12 Sep 2023
 | 12 Sep 2023
Status: this preprint is open for discussion.

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

Stijn Glorie, Sarah Gilbert, Martin Hand, and Jarred Lloyd

Abstract. In-situ Rb–Sr geochronology using LA-ICP-MS/MS technology allows rapid dating of K-rich minerals such as micas (e.g. biotite, muscovite, phlogopite) and K-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 for Rb–Sr dating. We demonstrate that downhole fractionation trends (DHF) 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 NIST-610 glass as the primary reference material (RM) and a natural mineral with similar DHF characteristics to the analysed samples as secondary RM to correct for matrix-dependent fractionation. In this work, MDC phlogopite (the source mineral for Mica-Mg nano-powder) was used as the secondary RM, consistently producing accurate Rb–Sr dates for a series of natural biotites and K-feldspars with well-characterized expected ages. However, biotite from the Banalasta Adamellite, Taratap Granodiorite and Entire Creek pegmatite are also suitable secondary 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.

Stijn Glorie et al.

Status: open (until 24 Oct 2023)

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Stijn Glorie et al.


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Short summary
Radiometric dating methods, involving laser-ablation as sample introduction, require robust calibrations to reference materials with similar ablation properties to the analysed samples. In the case of the Rubidium-Strontium dating method, calibrations are often conducted to a nano-powder with different ablation characteristics than crystalline minerals. We describe the limitations of this approach and recommend an alternative calibration method, involving natural minerals.