Evaluating uncertainties in the calibration of isotopic reference materials and multi-element isotopic tracers (EARTHTIME Tracer Calibration Part II). (1st September 2015)

• Record Type: Journal Article
• Authors: McLean, Noah M.; Condon, Daniel J.; Schoene, Blair; Bowring, Samuel A.
• Is Part Of: Geochimica et cosmochimica acta. Volume 164(2015:Sep. 01)
• Page Start: 481
• Page End: 501
• Publication Date: 2015-09-01
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.02.040
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: A statistical approach to evaluating uncertainties in the calibration of multi-element isotopic tracers has been developed and applied to determining the isotopic composition of mixed U–Pb ( 202 Pb– 205 Pb– 233 U– 235 U) tracers used for accurate isotope dilution U–Pb geochronology. Our experiment, part of the EARTHTIME initiative, directly links the tracer calibration to first-principles measurements of mass and purity that are all traceable to SI units, thereby quantifying the accuracy and precision of U–Pb dates in absolute time. The calibration incorporates new more accurate and precise purity measurements for a number of commonly used Pb and U reference materials, and requires inter-relating their isotopic compositions and uncertainties. Similar methods can be used for other isotope systems that utilize multiple isotopic standards for calibration purposes. We also detail the inter-calibration of three publicly available U–Pb gravimetric solutions, which can be used to bring the same first-principles traceability to in-house U–Pb tracers from other laboratories. Accounting for uncertainty correlations in the tracer isotope ratios yields a tracer calibration contribution to the relative uncertainty of a 206 Pb/ 238 U date that is only half of the relative uncertainty in the 235 U/ 205 Pb ratio of the tracer, which was historically used to approximate the tracer related uncertainty contribution to 206 Pb/ 238 U dates. The tracer uncertainty contribution to 206 Pb/ 238 U dates has in this way been reduced to <300 ppm when using the EARTHTIME and similarly calibrated tracers.