Sulfur isotope signatures in the lower crust: A SIMS study on S-rich scapolite of granulites

Abstract

Scapolite is an important reservoir for volatiles in the deep crust and provides unique insights into the S isotope signatures at themantle/crust interface. Herewe document the first scapolite referencematerial (herein referred to as CB1) for in situ S isotope analysis. The chemical and isotopic composition of this euhedral, S-rich scapolite megacryst was characterized via LA-ICP-MS, EPMA, SIMS, and bulk fluorination gas source isotope ratio mass spectrometry. The CB1 scapolite is isotopically homogeneous and our results show that crystal orientation does not affect in situ S isotope SIMS analysis. This makes CB1 an ideal primary calibration standard for in situ analysis of S isotope ratios (36S/32S, 34S/32S and 33S/32S) in scapolite. With this reference material in hand, we then applied in situ SIMS analysis of S isotopes for the first time on scapolite in granulite samples from the lower crust/upper mantle. The analysed sample suite comprises rocks from classic granulite xenolith locations in southeastern Australia, aswell as a sample fromthe high-grade suture zone of the Dahomeyides in south-eastern Ghana. The results show that scapolites in the lower crust have δ34S values between ~−0.5 and +4 (‰ VCDT). These values fallwithin the range of S isotope signatures present in mantle rocks and provide no evidence for the recycling of seawater-derived S into the lower crust. We propose that scapolite formed during granulite facies metamorphismof igneous cumulates,where Swas sourced fromprecursor igneous sulfides. Sulfur isotope heterogeneities between individual scapolite grains in some of the studied samples may reflect non-uniform Sisotope compositions of igneous S-phases, which precipitated from mantle-derived melt. Show more