths of ∼670 and ∼920 km, respectively. These findings provide mineral physics explanations to the distinct seismically-detected VS radial anisotropies at the topmost lower mantle near subducted slabs, especially in the Tonga-Kermadec subduction region.