Abstract. Chemical abrasion is a technique that combines thermal annealing and partial dissolution in hydrofluoric acid (HF) to selectively remove radiation-damaged portions of zircon crystals prior to U–Pb isotopic analysis, and it is applied ubiquitously to zircon prior to U–Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS). The mechanics of zircon dissolution in HF and the impact of different leaching conditions on the zircon structure, however, are poorly resolved. We present a microstructural investigation that integrates microscale X-ray computed tomography (µCT), scanning electron microscopy, and Raman spectroscopy to evaluate zircon dissolution in HF. We show that µCT is an effective tool for imaging metamictization and complex dissolution networks in three dimensions. Acid frequently reaches crystal interiors via fractures spatially associated with radiation damage zoning and inclusions to dissolve soluble high-U zones, some inclusions, and material around fractures, leaving behind a more crystalline zircon residue. Other acid paths to crystal cores include the dissolution of surface-reaching inclusions and the percolation of acid across zones with high defect densities. In highly crystalline samples dissolution is crystallographically controlled with dissolution proceeding almost exclusively along the c axis. Increasing the leaching temperature from 180 to 210 ∘C results in deeper etching textures, wider acid paths, more complex internal dissolution networks, and greater volume losses. How a grain dissolves strongly depends on its initial radiation damage content and defect distribution as well as the size and position of inclusions. As such, the effectiveness of any chemical abrasion protocol for ID-TIMS U–Pb geochronology is likely sample-dependent. We also briefly discuss the implications of our findings for deep-time (U-Th)/He thermochronology.

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.

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