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

Abstract

Mixed [superscript 235]U–[superscript 233]U–[superscript 205]Pb(–[superscript 202]Pb) 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 [superscript 206]Pb/[superscript 238]U 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, [superscript 238]U/[superscript 235]U 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 [superscript 238]U and [superscript 235]U decay constants (±0.11 and 0.14% 2σ). The calibration approach of the mixed EARTHTIME [superscript 235]U–[superscript 233]U–[superscript 205]Pb(–[superscript 202]Pb) 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.