The age and Pb loss behaviour of zircons from the Isua supracrustal belt as determined by ion microprobe

Ion microprobe U sbnd Th sbnd Pb analyses of zircons from a conglomeratic metavolcanic unit in the Isua supracrustal belt of West Greenland show that their magmatic age is 3807 ± 1Maσ and failed to detect any older zircon cores or xenocrysts. Low U areas within zircons, at a 30 μm scale, lost only small amounts of radiogenic Pb and only in the recent past, whereas high U areas within grains lost much greater proportions of their Pb, both recently and as early as the Archaean. Some areas in the zircons have concordant U sbnd Pb and Th sbnd Pb ages. The ion-probe results agree with earlier determinations for the Isua supracrustals and clarify minor discrepancies between previous multigrain and single crystal analyses.