The 4th International Conference on Matter and Radiation at Extremes (ICMRE 2019)

559 / 2019-03-20 17:06:01

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

Akimotoite; Thermoelasticity; Multiple discontinuities; Seismic anisotropies

Plenary track > High Pressure Physics and Materials Science

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). Our results agree well with previous experimental results and provide more data at the mantle conditions. Combing with our calculated data of other minerals, we investigated the VP, VS, and density contrasts of the akimotoite-related transitions. The velocity contrasts between akimotoite and bridgmanite are only about 1/3 of those between majorite and bridgmanite. Since the transformation from akimotoite to bridgmanite is as broad as the one from majorite to bridgmanite, multiple discontinuities around ~660 km depth in subduction zones disclosed by many seismological studies are probably irrelevant to akimotoite-bridgmanite transition. Instead, the decomposition of pyrope, occurring at cold subduction zones owing to the inhibition of the pyroxene-garnet transformation below ~1823/1673 K, has a large impendence contrast and a sharp phase boundary, and therefore could be a more reasonable explanation for the discontinuity at the depth of ~700-750 km. Furthermore, the coexisting high-pressure enstatite could directly transform to akimotoite at the depth of ~600 km. Our studies indicate that the transformation can increase VP, VS, and density by 10.1%, 14.8%, 9.9%, respectively, and thus can interpret another discontinuity at the depth of ~600 km in some subduction zones. The anisotropies of akimotoite are significantly higher than those of the other major minerals at the base of the mantle transition zone. The strong anisotropy and crystallographic preferred orientation of akimotoite could be the origin of seismic anisotropy detected in many subduction zones.