Mixed 235U{\textendash}233U{\textendash}205Pb({\textendash}202Pb) tracers for U{\textendash}Pb isotope-dilution isotope ratio mass spectrometry have been prepared under the auspices of the EARTHTIME Initiative. The methods and results for the preparation and calibration of the U/Pb ratio and isotopic abundances are given, and the various sources of uncertainty are discussed and quantified. The accuracy of the EARTHTIME U{\textendash}Pb tracer isotopic composition can be traced back to SI units via a series of assay and isotopic composition reference materials combined with the experiments described herein. The parameters used in calculating U/Pb ratios (and inferentially U{\textendash}Pb dates) have correlated uncertainties that result in a total uncertainty contribution to 206Pb/238U dates of {\textpm}\<0.03\% (95\% confidence). For suitable terrestrial materials such as zircon, when other sources of uncertainty have been minimised (e.g., open-system behaviour, 238U/235U variation, intermediate daughter product disequilibrium, common Pb, etc.) the U{\textendash}Pb tracer calibration uncertainty is a limiting factor in the accuracy of U{\textendash}Pb geochronology {\textendash} but less so than the uncertainty in the 238U and 235U decay constants ({\textpm}0.11 and 0.14\% 2σ). The calibration approach of the mixed EARTHTIME 235U{\textendash}233U{\textendash}205Pb({\textendash}202Pb) tracers, in addition to updated values for reference materials (e.g., mixed gravimetric reference solutions), and parameters (e.g., Pb reference material assay), can be applied to other laboratory-specific U{\textendash}Pb tracers and will facilitate the generation of accurate and directly inter-comparable U{\textendash}Pb data.
Free poster associated with the Mclean et al. and Condon et al. papers showing the traceability of ID-TIMS U-Pb geochronology to SI units.
}, year = {2015}, journal = { Geochimica et Cosmochimica Acta}, volume = {164}, pages = {464 - 480}, month = {09/2015}, isbn = {0016-7037}, url = {https://doi.org/10.1016/j.gca.2015.05.026}, doi = {10.1016/j.gca.2015.05.026}, language = {eng}, }