MONITORING THE DEFICIT OF 39AR IN IRRADIATED CLAY FRACTIONS AND GLAUCONITES: MODELING AND ANALYTICAL PROCEDURE

Abstract

Clay fractions and glauconite (clays) lose 30-80% of the 39Ar produced by the nuclear transformation 39K(n, p)39Ar during irradiation in a reactor. As a result it has not been possible to date clays by the 40Ar-39Ar step-heating technique. New observations on 39Ar loss in clays are presented, and the transport behavior of the 39Ar recoils in the elementary clay particle was simulated by the Monte-Carlo method. A new procedure for dating clays by the 40Ar-39Ar step-heating technique is proposed. The use of an aliquot of the same clay, along with its K-Ar age, is suggested as a specific monitor of the irradiated sample, enabling stepwise 40Ar-39Ar analysis. This specific monitor monitors, without changes in the experimental setup, the integral of 39Ar production from 39K during irradiation and its retention in the solid substrate. The new procedure was successfully demonstrated on the geochronological standard glauconite GL-O.