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

Abstract

Laser ablation tandem mass spectrometry is a burgeoning field for in situ Rb-Sr geochronology. Here, we determined simultaneous isotope ratios of 87Sr/86Sr and 87Rb/86Sr in metamorphic biotite from western Maine, using an ESL™ imageGEO™193 excimer laser ablation system coupled to a Thermo Scientific™ Neoma™ MC-ICP-MS/MS. Measurements were made on Faraday cups with Rb+ at mass 87; Sr isotopes were reacted with SF6 gas and measured as SrF+ at masses 103–107. Twenty-two laser spots in biotite from a single sample yield a "traditional" Rb-Sr isochron date of 289 ± 6 Ma. Time-resolved signals reveal significant zoning in 87Sr/86Sr and 87Rb/86Sr within single spot analyses, which were used to cons..