Mobilization of radiogenic Pb in zircon revealed by ion imaging: Implications for early Earth geochronology

Abstract

Zircon is arguably the most commonly used geochronometer, but the reliability of agesobtained requires a full understanding of processes that might compromise the integrity of itsU-Pb systematics. Here we present the results of a multifaceted ion microprobe study of zircongrains from the Napier Complex, East Antarctica, a region affected by pervasive high-temperaturemetamorphism at 2.5 Ga, and from which previous zircon geochronological interpretationshave been problematic. Both U-Pb spot analysis (~15 µm) and high spatial resolution (~2 µm) scanning ion imaging of Pb isotopes have been applied in an attempt to quantify the effectsof metamorphism. Spot analyses spread along concordia yielding 207Pb/206Pb ages from 2.5 Gato 3.9 Ga, with the oldest grains reversely discordant. Ion images of uranogenic Pb reveal a surprisingmicrometer-scale patchy distribution that is unrelated to crystal morphology or damage.The 207Pb/206Pb ratios within these subdomains correspond to apparent zircon ages as oldas 4.2 Ga. These are interpreted as artifacts of ancient redistribution of radiogenic Pb, a processthat can generate meaningless ages, and are not relicts of ancient (including Hadean) zircon.Scanning ion imaging thus facilitates identifi cation of unsupported radiogenic Pb and enablestesting of the validity of old ages from zircon known to have a long and complicated history.