Rubidium Isotope Ratios of International Geological Reference Materials

In this study we determined rubidium isotope ratios in twenty-one commonly used international geological reference materials, including igneous, sedimentary and metamorphic rocks, as well as an IAPSO seawater reference material. All δ⁸⁷Rb results were obtained relative to the NIST SRM 984 reference material. For most reference materials, Rb was purified using a single column loaded with Sr-spec resin. For reference materials containing low Rb but high mass fractions of matrix elements (such as basic rock and seawater), Rb was purified using two-column chromatography, with the first column packed with AGMP-50 resin and the second column packed with Sr-spec resin. Two methods for instrumental mass bias correction, sample-standard bracketing (SSB) mode, and the combined sample-standard bracketing and Zr internal normalisation (C-SSBIN) method, were compared for Rb isotopic measurements by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The long-term reproducibility of Rb isotopic measurements using both methods was similar, better than 0.06‰ (2s, standard deviation) for NIST SRM 984. Significant Rb isotopic fractionation was observed among the reference materials, with an overall variation in δ⁸⁷Rb values of approximately 0.5‰. The δ⁸⁷Rb values of igneous rocks ranged from -0.28‰ to +0.06‰, showing a trend from heavier isotopic compositions in mafic rocks to lighter δ⁸⁷Rb values in the more evolved felsic rocks. The sedimentary and metamorphic rocks had Rb isotope ratios similar to those of igneous rocks. The δ⁸⁷Rb values of the reference materials related to low-temperature geological processes showed a wider range than those of high-temperature processes. Notably, the IAPSO seawater reference material had a δ⁸⁷Rb value of +0.14‰, which deviated from that of igneous rocks, and represents the heaviest reservoir of Rb isotopes found thus far on Earth. The comprehensive dataset presented here has the potential to serve for quality assurance purposes, and provide a framework for interlaboratory comparisons of Rb isotope ratios.