Tracing sulfur sources in the crust via SIMS measurements of sulfur isotopes in apatite

Abstract

We present a refined approach for acquiring sulfur (S) isotope compositions (33S/32S, 34S/32S) in apatite by secondary ion mass spectrometry (SIMS), including the characterisation of new reference materials. In order to test the method, we analyzed potential apatite reference samples for their S isotope ratios via three different bulk methods. The investigated apatite samples contain S concentrations between ~160 μg/g and 3100 μg/g and their 34S/32S (δ34S) ratios deviate by more than 25‰ from the Vienna-Canyon Diablo Troilite (VCDT) standard. We identified four candidates as new primary reference materials for routine SIMS S isotope measurements of apatite. Based on ICP-MS, EA-IRMS, and fluorination analyses, recommended S isotope values are +12.27± 0.22 (2σ) ‰ δ34S for SAP1, +14.02 ± 0.22 (2σ) ‰ δ34S for Big1, −1.06 ± 0.80 (2σ) ‰ δ34S for Durango-A, and −1.39 ± 0.48 (2σ) ‰ for Durango-B. By selecting one of those four primary standards for SIMS analysis, the S isotope values of the other reference materials and additional tested apatite specimens can be reproduced to within 1‰. Under optimized SIMS conditions, single spot uncertainty for δ34S that combines the within-spot precision and the repeatability of measurements of the primary apatite reference material during an analytical session is ±0.4‰ (95% CI). We also show that in apatite with S > 1000 μg/g, SIMS analysis permits the detection of mass-independent S isotope signatures (i.e., Δ33S) that are larger than ~1.0‰ if an average of multiple grains is used, and larger than ~1.5‰ for a single analytical point. Furthermore, our study shows that apatite can record S isotope signatures from extremely diverse environments, making this near-ubiquitous mineral a key candidate for tracing S source reservoirs and to track the pathway of magmatic-hydrothermal fluids in a wide range of geological settings.