Metrology and traceability of U-Pb isotope dilution geochronology (EARTHTIME Tracer Calibration Part I)

Mixed ²³⁵U-²³³U-²⁰⁵Pb(-²⁰²Pb) tracers for U-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-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-Pb dates) have correlated uncertainties that result in a total uncertainty contribution to ²⁰⁶Pb/²³⁸U dates of ±<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-Pb tracer calibration uncertainty is a limiting factor in the accuracy of U-Pb geochronology - but less so than the uncertainty in the ²³⁸U and ²³⁵U decay constants (±0.11 and 0.14% 2σ). The calibration approach of the mixed EARTHTIME ²³⁵U-²³³U-²⁰⁵Pb(-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-Pb tracers and will facilitate the generation of accurate and directly inter-comparable U-Pb data.