Sulfur isotope composition of metasomatised mantle xenoliths from the Bultfontein kimberlite (Kimberley, South Africa): Contribution from subducted sediments and the effect of sulfide alteration on S isotope systematics

A. Giuliani, Marco Fiorentini, Laure Martin, J. Farquhar, D. Phillips, W.L. Griffin, Crystal Laflamme

Research output: Contribution to journalArticle

27 Citations (Scopus)

Abstract

© 2016 Elsevier B.V. Sulfur isotopes are a powerful geochemical tracer in high-temperature processes, but have rarely been applied to the study of mantle metasomatism. In addition, there are very limited S isotope data on sub-continental lithospheric mantle (SCLM) material. For cratonic regions, these data are restricted to sulfide inclusions in diamonds. To provide new constraints on the S isotope composition of the SCLM and on the source(s) of mantle metasomatic fluids beneath the diamondiferous Kimberley region (South Africa), we investigated the S isotope systematics of five metasomatised mantle xenoliths from the Bultfontein kimberlite. Pentlandite and chalcopyrite in these xenoliths were analysed by in situ secondary-ion mass spectrometry (SIMS), with bulk-rock material measured by gas source isotope ratio mass spectrometry techniques. Based on previous studies, the xenoliths experienced different types of metasomatism to one another at distinct times (~180 and ~90-80 Ma). Contained pentlandite grains show variable alteration to heazlewoodite (i.e. Ni sulfide) + magnetite. The in situ S isotope analyses of pentlandite exhibit a relatively restricted range between -5.9 and -1.4‰d34S (compared to VCDT), with no statistically meaningful differences between samples. Chalcopyrite only occurs in one sample and shows d34S values between -5.4 and -1.0‰. The bulk-rock Ssulfide isotope analyses vary between -3.4 and +0.8‰d34S. Importantly, the only sample hosting dominantly fresh sulfides shows a bulk-rock d34S value consistent with the mean value for the sulfides, whereas the other samples exhibit higher bulk 34S/32S ratios. The differences between bulk-rock and average in situ d34S values are directly correlated with the degree of sulfide alteration. This evidence indicates that the elevated 34S/32S ratios in the bulk samples are not due to the introduction of heavy S (commonly as sulfates) and are best explained by isotopic fractionation coupled with the removal of light S during serpentinisation, when pentlandite
Original languageEnglish
Pages (from-to)114-124
JournalEarth and Planetary Science Letters
Volume445
Early online date19 Apr 2016
DOIs
Publication statusPublished - Jul 2016

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