U–Pb ID-TIMS dating of U-rich accessory minerals using a Phoenix-ATONA and 202Pb–205Pb–233U–236U tracer

Using zircon reference material TEMORA 2 and synthetic EARTHTIME age solutions (ET100, ET500 and ET2000) analysed at the new geochronology laboratory at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS), using a ²⁰²Pb–²⁰⁵Pb–²³³U–²³⁶U spike, we evaluate analytical precision and inter-laboratory reproducibility of ²⁰⁶Pb/²³⁸U dates as precision approaches the 0.01% level. Single analyses of samples containing ∼30–150 pg of radiogenic Pb routinely achieve a ²⁰⁶Pb/²³⁸U date precision of 0.02–0.05%, comparable to results from leading U–Pb geochronology labs in the EARTHTIME consortium. In contrast, comparisons between the absolute values of ²⁰⁶Pb/²³⁸U dates of reference materials determined at GIGCAS and those reported by EARTHTIME labs reveal significant inconsistencies, up to ca. 0.14%, which vary among reference materials. These inconsistencies are most plausibly attributed to long-term drift in solution U/Pb ratios caused by differential adsorption of Pb and/or U on storage container walls. We discuss the possible ways of overcoming this problem and the prospect of improving spike calibration, which are the necessary steps towards precision and accuracy of ²⁰⁶Pb/²³⁸U dates at the 0.01% level.