THE FUNDAMENTAL LIMITS OF U-PB ID-TIMS PRECISION: PROPAGATING UNDERLYING SYSTEMATIC UNCERTAINTIES

U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) zircon dates are among the most precise and accurate available, and as such they underpin much of the geologic timescale. U/Pb isotope ratio determinations by ID-TIMS can be made to very high precision, but they are affected by systematic uncertainties from several sources, including the isotopic standards used to calibrate instrumental mass fractionation, the isotopic composition (IC) and U/Pb ratio of gravimetric solutions used to calibrate the tracer solution, and the minor isotopic compositions of the tracer and blank that are subtracted from each analysis. When considered with decay constant uncertainties and corrections for intermediate daughter excess/deficiencies these provide a fundamental limitation on the precision achievable by ID-TIMS.

The ultimate accuracy of all ID-TIMS data can be traced back to several common sources. For instance, all TIMS Pb measurements relate back to the certified ²⁰⁸Pb/²⁰⁶Pb ratio of NBS982. This ratio is used to internally normalize measurements of the other isotope ratios of 982, which are used to calibrate ²⁰⁷Pb-²⁰⁴Pb and ²⁰²Pb-²⁰⁵Pb double-spikes then used to determine the relative IC of NBS981 and NBS983. This set of standards is employed both in the gravimetric solutions utilized in tracer calibration, and in determining the average instrumental Pb mass fractionation that is then applied to each measured ratio. The same principle applies to the gravimetrically determined ²³³U/²³⁶U ratio of IRMM3636, used to calibrate commonly used reference solutions such as U500 and CRM112a, which are then utilized for tracer and instrument calibration.

The EARTHTIME effort to calibrate community U-Pb tracers provides an opportunity to unravel the correlation between the tracer parameters and each source of systematic uncertainty. This information is incorporated in a new algorithm that propagates systematic uncertainties, canceling them where appropriate without neglecting their contribution to weighted mean calculations. Explicitly hanging U-Pb measurements on a common framework of standards and systematic uncertainties facilitates accurate inter-comparisons between radioisotopic chronometers and directs future efforts at sharpening this tool.