ZMT04 muscovite as a potential Rb–Sr geochronological reference material: preliminary characterization by ID-MS coupled with improved Sr-specific extraction chromatography

Abstract

This study addresses critical analytical challenges in Rb–Sr isotopic dating of high-Rb/Sr minerals such as muscovite, particularly matrix effects and complete Rb–Sr separation. We report systematic improvements to Sr-specific extraction chromatography, notably the use of an HCl–HNO₃ mixed acid medium, instead of HNO₃ alone, for column loading after HF–HNO₃ digestion, effectively preventing SrF₂ precipitation. Additionally, matrix elements were eluted using 7 mol L⁻¹ HNO₃, minimizing matrix interference while ensuring efficient Rb removal and high Sr recovery (>85%). Coupled with isotope dilution mass spectrometry (ID-MS), the procedure enables high-precision Rb–Sr analysis of samples with extremely high Rb/Sr ratios (e.g., muscovite, >100 to >1000). Reproducibility was validated using the potassium feldspar Rb–Sr standard NIST 607, yielding results consistent with published values. The method was applied to two randomly selected units (8–3, n = 15; 8–13, n = 9) from the 196-bottle batch of the K–Ar/Ar–Ar muscovite standard ZMT04. The preliminary characterization yielded Rb = 1227 ± 17 µg g⁻¹, Sr = 13.07 ± 0.18 µg g⁻¹, ⁸⁷Rb/⁸⁶Sr = 843.1 ± 3.8, and ⁸⁷Sr/⁸⁶Sr = 22.217 ± 0.089 (expanded uncertainties, k = 2). Combined with recently collected TPS muscovite from the same pegmatite, the data define a well-constrained isochron corresponding to an age of 1802.7 ± 1.4 Ma with an initial ⁸⁷Sr/⁸⁶Sr of 0.705. These results demonstrate the strong potential of ZMT04 as a high-⁸⁷Rb/⁸⁶Sr reference material for quality assurance in Rb–Sr geochronology using ID-TIMS and MC-ICP-MS.