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 to inferring the Earth’s paleo-environmental evolution. The authigenic mineral glauconite can potentially provide absolute stratigraphic ages for sedimentary sequences and constraints on paleo-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), andmineralogical (XRD) screening methods before in situ Rb-Sr dating via LA-ICP-MS/MS. The obtained glauconite ages (~101 to 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 to 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 paleo-environmental reconstructions and direct sediment dating.Esther Scheiblhofer, Ulrike Moser, Stefan Löhr, Markus Wilmsen, Juraj Farkaš, Daniela Gallhofer, Alice Matsdotter Bäckström, Thomas Zack, and Andre Balderman

The Influence of Spectral Interferences on Critical Element Determination with Portable X-Ray Fluorescence (pXRF)

Field portable X-ray fluorescence (pXRF) spectrometers are routinely used in mineral resources studies. To date, mineral resources studies have largely focussed on the application of pXRF to the exploration for deposits of base and precious metals. By contrast, studies using pXRF for the quantification of critical elements in geological materials are scarce since these elements are difficult to determine with energy-dispersive pXRF technology. This study explores the capability of pXRF spectrometers to detect and quantify critical elements (Ba, P, Nb, V, Co, REE, W, Bi, Hf, and Ta) in certified reference materials (CRMs). While precision of many critical elements is acceptable (&lt;20% RSD), accuracy can be poor (&gt;50% difference) when using pre-installed factory calibration software. Spectra collected during the pXRF measurements show that poor accuracy and false positives tend to be associated with spectral interferences. Distinct combinations of spectral interferences (line overlaps, Compton scattered peaks, and Si escape peaks) were observed in the different matrix types. Our results show that critical elements may be determined in common geological materials when pronounced peaks occur in the spectra and that matrix-match of standards and samples is essential. Hence, XRF spectra should be routinely reviewed to identify erroneous quantification due to spectral interferences