The record of planetary differentiation preserved in sulfide

Overview

Sulfide is a ubiquitous mineral in mafic and ultramafic rocks, both mantle rocks and mantle derived magmas. Sulfide, like zircon in silica-rich rocks, preserves information on both the time of its crystallisation and the chemistry of the melt from which it forms. Crucially yielding information that cannot be recovered by measuring the host rock rock alone. Both Os and Pb are chalcophile (have an affinity for sulfide) and are highly concentrated in this phase. Consequently, it is possible to obtain precise age information from individual sulfide grains, and such ages have been used to determine the timing of mantle melting events [1,2], and in magmatic rocks used to determine the age of eruption and the chemistry of the mantle source [3].

A number of other elements are also concentrated in sulfide, including Fe, Cu and Ni, and the stable isotopes of these elements potentially preserve information on a range of processes and the interaction of different parts of the Earth. From core formation and differentiation of the early Earth, to the melting that produces oceanic and continental crust at the present day [4-6] .

Recent work has shown that sulfides in oceanic basalts can preserve information that relates both to early crystallisation from primitive melts, when trapped as globules in olivine megacrysts – to final crystallisation from enriched melts, when preserved as sulfide in glass and crystalline matrix [7].

This project aims to reconstruct the record of mantle differentiation preserved in individual sulfide crystals, combining the age information preserved by long-lived radiogenic Os and Pb isotopes, with information on mantle chemistry, in particular that preserved by a range of metal stable isotopes. The focus of the analytical work will be on well-characterised mafic and ultramafic rocks, ranging from 3.8 Billion year old ultramafic rocks, to oceanic basalts recently formed at mid-ocean ridges.

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Image Captions

Sulfide globule in olivine megacryst from a mid-ocean ridge basalt

Methodology

The aim of this project is to reconstruct the chalcophile element chemistry of magmatic rocks and the mantle over Earth’s history. This will be achieved through measurement of (i) Os and Pb to retrieve information on the age of crystallisation and the nature of the source and (ii) metal stable isotopes, revealing information on both source and magmatic process. These data will be used to (iii) trace the source of different heterogeneities in the mantle, and the implications for crustal recycling and mantle depletion, (iv) place limits on the timescale of mantle depletion, (v) determine the role of mantle heterogeneity in controlling melting beneath mid-ocean ridges.

Project Timeline

Year 1

Training in the measurement of radiogenic and stable isotopes, sample petrography; Preliminary isotope measurements of sulfides from MORB and mantle rocks, fieldwork in Scotland; Complete year 1 Research Proposal and review

Year 2

Selection and characterisation of MORB and mantle samples; continued measurement of all samples; Fieldwork in Greenland. Prepare research for presentation/publication; attend Goldschmidt geochemistry conference.

Year 3

Completion of isotope work and interpretation and modelling of data, writing up. Presentation of results at national/international conferences

Year 3.5

Complete and submit thesis; finalise manuscripts for publication.

Training
& Skills

Training in the measurement of stable and radiogenic isotopes using high precision MC-ICP-MS and TIMs techniques at Durham and St Andrews. Geochemical characterisation of sulphides and coexisiting phases.

Fieldwork in Scotland and Greenland

Interpretation and modelling of isotope and elemental data to place new constraints on mantle melting and mantle evolution over Earth’s history

Presentation of research at both national and international geochemistry conferences

References & further reading

[1] Harvey et al., Ancient melt extraction from the oceanic upper mantle revealed by Re-Os isotopes in abyssal peridotites from the Mid Atlantic ridge. Earth Planet. Sci. Lett. 244 (2006) 606-621.[2] Burton et al., Unradiogenic lead in Earth’s upper mantle, Nature Geoscience (2012) 5, 570-573.[3] Gannoun et al., The scale and origin of the osmium isotope variations in mid-ocean ridge basalts Earth Planet. Sci. Lett. 259 (2007) 541-556.[4] Savage et al., Copper isotopic evidence for large-scale sulphide fractionation during Earths differentiation. Geochem. Persp. Let. (2015) 1, 53-64 | doi: 10.7185/geochemlet.1506[5] Elliott and Steele, The isotope geochemistry of Ni. Rev. Mineral. Geochem. 82 (2017) 511-542[6] Schuessler et al., The experimental calibration of the iron isotope fractionation factor between pyrrhotite and peralkaline rhyolitic melt, Geochim Cosmochim Acta, 71 (2007) 417-433.[7] Gannoun et al., Highly Siderophile Element and Os Isotope Systematics of Volcanic Rocks at Divergent and Convergent Plate Boundaries and in Intraplate Settings, Rev. Mineral. Geochem., 81, 651-724 (2016).

Further Information

For further information please contact Kevin Burton (kevin.burton@durham.ac.uk)

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