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

In a reconnaissance study, we investigated the potential of γ‐ray induced production of 38 Ar K from 39 K for geochronological applications. For this purpose, various age monitors commonly in use for the established 40 Ar/ 39 Ar‐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 38 Ar K was depressed, leading to low J 38 ‐values of (2.1–4.1) × 10 ‐6 and hence resulted in only minor 38 Ar excess when compared with atmospheric 38 Ar / 36 Ar 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 36 Ar in analysed CaF 2 reference materials, and calculated a limit for Ca‐interference on 38 Ar Ca of ( 38 Ar/ 36 Ar) Ca = 0.07 ± 0.03 (1 s ). In addition, we investigated a potential recoil redistribution of 38 Ar 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 40 Ar/ 37 Ar ratios for age determination and the possibility of 42 Ar production from 44 Ca, which would allow correction for Ca‐interference reactions on other Ar isotopes. This would be a pre‐requisite for dating extra‐terrestrial rocks.