High-pressure Sound Velocity of Perovskite-Enstatite and Possible Composition of Earths Lower Mantle

The compressional sound velocity v_p for enstitate of polycrystalline specimens was measured at pressures from 40 to 140 GPa by using the optical analyzer techniques under shock loading. The dependence of v_p (in units of km/s) on Hugoniot pressure p (in units of GPa) can be described by: v_p = 3.079 - 0.691 lnp + 0.0941n²p. The v_p satisfies Birch's law: v_p = 6.213 + 1.212 ρ, where ρ is corresponding density, indicating that enstatite is stable throughout the conditions of the lower mantle. Here P wave velocity is 0.5% lower and S wave velocity is 2% higher than those of preliminary reference earth model, respectively. It is concluded that the lower mantle is mainly composed of perovskite-(Mg_1-x,Fe_x)SiO₃ and only small amount of (Mg_1-x,Fe_x)O is allowed in it.