Geochemical Journal, Vol. 52 (No. 3), pp. 281-286, 2018
Department of Chemistry, Faculty of Science, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo 171-8588, Japan
(Received July 26, 2017; Accepted October 29, 2017)
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.
Key words: zircon U-Pb dating, common Pb correction, initial disequilibria, Quaternary zircon, melt disequilibria