Equation of State and Observation of Partial Melting of B1 Phase MgO above 200 GPa from Shock Compression Experiments on Samples Preheated to 2300 K

MgO is one of the major constituents of the Earth's lower mantle.Thermodynamic properties and melting behavior of MgO at high pressureare important in geophysics and geochemistry. Yet, it remains poorly knownafter several decades of substantial theoretical and experimental efforts. To address this issue, we developed a technique for shock temperature andsound speed measurements in MgO single crystals preheated to 2300 K. Thefirst results were reported at the AGU Fall Meeting in 2013. Refinement ofthis type of measurement over the years eventually allowed us to reach andprobe for the first time the onset of B1 phase melting at 200 GPa. It wasdetected as a growing deviation of the true radiative temperature from thatpredicted by our most accurate equation of state (EOS) model for the B1phase of MgO constructed for the range of pressures and temperatures relevantto our studies.Unlike the first measurements that employed W foil between the hot Mo driverplate and hot MgO crystal, our final experiments with 2300 K initial targettemperature were done without any foil between hot Mo and MgO. Thateliminated multiple shocks in MgO at the early phase of the dynamiccompression process and increased the peak temperature of shocked sample by 400 K. Another significant improvement over the previous experiments wasshock front reflectivity measurements in a specially designed arrangementwith 5 layers of sapphire between the MgO sample and shock pyrometer. Thereflectivity values of 21%, well consistent with the most recent data fromour 1850 K experiments, were used to extract the true MgO shock temperaturesfrom the actually measured brightness temperatures.We also reanalyzed all our sound speed data for shock-compressed preheatedMgO using the newly constructed EOS for hot Mo and MgO and found a muchbetter agreement than reported earlier between the values measured and thosepredicted by our model. These findings including important details of ourmost recent measurements and data processing techniques will be discussed.Work supported by the U.S. NSF.