ACCURACY OF THE U-PB SYSTEM: TRACER CALIBRATIONS, DECAY CONSTANTS AND OTHER PARAMETERS

The accuracy of the U-Pb system as a chronometer is largely controlled by the following factors: (1) our ability to determine accurate U to radiogenic Pb ratios (²³⁸U/²⁰⁶Pb* and ²³⁵U/²⁰⁷Pb*) in samples, including correcting for common Pb; (2) how well we know the ²³⁸U and ²³⁵U decay constants, and (3) closed-system behaviour (with respect to U and Pb) of the material being analysed. Under the auspices of the EARTHTIME initiative a series of experiments have been carried out aimed at assessing each of these components of the U-Pb system via analyses of closed system minerals.

U-Pb analyses by isotope dilution thermal ionisation mass spectrometry (ID-TIMS) exploit mixed U-Pb tracers (²⁰²Pb±²⁰⁵Pb±²³³U±²³⁵U) that allow for accurate determination of the sample U/Pb ratio through independent U and Pb mass spectrometric measurements. We report a comprehensive tracer calibration exercise using multiple mixed U-Pb gravimetric reference solutions made from high-purity metals, and re-determination of the isotopic composition of uranium reference materials that are used to quantify mass fractionation during the calibration exercise and subsequent sample analyses.

High-precision ²³⁸U/²⁰⁶Pb* and ²³⁵U/²⁰⁷Pb* determinations made on closed system materials, such as zircon that has undergone the ‘chemical abrasion’ technique, afford an opportunity to quantify λ²³⁸U/ λ²³⁵U and thus determine λ²³⁵U with uncertainties exceeding those of the original counting experiments of Jaffey et al (1971). We will summarise two recent studies which arrive at a reproducible revision of the Jaffey et al. decay constants.

Finally, recent studies have demonstrated the potential for per-mil level variation in ²³⁸U/²³⁵U in low-temperature geochemical systems. The magnitude of this effect in high temperature petrologic systems and its implication for geochronology is unknown. We will present preliminary results of coupled U-Pb and ²³⁸U/²³⁵U measurements for a suite of closed system zircons as a means to assess the effect of natural variability in ²³⁸U/²³⁵U for high-precision geochronology.