Precise δ 29/28 Si and δ 30/28 Si determination on a high-resolution MC-ICP-MS

Abstract

The stable isotope composition of silicon (Si) has the potential to trace various (bio)geochemical processes in the Earth system. However, high-precision Si isotope analysis remains challenging, and few studies report 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 develop an efficient and convenient methods for Si purification and precise determination of silicon isotopes in carbonate rocks using high resolution MC-ICP-MS. A series of tests, such as volumes of alkali flux, the acidity of the final digestion solution, loading amount of Si, loading volumes of eluents, were conducted to ascertain the optimal conditions for sample digestion and Si purification.The effects of acidity and concentration mismatching and presence of potential cationic matrix elements on Si isotope measurements were also evaluated. The robustness of this method was demonstrated by determining igneous rocks reference materials with δ 30 Si values previously reported. Based on repeated measurements of Si isotope standards and reference materials, the long-term (over one year) intermediate precision was better than ± 0.09‰ (2SD, standard deviations). We additionally recommend seven reliable carbonate rocks 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.