P⿢V⿢T relation of MgO derived by simultaneous elastic wave velocity and in situ X-ray measurements: A new pressure scale for the mantle transition region

Simultaneous elastic wave velocity and in situ synchrotron X-ray measurements on polycrystalline MgO were carried out with a V/V0 between ⿱/40.9 and 1.0, which corresponds to the pressures up to 23.6 GPa in our P⿢V⿢T relation of MgO, and temperatures up to 1650 K using a multi-anvil apparatus. The experimentally observed adiabatic bulk modulus and unit-cell volume led to the first pressure-scale-independent P⿢V⿢T   relation of MgO. We obtained a pressure derivative of the isothermal bulk modulus (K⿲T0=4.35(10))(K⿲T0=4.35(10)) and a volume dependent Grñ/4neisen parameter (q0 = 1.5(5)), which reproduces the existing pressure-scale-independent data set (temperature dependence of elastic properties at ambient pressure and shock compression). Thus the present experimental and analytical procedure based on simultaneous elastic wave velocity and in situ X-ray diffraction measurements can give strong constraints on P⿢V⿢T equations of states (EoS) of minerals without any pressure scale. The present P⿢V⿢T EoS of MgO determined at pressure and temperature conditions comparable to the mantle transition region should be an important pressure scale to accurately determine the phase transition boundaries in this region. Application of the present pressure scale to the post-spinel transition boundary yields a transition pressure of 23.0 GPa at 1873 K, which is only marginally lower than that of the 660 km discontinuity.