Zircon U–Pb and Pb-isotope fractionation during stepwise HF acid leaching and geochronological implications

Two homogeneous zircons, zircon 91500 (Kuehl Lake, Ontario) and Phalaborwa zircon (South Africa), were analysed to study U and Pb behaviour during stepwise HF acid leaching. From the Phalaborwa zircon, ∼11% of radiogenic Pb and ∼7% of U were extracted into solution at the first leaching step and a reverse discordance was obtained. After extraction of ∼27.0% of the total radiogenic Pb budget and ∼24.5% of the U, the zircon residue gave concordant ages. It contained ∼75.8% of total radiogenic 206Pb budget, ∼76.4% of total radiogenic 207Pb budget, and ∼68.0% of total radiogenic 208Pb budget, indicating different mobility of Pb isotopes. The 208Pb*/206Pb* ratios of the solutions and the residue decrease systematically from ∼1.10 to ∼0.60. This pattern probably demonstrates that domains with different Th/U ratios were successively sampled during acid attack. In contrast to Phalaborwa zircon, only ∼1% of the total radiogenic Pb budget was extracted in leaching steps from each of three zircon 91500 fragments. Highly reverse discordant ages are observed in the first leaching step of the Phalaborwa zircon. This phenomenon is consistent with the recent studies of Mattinson [Partial dissolution analysis (PDF) of zircon: past, present, future. 11th Annual V. M. Goldschmidt Conference, Abstract volume, 3368.pdf. (2001)] and Davis and Krogh [Chem. Geol. 172 (2000) 41]. Due to the fractionations between U and Pb and between radiogenic Pb isotopes observed in the first leaching step, discordant domains or zonings of zircons have to be completely removed in the leaching steps to avoid any artificial high 207Pb*/206Pb* ratio in the residues. Evaluation of data from leaching studies shows that the extractability of Pb is positively correlated with the total radiogenic Pb budgets in zircons and subsequently with U contents and ages of zircons, indicating an important role of radiation damage. This correlation is consistent with the fact that old zircons with high U contents are much more susceptible to Pb loss than younger ones with low U contents. [Copyright &y& Elsevier]