1P-18 PDF
地質試料の感度法K-Ar年代測定に向けた新しいAr同位体分析システム
We present the outline and calibration results of new Ar isotope analytical system at Geological Survey of Japan for unspiked K-Ar dating of geological samples.
Unspiked K-Ar dating method (peak height comparison method/sensitivity method; Takaoka et al., 1989; Matsumoto et al., 1989) enables mass fractionation correction of initial Ar ratio, and this method has been applied successfully to young (<0.5 Ma) samples.
New Ar analytical system at GSJ consists of the gas extraction with Ta resistance furnace, purification line and MM5400Ar mass spectrometer. Calibration was conducted by using the biotite mineral standard (Sori 93 and FC3) in which radiogenic 40Ar concentration is known (Sudo et al., 1994), and the variation of 40Ar sensitivity is 2.4% within six months. The errors estimated with repeated analysis of the standard air Ar are 0.12% for 40Ar/36Ar (Faraday/Faraday), 0.43% for 38Ar/36Ar (Faraday/Faraday) and 0.20% for 38Ar/36Ar (EM/EM). The accuracy and precision of young K-Ar age data was assured by using age known sample in the order of 0.1 Ma. The obtained K-Ar ages agree well with the reported ages within the errors of about 7%, and the amount of sample necessary for analysis can be half or less.