25 octobre 2021, 13h amphithéâtre du CRPG | Yujin Jegal (CRPG-GéoRessources)
Development of reference materials for in-situ Rb-Sr dating by laser ablation ICP-MS/MS
Summary : Rb-Sr dating techniques have been widely used as a geochronological tool to date and constrain the cooling history of geological events. However, in-situ dating using the 87 Rb- 87 Sr beta decay system is severely limited by the isobaric overlap of 87 Rb on 87 Sr. Recent developments in in-situ Rb-Sr isotopic techniques by laser ablation inductively coupled plasma mass spectrometry equipped with a collision/reaction cell (LA-ICPMS/MS) can resolve this problem, if appropriate reaction cell gases are used (O 2 , N 2 O and SF 6 ). In-situ Rb-Sr dating of minerals by LA-ICP-MS/MS requires calibration with well-certified matrix-matched reference materials with precisely known Rb/Sr ratios and Sr isotopic compositions. However, such well-characterized reference materials for in-situ Rb-Sr dating, in particular Rb-rich minerals such as micas, are currently limited.
In this study, reference materials suitable for in-situ Rb-Sr geochronology of high-Rb minerals (i.e., micas and feldspars) are firstly established and characterized. Four CRPG geostandards, phlogopite Mica-Mg, biotite Mica-Fe, glauconite GL-O and potash feldspar FK-N, were selected as potential mineral standards based on their broad compositional range, covering the compositional variation of micas and feldspars from granitic rocks. I will present the measured 87 Sr/ 86 Sr and 87 Rb/ 86 Sr values of the standards obtained by the isotope dilution method using multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) and thermal ionization mass spectrometry (TIMS) for Rb and Sr isotope measurements, respectively. The second goal is to examine the selected Rb-Sr standards using an Agilent 8900 ICP-MS/MS coupled with a LA system and to use them as reference materials for dating of unknown samples. Towards this goal, the in-situ Rb-Sr isochron ages of micas from W ± Sn deposits in the French Massif Central will be compared against their already known U- Pb ages (Harlaux et al., 2018b).