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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities

Lead Supervisor: Sally Gibson, Earth Sciences

Co-Supervisor: Jason Day, Earth Sciences

Brief summary: 
This proposed PhD research explores fundamental aspects as to how volatiles, including those essential for life (such as water and carbon dioxide), are cycled through the solid Earth in deep time.
Importance of the area of research concerned: 
The outgassing of volatiles from Earth’s mantle regulates the surface climate through deep time and has shaped the habitability of our planet. A key but poorly understood reservoir that controls the outflux of volatiles (e.g. CO2, H2O) from Earth’s deep interior to its surface is the lithospheric mantle; this has the capacity to both store large quantities of deep-sourced volatiles and release them to its atmosphere during volcanism (Gibson et al., 2020). The origin of volatiles stored over deep time in this ancient reservoir is, however, controversial and fundamental outstanding questions include: (i) how much carbon is stored in the lithospheric mantle? (ii) are the volatiles introduced during melt-rock reactions involving oxidising carbonatitic melts/fluids released from subducted slabs at the upper-lower mantle transition zone (Sun and Dasgupta, 2019) or less oxidised silicate (proto-kimberlite melts) sourced from the deep convecting mantle (Foley et al., 2019)? (iii) How does the variable influx of multiple volatiles control oxidation state, and the formation and preservation of diamond in the lithospheric mantle?
Project summary : 
The transport of volatiles between the solid-Earth, oceans and atmosphere remains poorly constrained. This is especially true for the continents. The key objectives of this research project are to: (i) quantify the fluxes and depth distributions of volatiles and the associated oxidation state in Earth’s lithospheric mantle; (ii) establish how oxidation state relates to the passage of melts and fluids with variable CO2 and H2O contents; (iii) quantify the C budget of the lithospheric mantle. The research will also address the paradox that currently exists between models of kimberlite formation, which require the lower sub-cratonic lithosphere to be highly oxidised (e.g. Foley et al., 2019) and the general decrease in oxidation state with increasing depth calculated from analyses of mantle peridotites (Stagno et al., 2013; Miller et al., 2016).
What will the student do?: 
A critical aspect of the project is to address the cause of the wide scatter in oxygen fugacity determined for mantle peridotites entrained from similar depths, and establish if this permits the extrapolation of redox conditions into the underlying convecting mantle. The project will focus on the petrology (textures and mineralogy), together with the geochemistry of peridotite xenoliths entrained from the ancient lithospheric mantle ‘roots’ of continents by kimberlites of varying ages. An extensive set of peridotite xenoliths will be made available from the supervisors’ collections. Mantle phases in these xenoliths will be analysed for major, trace elements and volatile contents (C, H etc) together with Fe2+/Fe3+ ratios. Key volatile and non-volatile trace element ratios will act as proxies for the compositions of infiltrating melts and fluids. Pressure and temperature estimates derived from mineral chemistry will enable construction of stratigraphic profiles and geothermal gradients through the continental mantle and allow quantification of how the influx and distribution of volatiles and corresponding oxidation state has changed over billions of years.
References - references should provide further reading about the project: 
Gibson, S.A., Rooks, E.E., Day, J.A., Petrone, C.M., and Leat, P.T., 2020, The role of sub-continental mantle as both “sink” and “source” in deep Earth volatile cycles: Geochimica et Cosmochimica Acta, v. 275, p. 140–162, doi:10.1016/j.gca.2020.02.018.
Stagno, V., Ojwang, D.O., McCammon, C.A., and Frost, D.J., 2013, The oxidation state of the mantle and the extraction of carbon from Earth’s interior: Nature, v. 493, p. 84–88, doi:10.1038/nature11679.
Sun, C., and Dasgupta, R., 2019, Slab–mantle interaction, carbon transport, and kimberlite generation in the deep upper mantle: Earth and Planetary Science Letters, v. 506, p. 38–52, doi:10.1016/j.epsl.2018.10.028.
You can find out about applying for this project on the Department of Earth Sciences page.
Prof Sally Gibson
Department of Earth Sciences Graduate Administrator