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.