Recent Pb loss in zircon: A natural or laboratory-induced phenomenon?

Abstract

Monazite and zircon from a Proterozoic terrain in Antarctica had different isotopic responses to temporally discrete and geologically dissimilar Phanerozoic events. The monazite lost much of its Pb during upper-greenschist-facies metamorphism at ∼ 540 Ma, whereas the zircon did not respond to this event. In contrast, although potentially more susceptible to Pb loss, the monazite was not at all affected by a recent event which caused significant isotopic resetting of coexisting zircon. The different responses to the recent event are not a consequence of chemical processing during sample preparation and are unlikely to be related to mineral separation. A semi-quantitative correlation with the degree of alteration of feldspar to clay minerals indicates that the loss of Pb from zircon was induced by incipient weathering. Though recent Pb loss is easily identifiable in zircons with simple histories it is mostly very difficult to recognise in zircons with more complex histories. As the latter can generate geologically meaningless concordia ages, it is important to assess the extent of recent Pb loss in the interpretation of all UPb zircon discordia.