Optimization of irradiation parameters for 40Ar/39Ar dating by Argus VI multi-collector mass spectrometry

Abstract

In ⁴⁰Ar/³⁹Ar dating, ³⁹Ar_K generated is dependent on irradiation conditions, and the accumulation fast neutron fluence is a critical parameter for ⁴⁰Ar/³⁹Ar dating. With technological advancement, especially in the ultra-low background and multicollector technology, the boundary conditions to choose irradiation conditions have been greatly improved. Based on the detected and processed signal, the maximum boundary condition of ⁴⁰Ar*/³⁹Ar can be up to 5750, meaning that very low ³⁹Ar signals can yield considerable analytical accuracy. In this study, one biotite sample (ZBH-25) was irradiated for 4 hours to assess the modified irradiation conditions, and the high ⁴⁰Ar*/³⁹Ar ratio up to 4100 could yield ∼1% analytical uncertainty. This reveals that the new boundary condition is useful to choose fast neutron fluence, especially for very old or low K/Ca ratio samples.