Improved Sr-Specific Resin Separation Protocol and Development of High-Radiogenic Reference Materials for ID-MS Rb–Sr Geochronology

LA-ICP-MS/MS single-spot Rb–Sr analysis has become a powerful tool for in situ dating of ultra-radiogenic minerals such as muscovite. However, this technique relies critically on matrix-matched, highly radiogenic reference materials (RMs) for external calibration. Accurate characterization of such RMs, in turn, requires high-precision isotope dilution mass spectrometry (ID-MS).

Nevertheless, ID-MS analysis of highly radiogenic materials remains technically challenging. First, extremely high Rb/Sr ratios hinder complete separation of Sr from Rb, resulting in potential ⁸⁷Rb interference on ⁸⁷Sr during TIMS Sr isotope measurements. Second, very high ⁸⁷Sr/⁸⁶Sr ratios combined with low absolute ⁸⁶Sr abundances render analyses highly sensitive to Sr procedural blanks. Consequently, robust data quality control for ID-MS itself is essential, necessitating the availability of highly radiogenic RMs suitable for ID-MS validation. At present, however, such reference materials remain scarce.

In this study, we developed an improved Sr-specific resin separation protocol aimed at maximizing Sr recovery during ID-MS Rb–Sr analysis of highly radiogenic samples. The key modification involves dissolving HF-digested sample residues in a mixed acid of 7.5 M HNO₃ and 2.5 M HCl, rather than the conventionally used 3 M or 7 M HNO₃ alone, prior to loading onto the Sr-specific resin column. The mixed HCl–HNO₃ acid significantly enhances dissolution of fluoride-bearing residues following HF digestion. In contrast, when pure HNO₃ is used, more than 95% of Sr is coprecipitated with fluorides for muscovite samples, leading to substantial Sr loss. The improved protocol results in markedly increased Sr recovery and more reliable ID-MS measurements.

Using this optimized separation procedure, we conducted comprehensive ID-MS Rb–Sr analyses of two candidate reference materials: the pegmatite RM OU-9 (IAGeo Limited) and the muscovite Ar–Ar age RM ZMT04. OU-9 yields Rb and Sr mass fractions of 1227 ± 17 μg g⁻¹ and 13.07 ± 0.18 μg g⁻¹, respectively, with ⁸⁷Rb/⁸⁶Sr = 843.1 ± 3.8 and ⁸⁷Sr/⁸⁶Sr = 22.217 ± 0.089, corresponding to an Rb–Sr age of ~2650 Ma with an initial ⁸⁷Sr/⁸⁶Sr of ~1.2. ZMT04 contains 2494 ± 22 μg g⁻¹ Rb and 36.05 ± 0.25 μg g⁻¹ Sr, with ⁸⁷Rb/⁸⁶Sr = 832.4 ± 5.2 and ⁸⁷Sr/⁸⁶Sr = 32.60 ± 0.19, corresponding to an Rb–Sr age of ~1800 Ma with an initial ⁸⁷Sr/⁸⁶Sr of ~0.705. These results demonstrate that both materials have strong potential as highly radiogenic reference materials for ID-MS Rb–Sr geochronology.