U–Pb ID-TIMS geochronology using ATONA amplifiers

Abstract

We document the performance of new ATONA (‘aA to nA’) amplifiers installed on an Isotopx Phoenix thermal ionisation mass spectrometer (TIMS) at Princeton University and evaluate their suitability for high-precision analyses of Pb and U isotopes in pg- to ng-size samples characteristic for U–Pb geochronology. The new amplifiers are characterised by low and stable noise levels comparable to 10¹² to 10¹³ ohm resistors, response time <0.5 s, exceptional gain stability <1 ppm and a vast dynamic range theoretically allowing to quantify signals from aA (10⁻¹⁸ A) to nA (10⁻⁹ A) level. We measured a set of Pb standards, synthetic U–Pb solutions and natural zircons at currents of 2 × 10⁻¹⁶ to 2 × 10⁻¹² A (corresponding to intensities of 20 μV to 200 mV relative to a 10¹¹ ohm amplifier) to assess the utility of ATONA in replacing ion counting for the smallest samples. The results show a clear precision benefit of using ATONA-Faraday detection over Daly ion counting for ion currents of >10⁻¹⁴ A (1 mV relative to a 10¹¹ ohm amplifier or ca. 60 kcps). As such currents are routinely achievable for major Pb peaks of interest (^205–208Pb) in natural samples containing more than ca. 10 pg Pb* (radiogenic Pb), we expect ATONA-Faraday detection to find broad applications in U–Pb geochronology. Its practical use for low-blank, radiogenic samples continues to require ion counting for ²⁰⁴Pb, either with a fixed Faraday–ion counter gain or using a dynamic two-step (e.g. FaraDaly) method. Routine adoption of ATONA-Faraday collection in place of ion counting for most major Pb and U isotopes has the potential to increase sample throughput and precision, both improving the accessibility of isotope dilution (ID)-TIMS geochronology and pushing this technique towards better reproducibility.