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

Abstract

Ion microprobe UThPb analyses of zircons from a conglomeratic metavolcanic unit in the Isua supracrustal belt of West Greenland show that their magmatic age is 3807 ± 1 Ma σ 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 UPb and ThPb ages. The ion-probe results agree with earlier determinations for the Isua supracrustals and clarify minor discrepancies between previous multigrain and single crystal analyses. Show more