Comment on gchron-2022-19

Abstract

Chemical abrasion is a technique that combines laboratory 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. We find that most grains do not dissolve predominantly from rim-to-core. Acid frequently reaches crystal interiors via fracture networks spatially associated with radiation damage zoning and inclusions to dissolve higher U zones, material in the vicinity of fractures, and some inclusions. 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 °C to 210 °C results in deeper etching textures, wider acid paths, more complex internal dissolution networks, and greater volume losses. We discuss the implications of our findings for zircon ID-TIMS U-Pb geochronology and paired trace element analyses, radiation damage annealing models, and for using µCT for imaging radiation damage zoning for (U-Th)/He thermochronology.