Geophysics of Iron in the Earth's D" Layer and Core

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The Earth?s core and core-mantle boundary play a central role in the evolution and dynamic processes within the Earth. Seismological observations provide detailed descriptions of this most remote region in our planet, but interpretations of the many enigmatic seismic features await in-depth understanding of the mineral physics of iron and iron-rich compounds at extreme pressure-temperature (P-T) conditions where in-situ measurements are challenging. This project takes advantage of recent advances in high pressure synchrotron x-ray technology, pushes the envelope of current experimental capabilities, and integrates multiple in-situ probes in order to measure key properties and address fundamental geophysical questions surrounding the Earth?s core and D? layer. The experiments focus on taking an integrated approach in determining the high P-T elasticity of iron and iron-rich silicates. For iron, the equation of state, aggregate compressional and shear wave velocities, velocity anisotropy and lattice preferred orientation, and elastic tensor will be determined in-situ in a diamond anvil cell using a suite of complementary synchrotron x-ray techniques: hydrostatic x-ray diffraction (XRD), radial XRD, nuclear-resonant inelastic x-ray scattering, and phonon inelastic x-ray scattering. The results will address major issues such as inner core seismic anisotropy and will establish a benchmark for pure iron for constraint of theoretical calculations and comparison with iron alloys. For the core-mantle boundary, this project focuses on the elasticity of iron-bearing silicate post-perovskite. Although the significance of the discovery of silicate ppv at D? conditions is well recognized, due to experimental challenges, the amount of data on the properties of silicate post-pervoskite has been scarce. In-situ high P-T radial XRD measurements will be applied to this phase to provide the experimental foundation for understanding the unusual characteristics of the Earth?s D? layer, including the sharp discontinuity at the top of this layer, its lateral velocity variations, its seismic anisotropy, its VP-VS anti-correlation, and its ultralow-velocity zones.