LA-ICP-MS/MS-based Rb–Sr isotope mapping for geochronology

Abstract

This study introduces a new approach for in situ Rb–Sr dating that utilizes rapid line scans instead of static spot ablation, enabling the creation of two-dimensional ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr isotope ratio and Rb–Sr age maps. The data acquisition is conducted utilizing an ICP-MS/MS instrument with N₂O as the reaction gas, coupled to a 193 nm excimer laser via a low-aerosol-dispersion interface. This configuration allows for high repetition rates (>100 Hz) and sensitivities, enabling data acquisition at a high scanning speed and small laser beam size (3–4 μm). Notably, this approach requires just about 1/30 of the sample volume typically utilized in conventional spot ablation mode, while achieving similar levels of precision and accuracy. Line scan ablation is tested and compared to spot ablation on age-homogeneous crystalline muscovite and biotite, for which reference Rb–Sr age data is acquired through ID-TIMS. Results show that a key requirement for accurate Rb–Sr ages based on line scan analyses is matrix correction using chemically matched crystalline mica. By presenting Rb–Sr age maps of three naturally deformed mica samples, we highlight the potential of Rb–Sr mapping for extracting age data from rocks that exhibit complex metamorphic-metasomatic histories and microscale dynamic recrystallization. Additionally, we show that quantitative elemental information (Al, Fe, Si, Li) can be collected alongside Rb–Sr isotope data. This advancement offers a distinctly more insightful assessment of isotope mobility in natural systems, the timing of element enrichment processes and enables, in high-Rb/Sr rock systems, precise and accurate isotopic dating of intricate geological processes at small scales.