Abstract
UXe and UKr ages of pitchblendes and uraninites are compared with isotopic UPb ages and chemical UPb ages calculated from electron microprobe data. It is shown that the UXe and UKr systems are helpful for the interpretation of discordant UPb ages because systematic relations between the two types of dating methods are obtained. Three patterns are observed: 1. (1) U-oxides which are concordant with respect to UPb also yield identical UXe and UKr ages indicating closed-system behaviour of both radiogenic and fissiogenic uranium daughter products. 2. (2) Replacement of pitchblendes by later generations often results in finely intergrown, disseminated secondary pitchblende which yield discordant UPb but concordant fission gas ages. The latter generally coincide with UPb microprobe ages. It is concluded that noble gases are completely released during dissolution and recrystallisation, whereas part of the mobilized radiogenic Pb was coprecipitated in minerals adjacent to pitchblende, i.e. material which could not be separated for the UPb isotope analysis. 3. (3) Episodic Pb loss accompanied by loss of fission gases during which Xe and Kr fractionate. This fractionation thus indicates a perturbation of the UPb system. Examples are presented where alteration of pitchblende resulted in minor mobilisation of Pb (and U) due to in situ precipitation of secondary U phases. Thus concordant UPb ages were obtained. The strong depletion of the fission gases, the fractionation between Xe and Kr and microprobe analysis indicate, however, that severe mineral alteration took place at a later stage.
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