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

Author(s): Condon, Daniel J; Schoene, Blair; McLean, Noah M; Bowring, Samuel A; Parrish, Randall R

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Abstract: 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.
Publication Date: 1-Sep-2015
Electronic Publication Date: 19-May-2015
Citation: Condon, Daniel J., Blair Schoene, Noah M. McLean, Samuel A. Bowring, and Randall R. Parrish. "Metrology and traceability of U–Pb isotope dilution geochronology (EARTHTIME Tracer Calibration Part I)." Geochimica et Cosmochimica Acta 164 (2015): 464-480. doi:10.1016/j.gca.2015.05.026.
DOI: doi:10.1016/j.gca.2015.05.026
ISSN: 0016-7037
Pages: 464 - 480
Type of Material: Journal Article
Journal/Proceeding Title: Geochimica et Cosmochimica Acta
Version: Final published version. This is an open access article.

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