Precise δ29/28Si and δ30/28Si determination by high-resolution MC-ICP-MS

Abstract

The stable isotope compositions of silicon (Si) have the potential to trace various (bio)geochemical processes in Earth's systems. However, high-precision Si isotope analysis is challenging, and few studies have reported Si isotopes in low-Si rocks such as carbonate rocks, despite the intrinsic link between global silicon and carbon cycles. This scarcity is partly due to the lack of methods for Si purification and analysis of low-Si samples. In this study, we developed an efficient and convenient method for Si purification and precise determination of silicon isotopes in carbonate rocks using high resolution MC-ICP-MS. A series of conditions, such as volumes of alkali flux, acidity of the final digestion solution, loading amounts of Si, and loading volumes of eluents, were investigated to ascertain the optimal conditions for sample digestion and Si purification. The effects of acidity and concentration mismatch and presence of potential cationic matrix elements on Si isotope measurements were also evaluated. The robustness of this method was demonstrated by testing igneous rock reference materials and comparing with δ³⁰Si values previously reported. Based on repeated measurements of Si isotope standards and reference materials, the long-term (over one year) intermediate precision was found to be better than ±0.09‰ (2SD, standard deviations). We additionally recommend seven reliable carbonate rock reference materials for future Si isotope ratio measurements. As such, the reported method is ideally suited for Si isotopic analysis of multiple types of geological samples.