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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities

Lead Supervisor: Helen M. Williams, Earth Sciences

Co-Supervisors: Oliver Shorttle, Earth Sciences/Institute of Astronomy; Simon Matthews, University of Iceland and John Maclennan, Earth Sciences

Brief summary: 
This project will use novel stable isotopes in ophiolite sections and Icelandic volcanic rocks to discover the role that melt transport and melt-rock reaction processes play in creating short-wavelength chemical heterogeneity in the Earth's upper mantle.
Importance of the area of research concerned: 
Mid-ocean ridge basalts (MORB) provide a valuable window into the Earth's upper mantle. Many MORB are chemically and isotopically heterogenous at the crystal scale but it is unknown whether these variations reflect short-wavelength compositional and lithological variations in the depleted mantle or the processes of partial melting and melt transport. Major elements stable isotope systems (e.g., Fe, Mg, Ca, Si) are novel tracers of mantle lithology and transport processes. Many MORB display stable isotope signatures far more variable than predicted in partial melting models. These isotopic signatures may arise from preferential contributions of melt from more fusible pyroxenite lithologies, or isotopic fractionation during melt crystallisation and transport, in which isotopic disequilibrium may occur on short timescales. This project will focus on reconciling this ambiguity, which is essential to constraining the extent of compositional and lithological heterogeneity present in the depleted mantle, and the rates of melt-rock reaction and melt ascent through the mantle, which fundamental to understanding the evolution of the Earth’s interior.
Project summary : 
Ophiolites are exposures of igneous crust and residual upper mantle that provide a natural laboratory in which to investigate melt transport and melt-rock reaction processes. This project will use novel stable isotopes, in-situ major and trace element data from ophiolite melt/rock reaction zones to study the processes and timescales of melt transport and melt-rock reaction. Complementary studies will also be carried out on individual eruptions from Iceland, where the Mid‐Atlantic Ridge becomes subaerial. By combining these new data with models of mantle melting and melt transport it will be possible to unravel upper mantle lithological and compositional heterogeneity the role that melting and melt transport processes play in amplifying or muting these signatures.
What will the student do?: 
The project presents an exciting opportunity to use novel stable isotope tracers to unravel the roles that mantle lithology, melting and melt transport processes play in determining the geochemical variability displayed in mantle melts. The student will focus on well-characterised ophiolite sections and single eruptions from Iceland, which offer a unique opportunity to study magmas formed at high spatial and temporal resolution. Archive samples are available for analysis, but fieldwork can be incorporated into the project according to interest. With support from supervisors, the student will select well-characterised sample suites and profiles to analyse for different major element stable isotope systems. The result of these studies will be integrated into models of partial melting and melt transport, focussing on the potential for isotopic disequilibrium during melt focussing, transport and melt/rock reaction. Through this project the student will develop a series of powerful new geochemical tools of melting processes, mantle source region and lithological heterogeneity that can be used to gain new insights into the evolution of the Earth’s interior.
References - references should provide further reading about the project: 
Soderman, C. R., Matthews, S., Shorttle, O.,... & Williams, H. M. (2021). Heavy δ57Fe in ocean island basalts: A non-unique signature of processes and source lithologies in the mantle. Geochimica et Cosmochimica Acta, 292, 309-332.
B. L. Byerly, J. C. Lassiter (2014), Isotopically ultradepleted domains in the convecting upper mantle: Implications for MORB petrogenesis. Geology. 42, 203–206.
Lundstrom, C. C., Chaussidon, M., Hsui, A. T., Kelemen, P., & Zimmerman, M. (2005). Observations of Li isotopic variations in the Trinity Ophiolite: evidence for isotopic fractionation by diffusion during mantle melting. Geochimica et Cosmochimica Acta, 69(3), 735-751.
You can find out about applying for this project on the Department of Earth Sciences page.
Dr Helen Williams
Dr Oliver Shorttle
Dr John Maclennan
Department of Earth Sciences Graduate Administrator