Age determination of monazites using the new experimental chamber of the Cracow proton microprobe

The Cracow microprobe was applied to determine the age of small detritalmonazite crystals extracted from the uppermost carboniferous sandstone of Upper Silesia Coal Basin (southern Poland). The method applied follows the technique of chemical dating of U-Th minerals: assuming that the monazite crystal is a closed system and all Pb within grain is radiogenic, it is possible to determine the age of a specimen by solving an equilibrium equation which includes as computational factors the elemental contents of U, Th and Pb and decay times of uranium and thorium. Using this approach, thanks to rather fast Pb accumulation in a crystal, monazite older than 100 My may be dated with relative ease. The investigated specimens were clusters of crystals, 30-100 μm in size, polished and mounted in Epoxy resin. Due to different geological history, the age of individual crystals from one sample may differ significantly, therefore the application of a microprobe has been necessary to extract information concerning single monazite grains. Two age groups were identified: small cluster, connected with Cadomian tectonothermal event (579±30 My) and dominating cluster, connected with Variscan tectonothermal event (329±19 My, 353±41 My). After successful preliminary measurements carried out in a provisory experimental chamber, an optimized chamber was constructed and set into operation for regular studies. Together with results of these investigations, the constructional details of the new chamber are presented