Xenon geochronology of Schwarzwald pitchblendes

The uranium- /fission-xenon systems in eleven samples selected from three o ccurrences in the Schwarzwald were investigated (Menzenschwand, Wittichen, Mullenbach). Relationships between mineralogical properties, Xe release pro files and Xe-Xe age spectra provide criteria for sample suitability for U-X e and Xe-Xe dating and interpretation of the Xe-Xe age spectra. U leaching from pitchblendes was demonstrated for three of the Wittichen samples, indi cating also disturbed Lr-Pb relations. The results show that the U-Xe isoto pic system is a valuable geochronological tool for studying uranium mineral s. In samples of two deposits, fission Xe was well preserved and allowed us to determine Xe-Xe plateau ages: 301 +/- 11, 304; 9 and 279 +/- 10 Ma for Menzenschwand and 244 +/- 9 Ma for Wittichen pitchbrendes. These ages show the closure times of the U-Xe isotope systems and most probably correspond to the formation of these pitchblendes. They are in accord with published U -Pb data. Four other samples of these two deposits had very irregular spect ra. An attempt was made to interpret integrated Xe-Xe ages (U-Xe ages) in t hese samples in terms of alteration events. Little fission Xe was found in material from Mullenbach. There is a large time gap between the formation o f the hosting granites and the U-ore depositions (about 30 and 90 Ma, respe ctively) in Menzeaschwand and Wittichen.