Geochemical Journal, Vol. 49 (No. 6), pp. e15-e19, 2015
Daniele L. Pinti1*, Bassam Ghaleb1, Agnes Samper2 and Pierre-Yves Gillot3
1GEOTOP and Département des Sciences de la Terre et de l’atmosphère, Université du Québec à Montréal, CP 8888 succ. Centre-Ville, Montréal, QC, H3C 3P8, Canada
2UMSNH-IIM, Research Institute for Earth Sciences (INICIT), Morelia, Michoacan 58060, Mexico
3Laboratoire de Géochronologie Multi-Techniques - UMR8148 GEOPS, Université Paris-Sud, Orsay 91405, France
(Received July 29, 2015; Accepted September 4, 2015; Online published October 2, 2015)
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 (K2OAAS) 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/K2OAAS of 1.040 ± 0.060.
Key words: potassium, K-Ar dating, gamma spectrometry, atomic absorption spectroscopy, germanium well detector