Evaluation of temperature-time conditions for the chemical abrasion treatment of single zircons for U-Pb geochronology

- Optimal conditions for chemical abrasion of single zircons are annealing at 900 °C for 48 h and leaching at 190 °C for 15 h.
- Leaching at a lower temperature does not completely remove Pb loss.
- Annealing at higher temperature might restore crystallinity in zones that were affected by Pb loss.

Partial loss of radiogenic Pb and presence of older grains or cores (inheritance) are the main obstacles for accurate and precise U-Pb geochronology of magmatic zircons by ID-TIMS (isotope dilution thermal ionisation mass spectrometry). Sequential annealing and leaching (chemical abrasion, (Mattinson 2005)), restores crystallinity to zircons with low to medium radiation damage, and removes parts of the zircon with strongly radiation damaged crystal lattice, effectively eliminating Pb loss. The original version of chemical abrasion (Mattinson 2005) was designed for determination of precise 206Pb-238U plateau ages of multi-grain zircon populations, free from inheritance, using multiple dissolution steps. However, the main application of chemical abrasion is to date zircon populations potentially affected by both Pb loss and inheritance, which is commonly achieved by analysis of single zircon grains. Chemical abrasion of single zircon grains involves only one high temperature leaching step, since stepwise leaching is rarely possible or practical due to the small sample size. Despite the widespread use of single zircon chemical abrasion, the optimal temperature and duration of annealing and single step leaching have not been reported thus far. Furthermore, the temperature and time durations of chemical abrasion (apparently an ad-hoc adaptation of the findings of multi-grain, multi-step leaching experiments) vary substantially between published geochronological studies, leaving the possibility of incomplete removal of Pb loss and hence a potential bias in comparison of the ages. Here we report the results of a systematic study aimed at determination of optimal conditions for chemical abrasion of single zircon grains to ensure complete removal of zones that were affected by Pb loss in a single high temperature leaching step. The tests were performed on three different zircon standards with various ages (~ 418 to 3465 Ma) using a three step partial dissolution technique. The optimal conditions for low to moderately radiation damaged zircons are annealing at 900 °C for 48 h and subsequent leaching at 190 °C or slightly higher in concentrated HF in a pressurized vessel for 15 h. Leaching at a lower temperature does not completely remove Pb loss. Annealing at higher temperature might be restoring crystallinity in the parts of zircon crystals that were affected by Pb loss.