A practical method for calculating the U-Pb age of Quaternary zircon: Correction for common Pb and initial disequilibria

Abstract

To obtain accurate crystallization ages from Quaternary zircon, it is necessary to consider the contributions of common Pb and initial disequilibria caused by Th/U and Pa/U fractionation in the zircon-melt system. The disequilibrium in the melt also has influence on the resulting age. This study presents a method for calculating corrected U-Pb ages of Quaternary zircon. In the proposed method, initial disequilibria and common Pb are comprehensively accounted for by a single correction based on a modified Tera-Wasserburg concordia diagram (modified 207Pb method). A bisection method is used to quantitatively evaluate the influence of all factors employed in the correction on the resulting U-Pb age and its uncertainties. If the contribution of Pa/U partitioning is ignored (i.e., when using the conventional 207Pb method), the systematic age error for a 100 ka zircon is ∼9.4%. Melt disequilibria on 230Th and 231Pa also have significant influence on the calculated age, when it is non-negligible (e.g., basaltic rocks). Consequently, we suggest that the dating of very young zircon (<100 ka) with the 207Pb method requires precise estimation of both Th/U and Pa/U partitioning in the zircon-melt system.