Extracting K-Ar ages from shales: a theoretical test

A computing exercise is presented to demonstrate how far the plots of illite 'mixed ages' should depart from the straight line in geologically feasible situations. Using the standard K-Ar age equation, the 40Ar* contents of Miocene age diagenetic materials mixed with detrital materials are determined. Based on the results of this exercise, the following conclusions are derived: variable shapes of the mixed age plots are to be expected; heterogeneous ages of the detrimental components are more favorable for dating diagenesis; the simulation technique can be used to predict the shape of the mixed-age plot; attempts to use the mixed-age plot technique should be based on data from the widest compositional range possible; and the problem of a non-linear shape of the mixed-age plot can be avoided if the sample contains only an illitic material or if the dilutant phases can be quantified.