Interpretation of discordant U‐Pb zircon ages: An evaluation

Abstract

The most widely used technique for the determination of high precision mineral growth ages in igneous and metamorphic rocks is dating of zircons with the U‐Pb method. The interpretation of these ages, particularly in metamorphic settings, is hampered by an incomplete understanding of the common phenomenon of partial Pb‐loss in zircon. In principle, this Pb‐loss may occur in four very different ways: diffusion in metamict zircon, diffusion in pristine zircon, leaching from metamict zircon and recrystallization of metamict zircon. Here it is argued that, under conditions common in the continental crust, Pb‐loss is only possible in partially to strongly metamict zircons. Pb‐diffusion in the pristine zircon lattice is insignificant up to temperatures of at least 1000 °C. Pb‐loss is only possible if the zircons experienced a time interval below their annealing temperature of about 600–650 °C, because only below this temperature can the lattice damage through α‐decay and spontaneous fission accumulate. Zircons that remain above this temperature do not lose Pb by diffusion and will stay closed systems. Complete resetting of the U‐Pb system in zircon under crustal conditions is only possible through dissolution and reprecipitation of zircon. Partial resetting results from recrystallization, leaching or diffusion in metamict zircon. As a consequence, special care has to be taken to interpret lower intercepts on concordia diagrams defined by discordant U‐Pb data. Lower intercept ages may be significant only if they are defined by zircons with low U‐content (<100 p.p.m.) or if confirmed by other geochronological methods. In addition, the accuracy of the lower intercept should be confirmed by abrading the zircon fractions that define the discordia.