High-pressure sound velocity of perovskite-enstatite and the possible composition of the Earth's lower mantle

The compressional sound velocity V-p for enstatite of polycrystalline specimens were measured at pressures from 40 to 140 GPa using the optical analytical techniques under shock loading. The dependence of V-p (in km/s) on Hugoniot pressure (P, in GPa) can be described by in V-p = 3.079 - 0.691 In(P) + 0.094(InP)(2). V-p satisfies Birch's law: V-p = 4.068 + 1.677 rho, where rho is corresponding density, which indicated that enstatite is stable throughout the conditions of the lower mantle. The wave velocity P is 0.5% lower and the wave velocity S is 2% higher than that of PREM respectively. We concluded that the lower mantle is mainly composed of perovskite-(Mg1-x, Fe-x) SiO3 and only a small amount of (Mg1-x, Fe-x) O is allowed in it.