Chemical abrasion: the mechanics of zircon dissolution

Abstract

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.

Effect of chemical abrasion of zircon on SHRIMP U/Pb, δ18O, Trace element, and LA-ICPMS trace element and Lu-Hf isotopic analyses

“…In other words, the dissolution of discordant zircon visible in the form of dissolved zones and channels is not the only change to the zircon; the remaining visually intact material also undergoes a subtle change in U-Pb ratio as a result of chemical abrasion. McKanna et al (2023a) show that chemical abrasion removes material from the mm 540 scale all the way down to the submicron resolution of their SEM images, which is consistent with our SHRIMP U-Pb data.…”

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

“…Rims are enriched in actinides and radiation damage relative to cores. Raman data suggest that grains have accumulated intermediate-tohigh radiation damage densities with equivalent alpha doses ranging from 6´10 17 to 2´10 18 α/g (McKanna et al, 2023). There is no previous U-Pb geochronology for zircon from this sample, however, Pb loss is common in similarly aged zircon from the Pilbara craton (MacLennan, 2019).…”

“…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.…”

“…Zircon grains separated from SAM-47 are euhedral, brown, and translucent. Crystals display fine-scale concentric growth zones (McKanna et al, 2023). Rims are enriched in actinides and radiation damage relative to cores.…”

“…Many studies have now shown that chemically abraded zircon samples often exhibit residual Pb-loss. This challenge is widely recognized in the ID-TIMS U-Pb community and has prompted new investigations into the effects of different annealing and leaching conditions on geochronological outcomes (Huyskens et al, 2016;Widmann et al, 2019), new statistical approaches for evaluating overdispersed U-Pb datasets (Keller, 2023), and microstructural studies of chemically abraded zircon (McKanna et al, 2023).…”