Jegal, Y.; Zimmermann, C.; Reisberg, L.; Yeghicheyan, D.; Cloquet, C.; Peiffert, C.; Gerardin, M.; Deloule, E.: Mercadier, J.
Geostandards and Geoanalytical Research, 2022, 46, 4, 645-671
Voir en ligne : https://onlinelibrary.wiley.com/doi/10.1111/ggr.12456
Abstract :
We present Rb and Sr mass fraction and87Sr/86Sr isotope ratio measurement results for four reference materials (RMs)obtained from the Service d’Analyse des Roches et des Mineraux (SARM), Nancy, France: Mica-Mg, Mica-Fe, GL-O andFK-N. These four RMs have different chemical compositions spanning the range of those of most K-bearing feldspars andmicas, making them potential calibration materials forin situRb-Sr dating of natural minerals by LA-ICP-MS/MS. Selectedgrains and flakes from the four RMs present variable degrees of heterogeneity observable by SEM-EDS and EPMAimaging, and chemical mapping. This heterogeneity is mainly related to inclusions of minerals within flakes and grainsand to chemical substitutions linked to crystallographic control and alteration processes. The Mica-Mg RM is the leastaffected. The powders available at the SARM were analysed by ID-TIMS (87Sr/86Sr and Sr) and ID-MC-ICP-MS (Rb) afterdigestion and separation. The mean87Rb/86Sr ratios are 155.64.7% (2s, as for other RMs) for Mica-Mg,181514% for Mica-Fe, 36.211% for GL-O and 69.95.9% for FK-N. The mean87Sr/86Sr ratios are1.86220.36% (2s, as for other RMs) for Mica-Mg, 7.9913% for Mica-Fe, 0.753050.12% for GL-O, and1.21140.17% for FK-N. The four RMs each show dispersion in87Sr/86Sr and Rb and Sr mass fractions, to degrees thatdiffer between RMs and that reflect the heterogeneity of their original crystals. The most heterogeneous RMs are GL-O andMica-Fe. The calculated mean Rb-Sr isotopic ages are 52124 Ma for Mica-Mg, 28755 Ma for Mica-Fe,89.29.9 Ma for GL-O and 51230 Ma for FK-N. The proposed age for Mica-Fe may be unreliable due to theelevated dispersion of individual analysis linked to the highly radiogenic composition of the biotite and to the presence ofnumerous mineral inclusions. We recommend use of these proposed working values of87Sr/86Sr and87Rb/86Sr ratiosand associated uncertainties when using the four RMs forin situRb-Sr dating by LA-ICP-MS/MS. The availability of thesefour well-characterised RMs will allow progress in the development and application of the Rb-Sr dating approach byLA-ICP-MS/MS.