LA‐ICP‐MS/MS Single‐Spot Rb‐Sr Dating

Abstract

Due to developments in reaction cell technologies, it is now possible to collect large 87 Rb/ 86 Sr and 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr compositions rather than one fixed value: (1) 0.703 AE 0.003 (for mantle derived magmatic rocks), (2) 0.715 AE 0.015 (for evolved magmatic rocks) and (3) 0.730 AE 0.030 (for crustal rocks). Modelling shows that the 87 Sr/ 86 Sr 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 [ 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 (H ögsbo) and a nano-powder tablet (Mica-Fe) that we further propose using as secondary reference materials for Rb-Sr geochronology.

Revisiting Glauconite Geochronology: Lessons Learned from In Situ Radiometric Dating of a Glauconite-Rich Cretaceous Shelfal Sequence

“…The sample and standard average count rates (CPS) of all elements and relevant isotopes were processed with Iolite-Igor Pro © ( [47] to determine all important isotopic and element ratios and to enhance the accuracy of the results by adjusting peaks for each measured sample in terms of the program-calculated ratios. A data reduction scheme (DRS) was applied to all ratios, such as Rb 87 /Sr 86 and Sr 87 /Sr 86 ratios, which considers 2 sigma errors as well as an 87 Sr/ 86 Sr initial of 0.7074 (i.e., Cenomanian seawater 87 Sr/ 86 Sr composition [48]) and the primary reference material BCR-2G as well as the secondary standard NIST SRM 610 [39,49]. The DRS and all calculations therein are reported in Rösel and Zack [49].…”

“…A data reduction scheme (DRS) was applied to all ratios, such as Rb 87 /Sr 86 and Sr 87 /Sr 86 ratios, which considers 2 sigma errors as well as an 87 Sr/ 86 Sr initial of 0.7074 (i.e., Cenomanian seawater 87 Sr/ 86 Sr composition [48]) and the primary reference material BCR-2G as well as the secondary standard NIST SRM 610 [39,49]. The DRS and all calculations therein are reported in Rösel and Zack [49]. Sample and standard ages, internal uncertainties (2 sigma errors) and mean square of the weighted deviates (MSWD) were calculated from isochrons produced via the online version of IsoplotR [50][51][52] and using the updated Rb decay constant of 1.3975 ± 0.0045 (10-11 y −1 ) reported by Villa et al [53].…”

Constraining the geothermal parameters of in situ Rb–Sr dating on Proterozoic shales and their subsequent applications

“…Isochron and single-spot ages were calculated with ISOPLOTR (Vermeesch, 2018). Singlespot ages were calculated using the isochron intercept as their initial 87 Sr / 86 Sr ratios (Vermeesch, 2018;Nebel, 2014;Rösel and Zack, 2022). In addition, kernel density estimation (KDE) graphs (Vermeesch, 2012), cumulative age distribution (CAD) plots (Vermeesch, 2007), and multidimensional scaling (MDS) graphs (Vermeesch, 2013) were also constructed using ISOPLOTR (Vermeesch, 2018) to differentiate between the population of single-spot ages from each sample.…”

“…However, it should be noted that nanometre-or micrometre-sized mineral intergrowths of different origins still provide challenges when large spot sizes are used. The application of similar setups with other beta-decay dating systems have also yielded promising results (Tamblyn et al, 2021;Ribeiro et al, 2021;Harrison et al, 2010;Hog-malm et al, 2019;Brown et al, 2022;Simpson et al, 2022;Scheiblhofer et al, 2022;Rösel and Zack, 2022;Gorojovsky and Alard, 2020;Bevan et al, 2021).…”

In situ 87Rb–87Sr analyses of terrestrial and extraterrestrial samples by LA-MC-ICP-MS/MS with double Wien filter and collision cell technologies

“…Strategies have been developed to minimize these interferences, the most notable being (1) focusing on low Rb samples such as carbonate, apatite or plagioclase and correcting for possible isobaric interferences via measurement of the 85 Rb and peak-stripping assuming a constant value for 85 Rb/ 87 Rb, 32–39 (2) using resonant ionization mass spectrometry (RIMS) to only ionize Sr by using lasers with wavelengths tuned to a specific electronic transition of Sr such that Rb is not ionized, 4,40–43 and (3) using a reactive gas such as N 2 O, O 2 , CH 3 F, or SF 6 in a collision/reaction cell (CRC) to react Sr + but not Rb + , with the former analyzed as molecular ions SrO + or SrF + . 44–53…”