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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities
 

Lead Supervisor: Oliver Shorttle, Earth Sciences

Co-Supervisor: Helen M. Williams, Earth Sciences and David Neave, University of Manchester

Brief summary: 
Earth’s uniquely oxygen-rich surface environment is a result of a uniquely oxidized mantle; this project will investigate the co-evolution of these two terrestrial reservoirs.
Importance of the area of research concerned: 
Earth’s mantle is constantly interacting with our planet’s oceans and atmosphere through volcanism. At mid-ocean ridges, this juxtaposes juvenile basalt with cold ocean water, driving hydrothermal circulation and supplying nutrients to the oceans. At arc volcanoes huge fluxes of gas are injected into the atmosphere, perturbing the climate on short and long timescales. At each of these interfaces, the composition of the vast silicate interior of the planet affects the surface environment, and a key property in deciding the nature of the interaction is how oxidising the mantle is (its oxygen fugacity): this determines the speciation of volcanic gases entering the atmosphere. Despite the importance of mantle oxygen fugacity in determining planetary properties, its measurement and interpretation has in recent years proved controversial. In particular, it is unclear how the oxygen fugacity systematics of oceanic basalts link to other elemental and isotopic indices of mantle source composition. This project will expand upon recent observational and theoretical campaigns to improve the measurement and interpretation of the oxygen fugacity of natural mantle-derived samples.
Project summary : 
The oxidation state of Earth’s mantle is key in determining the outcome of mantle-atmosphere interaction. Earth’s mantle is peculiarly oxidised in a solar system context, and coincidently or not, Earth also has an oxygen-rich atmosphere. This project will investigate the present-day mantle oxygen fugacity to understand what features of the Earth-system have led to its current state. Synchrotron Fe-XANES data will be studied on basalts from prime natural laboratories (e.g., Iceland) for understanding the controls on mantle oxygen fugacity. These data will be combined with new thermodynamic modelling to constrain how basalt oxygen fugacity relates to their mantle source regions. Together, this approach will develop a new perspective on how oxidising Earth’s modern mantle is, and the factors that have caused it to end up distinct from other solar system bodies.
What will the student do?: 
This project will combine data analysis and thermodynamic modelling, with the scope for new synchrotron analyses to be made and new fieldwork to collect samples. In the first instance, data analysis will draw on existing Fe-XANES datasets collected from several key localities around the world. These datasets target regions with known mantle source compositional and thermal anomalies such that the effects of source and process in setting basaltic oxygen fugacity can be disentangled. New redox-sensitive trace element and isotopic data will be collected on these samples, to provide a complementary perspective on mantle oxygen fugacity. These data will be investigated through novel thermodynamic modelling of the controls on mantle oxygen fugacity, making predictions of basalt composition under different conditions.
References - references should provide further reading about the project: 
Williams, H.M. et al.. 2012. Isotopic evidence for internal oxidation of the Earth’s mantle during accretion, vol.321-322. doi:10.1016/j.epsl.2011.12.030.
Cottrell, E. & Kelley, K.A.. 2013. Redox heterogeneity in mid-ocean ridge basalts as a function of mantle source. Science, vol. 340. doi:10.1126/science.1233299.
Shorttle, O., et al., 2015. Fe-XANES analyses of Reykjanes Ridge basalts: implications for oceanic crust’s role I the solid Earth oxygen cycle. Earth and Planetary Science Letters, vol. 427. doi: 10.1016/j.epsl.2015.07.017.
Applying
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