Elasticity of akimotoite under the mantle conditions: Implications for multiple discontinuities and seismic anisotropies at the depth of ∼600–750 km in subduction zones

The equation of state and elastic properties of akimotoite at simultaneously high pressures and high temperatures are obtained using first-principles calculations based on the density functional theory (DFT). The calculated results agree with the available experimental data. Combining our results with the elastic data of other minerals, we estimated the VP, VS, and density contrasts caused by the akimotoite-related transitions. The velocity contrasts between akimotoite and bridgmanite are 4.6% and 8.3% for VP and VS, respectively, which are only about half of those between majorite and akimotoite. Moreover, because both the akimotoite-bridgmanite and majorite-akimotoite transitions have broad phase boundaries, these two phase transitions may not contribute to multiple discontinuities around ∼660 km depth in subduction zones as detected by seismic studies. Instead, the decomposition of pyrope into bridgmanite and corundum, which would occur in cold subduction zones with a sharp phase boundary and a large impedance contrast due to the inhibition of the pyroxene-garnet transformation at relatively low temperatures, could be a more reasonable explanation for the discontinuity at ∼700–750 km in subduction zones. Furthermore, the transformation from high-pressure clinopyroxene to akimotoite at the depth of ∼600 km can increase the VP, VS, and density by 10.1%, 14.8%, and 9.9%, respectively, indicating that the phase transition may account for the local discontinuity at ∼600 km in some subduction zones. In addition, the anisotropies of akimotoite are significantly higher than those of other major minerals at the base of the mantle transition zone and could be the origin of the seismic anisotropies detected in some subduction zones.