Testing Nano‐Powder and Fused‐Glass Mineral Reference Materials for In SituRb‐Sr Dating of Glauconite, Phlogopite, Biotite and Feldspar via LA‐ICP‐MS/MS

Abstract

Reference materials (RMs) with well-characterised composition are necessary for reliable quantification and quality control of isotopic analyses of geological samples. For in situ Rb-Sr analysis of silicate minerals via laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS) with a collision/reaction cell, there is a general lack of mineral-specific and matrix-matched RMs, which limits wider application of this new laser-based dating technique to certain minerals. In this work, pressed nano-powder pellets (NP) of four RMs, GL-O (glauconite), Mica-Mg (phlogopite), Mica-Fe (biotite) and FK-N (K-feldspar), were analysed and tested for in situ Rb-Sr dating, complemented by isotope dilution (ID) MC-ICP-MS Rb-Sr analyses of GL-O and Mica-Mg. In addition, we attempted to develop alternative flux-free and fused 'mineral glasses' from the above RMs for in situ Rb-Sr dating applications. Overall, the results of this study showed that among the above RMs only two NP (Mica-Mg-NP and GL-O-NP) were suitable and robust for in situ dating applications. These two nano-powder reference materials, Mica-Mg-NP and GL-O-NP, were thus used as primary RMs to normalise and determine Rb-Sr ages for three natural minerals: MDC phlogopite and GL-O glauconite grains, and also Mica-Fe-NP (biotite). Our in situ analyses of the above RMs yielded Rb-Sr ages that are in good agreement (within 8%) of published ages, which suggests that both Mica-Mg-NP and GL-O-NP are suitable RMs for in situ Rb-Sr dating of phlogopite, glauconite and biotite. However, using secondary RMs is recommended to monitor the quality of the obtained ages.

Microscale Petrographic, Trace Element, and Isotopic Constraints on Glauconite Diagenesis in Altered Sedimentary Sequences: Implications for Glauconite Geochronology

“…Consistent with Gorojovsky and Alard's (2020) and Redaa et al. (2022) results, normalization to Mica‐Mg and NIST 610 returned the most accurate and reproducible results for 87 Rb/ 86 Sr and 87 Sr/ 86 Sr of the standards, respectively, and is the preferred approach used throughout this study. Errorchons were constructed using the IsoplotR package (Vermeesch, 2018), utilizing the 87 Rb decay constant reported by Villa et al.…”

“…Here we address this research gap by studying glauconite-bearing, partially altered marine sedimentary sequences of the lower Cambrian age of the Arrowie Basin (South Australia) and the Amadeus Basin (Central Australia). We combine detailed petrographic characterization of glauconite grains by SEM-EDS mineral mapping (Rafiei et al, 2020), in situ laser ablation Rb/Sr dating (Gorojovsky & Alard, 2020;Redaa et al, 2021Redaa et al, , 2022Zack & Hogmalm, 2016) and simultaneous trace element geochemical fingerprinting. The aim of this study is to assess the microscale impact of diagenesis on glauconite embedded in partially altered sedimentary sequences and implications for glauconite geochronology.…”

LA-ICP-MS/MS-based Rb–Sr isotope mapping for geochronology

“…4 and Table 7). This bias is in the order of magnitude that has been observed in other studies, 29–31 where it is ascribed to matrix effects resulting from the use of a non-matrix matched primary reference material ( e.g. NIST610 glass or pressed nanopowder Mica-Mg).…”

“…This observation supports earlier conclusions on the general matrix mismatch between crystalline mica and pressed mica nanopowders. 21,27,29…”

“…9 Although these RM are frequently used in geochronological applications, none provides a proper matrix match to single-crystal mica, which is also the case for the recently tested fused mica mineral glasses. 29 It has been shown that a lack of matrix match between RM and sample may negatively affect dating accuracy. Systematic matrix age biases of as much as 10% have been observed.…”