Origin and Significance of Age Variability in the Glauconite Reference Material GL-O: Implications for In Situ Rb-Sr Geochronology

Stratigraphic ages from conventional glauconite geochronology are commonly younger than those obtained via high temperature chronometers. The widely used glauconite reference material GL-O, for example, has a K-Ar age (95.03 ± 1.11 Ma) ~ 5 Ma younger than its expected stratigraphic age. To identify the influences on glauconite ages and assess the suitability of GL-O as a reference material for in situ Rb-Sr geochronology, we separated GL-O grains based on colour and morphology. Each fraction was characterised petrographically and compositionally before in situ Rb-Sr dating. Separate aliquots were dated via conventional isotope dilution (ID) Rb-Sr geochronology. We find a ~ 10 Ma spread in the in situ Rb-Sr ages of GL-O fractions, where more rapid maturation and isotopic closure of darker grains yields ages closer to the depositional age, whereas smaller, more porous light green grains show evidence for delayed maturation leading to continued Rb uptake during burial as well as Sr isotope exchange with connate fluids. Discrepancies between ID and in situ Rb-Sr ages are explained by (i) core-rim age zonation, (ii) the presence of alteration resistant, Sr-rich apatite inclusions, (iii) differences in laser-induced fractionation. We recommend additional purification steps before use of GL-O as reference material for in situ Rb-Sr geochronology.