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.