High pressure elastic anisotropy of MgSiO3 perovskite and geophysical implications

Using plane wave pseudopotential method within the local density approximat ion (LDA), we calculate single-crystal elastic constants (c(ij)) of orthorh ombic MgSiO3 perovskite, generally accepted to be the major component of th e lower mantle, as a function of pressure up to 150 GPa. Our results are in excellent agreement with experimental data at zero pressure and compare fa vorably with other pseudopotential predictions over the pressure regime stu died. Here we use our elastic constants to calculate anisotropy of seismic wave velocities as a function of pressure (depth). MgSiO3 perovskite is sho wn to be highly anisotropic in all portions of the lower mantle and the nat ure of anisotropy changes significantly with depth. The absence of signific ant seismic anisotropy in most of the lower mantle suggests that MgSiO3 per ovskite assumes nearly random orientation in most of this region. Anisotrop y at the topmost lower mantle suggested by some studies can be attributed t o the preferred orientation of perovskite. However, anisotropy in the D " l ayer is difficult to be attributed to preferred orientation of perovskite. Some other mechanisms including the presence of the aligned melt pockets an d/or lattice preferred orientation of magnesiowustite are needed. (C) 1998 Elsevier Science B.V. All rights reserved.