U-Pb isotopic study of relict zircon inclusions from Muong Nong-type tektites

Isotopic composition of U and Pb were determined for relict zircon inclusions recovered from Muong Nong-type tektites from the Australasian and North American strewn fields. These inclusions are generally opaque white with granular textures, probably representing mechanical disagregation and/or local melting and recrystallization. All zircons have strongly discordant U-Pb isotopic ratios and the isotopic ratios of most of the zircons are dominated by common Pb. This is interpreted as being the result of complete resetting of the isotopic system during the impact event and exchange with Pb from the target sediments. Ages that may represent the target rocks were only retained by one sample, representing a zircon that was extracted from a Muong Nong-type georgiaite. A value of 0.6 Ga was obtained, which is similar to the Nd and Sr model ages obtained for the North American tektites. Our results indicate that the U-Pb isotopic system in almost all zircons from tektites has been totally reset during the impact event; this is in contrast to zircons extracted from impact melt rocks at some impact structures and/or distal ejecta, some of which are only partly reset. Copyrigh Π2001 Elsevier Science Ltd.