CSEDI: Integrated seismic, geodynamic, and mineral physics studies of scatterers and other multi-scale structures in Earth’s lower mantle

California Institute Of Technology, Pasadena CA

Investigators

Abstract

The behavior of materials in the Earth’s mantle constrains the flows that drive plate tectonics. Voluminous volcanic eruptions driven by deep mantle sources are thought to have caused global environmental changes. Dramatic compositional and thermal changes occur at the core-mantle boundary (CMB), about 3000 km below the Earth’s surface. These changes exert a primary influence on the cooling of the planet. They also influence the core dynamics, the Earth’s magnetic field, and 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 combines 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 materials anticipated to exist in of oceanic crustal material sinking to the CMB using powerful x-rays and infrared light at national synchrotron facilities. Taking advantage of recent advances in computational facilities, they simulate the interaction of candidate oceanic crustal materials with deep-mantle materials brought together by tectonic forces acting throughout Earth’s history to produce complex structures in the deep mantle. The results of the models will be compared with seismic observations of the Earth’s interior, testing our understanding of the dynamics of the deep Earth. The project provides support for the training of graduate students at the California Institute of Technology and fosters international collaboration with Australia. 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 to the tectonic scale as linked to the temporal dimension through dynamics. This is accomplished through the measurement of the thermoelastic properties of deep Earth phases as compared to seismically observed structures predicted by mantle dynamics arising from reconstructions of Earth’s plate tectonic history. The researchers will conduct a systematic study of the Pacific large low seismic velocity province (LLSVP) and proximal surroundings. 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 and rheological properties. 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 3-D shapes with distinctive anisotropy and affect D" topography? If hydrous phases can be transported into the lowermost mantle, are they seismically detectable and do they play a role in the stability of a thermo-chemical pile? This work will be accomplished through the training of three graduate students in cutting-edge techniques, and the collaborative project will enhance partnerships with Australian National University. Outreach activities through the Earthquake Fellows Program will include high school students from backgrounds underrepresented in the geosciences. 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.