High Rb/Sr Muscovite from the Koktokay Highly Fractionated Granites: Implications for Rb–Sr Chronology and LA-ICP-MS/MS In Situ Reference Material Development

Highly fractionated granites are important hosts for rare metals (Li, Be, Nb, Ta, Rb, Cs, Zr, Hf, etc.). Dating the mica minerals abundant within them can more accurately constrain the timing of rare metal mineralization. To investigate the episodes and chronology of rare metal mineralization in the Koktokay region, we conducted precise Rb-Sr dating by Isotope Dilution Mass Spectrometry (ID-MS) on muscovite separates, along with apatite and whole-rock powder, from six samples collected from this highly fractionated granite. The obtained age for this highly fractionated granite is 200.86 ± 0.29 Ma(2σ, n = 21), with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.805598 ± 0.000069. This age is younger than the main Triassic magmatic phase (ca. 210 Ma) but falls within the mineralization window of the No. 3 pegmatite vein (220-175 Ma). Meanwhile, the initial Sr ratio is significantly higher than that of typical crust-derived granites (~0.720), indicating derivation from a highly evolved source.

In recent years, laser ablation (multi-collector) inductively coupled plasma tandem mass spectrometry (LA-(MC)-ICP-MS/MS) techniques for in-situ Rb-Sr analysis have developed rapidly and become indispensable tools in geological research. However, the accuracy of LA-ICP-MS/MS Rb-Sr dating relies on matrix-matched reference materials to correct for instrumental drift (e.g., sensitivity changes) and elemental fractionation effects during analysis. For high Rb/Sr systems, there is still a lack of reference materials with high ⁸⁷Rb/⁸⁶Sr and high ⁸⁷Sr/⁸⁶Sr ratios.

The six muscovite samples analyzed in this study exhibit exceptionally high ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios. Electron Probe Microanalysis (EPMA) shows that these micas have high and stable Al and K contents of 10.755 ± 0.0373% and 33.39 ± 0.16%, respectively. Backscattered Electron (BSE) imaging confirms their homogeneous major element composition, devoid of impurities.

Given their exceptionally high ⁸⁷Rb/⁸⁶Sr ratios, we further analyzed these six muscovite samples using LA-ICP-MS/MS. The measured ages range from 190 Ma to 200 Ma. Rb concentrations vary from 2300 to 3700 µg/g, while Sr concentrations are low, at 2.5 and 3.5 µg g⁻¹. Among these samples, four exhibit extremely high ⁸⁷Rb/⁸⁶Sr ratios (28000-35000) and ⁸⁷Sr/⁸⁶Sr values (80-100). In contrast, the remaining two samples show moderately high ⁸⁷Rb/⁸⁶Sr ratios (11000-12000) and ⁸⁷Sr/⁸⁶Sr values (30-35). Based on the ID–MS isochron results and detailed LA-ICP-MS/MS data, all six muscovite samples show potential as candidate reference materials for LA-ICP-MS/MS Rb-Sr dating. Further detailed and systematic work is required to rigorously evaluate and validate their suitability.