Calibration methods for laser ablation Rb-Sr geochronology: comparisons and recommendation based on NIST glass and natural reference materials

Abstract

In situ rubidium–strontium (Rb–Sr) geochronology, using laser ablation–inductively coupled plasma– tandem mass spectrometry (LA-ICP-MS/MS) technology, allows rapid dating of K-rich minerals such as micas (e.g. biotite, muscovite, and phlogopite) and K-feldspar (potassium-containing feldspar). While many studies have demonstrated the ability of the method, analytical protocols vary significantly, and to date, no studies have provided an in-depth comparison and synthesis in terms of precision and accuracy. Here we compare four calibration protocols based on commonly used reference materials (RMs) for Rb–Sr dating. We demonstrate that downhole fractionation trends (DHFs) for natural biotite, K-feldspar, and phlogopite contrast with that for the commonly used Mica-Mg nano powder reference material. Consequently, Rb–Sr dates calibrated to Mica-Mg can be up to 5% inaccurate, and the degree of inaccuracy appears to be unsystematic between analytical sessions. Calibrating to Mica-Mg also introduces excess uncertainty that can be avoided with a more consistent primary calibration material. We propose a calibration approach involving (1) NIST-610 glass as the primary reference material (PRM) for normalisation and drift correction and (2) a natural mineral with similar DHF characteristics to the analysed samples as matrix correction RM (MCRM) to correct the Rb=Sr ratio for matrix-induced offsets. In this work, MDC phlogopite (the source mineral for Mica-Mg nano powder) was used as the MCRM, consistently producing accurate Rb–Sr dates for a series of natural biotites and K-feldspars with well-characterised expected ages. Biotite from the Banalasta Adamellite, Taratap Granodiorite, and Entire Creek pegmatite are also suitable RMs for Rb=Sr ratio calibration purposes, with consistently < 1:5% fully propagated uncertainties in our methodological approach. Until calibration using isochronous natural standards as the primary RM becomes possible in data reduction software, the two-step calibration approach described here is recommended.