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E325: The redox structure of the Icelandic plume: new constraints from iron stable isotopes (Lead Supervisor: Helen Williams, Earth Sciences)

Supervisors: Helen Williams (Earth Sciences), John Maclennan (Earth Sciences), Oliver Shorttle (Earth Sciences) and Julie Prytulak (Imperial, London)

Importance of the area of research:

The oxidation state of the Earth’s mantle has been key in coupling the physical and chemical evolution of our planet’s interior to that of its oceans and atmosphere. However, there is little consensus on the redox structure of the mantle. This controversy has arisen from the difficulty in reconciling constraints on mantle oxidation state derived from redox-sensitive trace elements such as vanadium with measurements of iron oxidation state (Fe3+/SFe). A major issue is that variations in Fe3+/SFe can be modified by magmatic differentiation, degassing and post-eruptive alteration processes.  Iron isotopes are a new tool that can be applied to this problem, as theory predicts that stable isotope fractionation will be driven by changes in chemical bonding environment and elemental oxidation state, and because the high abundance of Fe in mafic rocks renders this system insensitive to post-magmatic alteration [e.g. 1, 2].

Project summary:

The project will use iron stable isotopes in conjunction with Fe3+/SFe and trace element measurements to understand how mantle oxidation state varies between source lithologies and the presence of recycled and depleted source components. We will focus on the mantle sampled by the Reykjanes Ridge and Iceland plume, as these areas are well understood and encompass both local-scale chemical variations and longer-wavelength features. In order to distinguish mantle mineralogical variations from variations in oxidation state, we will first focus on lava suites considered to be derived from sources of similar source lithologies before tackling complex suites such as the Reykjanes Ridge, where variations in mantle mineralogy may be coupled to Fe3+/SFe.

What the student will do:

The student will initially carry out iron isotope analyses of samples at the bulk-rock and mineral separate scale, initially working on archive samples after which they will select target areas for fieldwork and new sample collection. We will study the eruptions of Borgarhraun and Gaesafjoll in north Iceland and Stapafell and Midfell in the south. They are primitive and have extreme trace element and isotopic compositions indicative of the presence of pyroxenitic components. These samples will be analysed for Fe3+/SFe, using either wet chemistry techniques or XANES. We will complement this work with a high-resolution suite from the Reykjanes Ridge, where systematic variations in trace elements and Fe3+/SFe are documented [3]. The student will use these data to determine the controls that source mineralogy, partial melting and magmatic differentiation processes exert on Fe3+/SFe, and isotopic and trace element proxies for melt oxidation state.

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:

[1] Williams, H. M., McCammon, C.A., Peslier, A. H., Halliday, A. N., Teutsch, N., Levasseur, S. and Burg, J.-P., 2004. Iron isotope fractionation and the oxygen fugacity of the mantle. Science 304 (5677), 1656-1659.

[2] Williams, H. M. and Bizimis, M. "Isotopic fingerprinting of mantle mineralogy." (2014) Earth and Planetary Science Letters 404, 396–407

[3] Shorttle, O., Moussallam, Y., Hartley, M., Maclennan, J., Edmonds, M., Murton, B., 2015 Fe-XANES analyses of Reykjanes Ridge basalts: Implications for oceanic crust’s role in the solid Earth oxygen cycle EPSL 427, 272-285.

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