Characterisation of Reference Materials forIn SituRb‐SrDating byLA‐ICP‐MS/MS

Abstract

We present Rb and Sr mass fraction and 87Sr/86Sr isotope ratio measurement results for four reference materials (RMs) obtained from the Service d'Analyse des Roches et des Minéraux (SARM), Nancy, France: Mica‐Mg, Mica‐Fe, GL‐O and FK‐N. These four RMs have different chemical compositions spanning the range of those of most K‐bearing feldspars and micas, making them potential calibration materials for in situ Rb‐Sr dating of natural minerals by LA‐ICP‐MS/MS. Selected grains and flakes from the four RMs present variable degrees of heterogeneity observable by SEM‐EDS and EPMA imaging, and chemical mapping. This heterogeneity is mainly related to inclusions of minerals within flakes and grains and to chemical substitutions linked to crystallographic control and alteration processes. The Mica‐Mg RM is the least affected. The powders available at the SARM were analysed by ID‐TIMS (87Sr/86Sr and Sr) and ID‐MC‐ICP‐MS (Rb) after digestion and separation. The mean 87Rb/86Sr ratios are 155.6 ± 4.7% (2s, as for other RMs) for Mica‐Mg, 1815 ± 14% for Mica‐Fe, 36.2 ± 11% for GL‐O and 69.9 ± 5.9% for FK‐N. The mean 87Sr/86Sr ratios are 1.8622 ± 0.36% (2s, as for other RMs) for Mica‐Mg, 7.99 ± 13% for Mica‐Fe, 0.75305 ± 0.12% for GL‐O, and 1.2114 ± 0.17% for FK‐N. The four RMs each show dispersion in 87Sr/86Sr and Rb and Sr mass fractions, to degrees that differ between RMs and that reflect the heterogeneity of their original crystals. The most heterogeneous RMs are GL‐O and Mica‐Fe. The calculated mean Rb‐Sr isotopic ages are 521 ± 24 Ma for Mica‐Mg, 287 ± 55 Ma for Mica‐Fe, 89.2 ± 9.9 Ma for GL‐O and 512 ± 30 Ma for FK‐N. The proposed age for Mica‐Fe may be unreliable due to the elevated dispersion of individual analysis linked to the highly radiogenic composition of the biotite and to the presence of numerous mineral inclusions. We recommend use of these proposed working values of 87Sr/86Sr and 87Rb/86Sr ratios and associated uncertainties when using the four RMs for in situ Rb‐Sr dating by LA‐ICP‐MS/MS. The availability of these four well‐characterised RMs will allow progress in the development and application of the Rb‐Sr dating approach by LA‐ICP‐MS/MS.

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

“…Reference materials were NIST610 as the primary calibration standard and the in-house secondary RM, which are single crystal mica, namely SagaB biotite, Phalaborwa biotite and Harvard 98973 muscovite. In run GÄD08, the pressed mica nanopowder MicaMg 9,26 has been measured in addition. A comprehensive overview of the essential ICP and laser settings used in each session is presented in Tables 1 and 2.…”

“…To correct for elemental and mass-dependent isotopic fractionation, reference materials (RM) with a well-known and homogeneous 87 Rb/ 86 Sr and 87 Sr/ 86 Sr distribution are applied. For in situ Rb-Sr dating, NIST 610 glass 25 and MicaMg-NP 9,26 are the most widely used. 15,20,21,27,28 The latter is a nely ground pressed powder of phlogopite from a pyroxenite from southern Madagascar that displays a homogeneous age signal (519.4 ± 6.5 Ma).…”

Single Spot Rb‐Sr Isochron Dating of Biotite by LA‐MC‐ICP‐MS/MS

“…Reference values for MicaMg from Jegal et al . (2022) for 87 Rb/ 86 Sr (155.6 ± 7.3) and 87 Sr/ 86 Sr (1.8622 ± 0.0067) were applied to all sample measurements.…”

“…The b coefficient was calculated from 88 Sr/ 86 Sr given the conventionally used true value of 8.375209 ( 86 Sr/ 88 Sr = 0.1194, Nier 1938). Reference values for MicaMg from Jegal et al (2022) for 87 Rb/ 86 Sr (155.6 AE 7.3) and 87 Sr/ 86 Sr (1.8622 AE 0.0067) were applied to all sample measurements.…”

Recent advances in petrochronology: from dates to ages and rates of deep orogenic processes

“…Among metamorphic rock-forming minerals, mica and garnet are perhaps the most popular for dating. In the past 20 years, the 39 Ar-40 Ar radiometric method has been predominantly used to date mica, by opposition to Rb-Sr, until recently not accessible in situ and still hampered by the availability of homogeneous standard materials Jegal et al [2022]. However, in recent years, the addition of a reaction cell between the quadrupoles in the MS device has enabled the use of reacting gases of various reaction affinities with isobaric elements to resolve isobaric interferences at m = 87.…”