ID-TIMS U–Pb geochronology at the 0.1‰ level using 1013 Ω resistors and simultaneous U and 18O/16O isotope ratio determination for accurate UO2 interference correction

Abstract

We document recent advances in analytical protocols that employ 10¹³ Ω resistors in the Faraday cup amplifier feedback loop for high-precision U–Pb geochronology by isotope dilution thermal ionisation mass spectrometry (ID-TIMS). We present a direct comparison of high-precision U–Pb data obtained using different detector systems, including dynamic peak hopping on a secondary electron multiplier and static multicollection routines that use 10¹³ Ω resistors in the amplifier feedback loop. The high-ohmic resistors also allow the precise and accurate measurement of the minor ²⁷²(UO₂) isotopologues (mostly ²³⁸U¹⁸O¹⁶O) in a static multicollection routine, which permits precise and accurate determination of the ¹⁸O/¹⁶O ratio of UO₂ molecules during the U-isotope ratio measurement without compromising signal intensity on the more abundant UO₂ isotopologues. This enables the precise determination of U abundances in zircons by isotope dilution with within-run correction of isobaric interferences from the minor UO₂ isotopologues, thereby eliminating one of the major sources of uncertainty in ultra-high-precision U–Pb data sets. This approach permits the determination of single U–Pb dates with uncertainties <0.2‰ and corresponding weighted mean dates with uncertainties <0.1‰ (n ∼ 6 to 10). Furthermore, the ability to use different combinations of detector setups for the same analysis within the same mass spectrometer allows for direct comparison of independently calibrated detector systems.