Thermal expansion of silicate perovskite and stratification of the Earth's mantle

Abstract

A pressure of 24 GPa and temperatures of about 2,000–3,000 K correspond to the observed 670-km seismic discontinuity which separates the upper and lower mantle. In such conditions the major minerals of the Earth's upper mantle, olivine ((Mg, Fe)₂SiO₄), pyroxene ((Mg, Fe)SiO₃) and garnet ((Mg, Fe, Ca)₃Al₂Si₃O₁₂) transform to a distorted (orthorhombic) perovskite-structured mineral ((Mg, Fe)SiO₃), or to a perovskite-dominated assemblage (refs 1–4). Because silicate perovskite is stable to at least 70 GPa, it is thought to be the most abundant mineral in the lower mantle and, possibly, in the entire Earth. Despite its importance, silicate perovskite was only discovered in 1976, and little is known about its physical properties because of the difficulty in achieving the conditions of pressure and temperature required for the synthesis of this phase. For example, the value of the thermal expansion coefficient of perovskite provides an important constraint on possible compositional models for the lower mantle^5–7. We have recently produced sufficient amounts of (Mg_0.9, Fe_0.1)SiO₃ perovskite to measure its zero-pressure thermal expansion to 840 K by X-ray diffraction. At high temperatures, the average thermal expansion coefficient is 4×l0⁻⁵K⁻¹. Such a large value for the thermal expansion coefficient implies that standard models of upper mantle composition, such as pyrolite or garnet peridotite (Mg value ≍ 0.89), yield zero-pressure densities that are about 2% lower than that of the density of the lower mantle extrapolated to zero pressure conditions. This result suggests that the upper and lower mantle are chemically distinct, in accord with layered models of the thermal and convective state of the mantle.