In situ measurement of sulfur isotope ratios in sulfide samples with LA-ICP-MS/MS using N2O and He reaction gas

Abstract

Sulfur isotope signatures (δ³⁴S) in sulfide minerals such as pyrite and pyrrhotite may reflect the specific geological conditions at their genesis. Understanding the δ³⁴S variability can help track (bio)–geochemical processes, from ore formation to finding evidence of early life. However, as sulfide mineral growth can occur at various stages of rock history, traditional bulk S isotope analysis can incorporate mixed geochemical signals generated by unrelated processes. In situ analytical techniques can be used to investigate compositional changes in δ³⁴S caused by early environmental or secondary processes. In this study, we aim to characterize δ³⁴S variability in pyrite and pyrrhotite using laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS) while introducing a mixture of N₂O and He in the reaction chamber to remove polyatomic interferences at m/z = 32 and m/z = 34. Alongside tuning the respective laser and ICP parameters, we employ a self-developed signal-smoothing device consisting of coiled thermoplastic elastomer (TPE) tubing and a cyclonic spray chamber to achieve better signal stability. In this way, we propose a new, fast, in situ screening approach for measuring the δ³⁴S of sulfides.