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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities
 

Lead Supervisor: Helen Williams, Earth Sciences 

Co-Supervisor: Heye Freymuth, Earth Sciences and Oliver Shorttle, Earth Sciences

Importance of the area of research concerned: 
The onset of plate tectonics and recycling of surface material into the mantle had a major impact on the composition of the continental crust and the Earth’s mantle. Yet the timing and extent of recycling as well as the composition of material recycled into the mantle is not well constrained. Low temperature geochemical processes at the Earth’s surface generate isotopically distinctive reservoirs some of which are transported into the mantle via subduction. This process changes the chemical composition of the mantle and as traditionally been traced via radiogenic isotope systems. Novel stable or quasi-stable isotope systems open up a new avenue to provide a more detailed view on the evolution of the crust-mantle system (e.g. Andersen et al. 2015) but precise analytical techniques have only recently become available. The high temperature geochemistry of most stable isotope systems is therefore still largely unexplored despite their great potential to become powerful tools for tracing large-scale geochemical cycles in the deep Earth.
Project summary : 
This project will explore the changing composition of the Earth’s mantle and crust from the perspective of novel isotopic tracers. Recently, differences in the 238U/235U in OIB and MORB have been related to recycling of surface material and the different ages of the OIB and MORB sources (Andersen et al. 2015). Similar effects might be expected for stable isotope systems such as the transition metals (e.g. δ98Mo) and δ138Ba (e.g.Freymuth et al. 2015, Nielsen et al. 2018). Mantle-derived samples representing a time-series of Earth’s history will be used to characterise the isotopic evolution of the mantle in these systems and relate them to the history of plate tectonics and the formation of continental crust via geochemical modeling.
What will the student do?: 
The student will analyse volcanic rocks from different stages of Earth’s history for their isotopic compositions. The work will focus on novel isotopic tracers. Some of the analytical techniques have recently been established in Cambridge. Others will be set up during the course of the PhD with full support from supervisors. The student will use the newly generated data to produce geochemical models for the changing composition of the mantle and crust.
References: 
Andersen, M. et al. (2015) The terrestrial uranium isotope cycle. Nature 517 p 356-359
Freymuth, H. et al. (2015) Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc. Earth and Planetary Science Letters 432 p 176-186
Nielsen, S. et al. (2018) Barium isotope evidence for pervasive sediment recycling in the upper mantle. Science Advances 4, p1-8
Applying
You can find out about applying for this project on the Department of Earth Sciences page.