APPLICATION OF U-XE AND U-KR SYSTEMS FOR DATING U MINERALS: A KEY FOR INTERPRETING DISCORDANT U-PB AGES

Abstract

U-Xe and U-Kr ages of pitchblendes and uraninites are compared with isotopic U-Pb ages and chemical U-Pb ages calculated from electron microprobe data. It is shown that the U-Xe and U-Kr systems are helpful for the interpretation of discordant U-Pb ages because systematic relations between the two types of dating methods are obtained. Three patterns are observed:(1) U-oxides which are concordant with respect to U-Pb also yield identical U-Xe and U-Kr ages indicating closed-system behaviour of both radiogenic and fissiogenic uranium daughter products.(2) Replacement of pitchblendes by later generations often results in finely intergrown, disseminated secondary pitchblende which yield discordant U-Pb but concordant fission gas ages. The latter generally coincide with U-Pb 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 U-Pb isotope analysis.(3) Episodic Pb loss accompanied by loss of fission gases during which Xe and Kr fractionate. This fractionation thus indicates a perturbation of the U-Pb 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 U-Pb 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.