CSEDI: Ultra-High Velocity Zones (UHVZs) at the core-mantle boundary

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Nearly half way to the center of the Earth, 2900 km deep, lies the boundary between the rocky mantle and the molten iron-rich outer core. There, at the core-mantle boundary (CMB), small enigmatic structures of tens of km in size were recently observed on the mantle side. The CMB is a critical boundary. The heat conducted away from the core by mantle materials contributes to power the Earth's magnetic field which shields us from the solar wind. Mantle thermal convection, which drives plate tectonics and associated hazards, might have brought unique materials to the CMB; possibly large sections of the Earth's crust and upper mantle. Characterizing the newly observed structures is important, because they hold clues on the planet dynamics, chemical evolution and present-day state. Here, the researchers use seismology - the study of seismic-wave propagation within the Earth - to image and characterize these structures. They are called ultra-high velocity zones, because seismic waves passing through them exhibit remarkably high velocities. To identify their origins, the team studies the convective properties of the mantle using geodynamic modeling. The researchers also carry out experiments and computational calculations to investigate the properties of candidate materials at the extreme pressure and temperature of the deep mantle. These data help to constrain and interpret seismic observations, lifting the veil on the nature of ultra-high velocity zones. This project provides support to an early-career scientist, a postdoctoral associate and two female graduate students. It also promotes training for an undergraduate student and educational outreach to the public at yearly events and via the internet. This multidisciplinary project frames the detection, imaging, modeling, and characterization of ultra-high velocity zones (UHVZs). The team's preliminary work finds them in spots beneath the Cocos and Caribbean tectonic plates, a region located beneath past and present active subduction zones. The researchers focus on mapping and modeling UHVZs there and in other geographical locales, analyzing seismic waves that reflects off of Earth's core (named ScS, ScP, and PcP waves). In addition, they explore compositional possibilities by performing high-pressure experiments and calculations. These involve materials introduced from above, such as subducted ocean crust and sediments, and from below (core-mantle interactions). Experiments on phase relations and properties of minerals are carried out using multi-anvil press and diamond-anvil cell setups at synchrotron national facilities. They explore a large range of pressure, up to the extreme pressure of 135 GPa (~1.3 million atm) prevailing in the lowermost mantle. Experiments are combined with calculations leading to estimates of expected velocities, which are compared with seismological results. In parallel, geodynamic modeling investigates the dynamical behavior of compositional input from subduction or the CMB. This provides a framework guiding the experimental work and the interpretation and modeling of seismic observations. UHVZs may have important effects on a number of deep mantle phenomena, including heat and chemistry exchange between the core and mantle. They may relate to important geodynamical cycles at the origin of the previously documented ultra-low velocity zones and/or thermochemical piles. Investigating the newly discovered CMB structures may provide insights on larger scale processes, like whole mantle convection and the evolution of mantle chemistry.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.