Mantle circulation and the Wilson cycle: charting the fate of isotopically distinct shallow mantle through successive ocean closures

This proposal concerns the Earth's mantle - a huge yet little-understood part of the planet that lies hidden from view beneath the crust. Geoscientists have long thought that it circulates slowly like simmering treacle, dissipating heat generated at depth (e.g. Holmes 1928). But even after all these years, we still don't fully understand how this dynamic process works, despite its fundamental importance to our planet. Workers who model the processes numerically suggest that circulation of mantle material is sufficiently vigorous to blend it all together to make the same chemical composition. But workers who analyse the chemistry of ocean floor igneous rocks that were sourced in the mantle, think the mantle has not been thoroughly mixed, because they find rocks of different chemistries. These two views are very different. Maybe the mantle has become better mixed than it used to be? Perhaps something is missing in the dynamic models so that they are giving us the wrong story? I plan to probe this discrepancy about how the mantle mixes by bringing together the two approaches - I shall investigate the chemistry of ancient ocean floor rocks and use computing to model the physical processes. First, I shall consider isotope chemistry. Isotope ratios can tell us about the mantle underneath mid-ocean ridges where they formed. If they are the same wherever we collect them, the mantle must be really well mixed as computer models suggest, but if they differ from one ocean to another then the mantle must not be thoroughly mixed. It's already possible to distinguish rocks in this way between the present-day Indian, Pacific or Atlantic oceans. Rocks of the Indian Ocean are the most distinctive, suggesting that the shallow mantle here, or at least significant parts of it, have not mixed with other parts of the mantle for 1000 million years (because of the very long time periods for radioactive decay of elements). To take this story much farther back in time, I will generate a new data set of high-precision hafnium-neodymium-lead (Hf-Nd-Pb) isotopic signatures from ancient oceanic rocks. However oceans open and close through geologic history and to find ancient oceans can be difficult; they only leave behind fragments for the geological record. I have chosen the area from the western Alps to China because (1) distinctive 'Indian Ocean type' rocks, up to 350 million years old, have survived at least two ocean closures, (2) this region has a large number of well-preserved oceanic rocks scattered throughout it, and (3) these rocks relate to three successive oceans (Palaeo-Tethys, Neo-Tethys, and the Indian) that all formed at about the same place on the Earth's surface, from 550 million years ago to the present. I will use this new picture of how a vast region of the mantle has changed over time, as a framework on which to hang 3D numerical models. Run on computer clusters to provide massive computing power, these will run Earth-like simulations of mantle convection linked to well-constrained histories of plate motions. Internally, tracers following convection paths will record the passage of the Indian Ocean-type mantle through space and time. By iteratively combining geochemistry with model simulations, I hope to achieve a much better understanding of the dynamics of the Earth's interior. This will provide a step-change in our understanding of mantle circulation both for future modellers and workers considering the long-term physical and chemical evolution of the mantle. For example, it will help us to understand the forces that drive plate tectonics, the process responsible for most geological activity including earthquakes, volcanism, and mountain building. It shall launch me on a career path integrating my isotope expertise with new exciting geodynamic modelling in the study of big mantle dynamic questions and will make concrete moves in bringing together disparate disciplines of this fundamental area of Earth science.