05 Jul 2023
 | 05 Jul 2023
Status: a revised version of this preprint is currently under review for the journal GChron.

Geochronological and Geochemical Effects of Zircon Chemical Abrasion: Insights from Single-Crystal Stepwise Dissolution Experiments

Alyssa J. McKanna, Blair Schoene, and Dawid Szymanowski

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 affects zircon U-Pb and trace element systematics in a series of 4-hour, single-crystal stepwise dissolution experiments at 180 °C and 210 °C. Microtextural data for the zircon samples studied is presented in a complementary paper by McKanna et al. (2023). We find that stepwise dissolution at 210 °C is more effective at eliminating U, common Pb (Pbc), and light rare earth element (LREE) enriched material affected by open system behavior; reduces the presence of leaching-induced artefacts 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-hour leaching step. To better understand the causes of Pb-loss in zircon, we calculate time-integrated alpha dose estimates for leachates and residues from measured radionuclide concentrations to determine: 1) the alpha dose of the material dissolved at the two leaching conditions, and 2) the apparent minimum alpha dose required for Pb loss to occur: ≥ 6 × 1017 α/g. We conclude that a single 8 h leaching step at 210 °C should yield crystallization ages in the majority of zircon and that this can be used as an effective approach for routine analysis. However, Ultimately, the effectiveness of any chemical abrasion protocol will be sample-dependent. By framing Pb loss and zircon solubility in terms of alpha dose, however, workers can better tailor the chemical abrasion process to specific zircon samples to improve the accuracy and precision of U-Pb results.

Alyssa J. McKanna et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gchron-2023-18', Anonymous Referee #1, 07 Aug 2023
  • RC2: 'Comment on gchron-2023-18', Fernando Corfu, 04 Sep 2023

Alyssa J. McKanna et al.

Alyssa J. McKanna et al.


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Short summary
Acid leaching is used to remove radiation-damaged portions of zircon crystals prior to U-Pb dating to improve the accuracy of datasets. We test how the temperature and duration of acid leaching affects geochronological and geochemical outcomes. We build a framework that relates radiation damage, zircon solubility, and Pb loss that will ultimately help researchers to tailor their acid leaching approach to different zircon samples.