Interpretation of discordant U‐Pb zircon ages: An evaluation

Abstract

A B S TR A CT 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 a-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.

U-Pb evidence for a polyorogenic evolution of the HP-HT units of the NW Iberian Massif

“…Interpretation of zircon U Pb data in metamorphic rocks is often complex (see review in Mezger and Krogstad 1997). In the present study, we dated zircons from a leucosome in order to constrain its crystallisation age and zircons from an eclogite-facies paragneiss (GCH-99-1) and from a granulite-facies paragneiss .…”

“…In this case we consider that the lower intercept is meaningful (cf. Discussion in Mezger and Krogstad 1997) and represents an event at ca. 470 480 Ma or slightly older.…”

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

“…A number of studies have found that zircon chemical and isotopic signatures can be altered by various mechanisms: Pb can be locally lost (e.g. Mezger and Krogstad 1997;Geisler et al 2002;McFarlane et al 2005) or gained (e.g. McFarlane et al 2005), or Pb can be redistributed at sub-micron scale (e.g.…”

SHRIMP dating of zircons from high-grade metasediments of the Schwarzwald/SW-Germany and implications for the evolution of the Moldanubian basement

“…This is relevant for discussions of the geological relevance of zircon age data from high-grade metamorphic zircon. The U-Pb stability of the low U-Th relatively crystalline zircons is in accord with the very low diffusivities of U, Th and Pb (Cherniak et al 1997;Lee et al 1997) and the very high closure temperatures for Pb loss in zircon (Lee et al 1997), indicating that volume diffusion in intact zircon crystals is not the dominant process of Pb loss (Mezger and Krogstad 1997).…”