Investigating Mantle Mixing and Chemical Layering through a Comprehensive Understanding of Transition Zone Seismic Discontinuities

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Transition zone seismic discontinuities (TZSDs), manifestations of mineral phase transitions or/and compositional changes between the upper mantle and the lower mantle, hold the key to resolve the mystery of mass and heat transport in the Earth's mantle and the long-term evolution of the Earth's interior.However, seismic characterizations of TZSDs are typically incomplete because of the limit in the data frequency bandwidth and sensitivity relevant to TZSDs. We innovate a simple, effective and high resolution probing of mantle discontinuity through examination of broadband forward and backward scattering waves in the context of the teleseismic receiver function method. This approach will allow us to comprehensively characterize TZSDs beneath the continents, including properties such as discontinuity topography, sharpness and gradient, shear velocity jump and density jump.To date, there has been no single study that is capable of simultaneously determining these essential seismic properties in the TZSDs. These renewed descriptions of TZSDs will be used to explore outstanding questions including mineralogical models of the transition zone and the presence of volatile/melt. In particular, we aim to address how current and past subduction determine short-term and long-term mantle mixing and whether such a mixing process may in turn shape slab sinking dynamics.A series of outstanding questions can be much better addressed with our new seismic observations:Did long-term mixing of billions of years result in apparent chemical layering as indicated in geodynamic models?What are the degree and the length scale of lateral heterogeneity if such a chemical layering exists?Is it possible that primordial structure may survive long term mixing and become trapped in the transition zone?Is the transition zone potentially a relatively shallow reservoir for long-term storage and geochemical evolution of basalt?Does chemical layering or large-scale primordial structure dictate the slab sinking dynamics?Does modern and ancient subduction recycle water into the deep mantle and transition zone?Does hydrated transition zone induce convective instability and contribute to intraplate volcanism?In the proposed work, we will use an innovative and effective observation with broadband forward and backward scattering waves to provide a comprehensive characterization of TZSDs, including properties such as discontinuity topography, sharpness, velocity and density jumps across the boundaries, and the gradient above/below the discontinuities. These unprecedentedly rich observations will provide renewed constraints on fundamental processes relevant to the Earth's interior and evolution.