Rubidium Isotope Measurements of Low-Rb Geological Materials by MC-ICP-MS

The rubidium (Rb) isotope system has the potential to trace planetary evolution, magmatic-fluid interaction and chemical weathering. These applications are based on Rb isotope measurement results with precisions fit for purpose, but measurements of low-Rb geological materials are challenging due to large sample consumption and overload of ion-exchange resin. Here we developed a measurement procedure for Rb isotope data (δ⁸⁷Rb_SRM984) of low-Rb geological materials using MC-ICP-MS. Using an Aridus III desolvator and Ni standard sampler + Ni X skimmer cone combination, the Rb loading amount was reduced significantly to 20 ng. A comparison between two methods for instrumental mass-bias correction, the sample-standard bracketing and combined sample-standard bracketing and internal (Zr) normalisation (C-SSBIN), shows that C-SSBIN could produce Rb isotope data with better intermediate measurement precisions but strictly restricted to optimal Zr/Rb ratio. The robustness of this method was demonstrated by monitoring δ⁸⁷Rb_SRM984 data of two in-house Rb isotope standards, replicates, some reference materials with δ⁸⁷Rb_SRM984 values previously reported, and element-doped and matrix-spiked synthetic solutions. Based on repeated measurements of Rb isotope standards and reference materials, the long-term (over one year) intermediate precision was better than 0.05‰ (2s, standard deviations). We additionally recommend thirteen reliable reference materials for future Rb isotope ratio measurements.