A natural plagioclase reference material for microbeam Sr isotopic analysis

Abstract

High-quality Sr isotope data are critically dependent on the quality of reference materials that are used for instrumental calibration and data monitoring during LA-MC-ICP-MS analysis. A suite of natural plagioclase samples (BDL-1, BDL-2 and BDL-3) from the Badaling intrusive complex, Beijing were investigated here to test their suitability as a plagioclase Sr isotope reference material. Major and trace element analyses by EPMA and LA-ICP-MS showed that plagioclase crystals separated from two troctolites (BDL-1 and BDL-3) and one gabbro (BDL-2) were all characterized by extremely low Rb/Sr ratios (0.00012–0.00047), and low heavy rare earth elements (HREEs) and high Sr contents (2377–2836 μg g⁻¹). A total of 200 LA-MC-ICP-MS analyses for plagioclases BDL-1, BDL-2 and BDL-3 showed consistent and homogeneous Sr isotopic compositions with an average ⁸⁷Sr/⁸⁶Sr ratio of 0.70535 ± 0.00007 (2SD), which was in good agreement with solution-based data determined by TIMS or SN-MC-ICP-MS (0.70533 ± 0.00004; 2SD, n = 12), thus suggesting that BDL plagioclase can be used as a potential reference material for microbeam Sr isotopic analysis. Compared to the other available plagioclase Sr isotopic reference materials, the large quantities, long-term availability, high Sr content and extremely low isobaric interference (Rb, Er and Yb) of BDL plagioclase make it especially suitable for instrumental calibration and to adjust for inter-laboratory bias of the measured ⁸⁷Sr/⁸⁶Sr ratios.