Mantle oddities: A sulphate fluid preserved in a MARID xenolith from the Bultfontein kimberlite (Kimberley, South Africa)

Sulphur in the lithospheric mantle is concentrated in sulphide minerals, with limited evidence for the occurrence of sulphate phases. Here we describe an unusual assemblage of celestine (SrSO₄), clinopyroxene and minor phlogopite, pectolite, sphene, apatite, barite (BaSO₄) and Ca-Sr carbonates in a MARID mantle xenolith sampled by the Bultfontein kimberlite (Kimberley, South Africa). This assemblage occurs in veins that pervasively traverse the xenolith, indicating that celestine and the other vein minerals crystallised from a fluid. In the MARID host rock, K-richterite is resorbed where in contact with celestine and is overgrown by clinopyroxene. Celestine hosts the other metasomatic vein phases, but also occurs as inclusions in euhedral clinopyroxene, suggesting co-precipitation of these minerals. Celestine was partly replaced by serpentine during alteration by hydrous fluids after kimberlite emplacement in the upper crust. Celestine has relatively radiogenic Sr isotopes (⁸⁷Sr/⁸⁶Sr = 0.70677), which overlap those of K-richterite in the MARID host rock and fall within the range of other MARID and phlogopite-K-richterite peridotites sampled by southern African kimberlites. Celestine displays S isotopes (δ³⁴S = + 5.9%) that are slightly heavier than typical mantle values (δ³⁴S = 0%) and there is no evidence of mass-independent fractionation (Δ³³S = -0.01%). The texture and chemical composition of the metasomatic phases indicate that the MARID rock was infiltrated by a sulphate fluid enriched in Sr, Ba, Na and Ca, with lesser P, Ti, LREE, CO₂ and F. The similar Sr/Ba ratios and Sr isotopic compositions of celestine and K-richterite suggest that K-richterite breakdown contributed to the alkali enrichment of the sulphate fluid. A mantle origin for the sulphate fluid is supported by (i) comparisons between the Sr-S isotopic compositions of celestine and the host kimberlite, crustal and mantle lithologies from the area, and (ii) alteration of celestine by late-stage hydrous fluids. Consequently, the celestine-bearing veins provide the first evidence for the occurrence of sulphate-dominated fluids in the Earth's mantle.