Zircon Th–Pb dating by secondary ion mass spectrometry

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 Pb-208 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 x 10(18) per g may tend to lose radiogenic Pb-208.