Thermoelasticity of CaSiO3 perovskite and implications for the lower mantle

Abstract

We report measurements on thermal expansion of CaSiO3 perovskite within the stability field and its room temperature volume‐pressure behavior, as well as in situ determination of the perovskite stability field. The phase boundary between CaSiO3 perovskite and the lower‐pressure phase assemblage Ca2SiO4 + CaSi2O5 was determined at temperatures between 1200 and 1600 K, using NaCl as the pressure standard. At 1590(20) K, the boundary is located at about 11.3(3) GPa with a positive slope, in close agreement with previous quench studies. At 300 K, the perovskite structure remains metastable at pressures ∼ 2 GPa, below which the sample transforms into an amorphous phase, with an anomalous volume decrease of the remaining perovskite. The volume data above 2 GPa are fit using a second‐order Birch‐Murnaghan equation of state, yielding V0 = 45.83 (7) Å3 and K0 = 280(23) GPa. Over a wide temperature range of 600–1600 K, the average thermal expansion of CaSiO3 perovskite is α = 2.69(8) and 2.56(8)×10−5 K−1 at 10.6 and 11.7 GPa, respectively, with little temperature dependence. These data are combined with results on MgSiO3 perovskite to examine constraints on lower mantle composition. The predicted density of CaSiO3 perovskite is similar to that of PREM and the bulk modulus similar to that of (Mg,Fe)SiO3 perovskite under lower mantle conditions. Thus, including CaSiO3 yields an Fe/(Mg+Fe) ratio of 0.12(1) and an (Mg+Fe+Ca)/Si ratio of 1.7(3) for the upper portion of the lower mantle.