Variable Behaviors of 3D Subducted Slabs and Their Influence On The Thermal and Chemical Heterogeneities In Earths lowermost Mantle

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The Earth’s outermost shell comprises several coherent pieces ("tectonic plates") that continually collide, pull apart, or slide past each other. Denser plates may dive below other plates (or "subduct") rather than colliding head on, and we can see these subducted "slabs" in images of the deep Earth made with seismic waves (analogous to a hospital CT scan). These seismic images show that subducted slabs have a variety of shapes, and near the bottom of the mantle they are associated with piles of anomalous material that is likely hotter and compositionally different from either the slabs or the surrounding mantle. Li and his team will perform many state-of-the art computations using supercomputers to simulate the behavior of subducted slabs and their interactions with their surroundings. This project will lead to a comprehensive understanding of why slabs have different shapes and how they influence the chemistry, structure, and temperatures of the deep mantle. Seismic studies have revealed a variety of morphologies of subducted slabs in the deep mantle. The subduction of slabs to the lowermost mantle and the resulted thermal/chemical heterogeneities at this depth are often thought to be responsible for the wide-spread seismic anomalies (e.g., ultra-low velocity zones, scatterers) outside the two large low velocity provinces (LLVPs). However, it remains unclear what controls slab morphology and how subducted slabs with different morphology control the formation and distribution of thermal/chemical heterogeneities in the relatively cold regions of the lowermost mantle outside the LLVPs. This project aims to address these questions using 3-dimensional (3D) models with Earth-like single-sided subduction. The team will first exhaustively explore the parameter space in 3D purely thermal models to build a library of models that show different scenarios of slab dynamics and morphology in the lowermost mantle. After that, the team will select representative models for different slab scenarios and introduce multiple compositional components (e.g., oceanic crust, depleted lithosphere, LLVP materials) to the model domain, and perform 3D high-resolution (up to a few km) thermochemical models, to study how subducted slabs with different dynamics and morphology affect the formation of hot thermal anomalies, and the segregation, accumulation, and distribution of subducted oceanic crust in the relatively cold regions of the lowermost mantle outside the LLVPs. This project will fundamentally advance our understanding of the long-term temporal variation of fine-scale 3D slab morphology and dynamics, and their contributions to the thermal and chemical heterogeneities and thus seismic heterogeneities in Earth’s lowermost mantle, especially outside the LLVPs.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.