Abstract

Glauconite dates obtained by conventional K-Ar and 40Ar/39Ar age spectrum techniques from Palaeocene horizons in Kent are examined against a Palaeogene K-Ar time-scale derived from the Tertiary igneous rocks of the North Atlantic region. Isotope ratio plots (correlation diagrams) are used to analyse and test the validity of the results. The boundary ages suggested by the dating of North Atlantic igneous rocks are: base Danian –66Ma, base Thanetian ~60Ma, base Lower Sparnacian ~56Ma, base Upper Sparnacian ~54 Ma, base Ypresian ~53 Ma and base Lutetian ~49 Ma (when calculated using Geological Society of London 1964 standard constants). A variety of natural deficiencies may impair the dating of authigenic glauconite grains, some specific to glauconite in addition to those generally applicable in K-Ar dating. These shortcomings prevent direct use of the majority of unconfirmed glauconite dates in the definition of the standard K-Ar time-scale. Nevertheless, accurate and precise glauconite dates can be obtained from ideal glauconite dating samples. It is suggested that ideal glauconite dating samples can be recognised and confirmed as such from the character of the 40Ar/39Ar step heating correlation diagrams obtained from them and that careful experimental procedures are essential if argon loss error in the laboratory is to be avoided.

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