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

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‰δ³⁴S (compared to VCDT), with no statistically meaningful differences between samples. Chalcopyrite only occurs in one sample and shows δ³⁴S values between -5.4 and -1.0‰. The bulk-rock S_sulfide isotope analyses vary between -3.4 and +0.8‰δ³⁴S. Importantly, the only sample hosting dominantly fresh sulfides shows a bulk-rock δ³⁴S 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 δ³⁴S 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 is altered to S-poor mixtures of heazlewoodite and magnetite. Available bulk S_sulfide isotopic data for SCLM peridotite xenoliths are dominated by positive δ³⁴S values, which contrasts with the negative values of the sulfides. These results imply that the mantle S isotope values from bulk peridotite samples are commonly modified by isotopic fractionation during serpentinisation. Therefore, the S isotopic composition of the SCLM may require revision. The limited isotopic variability shown by sulfides in the Bultfontein mantle xenoliths is probably due to intermittent tapping of a mantle source with a relatively restricted S isotope composition. While the asthenospheric mantle (δ³⁴S≤-1.4‰) is a viable candidate, δ³⁴S values as low as -5.9‰ require input from recycled crustal material, possibly represented by the sulfur reservoir with negative δ³⁴S signature that is missing in the >500 Ma sedimentary record and could have been subducted and stored in the Earth's mantle.