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

In a reconnaissance study, we investigated the potential of gamma-ray induced production of Ar-38(K) from K-39 for geochronological applications. For this purpose, various age monitors commonly in use for the established Ar-40/Ar-39-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 Ar-38(K) was depressed, leading to low J(38)-values of (2.1-4.1) x 10(-6) and hence resulted in only minor Ar-38 excess when compared with atmospheric Ar-38/Ar-36 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 Ar-36 in analysed CaF2 reference materials, and calculated a limit for Ca-interference on Ar-38(Ca) of (Ar-38/Ar-36)(Ca) = 0.07 +/- 0.03 (1s). In addition, we investigated a potential recoil redistribution of Ar-38 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 Ar-40/Ar-37 ratios for age determination and the possibility of Ar-42 production from Ca-44, which would allow correction for Ca-interference reactions on other Ar isotopes. This would be a pre-requisite for dating extra-terrestrial rocks.