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.