ABSTRACT

Since its formation billions of years ago, Earth has been slowly cooling via convection, where warmer material rises to the surface and cold tectonic plates plunge deep into the interior. Understanding this process is important for a wide range of problems, such as determining the factors that drive plate tectonics to sustaining deep water and carbon cycles that stabilize the atmosphere, hydrosphere, and climate throughout Earth?s history. This is particularly relevant for society, both because plate tectonic processes are the driving forces behind hazards such as earthquakes and volcanoes and because our climate and atmosphere make Earth habitable. This project studies large scale convection and volatile pathways in the solid Earth by using earthquake data recorded at global seismic stations paired with geochemical estimates of volatiles from rocks collected at mid-ocean ridges where oceanic plates are diverging. Although the mantle must rise in these locations to replace the mass of the plates as they diverge, the upwellings are typically considered to be small in scale, not necessarily tied to the larger whole Earth convective system. However, recent seismic imaging and the researcher's own comparisons between geochemical data and geophysical data suggest that in some regions upwellings beneath mid-ocean ridges may connect to the lower mantle with broad implications for the understanding of volatiles on Earth, their pathways, and abundances. An outreach program will increase diversity in the Earth Sciences by designing an interactive digital tool that explores and explains Earth?s deep volatile cycle and its connection to physical rocks samples. It will be designed for various levels, useable by experts, tour guides, K-12 school groups, self-directed students, the public, and explorers online. The tool will be delivered via large portable touchscreen interfaces in the marine sample repositories at U. Rhode Island and Woods Hole Oceanographic Institution, which store many of the rock samples to be studied by this project. Together the facilities reached ~15,000 visitors in the past 5 years, which included the COVID pandemic. The content will also be displayed at the WHOI Discovery Museum in the rotating exhibit section and will be made available on a website hosted through Volcano@URI. Finally, the project will provide training for 2 graduate students in cutting-edge methodologies in geochemical analyses and global seismic analyses.

Water is essential to life on Earth. It also plays a large role in studies of Earth?s deep interior and its evolution. It is thought to be an important factor in the existence of plate tectonics, the formation of the continents, and the initiation of volcanism and earthquakes. The mantle transition zone (MTZ), which separates the upper from the lower mantle from ~410 to ~660 km depth, is key to Earth?s hydration cycle in that it is thought to have the capacity to store ocean(s) of water. Yet, determining the exact locations and pathways of the hydration across the transition zone has proven challenging. Plumes and subduction zones are thought to be the main conduits of hydration between the upper and the lower mantle. Ridges are typically assumed to be relatively independent of Earth?s large-scale whole mantle convection patterns. However, ridges do release water from the mantle during mid-ocean ridge basalt (MORB) emplacement. Although MORB water contents are relatively low, ridges represent the longest continuous plate boundaries on the planet, and so the total mass of water passing through the ridge system is substantial. Ridges also represent an accessible window into the water content of much of the upper mantle. The researchers have made some preliminary comparisons of available information, namely global seismic models and compiled water contents from previously analysed ridge samples. They find that in some locations MORB samples with higher water content correspond to locations where seismic observables may suggest enhanced hydration in the MTZ. They also find many ridge segments have too little geochemical data to constrain any trend. This preliminary work is promising and demands additional attention and investigation. This project is co-funded by the Geophysics and Marine Geology and Geophysics programs.

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.

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