Single-crystal elasticity of (Al,Fe)-bearing bridgmanite up to 82 GPa

Thermoelastic properties of mantle candidate minerals are essential to our understanding of geophysical phenomena, geochemistry, and geodynamic evolutions of the silicate Earth. However, the lower-mantle mineralogy remains much debated due to the lack of single-crystal elastic moduli (C-ij) and aggregate sound velocities of (Al,Fe)-bearing bridgmanite, the most abundant mineral of the planet, at the lower mantle pressure-temperature (P-T) conditions. Here we report single-crystal C-ij of (Al,Fe)-bearing bridgmanite, Mg0.88Fe0.1Al0.14Si0.90O3 (Fe10-Al14-Bgm) with Fe3+/Sigma Fe = similar to 0.65, up to similar to 82 GPa using X-ray diffraction (XRD), Brillouin light scattering (BLS), and impulsive stimulated light scattering (ISLS) measurements in diamond-anvil cells (DACs). Two crystal platelets with orientations of (-0.50, 0.05, -0.86) and (0.65, -0.59, 0.48), that are sensitive to deriving all nine C-ij, are used for compressional and shear wave velocity (v(P) and v(S)) measurements as a function of azimuthal angles over 200 degrees at each experimental pressure. Our results show that all C-ij of singe-crystal Fe10-Al14-Bgm increase monotonically with pressure with small uncertainties of 1-2% (+/- 1 sigma), except C-55 and C-23, which have uncertainties of 3-4%. Using the third-order Eulerian finite-strain equations to model the elasticity data yields the aggregate adiabatic bulk and shear moduli and respective pressure derivatives at the reference pressure of 25 GPa: K-S = 326 +/- 4 GPa, mu = 211 +/- 2 GPa, K-s ' = 3.32 +/- 0.04, and mu ' = 1.66 +/- 0.02 GPa. The high-pressure aggregate v(S) and v(P) of Fe10-Al14-Bgm are 2.6-3.5% and 3.1-4.7% lower than those of MgSiO3 bridgmanite end-member, respectively. These data are used with literature reports on bridgmanite with different Fe and Al contents to quantitatively evaluate pressure and compositional effects on their elastic properties. Comparing with one-dimensional seismic profiles, our modeled velocity profiles of major lower-mantle mineral assemblages at relevant P-T suggest that the lower mantle could likely consist of about 89 vol% (Al,Fe)-bearing bridgmanite. After considering uncertainties, our best-fit model is still indistinguishable from pyrolitic or chondritic models.