ATTEMPTS TO CALIBRATE CHEMICAL ABRASION: EFFECTS ON ZIRCON CRYSTAL STRUCTURE, CHEMICAL COMPOSITION, AND U/PB AGE

Reliable and meaningful U/Pb dates require the control of factors that cause age dispersion within a zircon population. However, especially the recognition and mitigation of post-crystallization loss of radiogenic lead (Pb*) is a still an unsolved problem in high-precision U/Pb-CA-ID-TIMS dating. The loss of radiogenic lead preferentially occurs along altered zones of the crystal lattice as a consequence of the structural damage produced by Uranium decay that results in too young ages. Yet, the rejection of young ages from a zircon population due to suspected Pb-loss can be difficult to justify since it is hard to prove that Pb-loss is the cause of the young age. The introduction of procedures such as chemical abrasion (CA) on single zircon grains has significantly reduced the impact of Pb-loss. However, there is no guarantee that Pb-loss can be completely removed from a zircon. Unfortunately, the CA technique is applied by different laboratories using slightly different procedures making comparisons between laboratories more difficult. Furthermore, this method is mostly empirical and used without a detailed understanding of how (1) the applied annealing temperature and (2) the temperature and duration of partial dissolution affect more or less decay-damaged zones.

We will present an experimental approach to quantify the effects of chemical abrasion on the zircon chemistry and its crystal structure. For this purpose, we have chosen the natural reference zircon Plešovice, due to its known variation in trace element concentrations and the presence of domains rich in actinides. Plešovice grain fragments were annealed for 48h at 900°C. Eight aliquots were separated and attacked in concentrated hydrofluoric acid at 180 °C and 210°C for 6h, 12h, 18h, and 24h. We performed Raman spectroscopy, EMPA, CL imaging and LA-ICP-MS trace element analysis on each of these fractions. The zircon treated by CA are compared to untreated and only annealed zircons.

We will propose improved protocols for the chemical pre-treatment of zircon based on the state of the crystal structure and chemistry. We further demonstrate that Plešovice zircon cannot be considered as a homogenous standard at the current level of precision achieved by CA-ID-TIMS dating due to natural age variation on a sub-million-year scale.