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

Publication Year
2015

Type

Journal Article
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 (202Pb–205Pb–233U–235U) 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 206Pb/238U date that is only half of the relative uncertainty in the 235U/205Pb ratio of the tracer, which was historically used to approximate the tracer related uncertainty contribution to 206Pb/238U dates. The tracer uncertainty contribution to 206Pb/238U dates has in this way been reduced to <300ppm when using the EARTHTIME and similarly calibrated tracers.

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Journal
Geochimica et Cosmochimica Acta
Volume
164
Pages
481 - 501
Date Published
09/2015
ISBN
0016-7037
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