Geochronological and geochemical effects of zircon chemical abrasion: insights from single-crystal stepwise dissolution experiments

Abstract

Abstract. Chemical abrasion in hydrofluoric acid (HF) is routinely applied to zircon grains prior to U–Pb dating by isotope dilution thermal ionization mass spectrometry (ID-TIMS) to remove radiation-damaged portions of grains affected by Pb loss. Still, many chemically abraded datasets exhibit evidence of residual Pb loss. Here we test how the temperature and duration of chemical abrasion affect zircon U–Pb and trace element systematics in a series of 4 h, single-crystal stepwise dissolution experiments at 180 and 210 ∘C. Microtextural data for the zircon samples studied are presented in a companion paper by McKanna et al. (2023). We find that stepwise dissolution at 210 ∘C is more effective at eliminating material affected by open-system behavior and enriched in U, common Pb (Pbc), and light rare earth elements (LREEs); reduces the presence of leaching-induced artifacts that manifest as reverse discordance; and produces more consistent and concordant results in zircon from the three rocks studied. We estimate that stepwise dissolution in three 4 h steps is roughly equivalent to a single ∼ 8 h leaching step due to the insulating properties of the PTFE sleeve in the Parr pressure dissolution vessel, whereas traditionally labs utilize a single 12 h leaching step. We conclude that a single 8 h leaching step at 210 ∘C should remove Pb loss effects in the majority of zircon and that this can be used as an effective approach for routine analysis. Further, we calculate time-integrated alpha doses for leachates and residues from measured radionuclide concentrations to investigate (1) the alpha dose of the material dissolved under the two leaching conditions and (2) the apparent minimum alpha dose required for Pb loss susceptibility: ≥ 6×1017 α g−1.