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

摘要

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 V-0=46.56(1) Angstrom(3), K-T0=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(partial derivativeK(T)/partial derivativeT)(P)=-0.046(5) GPa K-1 and alpha=a+bT with values of a=1.26(11)x10 K-5(1) and b=1.29(17)x10 K-8(2). A fit to the thermal pressure EOS gives alpha(0)=1.62(9)x10(-5) K-1, (partial derivativeK(T) /partial derivativeT)(V)=-0.027(4) GPa K-1 and (partial derivative(2)P/partial derivativeT(2))(V)=27(5)x10(7) GPa K-1. The lattice dynamical approach by Mie-Gruneisen-Debye EOS yielded gamma(0)=1.33(6), q=6.1(8)and theta(0)=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 (partial derivativeK(T) /partial derivativeT) (V)not equal0 and qnot equal1. 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.