Tracking element transfer during subduction initiation using the isotopic composition of ‘chalcophile’ elements

Overview

Subduction of one tectonic plate beneath another is the primary driving force for plate tectonics on our planet. Furthermore, subduction zones are the main location of mass transfer between the surface and interior of the Earth. The processes that occur at subduction zones thus impact the geochemical cycling of all elements.

Perhap the most enigmatic aspect of subduction is how it begins. Processes like continental rifting and eventual ocean basin formation have accessible, real world analogues (e.g., East African rift). This is not the case for subduction zones.

International Ocean Discovery Program (IODP) Expedition 352 took place in 2014 and recovered the most complete, in situ record of magmatism at the onset of subduction (Reagan et al., 2017). The recovered core is exceptional for its preservation of fresh glass, which implies that limited to no secondary alternation has affected the quenched melt – this lack of well preserved primary compositions has hindered many geochemical investigations of early subduction products.

This project focussed on the first two distinct lithologies that are produced during subduction initiation: forearc basalts (FAB) and boninites. FAB closely resemble mid-ocean riddge basalt (MORB) with little indication of a subduction influence. Intrigingly, formation of these MORB-like rocks is suddenly (geologically speaking) followed by boninite – a fluid and volatile rich magma type with extreme enrichments in fluid mobile elements, presumably all dervied from the newly subducting slab (e.g., Li et al., 2019; Maunder et al., 2020). The close spatial and temporal association of the FAB and boninites (within 30 km and ~800 kyr) combined with high resolution recovery on Exp 352 offers an unrivalled opportunity to examine the very first element transfer from the subducting slab and how that might evolve as subduction establishes itself.

The focus of this PhD is on the so-called chalcophile elements (e.g., Barnes, 2016) and their isotopic systematics, in particular copper (Cu), thallium (Tl) and sulphur (S). Subduction has a strong concentrative effect on these elements (most giant chalcophile ore deposits are assocaited with subduction), and furthermore, S is a much more effective oxidising agent than water or CO2. Thus mapping chalcophile pathways – the source(s) of these elements, and where, how and why they were transported – has implcaitions for constraining both the physical parameters of subduction initiation, but may also yield insight to conditions that favour chalcophile concentration.

The extensive existing chemical and petrologic charaterisation of the IODP Exp 352 drill samples (eg., Li et al., 2019 ; Shervais et al., 2019, 2020) provide the necessary context in which to evaluate chalcophile element behavior, isotopic fractionation, complexation and fluid transfer.

Specifically, this project aims to:
• Determine the relative budget of chalcophile elements derived from the slab and mantle in FAB and evolving boninites
• Determine the magnitude and direction of isotopic fractionation associated with chalcophile fluid complexation
• Evaluate whether the oxygen fugacity of the evolving subduction system can be derived in a self-consistent manner from combined Cu-Tl-S isotopes.

Click on an image to expand

Image Captions

Figure 1. Schematic of subduction initiation from the Bonin Forearc (Shervais et al. 2020, in press)

Methodology

The student will initially work on samples from IODP 352. If time permits, this may be expanded to include ophiolite sequences interpreted to record subduction initiation. There may be the opportunity for further fieldwork in Cyprus, pandemic permitting. However, the project does not depend on fieldwork.

All IODP samples have been characterised for major, trace and some radiogenic isotope systems. The student’s research will focus on determining stable isotope compositions of elements with chalcophile affinity (Cu, S, Tl). The student will use a combination of cutting-edge techniques and the unique sample set to achieve the research objectives.

Specifically, they will use methods including:
• Sample purification by column chemistry and measurement via MC-ICP-MS for Cu and Tl at Durham, Earth Sciences
• S species will be extracted from whole-rock powders using a Cr reduction procedure. These will then be converted to SO2 and SF6 for IRMS of major and minor S isotopes, repectively. Possible applications for SIMS in situ S isotopes at Edinburugh facility will be possible to further interrogate whole-rock isoptopic trends

Project Timeline

Year 1

Literature review, training in clean laboratory procedures, column chemistry, and mass spectrometry for Cu and Tl isotopes. Write/defend research proposal. Attend national Geochemistry Research in Progress and Volcanic Magmatic Studies Group conferences.

Year 2

Sample and data processing, main geochemical approaches (ICP-MS, MC-ICP-MS, IRMS). Time spent at St. Andrews to undertake S isotope analyses on selected samples. Further develop writing skills and manuscript preparation for publication. Attend national Geochemistry Research in Progress and Volcanic Magmatic Studies Group conferences.

Year 3

Synthesise and model datasets; attend international conference(s); publication and thesis writing.

Year 3.5

Complete and submit thesis; finalise manuscripts for publication.

Training
& Skills

This project would suit a student with a degree in Earth Sciences, Chemistry (or a related field) and strong interests in subducion zones, igneous process, trace metal and isotope geochemistry.

Excellent time management skills coupled with strong numerical, verbal and written communication are important. Previous analytical experience would be an advantage but is not essential. Training will cover a wide range of cutting-edge geochemical methods, including ion chromatography and isotope ratio measurement. The student will be ecourages to participate in relevant workshops for both project specific and general transferable skills.

The student will join the vibrant Durham Isotope Group, which includes research students and postdocs from the Earth Science, Geography and Archaeology departments. They will also benefit from interactions with the St. Andrews Isotope Geochemistry Group (StAIG) which will help develop complementary research skills and widen their network of collaborators.

They will attend national and international conferences, networking events and outreach activities, developing an important network for feedback and future employment.

References & further reading

Barnes, S.-J. 2016. Chalcophile Elements. Encylopedia of Geochemistry, W.M. White (ed.) doi:10.1007/978-3-319-39193-9_220-1.

Maunder, B., Prytulak, J., Goes, S, Reagan, M.K. 2020. Rapid subduction initiation and magma genesis driven by vertical forces. Nature Communications, doi.org/10.1038/s41467-020-15737-4

Li, H., Taylor, R.N., Prytulak, J., Kirchenbaur, M., Shervais, J., Godard, M., Reagan, M., Ryan, J., Pearce, J.A. 2019. Radiogenic isotopes document the start of subduction in the Western Pacific. Earth and Planetary Science Letters, 518, 197-210.

Reagan, M.K., Pearce, J.A., Petronotis, K., Almeev, R., Avery, A.J., Carvallo, C., Chapman, T., Christenson, G.L., Ferre, E.C., Godard, M., Heaton, D.E., Kirchenbaur, M., Kurz, W., Kutterolf, S., Li, H., Li, Y., Michibayashi, K., Morgan, S., Nelson, W.R., Prytulak, J., Python, M., Robertson, A.H.F., Ryan, J.G., Sager, W.W., Sakuyama, T., Shervais, J., Shimzu, K., Whattam, S.A. 2017. Subduction initiation and ophiolite crust: new insights from IODP Drilling. International Geology Review, doi.org/10.1080/00206814.2016.127642.

Shervais, J.W., Reagan, M.K., Haugen, E., Almeev, R., Pearce, J., Prytulak, J., Ryan, J.G., Whattam, S., Godard, M., Chapman, T., Li, H., Nelson, W.R., Kirchenbaur, M., Heaton, D., Kurz, W., Shimizu, K., Sakuyama, T., Li, Y., Vetter, S.K. 2018. Magmatic response to subduction initiation, Part I: Fore-arc basalts of the Izu-Bonin Arc from IODP Expedition 352. G-cubed, doi.org/10.1029/2018GC007731.

Shervais, J.W., Reagan, M.K., Godard, M., Prytulak, J., Ryan, J.G., Pearce, J., Almeev, R., Li, H., Haugen, E., Chapman, T., Kurz, W., Nelson, W.R., Heaton, D., Kirchenbaur, M., Shimizu, K., Sakuyama, T., Vetter, S.K., Li, Y., Whattam, S. 2020. Magmatic response to subduction initiation, Part II: Boninites and related rocks of the Izu-Bonin Arc from IODP Expedition 352. G-cubed, in press.

Further Information

For further information and informal enquiries, please contact Julie Prytulak, Durham University (julie.prytulak@durham.ac.uk)

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