Sulphur isotopes (δ34S and ∆33S) in sulphides from cratonic mantle eclogites : A glimpse of volatile cycling in ancient subduction zones

Burness, S. ; Thomassot, E. ; Smart, K.A. ; Tappe, S. (2021) Earth and Planetary Science Letters, 572, 117118

Multiple sulphur isotopic compositions of sulphides from Kaapvaal craton mantle eclogites allow to elucidate the recycling of sulphur into the deep Earth and to differentiate between recycled crust and mantle origins of eclogite-hosted sulphides, including the precious metals that they capture. We present multiple sulphur isotope ratio measurements by secondary ion mass spectrometry for sulphides from a collection of mantle-derived eclogite xenoliths from Proterozoic and Mesozoic kimberlite occurrences in South Africa (Premier, Roberts Victor, Jagersfontein). Previous work established that the host eclogites have elemental and oxygen isotopic compositions in support of seawater-altered oceanic lithosphere protoliths, and for many of these xenolith suites Archean ages have been suggested.

The eclogite-hosted sulphides have δ34S values from −5.7 to +29°/°°, with the upper end of this wide range representing the highest-ever recorded δ34S composition of material derived from the Earth’s mantle. The ∆33S values range from −0.29to +0.18°/°° and do not record significant mass-independent sulphur isotope fractionation, i.e., there is no compelling S-MIF signature. Most of the sulphide grains have δ34S values that fall within a range between −6 and +4°/°° and they probably retain an isotopic record of sulphides that formed originally within altered oceanic crust. In contrast, the highly positive δ34S values from +13 to +29°/°° detected in sulphide grains from a single eclogite xenolith are similar to those of marine sulphates, which were probably a minor sulphur component of the oceanic crustal protolith. The lack of a significant S-MIF signature in the eclogitic sulphides that show δ34S evidence for a recycled crust origin implies that this sulphur component stems from a <2.4Ga post-Archean surficial reservoir. This finding suggests that the cratonic mantle eclogites may have formed from post-Archean oceanic crust (e.g., Paleoproterozoic eclogite protoliths), or– as is preferred here– the ‘surficial’ sulphur was introduced into the cratonic root during relatively young metasomatic events and is thus unrelated to eclogite petrogenesis and Archean continent formation.