SP-IM and SP-SP: two potential natural sphalerite reference materials for in situ Zn and S isotope analysis by LA-MC-ICP-MS

Abstract

Reliable matrix-matched reference materials are essential for accurate and precise in situ isotope analysis of sulfide minerals by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). Sphalerite is one of the most important Zn-bearing sulfides in economic geology and isotope geochemistry, yet well-characterized natural sphalerite reference materials for in situ Zn and S isotope analysis remain scarce. In this study, two natural sphalerite materials, designated SP-IM and SP-SP, were systematically evaluated as candidate reference materials for in situ Zn and S isotope analysis by fs-LA-MC-ICP-MS. Their mineralogical and chemical characteristics were investigated by electron probe microanalysis (EPMA) and laser ablation ICP-MS trace element analysis. Both materials are dominated by Zn and S, contain low Fe contents, and display generally simple internal textures. Representative time-resolved signals indicate stable ablation behaviour during LA-MC-ICP-MS analysis. Repeated LA-MC-ICP-MS analyses yielded mean δ66ZnJMC-Lyon values of 0.12 ± 0.08‰ (2SD, n = 234) for SP-IM and −0.02 ± 0.10‰ (2SD, n = 185) for SP-SP, and mean δ34SV-CDT values of −2.07 ± 0.25‰ (2SD, n = 264) and 3.85 ± 0.32‰ (2SD, n = 191), respectively. These values agree well with the independently determined bulk isotope compositions obtained by solution MC-ICP-MS and EA-IRMS. In situ isotopic homogeneity was further assessed by repeated analyses of eight fragments from each material and by one-way ANOVA. The calculated F values for both Zn and S isotope data are lower than the corresponding critical F values, indicating no statistically significant between-fragment isotopic heterogeneity at the analytical scale investigated. These results indicate that SP-IM and SP-SP are suitable candidate reference materials for routine in situ Zn and S isotope analysis of sphalerite and may be useful for method development, routine quality control, and future inter-laboratory comparison in sulfide isotope studies.