Origin and significance of age variability in the glauconite standard GL-O: Implications for in situ Rb-Sr geochronology

Description

The K- and Fe-rich marine authigenic clay mineral glauconite has been widely used for absolute radiometric dating of marine sedimentary rocks. Problematically, the most widely employed glauconite standard, GL-O, has a K-Ar age (95.03 ± 1.11 Ma) that is ~4 Ma younger than its expected stratigraphic age. More generally, sediment depositional ages obtained via glauconite geochronology are commonly younger than those obtained via high temperature chronometers. To identify the reasons for these discrepancies, and to assess the suitability of GL-O as a standard for in situ Rb-Sr geochronology, we separated GL-O grains based on colour, morphology and evidence of oxidation. Each fraction was characterized petrographically and for major oxide composition, followed by in situ Rb-Sr dating and trace element characterization via laser ablation - inductively coupled plasma - tandem mass spectrometry. Separate aliquots were also dated via conventional isotope dilution (ID) Rb-Sr geochronology. The dataset uploaded here includes major oxide element composition from electron microprobe (EMP) analysis, all elemental and isotopic results from LA-ICP-MS/MS analyses, and summary downhole fractionation (DHF) trends in 87Rb/86Sr from LA-ICP-MS/MS analyses for the analysed GL-O grains. DHF trends were calculated for calcite and apatite-inclusion free analytical spots only.

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