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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities

Lead Supervisor: Oliver Shorttle, Earth Sciences

Co-Supervisors:  John Maclennan, University of Cambridge and Margaret Hartley, University of Manchester

Importance of the area of research concerned: 
Earth’s environmental history is a story of carbon regulation, whereby feedbacks in the fluxes of carbon between atmospheric and rock reservoirs have sustained a habitable planet for over 4 billion years. At the heart of the terrestrial carbon cycle is volcanic degassing of mantle carbon, which acts as a long-term forcing on Earth’s climate. However, the amount and distribution of carbon in the Earth’s vast silicate interior remains highly uncertain, in part due to the very property that makes carbon such an important element: its tendency to enter the gas phase. This means that basalts, our best tool for interrogating mantle chemistry, have frequently lost their primary CO2 before we can measure it. This work will therefore focus on deciphering the degassing history of basalts in order to constrain the distribution of carbon in Earth’s mantle, focusing on basalts from three key localities which sample shallow mantle, deep mantle, and recycled crustal material.
Project summary : 
The project will ask the question 'How reliable are magmatic archives as recorders of mantle carbon content?'. There are significant limitations to the most ubiquitous approaches of estimating the carbon content of Earth's mantle: Using seawater noble gas contents provides a bulk upper mantle estimate, not easily resolving the constituent mantle sources; whilst C/trace-element ratios in melt inclusions are frequently lowered by C degassing. This project will provide new constraints on the distribution and cycling of C within the mantle by employing C isotopes as an independent tracer of degassing, and by targeting for analysis eruptions with geochemical signatures from: the lower mantle (Iceland); crustal recycling (The Canaries); and the archetypal depleted upper mantle.
What will the student do?: 
This project will combine field work, geochemical micro-analysis, and modelling. Melt inclusions from three eruptions will be investigated for their volatile, trace element and C isotope composition: Icelandic eruptions will be targeted because of their extreme geochemical variability and association with primordial noble gas signatures; the Canary island lavas will be targeted because it has been argued that recycled crustal C is present in its source region; whilst, melt inclusions from the a mid-ocean ridge locality will be used as a key reference point for quantifying upper mantle C content. The student will undertake fieldwork in Iceland to collect new material for analysis and draw on existing collections for the Canary Islands and mid-ocean ridges. Sample characterisation will be carried out at Cambridge's micro-analytical suite, comprising SEM, EPMA and Raman analysis. Trace elements and isotopes will be measured with the Edinburgh NERC ion probe through a separate application. Models of carbon degassing and the isotopic effects of this will be developed and applied to each eruption to estimate its pre-degassed carbon content.
Matthews, S., Shorttle, O., Rudge, J.F. & Maclennan, J. 2017. Constraining mantle carbon: CO2-trace element systematics in basalts and the roles of magma mixing and degassing. Earth and Planetary Science Letters, vol. 480, pp.1-14. doi:10.1016/j.epsl.2017.09.047
Saal, A., Hauri, E.H., Langmuir, C.H., Perfit, M.R. 2002. Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth’s upper mantle. Nature, vol. 419, pp.451-455.
Marty, B., 2012. The origins and concentrations of water, carbon, nitrogen and noble gases on Earth. Earth and Planetary Science Letters, vol.313-314, pp.56-66.
You can find out about applying for this project on the Department of Earth Sciences page.