Pb/Pb And U/Pb Dating By NanoSIMS With The Radio-Frequency Ion Source Using 16O2- As The Primary Species

Abstract

: NanoSIMS features a capability of high spatial resolution, which allows for Pb/Pb and U/Pb dating at the submicron scale, using a recently established radio-frequency (RF) oxygen ion source. However, the secondary ion yield of Pb + is relatively low when using 16 O - as the primary beam species compared to that obtained using large-geometry secondary ion mass spectrometry (LG-SIMS), which limits the precision of Pb/Pb and U/Pb dating. In this study, using 16 O 2- as the primary species with an RF oxygen source on a CAMEA NanoSIMS 50L ion probe, we acquired Pb sensitivity in zircon of approximately 12 cps/nA/ppm. Analyses of the zircon reference materials M257 and OG1 resulted in Pb isotopic compositions consistent with their reference values and Pb/Pb age with an uncertainty of ~3 % for the target region of interest (ROI) smaller than an area of 2 μm × 2 μm. Pb/U age with an uncertainty < 2.2% was acquired via Pb/U correction based on the Pb/U vs. UO 2 /U power relationship or the Pb/UO vs. UO 2 /UO one. The adoption of 16 O 2- as the primary species for the RF ion source is as a possible approach for accurate U/Pb age determination. It scale, with low precious extraterrestrial