Pb isotope analysis on Phoenix-ATONA: assessment and minimisation of uncertainties

Abstract

We present the procedures for isotope analyses of Pb samples spiked with ²⁰²Pb-²⁰⁵Pb using the Phoenix-ATONA thermal ionisation mass spectrometer, and the ways of their optimisation for precision, accuracy and efficiency. We evaluate the stability of amplifier gains and baselines, Faraday cup efficiency and its influence on the accuracy of Pb-isotope ratios, and stability of Daly gains. We also explore several other issues related to Pb isotope analysis by TIMS, including the effect of radioactive decay on the isotopic composition of the spike, the relative performance and long-term stability of different silicagel emitters, and the patterns of isotope fractionation during evaporation of Pb from molten silica. We demonstrate that the two-step multicollector procedure where the data for all Pb isotopes are acquired in the main 30s step with ²⁰⁴Pb in Daly, and the Daly gain is calibrated in real time using the second 4 s step with ²⁰⁵Pb in Daly is suitable for a wide range of samples in terrestrial and extraterrestrial U-Pb geochronology. The achieved performance improvement, mainly due to less noisy and more stable amplifier baselines and the optimised way of their measurement is confirmed by analyses of SRM 981, SRM 982 and EarlyTime 1x Pb-isotope reference materials, and by U-Pb dating of minerals from plutonic angrite NWA 4590.