Construction of I-Xe and ⁴⁰Ar-³⁹Ar Dating System Using a Modified VG3600 Mass Spectrometer and the First I-Xe Data Obtained in Japan

Abstract

Improvements in ion collectors of the VG3600 mass spectrometer used for ⁴⁰Ar-³⁹Ar dating in the Radioisotope Center, University of Tokyo, have been performed. We also constructed a new compact vacuum line for extraction and purification of noble gases, and connected it to the mass spectrometer. The new noble gas mass spectrometry system enables us to measure all noble gases (He, Ne, Ar, Kr, and Xe) at low detection limits corresponding to 2×10^-15 cm³ STP or 5×10³ atoms of Kr and Xe. A newly designed W-coil furnace effectively heats small samples (1-100 mg) in a Mo-crucible with condition of low blank levels (10^-10 and 10^-15 cm³ STP for Ar and Xe). Smaller sample sizes required for ⁴⁰Ar-³⁹Ar dating have reduced activity levels. The system was preliminarily applied to I-Xe dating of neutron irradiated meteorites, Bjurböle, Allende and Yamato-74191, and discovered a late formation age of Yamato-74191 (26 Ma after Bjurböle) compared with the Allende meteorite.