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

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.

This is a blue-skies, curiosity-driven research project that aims at using new seismic observations for a comprehensive characterization of transition zone seismic discontinuities, which will provide key information on the composition and evolution of the Earth's mantle. Given the fundamental nature of this project, the main non-academic beneficiaries of this project will be school children, teachers and the wider public. The dynamics of the Earth's interior and its consequences - earthquakes, volcanoes, mountain building - are all topics that greatly interest these users, having a strong potential to help educate and motivate the public for STEM subjects. Ultimately, this project may also benefit the geophysics industry (e.g., hydrocarbon reservoir characterization), as the comprehensive imaging techniques developed in the project may be transferrable to small scale, industrial applications.

School children, teachers and the wider public will benefit from the project through a higher visibility of geophysics in schools, notably through a revived UCL "Seismology in schools" project. Two-way partnerships between the project's team and school teachers and students will enable students to carry out end-to-end seismic analyses, from ground motion observations to the construction of profiles of mantle's composition. In addition, teachers will be able to refresh their teaching materials and knowledge in geophysics. More generally, the wider public will benefit from building on existing outreach efforts at UCL (e.g., "Build your planet" interactive website and the GeoBus) and from new demonstration materials on the dynamics of the Earth's interior that will be used in regular outreach events and at UCL's Geology festival.

The geophysics industry will benefit from this project by being informed about the project's outputs and new seismic methods developed through regular discussions during the project. This will be enabled by the PI and co-I's extensive network of contacts and by their involvement in events with industry's participation (e.g., workshops on Big Data and industry's presentations at UCL). The outcomes of the project will be published in a popular science article in a journal accessible to the wide academic and non-academic community, such as the RAS journal 'Astronomy and Geophysics', the AGU Eos newspaper, New Scientist or NERC's Planet Earth magazine for wide ranging dissemination of the project.