P - V - T equation of state of stishovite to the mantle transition zone conditions

Abstract

In-situ synchrotron X-ray diffraction experiments were conducted using the SPEED-1500 multi-anvil press of SPring-8 on stishovite SiO2 and pressure-volume-temperature data were collected at up to 22.5 GPa and 1,073 K, which corresponds to the pressure conditions of the base of the mantle transition zone. The analysis of room-temperature data yielded V0=46.56(1) A3, KT 0=296(5) GPa and K ′ T =4.2(4), and these properties were consistent with the subsequent thermal equation of state (EOS) analyses. A fit of the present data to high-temperature Birch-Murnaghan EOS yielded (∂K T /∂T) P =−0.046(5) GPa K−1 and α= a + bT with values of a =1.26(11)×10–5 K–1 and b =1.29(17)×10–8 K–2. A fit to the thermal pressure EOS gives α0=1.62(9)×10−5 K–1, (∂ K T /∂ T) V =−0.027(4) GPa K−1 and (∂2P /∂T 2) V =27(5)×10–7 GPa K−2. The lattice dynamical approach by Mie-Gruneisen-Debye EOS yielded γ0=1.33(6), q =6.1(8) and θ0=1160(120) K. The strong volume dependence of the thermal pressure of stishovite was revealed by the analysis of present data, which was not detectable by the previous high-temperature data at lower pressures, and this yields (∂ K T /∂ T) V ≠0 and q ≠1. The analyses for the fictive volume for a and c axes show that relative stiffness of c axis to a axis is similar both on compression and thermal expansion. Present EOS enables the accurate estimate of density of SiO2 in the deep mantle conditions.