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 <300ppm when using the EARTHTIME and similarly calibrated tracers.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology