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, similar to11% of radiogenic Ph and similar to7% of U were extracted into solution at the first leaching step and a reverse discordance was obtained. After extraction of similar to27.0% of the total radiogenic Ph budget and similar to24.5% of the U, the zircon residue gave concordant ages. It contained similar to75.8% of total radiogenic Ph-206 budget, similar to76.4% of total radiogenic Pb-207 budget, and similar to68.0% of total radiogenic Pb-208 budget, indicating different mobility of Pb isotopes. The Pb-208*/Pb-206* ratios of the solutions and the residue decrease systematically fiom similar to1.10 to similar to0.60. This pattern probably demonstrates that domains with different Th/U ratios were successively sampled during acid attack. In contrast to Phalaborwa zircon, only similar to1% of the total radiogenic Ph 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) 4 1]. Due to the fractionations between U and Ph and between radiogenic Ph 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 Pb-207*/Pb-206* ratio in the residues. Evaluation of data from leaching studies shows that the extractability of Ph is positively correlated with the total radiogenic Ph 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. (C) 2002 Elsevier Science B.V. All rights reserved.