No significant crystal orientation effect during sulfur isotope analysis of marcasite using SIMS†

Abstract

The sulfur isotopic composition of marcasite (FeS₂, orthorhombic), a dimorph of pyrite (FeS₂, isometric), can be used to trace ore-forming fluid source(s) and ocean acidification events in deep time. However, matrix-matched standards for marcasite are scarce, and in practice, a pyrite standard is commonly used to calibrate the sulfur isotope ratio of marcasite during laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) and secondary ion mass spectrometry (SIMS) analyses. Recently, it has been suggested that it is feasible to calibrate the sulfur isotope ratio of marcasite with a pyrite standard during SIMS analysis, but the crystal orientation effect during sulfur isotope analysis of marcasite using SIMS remains unexplored. We investigated this topic by analyzing a natural marcasite sample (NJUMc-1). We analyzed NJUMc-1 using LA-MC-ICP-MS and SIMS. The analysis spots for LA-MC-ICP-MS and SIMS were very close to each other, and both spots were located in domains with the same brightness in backscattered electron images (BSE). The results derived from these two methods lie around a 1 : 1 line in a δ³⁴S_LA-MC-ICP-MSvs. δ³⁴S_SIMS binary diagram, indicating that SIMS data are comparable to LA-MC-ICP-MS data. However, there are some grains in which δ³⁴S_LA-MC-ICP-MS deviates from δ³⁴S_SIMS (“δ³⁴S_{LA-MC-ICP-MS_NJU} − δ³⁴S_SIMS” >+2‰ or <−2‰). This discrepancy possibly indicates a crystal orientation effect during SIMS analysis. To confirm whether there is a crystal orientation effect during SIMS analysis, we conducted a second round of LA-MC-ICP-MS analysis in Beijing and selected two sets of NJUMc-1 marcasite grains and conducted electron backscatter diffraction (EBSD) analysis. For the first set of NJUMc-1 grains, characterized by variable “δ³⁴S_{LA-MC-ICP-MS_Beijing} − δ³⁴S_SIMS” values, there is no correlation between crystal orientation and “δ³⁴S_{LA-MC-ICP-MS_beijing} − δ³⁴S_SIMS” values. For the second set of NJUMc-1 grains, characterized by “δ³⁴S_{LA-MC-ICP-MS_Beijing} − δ³⁴S_SIMS” values around 0‰, highly variable crystal orientations are observed. These findings suggest that if a crystal orientation effect exists during SIMS analysis, it should be no larger than 1.5‰.