P–V–T equation of state of CaAl4Si2O11 CAS phase

Abstract

The thermoelastic parameters of the CAS phase (CaAl4Si2O11) were examined by in situ high-pressure (up to 23.7 GPa) and high-temperature (up to 2,100 K) synchrotron X-ray diffraction, using a Kawai-type multi-anvil press. P–V data at room temperature fitted to a third-order Birch–Murnaghan equation of state (BM EOS) yielded: V0,300 = 324.2 ± 0.2 A3 and K0,300 = 164 ± 6 GPa for K′0,300 = 6.2 ± 0.8. With K′0,300 fixed to 4.0, we obtained: V0,300 = 324.0 ± 0.1 A3 and K0,300 = 180 ± 1 GPa. Fitting our P–V–T data with a modified high-temperature BM EOS, we obtained: V0,300 = 324.2 ± 0.1 A3, K0,300 = 171 ± 5 GPa, K′0,300 = 5.1 ± 0.6 (∂K0,T/∂T)P = −0.023 ± 0.006 GPa K−1, and α0,T = 3.09 ± 0.25 × 10−5 K−1. Using the equation of state parameters of the CAS phase determined in the present study, we calculated a density profile of a hypothetical continental crust that would contain ~10 vol% of CaAl4Si2O11. Because of the higher density compared with the coexisting minerals, the CAS phase is expected to be a plunging agent for continental crust subducted in the transition zone. On the other hand, because of the lower density compared with lower mantle minerals, the CAS phase is expected to remain buoyant in the lowermost part of the transition zone.