Title: Ex uno plures: the revolution of in situ Rb-Sr geochronology

Abstract: In situ single spot Rb-Sr isochron dating of micas by LA-MC-ICP-MS/MS represents a new analytical and geologic horizon for the Rb-Sr system. Single spot Rb-Sr isochrons in biotite from western Maine, USA, record three distinct events that are obscured by the traditional spot dating approach. The punctuated nature of the events recorded by the Rb-Sr system is somewhat at odds with the broad, long-lived cooling histories that are typically recorded by the Ar/Ar system. With their perfect cleavage and propensity to hold ions in interlayer sites, micas are not typically thought of as minerals that record multiple geologic events, and are considered to be sensitive to resetting by thermal and fluid-mediated processes. Here we show that very distinct isotope ratio and trace element zones are preserved in biotite, marking a departure from the conventional view of Rb and Sr retention in mica minerals. The prospects for single spot isochron dating combined with isotope ratio mapping by LA-MC-ICP-MS/MS open up a new frontier for mica geochronology.

 

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