Ar Isotope Analysis for K-Ar Dating Using Two Modified-VG5400 Mass Spectrometers-I : Isotope Dilution Method

Two noble gas mass spectrometry systems (MS-I and MS-II) have been installed in 1989, one of which was designed in our laboratory for Ar isotope analysis to measure K-Ar age by isotope dilution method using 38Ar-spike. The whole system was modified and improved to obtain high quality Ar data. The improved system composed of an Ar extraction and purification line and a Modified-VG5400 mass spectrometer enables us to determine precise Ar isotope composition easily and speedily irrespective to operator's skillfulness.Formulas calculating radiogenic 40Ar abundance, which takes account of correction for Ar isotopes of both spike and blank, are presented. For samples with young K-Ar age, formulas to estimate the radiogenic 40Ar abundance based on 40Ar/36Ar ratio are also proposed. Replicate analyses on many K-Ar standard samples show that the system can be applicable to samples with ages ranging 0.01 to 1500 Ma without any systematic analytical errors.