K-Ar and Rb-Sr ages were measured on rapidly cooled igneous rocks from Kinnekulle, Sweden (Ordovician bentonite), the Cerberean Cauldron, Australia (late Devonian felsic volcanics), Mount Dromedary, Australia (Cretaceous monzonite) and Cooma, Australia (Jurassic syenite). No systematic bias was detected between the ages determined by the two techniques, calculated using the constants recommended by the I.U.G.S. Subcommission on Geochronology. The K-Ar and Rb-Sr ages of the Jurassic syenite also were indistinguishable from its zircon U-Pb age. The Kinnekulle A 1 bentonite gives a minimum age of 446 ± 3 Ma for the early Caradocian Diplograptus molestus Zone. The Cerberean Cauldron volcanics give a minimum age of 367 ± 2 Ma for the early Frasnian. These data are consistent with a small upwards revision of the ages in Gale et al. 's 1979 time scale as proposed by Gale et al . (1980).

Dating geological samples by the 4øAr/a9Ar method involves an irradiation in a reactor during which interfering nuclear reactions also occur that require corrections to be made to the observed data. The (39Ar/37Ar)ca and (36Ar/37Ar)ca correction factors for HIFAR reactor agree with those reported for other reactors, whereas the (4øAr/39Ar)K correction factor (about 0.3) is more than an order of magnitude greater than that found for most other reactors. A positive correlation found between reported thermal/ fast neutron flux ratios and the (4øAr/39Ar)K correction factors for different reactors leads to the inference that significant 4øAr can be produced by the 4øK(n, p)4øAr reaction through interaction with thermal neutrons. This effect of thermal neutrons also causes substantial variation in (4øAr/39Ar)g as a function of position in the reactor. Cadmium shielding has been used to modulate these effects over about 2 orders of magnitude and thereby to reproduce artificially the entire range of (4øAr/39Ar)•t values reported for other reactors. Elimination of the thermal neutron interference (via Cd shielding _> 0.5 mm) can increase the range of useful neutron fiuencies by about an order of magnitude and also reduce corrections in young samples, where it can be relatively large.

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