High-precision zircon U/Pb geochronology by ID-TIMS using new 1013 ohm resistors

Abstract

Accessory mineral U–Pb geochronology by isotope dilution thermal ionization mass spectrometry (ID-TIMS) requires precise and accurate determinations of parent–daughter isotope ratios. The small sample size, particularly with respect to radiogenic Pb (Pb*), requires highly sensitive ion detection systems. Most studies therefore employ either secondary electron multipliers (SEMs) or Daly photomultipliers that provide low background noise and high sensitivity but have a limited linear range and require dynamic peak-hopping. We here evaluate the application of new 10¹³ ohm resistors in a Faraday cup amplifier feedback loop for the static collection of all Pb isotopes (sample and tracer) with ^{202,205,206,207,208}Pb measured on Faraday cups and ²⁰⁴Pb measured in the axial SEM of a Thermo Scientific™ TRITON™ Plus TIMS instrument. We demonstrate long-term stability of the amplifier gain calibration using a secondary Nd standard and test short- and long-term stability and reproducibility of amplifier baselines. Accurate calibration of static detector arrays is demonstrated by repeated analyses of synthetic and natural U–Pb standards (ET100, Temora-2 and AUS_Z7_5) with variable Pb* (0.551 to 699 pg) and comparison with conventional dynamic ion counting data. Excellent agreement between the two detector systems for all analysed standards suggests that our static measurement routine with 10¹³ ohm resistors produces accurate and precise U–Pb isotopic data with superior external reproducibility. We anticipate that this new technique will push the frontiers of high-precision U–Pb geochronology and may represent a crucial advancement in the quest towards inter- and intra-laboratory reproducibility at the 0.01% level.