Pressure and temperature dependence of elastic wave velocity of MgSiO 3 perovskite and the composition of the lower mantle

Abstract

Simultaneous measurements of compressional ( P) and shear ( S) wave velocities and unit cell volume (density) have been conducted on MgSiO 3 perovskite to 9.2 GPa and 873 K using ultrasonic interferometry in conjunction with X-ray diffraction. Finite strain analysis of current data allowed us to determine the elastic moduli and their pressure derivatives independent of the pressure measurement. The results are K S = 253(2) GPa, G = 173(1) GPa, ∂ K S / ∂P = 4.4(1) and ∂G/ ∂P = 2.0(1). The temperature derivatives of the bulk and shear moduli were tightly constrained from acoustic velocity measurements, yielding ∂ K S /∂ T = −0.021(2) GPa/K and ∂ G/∂ T = −0.028(2) GPa/K. P and S wave velocities and density profiles for a pyrolite compositional model consisting of magnesium silicate perovskite and magnesiowustite are calculated at lower mantle depths along a 1600 K adiabatic geotherm, the results agree with those of seismic model PREM within 0.5, 0.5 and 0.3%, respectively, from 800 to ∼2600 km. The effect of Fe and Mg partitioning between magnesium silicate perovskite and magnesiowustite on the calculated velocities and density is negligibly small compared to the effects of composition and temperature. Simultaneous constraints from P and S wave velocities and density support for a homogeneous lower mantle with pyrolitic composition.