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

In 40 Ar/ 39 Ar dating, 39 Ar K generated is dependent on irradiation conditions, and the accumulation fast neutron fluence is a critical parameter for 40 Ar/ 39 Ar 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 40 Ar*/ 39 Ar can be up to 5750, meaning that very low 39 Ar 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 40 Ar*/ 39 Ar 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. In 40 Ar/ 39 Ar dating, 39 Ar K generated is dependent on irradiation conditions, and the accumulation fast neutron fluence is a critical parameter for 40 Ar/ 39 Ar dating.