Elastic properties of stishovite and the CaCl2-type silica at the mantle temperature and pressure: an ab initio investigation

Abstract:

Stishovite is stable at 9~50GPa and further transformed into CaCl₂-type silica. It is estimated that stishovite makes up more than 20% of the subducted oceanic crust in the mantle transition zone and lower mantle. Therefore, the properties of stishovite and its polymorph are critical for us to understand the mantle convection. We calculated the elastic constants of stishovite and CaCl₂-type silica at high-temperature and -pressure using the new method developed by Wu and Wentzcovitch [1], which requires only tenth of computational time of the conventional first principles method. The elastic properties of stishovite show not only strong pressure dependence but also temperature dependence. By increasing temperature, the shear instability of stishovite is shifted to an elevated pressure with a slope of ~5.4±1.4 MPa/K. The softening of the shear modulus and the positive Clapeyron slope result in crossing of the sound velocities at different temperatures, which leads to the unusual positive temperature dependence of the sound velocities around the phase boundary. The transition from stishovite to the CaCl₂-type silica at the lower mantle’s temperature occurs at a depth far deeper than 1200 km and is accompanied by a velocity jump ~ 0.98±0.08 km/s in S wave velocity (V_S)  and ~ 0.45±0.15 km/s in P wave velocity (V_P). This transition is likely related to the seismic discontinuity at the depth ~1670 km in the vicinity of Mariana Island. The unusual positive temperature dependence of V_S of stishovite and strong anisotropy of stishovite and the CaCl₂-type silica around the phase boundary provide potential ways to identify the origin of the seismic discontinuity [2].

[1]. Wu, Z., Wentzcovitch, R. M., Phys. Rev. B 83, 184115 (2011)

[2] Yang, R., Wu, Z., Earth Planet. Sci. Lett. in press.