Whole-grain evaporation for 207Pb/206Pb-age-investigations on single zircons using a double-filament thermal ion source |
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| Authors: | Bernd Kober |
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| Institution: | (1) Laboratorium für Geochronologie, Ruprecht-Karls-Universität, Im Neuenheimer Feld 234, D-6900 Heidelberg, Federal Republic of Germany |
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| Abstract: | A technique has been developed and tested to analyse 207Pb/206Pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains ( 0.3 mm). The evaporation analyses are performed in the double-filament arrangement of a thermal ion mass spectrometer (ThIMS). The method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons. The evaporation of test samples results in 207Pb/206Pb apparent ages in good agreement with U/Pb ages known from literature: single zircons from a granite of the  Marble Mountains /California yield an age of crystallization of 1,410±30 Ma; Ceylon zircons from heavy-mineral bearing gravels yield 560± 40 Ma as age of crystallization of the pegmatitic gravel sources; individuals from a heterogeneous zircon population of a diatexite from the Southern Schwarzwald/SW-Germany indicate metamorphic zircon formation around 500 Ma and the existence of Middle-Proterozoic relics (1.95±0.05 Ga).The evaporation analyses revealed closed-system U/Pb evolution of the crystalline domains of all investigated zircons irrespective of discordancy-trends documented by U/Pb analyses on related zircon concentrates. Therefore the majority of discordia-lines derived from U/Pb isotope distributions of zircon samples are supposed to be due to phase mixing. Lead components from the crystalline domains are concordant end members of the mixing arrays. Open-system behaviour and U/Pb fractionation should be attributed only to phases with low Pb activation energies eg. metamict zircon domains or intergrown non-zircon minerals. |
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