This project will support the Caltech shock wave laboratory to undergo studies of silicate melts and the thermodynamics of melting in the lower mantle. The ultimate goal is to constrain the plausible temperature, composition, and relative buoyancy of partially molten regions that might constitute the ultra-low velocity zones (ULVZs) seismically observed at the base of the mantle, and thereby to place limits on the dynamical and chemical consequences of the existence of such zones. This program of equation of state (EOS) measurements and melting curve determinations is expected to provide the basis for extending phase equilibria to the pressures and temperatures of the Earth's whole mantle. These data will contribute to evaluation of the present state of the core-mantle boundary region and also calculation of the crystallization history of a possible early terrestrial magma ocean and the convective heat and mass flows accompanying such an event.
The first experimental program is a series of measurements of the shock velocity in preheated, encapsulated silicate liquids to directly determine the EOS of molten silicates. This technique is being extended throughout the lower mantle pressure range for the first time. The liquid compositions diopside, anorthite, and diopside-anorthite eutectic mix will be sufficient to provide a test of the theory of linear mixing of volumes. These density determinations will constrain whether such liquids would be neutrally buoyant at the core-mantle boundary, what their effect on seismic velocity might be, and how their melting curves and the rest of the phase diagram may behave with increasing pressure.
Second, five high-purity, low-porosity disks of polycrystalline MgSiO3 perovskite have been synthesized that are large enough to use as targets for Hugoniot equation of state experiments. These will be shocked to pressures between 50 and 250 GPa. The determination of pressure, volume, and internal energy in this material will provide the baseline needed to convert previous data on enstatite and MgSiO3 glass in the perovskite stability field into a precise value for the Gruneisen parameter of this material at high pressure and hence a constraint on the thermal gradient of the lower mantle. With sufficient compression, a tight constraint on the pressure derivative of the bulk modulus of perovskite will be obtained.
Third, the melting curve of the important lower mantle end member MgO, which is presently discrepant among existing experimental and theoretical studies by >2000 K, will be determined at various pressures through sound velocity determinations in shocked porous MgO samples.
