LA‐ICP‐MS/MS Single‐Spot Rb‐Sr Dating

Abstract

Due to developments in reaction cell technologies, it is now possible to collect large 87Rb/86Sr and 87Sr/86Sr data sets using LA‐ICP‐MS/MS. Until now, LA‐ICP‐MS/MS Rb‐Sr ages were constrained using conventional isochrons on a contiguous set of Rb‐Sr data. In this contribution, we present a straightforward procedure on how to measure, calculate and validate Rb‐Sr ages from individual laser spots even from detrital grains devoid of context. This approach does not only allow provenance studies of detrital micas, but also furthermore opens the opportunity to produce age maps with unprecedented spatial resolution, both for zoned single crystals and targeting different textural domains. The main challenge for calculating single‐spot Rb‐Sr ages is the estimation of the initial 87Sr/86Sr composition, if it cannot be constrained through analysis of a paragenetic phase. In this case, we propose to use a range of geologically relevant initial 87Sr/86Sr compositions rather than one fixed value: (1) 0.703 ± 0.003 (for mantle derived magmatic rocks), (2) 0.715 ± 0.015 (for evolved magmatic rocks) and (3) 0.730 ± 0.030 (for crustal rocks). Modelling shows that the 87Sr/86Sr ratio of the targeted material delineates the accuracy and precision of single‐spot Rb‐Sr ages. If the measured Sr isotopic composition is sufficiently radiogenic [87Sr/86Sr of > 1.0 for scenario (1) to > 4.5 for scenario (3)], the calculated age and its uncertainty are not significantly influenced by the initial 87Sr/86Sr composition. Using Mica‐Mg as the primary reference material, single‐spot Rb‐Sr dating was tested on biotite (Mount Dromedary, La Posta, McClure Mountain), muscovite (Högsbo) and a nano‐powder tablet (Mica‐Fe) that we further propose using as secondary reference materials for Rb‐Sr geochronology.