Revisiting glauconite geochronology: Lessons learned from in-situ radiometric dating of a glauconite-rich Cretaceous shelfal sequence

The scarcity of well-preserved and directly dateable sedimentary sequences is a major impediment for inferring the Earth’s palaeo-environmental evolution. The authigenic mineral glauconite can potentially provide absolute stratigraphic ages for sedimentary sequences and constraints on palaeo-depositional conditions. This requires improved approaches for measuring and interpreting glauconite formation ages. Here, glauconite from a Cretaceous shelfal sequence (Langenstein, Northern Germany) was characterized using petrographical, geochemical (EMP) and mineralogical (XRD) screening methods before in-situ Rb-Sr dating via LA-ICP-MS/MS. The obtained glauconite ages (~101–97 Ma) partly overlap with the depositional age of the Langenstein sequence (± 3 Ma), but without the expected stratigraphic age progression, which we attribute to detrital and diagenetic illitic phase impurities inside the glauconites. Using a novel age deconvolution approach, which combines the new Rb-Sr dataset with published K-Ar ages, we recalculate the glauconite bulk ages to obtain stratigraphically significant ‘pure’ glauconite ages (~100–96 Ma). Thus, our results show that pristine ages can be preserved in mineralogically complex glauconite grains even under burial diagenetic conditions (T < 65 °C; < 1500 m depth), confirming that glauconite could be a suitable archive for palaeo-environmental reconstructions and direct sediment dating.