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 the 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 of exceeding 99%. Matrix elements such as Rb, Ba, Eu, Yb, Ca were reduced to negligible levels, well below the matrix/Sr thresholds that would induce a significant analytical bias, as determined by doping experiments. δ88/86Sr and 87Sr/86Sr were simultaneously measured by the Zr-doped SSB method. The calibration strategy employed the optimized regression model, replying on the observed linear relationship between ln88Sr/86Sr and ln94Zr/90Zr. Through replicated measurement of Sr standard solutions (GSB Sr and NIST SRM 987) and geological reference materials (GRMs) (e.g., IAPSO, BHVO-2, AGV-2), long-term precision was better than 0.03‰ (2SD) for δ88/86Sr and 3 × 10-5 (2SD) for 87Sr/86Sr. These results were comparable to those acquired by 92Zr/90Zr correction and the DS technique, better than that of 91Zr/90Zr correction. The accuracy was verified by (i) near-zero ∆88/86Srmeasured-recommended (-0.014 ± 0.019‰, 2SD, n=23) and ∆87Sr/86Srmeasured-recommended (0.000007 ± 20, 2SD, n=23) for synthetic solutions made by mixing of 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 firstly for 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 δ88/86Sr and 87Sr/86Sr, especially when the presence of certain matrices (e.g., W, Cr, V) compromises the validity of 92Zr/90Zr and 91Zr/90Zr correction.