Mobility of trace elements and oxygen in zircon during metamorphism: Consequences for geochemical tracing

Abstract

This study addresses the mobility of the rare earth elements (REE), O, U and Pb during the formation of metamorphic zircon by overgrowth and coupled dissolution–reprecipitation. For this purpose, the texture and REE, δ18O and U–Pb compositions of zircon crystals from a metabasite, an anatectic granite and a metapelite sampled on Naxos Island (Greece) have been characterised. The behaviour of O and REE during the formation of the zircon metamorphic domains shows that two end-members can be distinguished:(i)Closed (or partially closed) system behaviour for oxygen and trace elements in zircon formed by coupled dissolution–reprecipitation. Metamorphic domains partially preserve the textural and geochemical characteristics of the precursor zircon. Chemical or isotopic equilibrium between the metamorphic zircon and the matrix is not completely reached for all the elements. Such zircons have to be used with caution for geochemical tracing. When re-equilibration is not complete, the U–Pb system is also not completely reset and the metamorphic domains yield old, scattered ages.(ii)Open system behaviour for oxygen and trace elements in zircon formed by overgrowth. These metamorphic zircons do not show an inherited shape and are devoid of relic magmatic zoning. Their composition reflects growth in an open system at the scale of the zircon grain with the fluid phase and the surrounding minerals. Because equilibrium between the metamorphic zircon and the matrix was thus favoured, zircon crystallisation can be more confidently related to the P–T history and fluid circulation events by geochemical tracing.