Source or Sink? Seismic Observations of Mass and Water Transport to and from the Mantle Transition Zone

Almost no other component plays as large a role on the dynamics of the Earth as water. It affects how the mantle melts, how easily it flows and its presence could even be the reason plate tectonics is able to operate. Water is continuously being returned to the mantle via subduction and is therefore inherently linked to the composition of the rocks that deliver it to depth. Alternatively, water may also have been trapped deep within the mantle during the formation of the Earth. Either way, given that at least one ocean mass has been subducted during the lifespan of the Earth there must also exist a feedback between the surface and interior. One region of particular importance is the mantle transition zone (between nominal depths of 410 km and 660 km), where the major minerals have the potential to store vast quantities of water (five times that of the oceans). Identifying regions of variable composition and excess hydration can therefore provide insights into mantle dynamics and the connection between the mantle and surface.

This project aims to seismically characterise the manifestation of variable composition and material flux at depth. For instance, due to the increased water storage capacity within the transition zone, partial melting is expected to occur when excessively hydrated material leaves it. The depth, sharpness and magnitude of the seismic discontinuities associated with the transition zone will also vary depending on the thermochemical characteristics. Key to understanding this is to investigate end-member tectonic settings (subduction zones, plumes and unperturbed mantle) using a variety of complimentary passive seismological probes using dense broadband seismic networks. Seismic observations will therefore form the basis of the project and will be performed at Cardiff in collaboration with Bristol. The project is also extremely complimentary to ongoing geodynamical modelling at Cardiff regarding the deep water cycle. Major questions that this project aims to address include:

How well mixed is the mantle?
How much water is entering/leaving the transition zone?
Is the transition zone a globally significant water reservoir?

The project would suit a student interested in understanding fundamental processes within the Earth through both observation and modelling. A strong background in geology/geophysics and experience with computer programming and data analysis would be extremely beneficial.