INSIGHTS INTO THE SULFUR ISOTOPE SIGNATURE OF THE LOWER CRUST: IN SITU S ISOTOPE ANALYSIS OF S-RICH SCAPOLITE VIA SIMS

The global sulfur (S) cycle has attracted vast interest across various disciplines, as S is a crucial element for a wide range of processes including the evolution of the atmosphere and potentially also the oxidation of the sub-arc mantle. One key process in global S (re-) cycling is the subduction of oceanic crust together with sediments and seawater and the subsequent release of S-bearing fluids from the subducted slab. It has been hypothesized that S derived from subducted slab material is recycled via the mantle into the overlying arc. However, it is not well understood how this subduction-derived S interacts with the mantle S reservoir, nor how S is transported via fluids through the sub-arc mantle and crust. The interface between the mantle and the crust has been poorly studied in terms of S isotope systematics. Here, we explore S isotope signatures in the lower crust via in situ (SIMS) S isotope analysis of S-rich scapolite hosted in granulites. S-rich scapolite has a restricted stability at conditions found in the lower crust, and is typically formed during granulite-facies metamorphism under oxidized conditions, where scapolite often occurs as the only S-bearing mineral. Therefore, scapolite is a suitable mineral to trace fluid fluxes from the subducting slab and mantle through to the overlying crust. In this study we present a large scapolite crystal (CB1) as a suitable standard for in situ S isotope analysis in scapolite due to its homogeneity and lack of inclusions. We applied SIMS analysis to several scapolite-bearing granulite xenoliths from the Delegate Breccia Pipes locality in New South Wales, Australia, and a scapolite-bearing granulite from Shai Hills, southeastern Ghana. Our results show that the S isotope signatures fall within the same range as typically found in the upper mantle (d³⁴S ~­­–0.3 to ~4.2 ‰). Given the mantle-like d³⁴S signature, a major seawater component (d³⁴S >>5) was not likely to be present at the time of scapolite formation. Our results, together with previous studies, challenge the concept that slab-derived fluids play a key role in enriching arc magmas in ³⁴S.