EXTRACTING K-AR AGES FROM SHALES: THE ANALYTICAL EVIDENCE

Abstract

The relationship between the K-Ar dates of mixed natural clays and the ratios of detrital and diagenetic components follows in most cases a linear trend, which agrees with the theory when the K2O contents of the end-members are similar. However, significant deviations from a linear trend may occur and take the form of poorly-defined concave curves. The absence of a linear trend is less likely to be due to the imprecise determination of the ratio between the two end-members than to heterogeneities of the natural samples which may arise from complex K-Ar evolutions of the detrital and diagenetic end-members. The straight-line approach is of limited use in the definition of diagenetic ages in shales by extrapolating linear trends to diagenetic end-members.