Elasticity of Corundum at High Pressures and Temperatures: Implications for Pyrope Decomposition and Al‐Content Effect on Elastic Properties of Bridgmanite

Abstract

AbstractThe thermodynamic and elastic properties of corundum at high pressures and temperatures are calculated based on density functional theory within the local density approximation. The calculated results agree well with the available experimental results and provide reliable data up to 80 GPa and 2,500 K. Our results confirm that C14 should be positive at ambient conditions. Elastic moduli of corundum, especially its shear modulus, exhibit noticeable nonlinear dependences with pressure and thereby result in the nonlinear pressure dependences of wave velocities. The decomposition of pyrope into the assemblage of bridgmanite and corundum is also investigated within the generalized gradient approximation. The calculated phase boundary is comparable to experimental results and has a positive Clapeyron slope of +2.1 MPa/K. Pyrope decomposition can result in VP, VS, and density jumps of 12.4%, 20%, and 9.8%, respectively. A small amount of pyrope dissociation can cause a large impendence contrast and may partially account for the observed discontinuities below 660 km at the subduction zone. Combining our calculations with results from the previous study, we suggest that the incorporation of Al into bridgmanite shows no discernable effect on its elastic moduli and wave velocities. This may impede the detection of Al content in bridgmanite through seismological methods.