Conjunctive K-Ca and Rb-Sr dating of glauconies

Abstract

The glaucony series (of which glauconitic mica is the K-rich end member) is one of the few widely occurring minerals in the sedimentary record. So their isotopic ages determined mainly by the K-Ar method represent a major database for the geologic time scale. Many K-Ar ages were, however, found to be too young and often also discordant with available corresponding Rb-Sr ages. This discrepancy may reflect the disparate responses of the chemically contrasting daughter elements, Ar and Sr to postdepositional processes, and renders either age equivocal. A potentially promising approach to evaluate the intrinsic potential of glauconies and other K-rich minerals for direct and reliable dating of sediments is the conjunctive use of the chemically very similar K-Ca and Rb-Sr systems. The relationship between K-Ca and Rb-Sr ages may show a coherence or regularity indicative of a geologically meaningful age like in the case of the well known chemically identical dual U-Pb decay systems. As a crucial first step, this report presents the first successful precise K-Ca dating in conjunction with Rb-Sr dating of an Ordovician glauconite using the same sample aliquot, chemical procedure and mass spectrometer.