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

摘要: 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 (238 U/206 Pb* and 235 U/207 Pb*) in samples, including correcting for common Pb;(2) how well we know the 238 U and 235 U 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 (202 Pb±205 Pb±233 U±235 U) 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.