Precise determination of Phanerozoic zircon Pb/Pb age by multicollector SIMS without external standardization

Abstract

Zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences. Modern microbeam techniques such as SIMS and LA‐ICPMS have been successfully applied to in situ U‐Pb zircon age determinations, at spatial resolutions of 20–30 μ m or less. Matrix‐matched calibration by external standardization of well‐characterized natural zircon references is a principal requirement for precise microbeam U‐Pb zircon age determination due to fractionation effects between Pb and U, which usually result in an external age error exceeding 1%. Alternatively, zircons with a closed U‐Pb system can be directly dated by measurement of 207 Pb/206 Pb isotopic ratio without external standardization, which has been a common practice for zircons older than 1.0 Ga, but not for relatively young (<1.0 Ga and particularly Phanerozoic) ones because of limitations of analytical precision. We describe in this paper a method of 207 Pb/206 Pb measurement on Phanerozoic zircons using a new generation of large radius magnetic sector multicollector Cameca IMS‐1280 SIMS. In combination with multicollector mode, a Nuclear Magnetic Resonance (NMR) magnet controller and oxygen flooding techniques, we achieve precisions of 207 Pb/206 Pb ratio of <0.1% and 0.1 ∼ 0.2%, propagating to Pb/Pb age errors <0.4% and 1–3% (excluding U decay constant uncertainties), for zircons of latest Neoproterozoic and late Paleozoic to Mesozoic age, respectively. Therefore, the multicollector SIMS is capable of direct determination of zircon Pb/Pb ages as young as Mesozoic age with uncertainties of geological significance. This technique is useful for direct dating of zircons in thin sections. Moreover, it has significance for dating of some other U‐rich minerals (i.e., baddeleyite and zirconolite) that are not suitable for SIMS U‐Pb dating by external standardization.