Thermoelasticity of CaSiO 3 perovskite and implications for the lower mantle

We report measurements on thermal expansion of CaSiO 3 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 CaSiO 3 perovskite and the lower‐pressure phase assemblage Ca 2 SiO 4 + CaSi 2 O 5 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 V 0 = 45.83 (7) Å 3 and K 0 = 280(23) GPa. Over a wide temperature range of 600–1600 K, the average thermal expansion of CaSiO 3 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 MgSiO 3 perovskite to examine constraints on lower mantle composition. The predicted density of CaSiO 3 perovskite is similar to that of PREM and the bulk modulus similar to that of (Mg,Fe)SiO 3 perovskite under lower mantle conditions. Thus, including CaSiO 3 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.