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

Abstract

Using zircon reference material TEMORA 2 and synthetic EARTHTIME age solutions (ET100, ET500 and ET2000) analysed at the new geochronology laboratory at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS) using 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 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 containers 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.