Precise determination of stable (δ88/86Sr) and radiogenic (87Sr/86Sr) strontium isotopic compositions using 94Zr/90Zr for mass bias correction

Abstract

The combination of stable and radiogenic strontium (Sr) isotopes offers a novel perspective for distinguishing between source and process signatures among geological samples. Here, we applied a combined Zr doping (⁹⁴Zr/⁹⁰Zr) and standard-sample-bracketing method (termed Zr-doped SSB) for mass bias correction during Sr isotope measurement. Prior to isotope analysis, Sr was purified using a two-step column procedure, achieving a total Sr yield exceeding 99%. Matrix elements such as Rb, Ba, Eu, Yb, and Ca were reduced to negligible levels, well below the matrix/Sr thresholds that would induce a significant analytical bias, as determined by doping experiments. δ^88/86Sr and ⁸⁷Sr/⁸⁶Sr were simultaneously measured by the Zr-doped SSB method. The calibration strategy employed the optimized regression model, relying on the observed linear relationship between ln⁸⁸Sr/⁸⁶Sr and ln⁹⁴Zr/⁹⁰Zr. Through replicate measurements of Sr standard solutions (GSB Sr and NIST SRM 987) and geological reference materials (GRMs) (e.g., IAPSO, BHVO-2, and AGV-2), long-term precision was found to be better than 0.03‰ (2SD) for δ^88/86Sr and 3 × 10⁻⁵ (2SD) for ⁸⁷Sr/⁸⁶Sr. These results were comparable to those acquired by ⁹²Zr/⁹⁰Zr correction and the DS technique and better than those obtained using ⁹¹Zr/⁹⁰Zr correction. The accuracy was verified by (i) near-zero Δ^88/86Sr_{measured-recommended} (−0.014 ± 0.019‰, 2SD, n = 23) and Δ⁸⁷Sr/⁸⁶Sr_{measured-recommended} (0.000007 ± 20, 2SD, n = 23) for synthetic solutions made by mixing NIST SRM 987 with Sr-free matrices from GRMs; (ii) consistency between the measured and literature values for GRMs. Using the established method, Sr isotopic compositions of 12 GRMs including igneous, metamorphic, and sedimentary rocks were reported for the first time to facilitate further inter-laboratory comparison and data quality control. The Zr-doped SSB method using ⁹⁴Zr/⁹⁰Zr can serve as a robust alternative for the simultaneous precise determination of δ^88/86Sr and ⁸⁷Sr/⁸⁶Sr, especially when the presence of certain matrices (e.g., W, Cr, and V) compromises the validity of ⁹²Zr/⁹⁰Zr and ⁹¹Zr/⁹⁰Zr correction.