K [sbnd]Ar isochron method and initial argon ratios

Abstract

40Ar 36Ar versus 40K 36Ar isochron plots of 3 sets of discordant K Ar age data yielded concordant isochrons with different initial 40Ar 36Ar ratios. These results indicate that (1) the initial 40Ar 36Ar ratio was homogeneous within each set of samples but not always equal to the present atmospheric argon ratio, (2) the standard method of non-radiogenic argon correction is, therefore, not always adequate (3) this isochron method can yield more meaningful ages and (4) excess argon can be due to high initial 40Ar 36Ar ratio.