A Reappraisal of the 40Ar/38Ar Dating Technique via Irradiation with γ-Rays

In a reconnaissance study, we investigated the potential of γ-ray induced production of ³⁸Ar_K from ³⁹K for geochronological applications. For this purpose, various age monitors commonly in use for the established ⁴⁰Ar/³⁹Ar-method were co-irradiated for 60 h at 17.6 MeV maximum energy in the ELBE facility, Dresden-Rossendorf, Germany. Because the available energy was low, total production of ³⁸Ar_K was depressed, leading to low J₃₈-values of (2.1–4.1) × 10^-6 and hence resulted in only minor ³⁸Ar excess when compared with atmospheric ³⁸Ar_/³⁶Ar ratios. In spite of these restrictions, ages of younger monitors could be reproduced within error, whereas older age reference materials showed discrepancies due to the low production rate. We observed Ca-derived contributions on ³⁶Ar in analysed CaF₂ reference materials, and calculated a limit for Ca-interference on ³⁸Ar_Ca of (³⁸Ar/³⁶Ar)_Ca = 0.07 ± 0.03 (1s). In addition, we investigated a potential recoil redistribution of ³⁸Ar by stepwise heating experiments, but could not quantify this further because of concurring processes. More work at higher photon energies is necessary to resolve other open issues, in particular the potential of utilising ⁴⁰Ar/³⁷Ar ratios for age determination and the possibility of ⁴²Ar production from ⁴⁴Ca, which would allow correction for Ca-interference reactions on other Ar isotopes. This would be a pre-requisite for dating extra-terrestrial rocks.