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.