Pressure calibration to 20 GPa by simultaneous use of ultrasonic and x-ray techniques

Simultaneous measurements of elastic P - and S -wave travel times, density (specific volume), and sample length using a multianvil apparatus interfaced with ultrasonic interferometry and x-ray diffraction and radiography have enabled simultaneous determination of elastic properties and absolute pressure at high pressures and high temperatures. Experimental procedures and data analyses are demonstrated using data collected on polycrystalline samples of ferropericalse (MgFe) and wadsleyite Mg2 Si O4 under quasihydrostatic conditions up to 20 GPa. Compared with those derived from the internal pressure standard in the same experiments based on equations of state of NaCl, the directly determined pressure at 20 GPa is ∼8% and ∼12% higher than those inferred from the Brown and Decker pressure scales, respectively. The discrepancy cannot be reconciled by uncertainties in the NaCl pressure scale and current experimental data. Until further results are available, the pressure determined in this study is believed to be a more accurate measurement of the effective pressure imposed on the sample than that inferred from NaCl based on previous pressure scales.