Towards higher precision SIMS U–Pb zircon geochronology via dynamic multi-collector analysis

Abstract

The U–Pb dating system has been widely used in geochronology because the system contains two independent parent/daughter pairs yielding three ages (i.e., ²³⁸U/²⁰⁶Pb, ²³⁵U/²⁰⁷Pb and ²⁰⁷Pb/²⁰⁶Pb ages) to internally check self-consistency. Among numerous U-bearing minerals, zircon has been recognized as the best mineral for U–Pb geochronology owing to its moderate U content, negligible initial unradiogenic Pb (or common Pb) and occurrence in a wide range of rock types. With the development of Secondary Ion Mass Spectrometry (SIMS) and in situ dating methods, the ²³⁸U/²⁰⁶Pb zircon age uncertainty could be achieved at an ∼1% level. However, the ²⁰⁷Pb/²⁰⁶Pb age uncertainty of Phanerozoic zircon is always very poor, when single-collector SIMS is used. The low level precision often hampers effective examination of concordance of young zircon between U–Pb and Pb/Pb ages, which is crucial to the data quality evaluation and chronological interpretations. In this study, we developed a hybrid “dynamic multi-collector U–Pb dating technique”. It takes advantage of both the static multi-collector mode and peak-hopping mono-collector mode. The technique is able to simultaneously measure with high-precision the ²⁰⁷Pb/²⁰⁶Pb ratio as in the static multi-collector mode without trade off in the analytical precision of the ²³⁸U/²⁰⁶Pb ratio of the conventional peak-hopping mono-collector mode. Four zircon reference materials (91500, M257, Temora and Plešovice) were measured to demonstrate that this new analytical protocol is able to achieve a higher precision for the ²⁰⁷Pb/²⁰⁶Pb age by a factor of two than the conventional mono-collector mode within the same working time. It is possible to simultaneously obtain the ²⁰⁷Pb/²⁰⁶Pb age and ²³⁸U/²⁰⁶Pb age with comparable quality to effectively evaluate the concordance of the U–Pb system for Phanerozoic samples.