Zircon Th–Pb dating by secondary ion mass spectrometry

Abstract

Zircon has been widely used as a geochronometer with the U–Pb decay system but rarely with the Th–Pb system. However, zircon from carbonatite contains very little U and has a high Th/U ratio, making Th–Pb dating preferable. As a one-dimensional system, a series of consistent Th–Pb ages can be used to date a geological event. In contrast, a wide variation in Th–Pb ages could result from Pb loss or multiple growth events, making it difficult to link to specific geological events. In this study, we described Th–Pb dating protocols on zircon and carried out Th–Pb measurements on seven zircon reference materials using a Cameca IMS 1280HR ion microprobe. The results demonstrated that these seven U–Pb zircon standards have similar absolute concentrations of common lead. The radiogenic ²⁰⁸Pb concentrations (depending on the Th content and age) determine the proportion of common lead and define the extent of variation in the Th–Pb system under certain analytical conditions. This relationship could be used as a criterion to evaluate whether it is a single population or not based on Th–Pb dating results of unknown zircons. By comparison, M257 and Qinghu zircons are suggested as the most suitable Th–Pb dating standards, with ID-TIMS U–Pb ages representing the best estimate of the Th–Pb reference ages. A zircon sample from the Wu dyke, an extremely rare earth element (REE)-rich carbonatite dyke in the Bayan Obo area, was dated with the established Th–Pb dating procedure and yielded an age of 1327 ± 20 Ma as the emplacement time. A pronounced relationship between apparent Th–Pb ages and corresponding Th contents (>1500 ppm) indicates that zircons with accumulated radiogenic doses of >1.7 × 10¹⁸ α per g may tend to lose radiogenic ²⁰⁸Pb.