Mechanism of late diagenetic alteration of glauconite and implications for geochronology

Glauconites – the most common authigenic minerals of sedimentary rocks suitable for radiometric stratigraphic dating – are known to give sometimes strongly rejuvenated ages (i.e. younger than stratigraphic ages of their host sediments). The glauconites separated from the Ediacaran/Cambrian sediments of the western part of the East European Craton (Baltica) were used to investigate the mechanism of age rejuvenation. All pure samples gave strongly rejuvenated (Paleozoic) ages, using both K-Ar and Rb-Sr dating, older than ages of Proterozoic illite-smectites from the same area. Their diverse characteristics were compared to younger glauconite samples of the same radiometric age as the stratigraphic age of their host sediment, and to younger glauconite samples with rejuvenated ages. It was established that the rejuvenated samples differ from the primary samples in their X-ray diffraction characteristics. Their 060 reflections are broader and displaced towards higher angles, an indication of chemical heterogeneity (layers with lower, variable Fe contents). This finding was confirmed by electron probe micro-analysis (less Fe and Mg, more Al in rejuvenated glauconites), and by Fourier transform infrared spectroscopy in the OH stretching region. Average boron contents are lower in the rejuvenated glauconites, and the boron isotope composition is quite variable, with no systematic trends observed. Modeling indicates that ancient seawater δ¹¹B was not much different from the present-day value, and that input of organic-derived boron to glauconite began at the sedimentary stage. All data combined are indicative of the mechanism of burial diagenetic alteration of glauconite: the growth of glauconitic 10 Å layers with lower iron contents at the expense of Fe-smectite layers present in the original glauconitic mineral crystallized during sedimentation, while the primary high-Fe glauconitic 10 Å layers are preserved during burial diagenesis. This mechanism is responsible for the observed chemical heterogeneity of many glauconitic samples, and for their mixed isotopic ages, intermediate between the ages of primary and diagenetic growth of 10 Å layers.