A Reappraisal of the40Ar/38ArDating Technique via Irradiation withγ‐Rays

Abstract

In a reconnaissance study, we investigated the potential of γ‐ray induced production of38ArKfrom39K for geochronological applications. For this purpose, various age monitors commonly in use for the established40Ar/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 of38ArKwas depressed, leading to lowJ38‐values of (2.1–4.1) × 10‐6and hence resulted in only minor38Ar excess when compared with atmospheric38Ar/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 on36Ar in analysed CaF2reference materials, and calculated a limit for Ca‐interference on38ArCaof (38Ar/36Ar)Ca = 0.07 ± 0.03 (1s). In addition, we investigated a potential recoil redistribution of38Ar 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 utilising40Ar/37Ar ratios for age determination and the possibility of42Ar production from44Ca, which would allow correction for Ca‐interference reactions on other Ar isotopes. This would be a pre‐requisite for dating extra‐terrestrial rocks.