ABSTRACT

We performed a calibration of a Ge-detector gamma ray spectrometer to measure the activity of ⁴⁰K in natural volcanic glass and minerals. The goal of this experimental study is to demonstrate that non-destructive measurements of ⁴⁰K in rocks and minerals can be obtained with a good precision, comparable at 2σ to that obtained by classical atomic absorption spectroscopy. This is the first step toward an enhanced K-Ar dating method in which both parent (⁴⁰K) and daughter (⁴⁰Ar*) nuclides are measured in the same sample aliquot. This method should minimize the undeterminable internal error produced by the natural chemical heterogeneity of the measured sample. The efficiency of the Ge-detector was calibrated at 1461 keV, corresponding to the emission energy of ⁴⁰K, on five geostandards of biotite, feldspar, trachyte, glauconite and a synthetic glass. Calculated efficiency ranges from 0.0403 to 0.0442 with a mean value of 0.0418 ± 0.0018. Comparison between K₂O contents calculated after measurements on the gamma ray detector (K₂Oγ) of nineteen natural volcanic mineral and groundmass samples and those obtained by classical atomic absorption spectroscopy (K₂O_AAS) are identical within 2σ uncertainties. Best results are obtained by calibrating the natural sample measurements with the efficiency obtained for the synthetic glass geostandard VS-N with a K₂Oγ/K₂O_AAS of 1.040 ± 0.060.