Long-term repeatability and interlaboratory reproducibility of high-precision ID-TIMS U-Pb geochronology

Age determination of minerals using the U-Pb technique is widely used to quantify time in Earth's history. A number of geochronology laboratories produce the highest precision U-Pb dates employing the EARTHTIME 202Pb-205Pb-233U-235U tracer solution for isotope dilution, and the EARTHTIME ET100 and ET2000 solutions for system calibration and laboratory intercalibration. Here, we report ET100 and ET2000 solution data from the geochronology laboratory of University of Geneva obtained between 2008 and 2021 and compare the most recent data with results from the geochronology laboratories of Princeton University and ETH Zürich. This compilation demonstrates that (i) the choice of the thermal ionization mass spectrometer model has no influence on precision and accuracy of the data; (ii) the often observed excess scatter of apparent ET100 solution 206Pb/238U dates can be mitigated by more careful tracer-sample equilibration; and (iii) natural zircon reference materials are not suitable for evaluating intra-laboratory repeatability and inter-laboratory reproducibility, since they combine several phenomena of natural system complexities (especially domains of different age within the same zircon grain, and residual loss of radiogenic lead in domains of high decay damage after chemical abrasion pre-treatment). We provide our best estimates of apparent dates for the ET100 solution (206Pb/238U date, 100.173 ± 0.003 Ma), for ET2000 solution (207Pb/206Pb date, 1999.935 ± 0.063 Ma), as well as for natural reference zircon Temora-2 (206Pb/238U date, 417.353 ± 0.052 Ma). These data will allow U-Pb laboratories to evaluate their analytical performance and to independently calibrate non-EARTHTIME tracer solutions in use.