U-Pb dating of Quaternary speleothems using LA & ID MC-ICPMS

Carbonate U-Pb dating techniques have broad applications. Recently, the U-Pb isotope diluent(ID)and laser ablation(LA)dating of carbonate developed rapidly. Yet the U-Pb LA dating technique has not been used to obtain ages of carbonate formed in the Quaternary, and the U-Pb ID technique for dating carbonate remains unavailable in China. In this context, the U-Pb LA & ID MC-ICPMS dating tourniquets have been developed in the Isotope Laboratory of Xi'an Jiaotong University and applied to establish chronology for a set of speleothems across the Quaternary. We have tested different carbonate reference materials using our LA MC-ICPMS method, and the age results are consistent within the quoted uncertainties. Here, we report the first set of high-precision LA U-Pb ages of Quaternary speleothem samples from Sanbao cave, Central China. We discussed the U-series initial disequilibrium corrections which is critical to the Quaternary carbonate samples and then highlighted the importance of the ID methods for high-precision measurements of both δ234U and Pb isotopes. Regarding the U-Pb ID method, the total Pb blank in our lab is now comparable to the world-class level(ca.10 pg). Our preliminary ID ages(e.g., 1.14 ±0.04 Ma, 2σ, n=5, MSWD=1)obtained from Quaternary speleothem samples are in agreement with the previous results from the U-Pb lab of Melbourne University, Australia. The age precisions maybe potentially improved substantially. In short, we have developed high-precision LA and ID U-Pb dating techniques in our lab, which are applicable to date carbonate samples formed in time period older than ca.640 ka. This will effectively propel our scientific researches further back to the deep-time beyond the U-Th dating range.