High precision Rb isotope measurements by MC-ICP-MS

Abstract

We present a high precision method to measure the Rb isotope composition of terrestrial rocks in this study. Rb was efficiently separated from matrix elements by a chromatographic technique using AG50W-X12 and Sr-spec resin columns. Rb isotope ratios were measured using a Thermo Scientific Neptune Plus MC-ICP-MS employing a sample-standard bracketing method with NIST SRM984 as the bracketing standard. Potential effects of acid molarities and concentration mismatch during instrumental analyses were rigorously evaluated. The δ⁸⁷Rb values of the two in-house standards ICP-Rb and USTC-Rb relative to SRM984 measured in our laboratory were −0.31 ± 0.05‰ (2SD, n = 276) and −0.15 ± 0.03‰ (2SD, n = 78), respectively. Analyses of synthetic standard solutions obtained identical δ⁸⁷Rb for the pure SRM984 solution (0‰) with a precision better than 0.05‰ (2SD). We further measured the Rb isotope compositions of 16 reference materials, including 11 igneous rock standards, 2 soil standards, 1 loess standard, and 2 sediment standards using this method. The Rb isotope data of these geological standards are helpful for future interlaboratory calibration and data quality control. Based on replicated analyses of solution standards and rock standards, our long-term external precision for δ⁸⁷Rb is better than 0.05‰ (2SD), which allows us to identify Rb isotope fractionation during various geological processes.