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. As the pressure increases, the temperature dependences of elastic moduli and wave velocities are suppressed significantly. At 0 GPa, V_P and V_S reduced 7.4% and 8.8% from 300 K to 2000 K respectively, while the corresponding reductions at 25 GPa are 3.2% and 3.5% respectively.

Combining our results with the elastic data of other minerals, we estimated the V_P, V_S, and density contrasts caused by the akimotoite-related transitions (Figure 1 and Table 1). The velocity contrasts between akimotoite and bridgmanite are 4.6% and 8.3% for V_P and V_S, respectively, which are only about a third of those between majorite and bridgmanite. Moreover, because both the akimotoite-bridgmanite and majorite-bridgmanite transitions have broad phase boundaries, these two phase transitions may not contribute to multiple discontinuities below ~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 due to the inhibition of the pyroxene-garnet transformation at relatively low temperatures, has a sharp phase boundary and could cause a large impedance contrast. Therefore, the decomposition of pyrope 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 V_P, V_S, 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 depth 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 (Figure 2). The crystallographic preferred orientation of akimotoite could be the origin of the seismic anisotropies detected at the base of the mantle transition zone in some subduction zones.