CSEDI: Integrated seismic, geodynamic, and mineral physics studies of multi-scale structures in the lowermost mantle

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The behavior of materials in the deep Earth constrains the flows that drive plate tectonics. Voluminous eruptions driven by deep mantle sources are thought to have caused global environmental changes. At the core-mantle boundary (CMB) – about 3000 km below the Earth’s surface - dramatic compositional and thermal changes occur. These changes exert a primary influence on the cooling of the planet. They also influence the core dynamics (hence Earth’s magnetic field) and impact mantle thermal convection. Yet, understanding the dynamics of the deep Earth is not trivial. Indeed, multidisciplinary efforts and state-of-the art techniques are required to tackle the complexity of the Earth system. Here, the researchers investigate enigmatic features observed at the core-mantle boundary. To unveil their origin, the team combine expertise in seismology, geodynamics, and experimental mineral physics. The researchers carry out experiments at the extreme pressures prevailing in the mantle. They measure the properties of deep Earth materials using powerful x rays and infrared light at national synchrotron facilities. Taking advantage of recent advances in computational facilities, they simulate the interaction of crustal materials with lower-mantle materials made of multi-scale structures. These materials are brought together by tectonic forces through Earth’s complex history. Outputs of the models are compared with seismic observations, hence gradually unveiling the dynamics of the deep Earth. The project provides support for graduate students at the California Institute of Technology. It also fosters international collaboration with Australia and the UK. 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 the observed complexity. With expertise in seismology, geodynamics and experimental mineral physics, the team connects 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). The researchers conduct a systematic study of the Pacific large low seismic velocity province (LLSVP) and proximal surroundings such as ultralow velocity zones (ULVZs). They use whole seismograms compared against synthetics generated from enhanced tomographic models and thermo-chemical convection models. The models integrate plate tectonic reconstructions constrained by observations and account for materials’ physical properties, including elastic tensors. The experiments assess the sources of the seismic signatures of candidate deep hydrous phases in subducted slab. They include: (1) shear wave speed measurements using inelastic x-ray scattering techniques; and (2) thermal equation of state and stability constraints using x-ray diffraction and synchrotron infrared spectroscopy at lower mantle conditions. The study addresses fundamental questions, such as: can the presence of subducted slabs deform LLSVPs into seismically resolvable 3D shapes (with distinctive anisotropy) and affects D" topography and chemically–distinct structures near the edges of LLSVPs? Are all ULVZs created equally? If hydrous phases can be transported into the lowermost mantle, are they seismically detectable and can they contribute to the stability of a thermo-chemical pile?This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.