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 construct single spot isochrons. Individual laser spots contain multiple isochronous subpopulations; some spots contain up to three distinct Rb-Sr isochrons that are decoupled from variations in Rb/Sr. Thirty-five isochron dates were determined using this "sub-spot" approach, with ⁸⁷Sr/⁸⁶Sr intercepts that systematically vary with Rb-Sr date; two-point isochrons were calculated for individual integrations (n = 780) based on these variable intercepts. Both methods yield age peaks at 303, 270 and 240 Ma. These data suggest that the Rb-Sr system has the potential to record multiple heating, cooling or fluid-alteration events spanning ~ 100 My within small domains in single biotite crystals.