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E333: Tracking the early stages of slab decarbonation using stable isotopes (Lead Supervisor: Helen Williams, Earth Sciences)

Supervisors: Helen Williams (Earth Sciences), Owen Weller (Earth Sciences), Baptiste Debret (Earth Sciences) and Marie-Laure Pons (Earth Sciences)

Importance of the area of research:

Understanding the fate of carbon during subduction has critical implications for understanding the oxygenation of the Earth’s atmosphere and oceans over the last 4 billion years. Considerable quantities of carbon are transported into the Earth’s deep interior by subduction, manifest in the existence of deep diamonds and other lower-mantle carbon bearing phases. The efficiency of this carbon burial is unknown and it has been argued that most of the slab’s carbon is lost to the overlying plate as hydrous carbonated fluids. However, recent studies have shown that carbonate-bearing fluids released during slab dehydration do not necessarily escape and may be retained in the subducting slab in the form of carbonated eclogites, which are ultimately subducted and recycled into the mantle. New ways of studying the slab decarbonation and carbonation reactions, the mass balance of carbon during subduction and carbon transport from and within the subducting slab and mantle wedge are therefore required.

Project summary:

The principal question this project will address is how much carbon is retained in the slab during subduction and buried in the deep mantle versus how much leaves the slab to contribute to the source region of arc lavas. The project will use novel stable isotope systems in conjunction with fieldwork and thermodynamic calculations to trace carbon-bearing fluids in subduction zones and understand the relationships between P-T-fO2-aCO2 conditions and carbonate solubility in slab-derived fluids. The student will combine fieldwork, geochemistry and petrology to study carbon bearing metasomatic contacts between subducted ultramafic rocks and carbonate sediments. Depending on the student’s interests, this project could expand to consider primitive arc lavas, and the degree to which their volatile contents are controlled by slab fluids, or ocean-island basalts, as a potential record of long-term carbon recycling into the deep mantle.

What the student will do:

The student will use a series of novel stable isotope systems that have been shown to be sensitive tracers of carbonate fluid transport and redox reactions in subduction zones (e.g. Fe, Zn, Mg) in order to obtain direct mass balance constraints on fluid transfer. The student will initially study carbon bearing metasomatic contacts via fieldwork in well-characterised regions such as the Eastern Alps and Japan and through the study of archive samples.  Isotopic analyses will be carried out at both the bulk sample scale, in order to constrain open-system processes such as decarbonation, and the mineral scale, in order to identify the carriers of isotopic anomalies. These samples will also be characterised for their major and trace element abundances, and potentially concentrations of volatile elements (via ion probe).  This work will be complemented by thermodynamic calculations in order to link the isotopic results with constraints on P-T-fO2-aCO2 and carbonate solubility.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Debret et al., “Isotopic evidence for iron mobility during subduction” (2016), Geology, v. 2, DOI: 10.1130/G37565.1

Galvez et al., “Graphite formation by carbonate reduction during subduction” (2013), Nature Geoscience, v. 6 p. 473, DOI: 10.1038/NGEO1827

Wang et al., “Tracing carbonate–silicate interaction during subduction using magnesium and oxygen isotopes” (2014) Nature Communications, DOI: 10.1038/ncomms6328.

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