Anisotropy of akimotoite: A molecular dynamics study

Elastic constants of akimotoite at high-temperatures and pressures are calculated by using molecular dynamics simulations to study the anisotropic properties of akimotoite. We find that akimotoite becomes more anisotropic with increasing temperature but less with increasing pressure. On the (0 0 1) face, S-wave splitting and azimuthal anisotropy will occur simultaneously while the P-wave velocity remains almost constant. Polarization directions of the two S-waves are neither strictly perpendicular nor parallel to the crystallographic face. The polarization direction of the fast S-wave is more parallel with the plane than that of the slow S-wave for all wave propagation directions. On the (1 0 0) face, no S-wave splitting will occur for waves passing along the crystallographic c-axis, and the P-wave velocity along the axis is smaller than that in other directions. Polarization anisotropy changes its sign with the variation of wave propagation directions, giving rise to the dependence of the relative magnitude of V_SH and V_SV on the seismic wave propagation directions. We further calculate the anisotropic properties of a mineral assemblage containing ringwoodite, akimotoite, and Ca perovskite to infer the effect that akimotoite may have on the anisotropy of the lower portion of the mantle transition zone. We find that the mineral assemblage is much less anisotropic than akimotoite, but possesses many anisotropy features of akimotoite. Comparison of the anisotropy features with seismological observations indicates that a vertical flow direction is required in areas close to the subduction zones, which may be caused by the downward penetration of slabs to the lower mantle as well as upwelling of fluids and light materials in the mantle wedge associated with deep dehydration reactions of the slabs.