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 InV p = 3.079-0.691 In( P ) + 0.094(lnP) 2 . V p satisfies Birch’s law: V p = 4.068 + 1.677 ρ , where ρ 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-(Mg 1-x , Fe x ) SiO 3 and only a small amount of (Mg 1-x , Fe x ) O is allowed in it.