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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities

Lead Supervisor: Helen M. Williams, Earth Sciences

Co-Supervisor: Oliver Shorttle, Earth Sciences/Institute of Astronomy; Simon Matthews, University of Iceland; Marie-Laure Bagard, Earth Sciences and Heye Freymuth, Earth Sciences

Brief summary: 
This project will use novel isotope systems to determine whether komatiites (unusual igneous rocks formed by the eruption of extremely hot magmas and largely restricted to the first billion years of Earth history) have origins in the Earth's lower mantle.
Importance of the area of research concerned: 
Komatiites are MgO-rich igneous rocks that were largely erupted during the Archean, where they provide unique insights into the oxidation state, composition and mineralogy of the mantle at that time. Komatiites are widely considered to have formed by large degrees of melting of hot mantle, and their declining abundance with age is often interpreted to reflect the secular cooling of the Earth’s interior. Komatiites are also widely used as direct samples of the Earth’s mantle, as it is assumed that the elemental and isotopic signatures of such high-degree melts should faithfully represent those of their mantle source regions. However, the exact melting processes, composition and mineralogy of komatiite mantle source regions are in fact not well understood, limiting the extent to which we can use these rocks to constrain mantle chemistry and melting processes through time. For example, it has controversially been suggested that the exceptionally low concentrations of incompatible elements in komatiites reflects an origin in the lower mantle, rather than exceptionally high degrees of melting in the upper mantle, with enormous implications for mantle chemical and thermal evolution.
Project summary : 
The overall goal of this project is to find new ways of constraining komatiite melting processes and source region mineralogy and evaluate how these may have changed over the course of Earth history. Specifically, this project will aim to determine whether the low incompatible element concentrations of komatiites reflect derivation by high degrees of melting of an upper mantle source already depleted by melt extraction, or whether these signatures could be created from melting of undepleted, ‘primordial’ lower mantle. Novel stable isotope systems have proved to be exceptionally powerful tracers of mantle melting processes and lithology and will be used in conjunction with mantle melting models to address the question of komatiite origins and hence the thermal and chemical evolution of the Earth’s mantle.
What will the student do?: 
The project will develop and use an exciting array of novel stable isotope tracers to answer fundamental questions about the origins of some of the Earth’s earliest mantle melts. The isotope systems to be studied will be selected on the basis of their predicted behaviour during upper and lower mantle melting as well as their resilience to post-magmatic alteration. With support from supervisors, the student will target key komatiite suites of different ages for analysis. Depending on initial results and the student’s interests, the project could then be taken in a range of directions. These include carrying out a comprehensive global study of komatiites to identify spatial-temporal variations in mantle source and melting processes or comparative isotope studies of younger high-degree mantle melts or residual mantle material. Alternatively, the student could take the project in a more theoretical direction and develop new models of lower mantle melting and crystallisation processes incorporating stable isotopes and redox effects. Depending on the student’s interests, this project could also include fieldwork, but all required sample sets are already in place.
References - references should provide further reading about the project: 
Williams, H. M., Matthews, S., Rizo, H., & Shorttle, O. (2021). Iron isotopes trace primordial magma ocean cumulates melting in Earth’s upper mantle. Science Advances, 7(11), eabc7394.
Sossi, P. A., Eggins, S. M., Nesbitt, R. W., Nebel, O., Hergt, J. M., Campbell, I. H., ... & Davies, D. R. (2016). Petrogenesis and geochemistry of Archean komatiites. Journal of Petrology, 57(1), 147-184.
McKenzie, D. (2020). Speculations on the generation and movement of komatiites. Journal of Petrology, 61(7), egaa061.
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