Formula omitted] dating of glauconites: Measured 39Ar recoil loss from well-crystallized specimens

Abstract

Nine glauconite samples with relatively high K concentrations and which appear to be well crystallized using normal X-ray diffraction techniques have been studied using the 40 Ar 39 Ar method. The glauconite 40 Ar 39 Ar apparent ages exceed their KAr, RbSr and, in most cases, stratigraphic ages by substantial amounts. 40 Ar 39 Ar release spectra sometimes yield plateaus but these apparent ages have no geological significance. The results indicate that 39Ar is lost by recoil from mineral grains during neutron irradiation, consistent with previously reported observations. The amount of 39Ar loss was measured by isotope dilution for four samples and varied from 29% to 17%. In contrast, radiogenic 40Ar is quantitatively retained during irradiation. The very fine blades which make up glauconite grains yield the mineral susceptible to large amounts of 39Ar loss and unsuitable for 40 Ar 39 Ar dating.