Elasticity and phase stability of pyrope garnet from ab initio computation

•High pressure evolution of pyrope reported experimentally is well explained.•Pyrope as well as grossular is found to be very isotropic.•Elasticity of pyrope is essential to understand mineralogy in the MTZ.

We study the high-pressure stability and elastic properties of Mg3Al2Si3O12 pyrope garnet using the density functional first principles computation method. Pyrope garnet is found to dissociate into an assemblage of MgSiO3 Mg-perovskite (Pv) and Al2O3 corundum (Cor) solid solutions at ∼19.7 GPa at static conditions. Then, this assemblage undergoes a phase transition to pyropic (Al-bearing) Pv at ∼65 GPa. The enthalpy of an assemblage of MgAl2O4 calcium ferrite (CF), MgPv, and stishovite (St) is less stable than that of MgPv plus Cor. A continuous reaction in the MgSiO3–Al2O3 system suggested by this study is consistent with previous experimental and computational studies but not with a recently modeled phase diagram. This implies that the formation of pyropic Pv could not cause any seismic scatterers in the mid-lower mantle. The investigated anisotropy of elastic velocities further indicates that pyrope garnet is a very isotropic mineral. Our results suggest that seismological anisotropy inferred in the upper mantle could not be due to garnet.