The behavior of deep planetary materials helps drive the flows that produce plate tectonics. Voluminous igneous eruptions driven by deep mantle sources are thought to have caused global environmental changes. Located approximately 3000 km below the surface of the planet, the core-mantle boundary region represents one of the most dramatic compositional and thermal boundary layers within our planet. Gradients across this boundary exert a primary influence on the cooling of the planet, on the dynamics of the core (and hence Earth's magnetic field), and on the dynamics of the mantle (perhaps expressed as large-scale volcanism at the Earth's surface). Major progress towards understanding the dynamics of the deep Earth system within various geoscience disciplines has been made possible through advances in experimental and computational facilities and techniques, instrumental resolution, and deployment of the USArray. Capitalizing on these major advancements and the team's individual expertise within seismology, geodynamics, and experimental mineral physics, the team will study the interaction of subducted slabs with multi-scale lower mantle structures in the framework of mantle flow. They will mentor next generation scientists working in their multi-disciplinary environment, which should improve understanding of how the planet works as a global system.
Seismologists have revealed that the mantle side of the CMB is extraordinarily heterogeneous, with km-scale fine structure that could harbor distinct chemical reservoirs. Thermal and chemical heterogeneity, solid-solid phase transitions, elastic anisotropy, variable viscosity, and melting are probably all required to explain this observed complexity. With individual expertise in seismology (Helmberger and Zhan), geodynamics (Gurnis), and experimental mineral physics (Jackson), they will connect the atomic scale (thermoelastic properties of deep Earth phases) to the tectonic scale (seismically observed structures and their dynamics) and link all processes to the temporal dimension (reconstruction of tectonic plate history). They will conduct a systematic study of the Pacific large low seismic velocity province (LLSVP) using whole seismograms compared against synthetics generated from existing enhanced tomographic models and compressible thermo-chemical convection with reasonable plate tectonic reconstructions. A detailed comparison of the Pacific and African LLSVPs will be done to test the impact of tectonic histories, presence of seismic anisotropy, and possible compositional and/or thermal differences. The experiments assess the sources of (1) seismic anisotropy through inelastic x-ray scattering and diffraction experiments on single crystals of (Mg0.22Fe0.78)O magnesiowüstite (2) seismic gradients and discontinuities through x-ray diffraction experiments on mid-oceanic ridge basalt phase assemblages. Their study will culminate with generating an updated global map of the CMB region. The fundamental questions they will address include: Can the presence of subducted slabs deform LLSVPs into seismically resolvable 3D shapes? Is iron-rich (Mg,Fe)O a source of observable seismic anisotropy, developed by flow of the mantle near subducting slabs? How do these slabs interact and affect D? topography and chemically-distinct structures near the edges of LLSVPs?
