A Critical Evaluation of U-Pb Calibration Schemes Used in SIMS Zircon Geochronology

Precision and accuracy in SIMS zircon geochronology strongly depend on the method of determination of the interelement ion ratios (e.g., Pb-206/U-238) from the measured secondary ion ratios (Pb-206+/U-238(+)). Six possible U-Pb calibrations (Pb/U-UO2/U, Pb/U-UO/U, Pb/UUO2/UO, Pb/UO-UO2/U, Pb/UO-UO/U, Pb/UO-UO2/UO) based on simple power law relationships, and Pb/UO2-related one-and two- (a power law) dimensional ones were compared using data acquired on the 91500 zircon reference material from one hundred measurement sessions, to determine the most statistically reliable scheme. Taking advantage of U oxide species (UO and UO2) over atomic U, due to their similar energy distribution to Pb and higher intensities, the data calibrated with Pb/UO-UO2/UO showed the smallest mean uncertainties and dispersions, and provided the best-fit calibration curve consistently. Although it was demonstrated with Temora 2 that the unknown zircon age was not changed significantly by different calibrations, its precision could be improved using the Pb/UO-UO2/UO calibration in zircon geochronology.