Thermoelastic behaviour of silicate perovskites: Insights from new high-temperature ultrasonic data for ScAlO 3

Abstract

(Mg,Fe,Al)(Si,Al)O 3 perovskite, stable only at pressures >20 GPa, is the dominant mineral of the Earth's lower mantle and thus controls its physical properties including seismic wave speeds. However, the development of a thorough understanding of the thermoelastic behaviour of this silicate perovskite is compromised by its marginal metastability on recovery at ambient conditions. Study of the close structural analogue ScAlO 3, stable at much lower pressure, has the potential to provide insight into the behaviour of its silicate cousin. Here, previous exploratory ultrasonic measurements of the compressional and shear wave speeds on a fine-grained polycrystalline specimen have been extended to 1000 K under 300 MPa confining pressure within an internally heated gas-medium high-pressure apparatus. The wave speeds and derived bulk and shear moduli vary approximately linearly with temperature with derivatives ( ∂K S / ∂T) P = −21.3(3) MPa K −1 and ( ∂G/ ∂T) P = −19.0(1) MPa K −1, respectively. The new data, along with previous measurements of thermal expansion and the pressure dependence of the elastic moduli, have been assimilated into the comprehensive internally consistent finite-strain model of thermoelastic behaviour proposed by [Stixrude, L., Lithgow-Bertelloni, C., 2005. Thermodynamics of mantle minerals—I. Physical properties. Geophys. J. Int. 162, 610–632]. Comparison of the newly constrained model for ScAlO 3 with emerging data for MgSiO 3 perovskite provides guidance in the analysis of the chemical composition and temperature of the Earth's lower mantle.