Our ability to interpret seismic observations including the seismic discontinuities and the density and velocity profiles in the earth's interior is critically dependent on the accuracy of pressure measurements up to 364 GPa at high temperature. Pressure scales based on the reduced shock-wave equations of state alone may predict pressure variations up to 7% in the megabar pressure range at room temperature and even higher percentage at high temperature, leading to large uncertainties in understanding the nature of the seismic discontinuities and chemical composition of the earth's interior. Here, we report compression data of gold (Au), platinum (Pt), the NaCl-B2 phase, and solid neon (Ne) at 300 K and high temperatures up to megabar pressures. Combined with existing experimental data, the compression data were used to establish internally consistent thermal equations of state of Au, Pt, NaCl-B2, and solid Ne. The internally consistent pressure scales provide a tractable, accurate baseline for comparing high pressuretemperature experimental data with theoretical calculations and the seismic observations, thereby advancing our understanding fundamental high-pressure phenomena and the chemistry and physics of the earth's interior.diamond-anvil cell ͉ high-pressure research ͉ pressure calibration ͉ thermodynamics ͉ x-ray diffraction T he earth has a layered internal structure with distinct boundaries. The boundaries of the five main layers (the upper mantle, the transition zone, the lower mantle, the liquid outer core, and the solid inner core) are well defined by the observed seismic velocity discontinuities at depths of 400, 670, 2,891, and 5,149 km (corresponding to pressures of 13.4, 23.8, 135.8, and 328.9 GPa, respectively) in a global average preliminary reference earth model (PREM) (1). The interpretation of these discontinuities requires experimental investigations of earth materials at high pressure and temperature. The seismic discontinuities near 400 and 670 km depth are commonly associated with the mineralogical phase transformations of (Mg,Fe) 2 SiO 4 from ␣-olivine to ␤-phase (wadsleyite) and from ␥-spinel (ringwoodite) to (Mg,Fe)SiO 3 -perovskite plus (Mg,Fe)O-magnesiowüstite, respectively (2). With the rapid increase in the use of broadband seismometers and seismic arrays, seismologists have been able to determine the depths of the 400-and 670-km discontinuities and their lateral variation with increasingly finer resolutions (3). To correlate the observed seismic variability with the compositional and thermal variations in the mantle, we have to be able to determine mantle phase transitions with high accuracy, better than 1% in pressure determination (i.e., Ϯ0.25 GPa at 25 GPa). Similarly, it is critically dependent on the accuracy in pressure determination whether or not the recently discovered postperovskite transition (4, 5) indeed occurs at the base of the lower mantle and accounts for a number of seismic anomalies observed in the DЉ region. Because the DЉ layer is observed in a narrow depth int...

[1] We have performed a series of experiments to investigate the compositional effect on the compression behavior of (Mg, Fe)O solid solutions at high pressure. The in-situ synchrotron X-ray diffraction data revealed abnormal volume contractions at about 40, 60, and 80 GPa for The equations of state of (Mg,Fe)O established in this study are directly applicable to the Earth's lower mantle in composition and pressure ranges and provide essential data for modeling the density profile of the lower mantle.

[1] The phase relations and density of a natural mid-ocean ridge basalt (MORB) were investigated from 28 to 89 GPa and 1600 to 2700 K by in situ X-ray diffraction measurements and chemical analysis of the quenched samples using transmission electron microscopy (TEM). We observed an assemblage of five phases up to 50 GPa, namely an aluminum-bearing magnesium perovskite phase, a calcium perovskite phase, a stishovite phase, the new aluminum-rich (NAL) phase, and a calcium ferrite-type phase. The NAL phase was no longer observed above 50 GPa. The phase proportions were obtained by Rietveld refinement of the in situ X-ray diffraction patterns. After the disappearance of the NAL phase beyond 50 GPa, the proportion of each phase remains constant up to 89 GPa. The density of MORB was calculated using the measured volumes, phase proportions, and chemical compositions of the coexisting phases. The thermoelastic parameters of the MORB sample were estimated from the fit of the measured densities at various pressure and temperature conditions. Resulting MORB density profiles were calculated for different subducting slab temperature profiles. MORB densities are 0.5% to 2% greater than those of the surrounding mantle over the entire lower mantle range, suggesting MORB likely subducts to the core-mantle boundary.