A new synthesis scheme of pyrite and chalcopyrite reference materials for in situ iron and sulfur isotope analysis using LA-MC-ICP-MS

Abstract

Microanalysis of Fe and S isotopes in sulfides by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) has been used as an important tool to identify material sources and cycling in geochemical and biological processes. However, the lack of matrix-matched reference materials has plagued the measurements of Fe and S isotope ratios for a long time. The synthesis of reference materials is an alternative solution. In this study, we investigated the traditional methods of pressed powder pellet and high-temperature melting (HTM), and a new plasma-activated sintering (PAS) to synthesize pyrite and chalcopyrite reference materials. The results indicated that when milling natural sulfides to ultra-fine powders, the homogeneity of Fe isotope can improve from 0.49–0.51‰ to 0.09–0.12‰ (2SD). However, the Fe signals for the pressed powder pellets were ∼1.4 times higher than those for natural pyrite and chalcopyrite because of the higher ablation rate in the pellets. The pyrite glass formed by HTM at 1200 °C presented a large variation of Fe isotope (0.65‰) due to the phase transformation from pyrite to pyrrhotite. For the new PAS method, the sintering temperature significantly influenced the physical and chemical properties of the synthesized pyrites and chalcopyrites. At the optimized conditions (600 °C for pyrite, 400–600 °C for chalcopyrite), the PAS-synthesized sulfides have a smooth surface and dense structure that are suitable for laser ablation analysis. Repeated measurements of Fe isotope and S isotope conducted on the PAS-synthesized pyrite and chalcopyrite by LA-MC-ICP-MS yielded highly consistent Fe and S isotope ratios. The reproducibilities of δ⁵⁶Fe_IRMM-014 were 0.27 ± 0.08‰ (n = 86), and −0.07 ± 0.11‰ (n = 93) and −0.06 ± 0.13‰ (n = 94) for PAS-synthesized pyrite and chalcopyrites, respectively, while those of δ³⁴S_V-CDT were 18.06 ± 0.25‰ (n = 75), and 10.35 ± 0.23‰ (n = 63) and 10.43 ± 0.29‰ (n = 62), respectively. The data demonstrated that the PAS-synthesized sulfides have a homogeneous distribution of Fe and S isotopes. Therefore, we recommended the combination of ultra-fine powders with PAS as an ideal synthesis scheme for the development of sulfide reference materials. Moreover, this scheme also can be used to synthesize other minerals to meet the requirements of matrix-matched reference materials for microanalysis.