Reconstructing Earth’s redox evolution with selenium isotopes in hydrothermal ore deposits

Overview

Selenium isotopes are newly emerging redox proxy that may advance our understanding of environmental evolution over the course of Earth’s history (Stueeken 2017). The aim of this project is to set up a new method for selenium isotope measurements and apply it to hydrothermal ore deposits. The selenium data will be combined with sulphur isotope measurements. Together these two proxies will shed new light on the redox state of ancient oceans during the time of ore emplacement.
In the modern ocean, selenium is mostly present as selenate (SeO42-) and selenite (SeO32-), which have similar properties as sulphate (SO42-). As seawater circulates through oceanic crust, these oxyanions become reduced to elemental selenium and selenide. Sulphate is reduced to sulphide. Importantly, these reduction reactions occur at a higher redox potential for selenium than for sulphur, and they impart large isotopic fractionations, which have been documented from modern hydrothermal ore deposits (Rouxel et al. 2004). Another important characteristic of selenium is that its marine reservoir is several orders of magnitude smaller than the sulphate reservoir. Selenium oxyanion reduction thus goes to completion more quickly. This difference is likely to be more extreme under suboxic ocean conditions, where selenium oxyanions are thermodynamically unstable while sulphate can still persist. It is therefore expected that ore deposits forming in less oxygenated marine basins record a lower range in selenium isotope ratios, because selenium oxyanion reduction would rapidly go to completion with no net isotopic effect. In contrast, sulphur isotopes may record moderate levels of fractionation, as documented, for example, from Proterozoic ore deposits (Farquhar et al. 2010).
Preliminary data from Archean and Phanerozoic ore deposits support this hypothesis (Stueeken et al. 2015). By combining selenium and sulphur isotope ratios, it may thus be possible to reconstruct redox conditions in ancient environments more accurately. Indeed, decoupling of the two systems has been previously been demonstrated (Koenig et al. 2019). However, until now, most analyses of selenium isotopes in the Precambrian rock record have focused on black shales, which contain relatively high levels of selenium bound to organic matter. The utility of this bulk rock dataset is limited, because assimilation of selenium into biomass imparts very little isotopic fractionation compared to oxyanion reduction to inorganic selenide (Johnson 2004). Abundant organic-bound selenium in black shales may therefore dilute isotopically fractionated inorganic selenide contained in sulphide minerals, if both phases are analysed simultaneously in a bulk sample. Focusing on pure sulphide minerals from ore deposits may thus provide a cleaner signature of redox processes in the distant past.
This project will focus on selected samples of well-characterised deposits from the Archean Sulphur Springs Group (Australia), the mid-Proterozoic McArthur basin (Australia), the Neoproterozoic Aberfeldy area (Scotland) and the Devonian Iberian Pyrite Belt (Spain). This sample set, combined with existing data from modern hydrothermal sulphides (Rouxel et al. 2004), will provide a first impression of potential temporal trends in selenium isotopes as the ocean became progressively more oxygenated over the course of Earth history. The Devonian ore deposit formed in a restricted marine basin that was probably anoxic during the dime of deposition. It will thus represent a potential analogue to Precambrian sites, but given the exposure of the deposit, it can be investigated in greater detail to better understand the relative partitioning of sulphur and selenium on a deposit scale.

Methodology

A number of samples from several hydrothermal ore deposits is already available. Additional samples will be supplied by collaborator Dr Dan Gregory and collected in the field from ore deposits in the Rio Tinto area, Southern Spain. An aliquot of each sample will be dissolved in acid under clean-lab conditions and analysed with a multi-collector inductively coupled plasma mass spectrometer. The analyses will be carried out with a newly purchased instrument (Sapphire, Nu Instrument) that possesses a collision cell. A major component of this research will therefore be the development of a new analytical technique.
Sulphur isotope measurements will be carried out by combustion with routine gas-source mass spectrometry (EA IsoLink MAT253, Thermo Finnigan). An aliquot of the solutions prepared for selenium isotope analyses will also be analysed for trace metal abundances by ICP-MS, which can provide additional information about the formation of the sample. Reflected light microscopy will be conducted on representative samples from each deposit.

Project Timeline

Year 1

Development of the selenium isotope method on the Sapphire instrument. Field trip to Southern Spain in April 2021. Complete microscopy of key samples.

Year 2

Complete method development. Publication of a methods paper. Begin selenium and sulphur isotope analyses. Present methodology at the GRIP conference.

Year 3

Complete isotopic measurements. Publication about coupled Se/S isotopes in different sulphide minerals from a selected deposit; publication of temporal trends in Se isotopes and their relationship to ocean redox conditions. Present results at the international Goldschmidt conference.

Year 3.5

Completion of publications and writing of the dissertation.

Training
& Skills

The student will gain extensive experience in a number of areas, including:
– clean lab techniques
– method development with an MC-ICP-MS
– gas source mass spectrometry
– economic geology
– hydrothermal processes

Transferrable skills will include scientific writing, project planning and organisation, quantitative analyses of complex datasets, and public oral presentations.

References & further reading

• Farquhar et al., 2010. Connections between sulfur cycle evolution, sulfur isotopes, sediments, and base metal sulfide deposits. Econ. Geol., 105(3), 509-533.
• Johnson, 2004. A review of mass-dependent fractionation of selenium isotopes and implications for other heavy stable isotopes. Chem. Geol., 204(3-4), 201-214.
• Koenig et al., 2019. Redox induced sulfur-selenium isotope decoupling recorded in pyrite. GCA, 244, 24-39.
• Rouxel et al., 2004. Subsurface processes at the Lucky Strike hydrothermal field, Mid-Atlantic Ridge: evidence from sulfur, selenium, and iron isotopes. GCA, 68(10), 2295-2311.
• Stueeken et al., 2015. The evolution of the global selenium cycle: Secular trends in Se isotopes and abundances. GCA, 162, 109-125.
• Stueeken, 2017. Selenium isotopes as a biogeochemical proxy in deep time. Rev. Mineral. Geochem., 82(1), 657-682.

Further Information

For additional information contact Dr Eva Stueeken (ees4@st-andrews.ac.uk)

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