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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities
Brief summary: 
This project will use novel stable isotopes in primitive mantle melts and lunar samples to determine whether the Earth's mantle experienced the exsolution and segregation of a separate iron sulfide liquid melt: the 'Hadean Matte'
Importance of the area of research concerned: 
Catastrophic planetary collisions during the Earth’s first 500 million years (the Hadean) provided enough energy to melt the planet’s interior, creating a largely molten mantle. Mantle cooling and crystallisation then determined the physical and chemical properties of the Earth. For example, the Earth’s mantle is depleted in S and other chalcophile (‘sulfur loving’) elements like Cu and Au. This depletion may reflect the exsolution of a dense and immiscible Fe-S melt, or ‘matte’, from the mantle. The causes and timing of matte exsolution are debated. One possibility is that matte exsolution took place in the Hadean, as the mantle cooled after the Moon-forming giant impact, reducing its capacity to store S. Another is that matte exsolution was triggered earlier, potentially multiple times, through the addition of S-rich or reduced planetary material to the Earth during accretion. Resolving these different scenarios is important because matte segregation has the potential to create dense reservoirs at the base of the Earth’s mantle that could act as stores of elements essential to life (e.g., S, Fe) and rare, economically-important metals.
Project summary : 
We do not understand the processes driving Fe-S matte exsolution, when and how this happened, and the implications for the evolution of the Earth’s interior. It is unknown whether any matte material formed was sequestered into the core or whether it remained in the mantle, either in reacted form or as a discrete lithology that could be sampled in later melting events, potentially releasing precious volatiles and metals. A major challenge in understanding Fe-S matte exsolution on Earth is that many chalcophile elements are also volatile, such that their depletion in the silicate Earth could also reflect volatile depletion and impacts as well as matte segregation. This project will take a novel approach and use the stable isotope signatures of chalcophile elements of differing volatility in samples of the Earth’s mantle to test different scenarios for Fe-S matte exsolution.
What will the student do?: 
The project presents an exciting opportunity to develop and use new stable isotope tracers based on chalcophile elements to answer planetary-scale problems. This project will first target ancient samples of the Earth’s mantle to answer the question of when the geochemical signature of Fe-S matte exsolution from the mantle can first be identified, and hence what the driving processes for this event were. The student will then carry out a program of stable isotope work on modern samples of the Earth’s mantle to verify the presence (or demonstrated absence) of stranded Fe-S matte material in their source regions. The analyses will then be integrated into models of mantle melting, sulfide saturation and exsolution. From these results it will be possible to determine how heterogeneously any residual Fe-S matte material is distributed through the mantle and if this has changed during the Earth’s evolution. Depending on the student’s interests, this project could also expand to consider lunar samples, as the Moon has a well-documented magma ocean stage and represents an alternative planetary setting where Fe-S matte exsolution could have taken place.
References - references should provide further reading about the project: 
Savage, P. S., Moynier, F., Chen, H., Shofner, G., Siebert, J., Badro, J., & Puchtel, I. S. (2015). Copper isotope evidence for large-scale sulphide fractionation during Earth’s differentiation. Geochemical Perspectives Letters, 1, 53-64.
Creech, J. B., Baker, J. A., Handler, M. R., Lorand, J. P., Storey, M., Wainwright, A. N., ... & Bizzarro, M. (2017). Late accretion history of the terrestrial planets inferred from platinum stable isotopes. Geochemical Perspectives Letters, 3(1), 94-104.
Rubie, D. C., Laurenz, V., Jacobson, S. A., Morbidelli, A., Palme, H., Vogel, A. K., & Frost, D. J. (2016). Highly siderophile elements were stripped from Earth’s mantle by iron sulfide segregation. Science, 353(6304), 1141-1144.
You can find out about applying for this project on the Department of Earth Sciences page.
Dr Helen Williams
Dr Oliver Shorttle
Department of Earth Sciences Graduate Administrator