Precise ID-TIMS U-Pb dating of single zircons using Pb-265-U-233-U-236 tracers : Isobaric oxide corrections for UO2+ with in-run measured O-18/O-16

Due to the interferences of U-236(16) O-18 O+ and U-236 O-18 O+ on U-236(18) O-16 O-2(+), a Pb-205-(233) U-(235) U spike rather than a Pb-205-(233) U-(236) U spike is usually used for the single grain zircon CA-ID-TIMS U-Pb dating worldwide. In order to use the Pb-205-(233) U-(236) spike for correcting the U isotope fractionation effect by the 233U 236U double spike, in this study, the impact of isobaric interferences of U-236(16) O-18 O+ and (UO16)-U-236-O-18 O+ on U-236(18) O-16 O-2(+) for the TIMS U isotope analysis is thoroughly addressed. We found that it is critical to ensure the sample is not over-spiked using the Pb-205-U-233-(236) U spike by controlling the amount of the spike to be added into the sample during the CA-ID-TIMS U-Pb analysis. Actually, when the U-238/U-236 ratio in the sample -spike mixture is not <0.5, the effect of the interferences of U-236(16) O-18 O+ and U-236(18) O-16 O+ on U-238(16) -2 on the final U-Pb dating result is not significant. In addition, the importance of the correct setting of the dead -time of SEM ( secondary electron multiplier) for the U and Pb isotope analyses for the CAID-TIMS U-Pb dating has also been fully addressed. Accordingly, a high precision CA-ID-TIMS U-Pb geochronology method for single zircons using Pb-205-U-233-(236) U tracers with in -run measured O-18/O-16 for UO2+ isobaric oxide corrections has been developed. Finally, a standard zircon-ZS was analyzed using the method, yielding a weighted mean Pb-206/(238) U age of 559. 91 0. 25Ma (2 sigma o-, analytical error), which is consistent with the previously reported date within analytical error. Due to the use of U-233-U-236 double spike for internal correction of U isotope fractionation effect, the analytical precision for this standard zircon is better than 0. 05% (2RSE), which is significantly better than that obtained by using the single Pb-205-U-235 tracer.