Sound velocities and elasticity of aluminous MgSiO₃ perovskite: Implications for aluminum heterogeneity in Earth's lower mantle

Aluminum has been reported to have a remarkably strong effect on the thermoelastic properties of MgSiO₃ perovskite. However, the sound velocities of aluminous MgSiO₃ perovskite have not been previously measured, even though this phase likely dominates most of the chemistry in Earth's lower mantle. Here we report the first sound velocity measurements on aluminous MgSiO₃ perovskite using Brillouin spectroscopy and obtain the following values for the room-pressure room-temperature adiabatic bulk and shear moduli: K_s = 252 ± 5 GPa and μ = 165 ± 2 GPa, respectively. The presence of 5.1 ± 0.2 wt.% A1₂O₃ in MgSiO₃ perovskite decreases the shear modulus by 5.6%. However, within experimental uncertainties, there is no discernable effect of aluminum on the bulk modulus. We find that variations in the aluminum content Of MgSiO₃ perovskite may provide an explanation for some observed lateral heterogeneity in Earth's lower mantle.