Abstract

Zircon is a key mineral in geochronology because of its chemical and physical durability and tendency to incorporate radioactive trace elements such as U and Th. Quantifying the partitioning of the actinide elements is critical to constrain initial non-secular equilibrium amounts of 230 Th in zircon. An excess or deficit of 206 Pb will be produced from such an initial excess/deficit of 230 Th from the secular equilibrium condition, which influences the calculated 206 Pb/ 238 U age (Schärer, 1984; Mattinson, 1973). However, there is no standard way to calculate Th/U partitioning ratios when applying age corrections to young igneous systems, making uncertainties hard to estimate. To address this problem, zircon was synthesized in oneatmosphere experiments using basaltic andesite, andesite, and rhyolite starting materials, doped with Zr, U, and Th. Different experimental temperatures and oxygen fugacity conditions (ΔQFM-4 to ΔQFM+4) were explored to examine non-equilibrium U and Th partitioning. U and Th concentrations in zircon crystals and coexisting melt were analyzed through EPMA. In our study, we specifically quantify the effects of sector zoning, fractional crystallization, oxygen fugacity, melt composition, and temperature on actinide element partitioning. By combing experimental and natural zircon data, we find temperature has the primary control on the partitioning of U and Th in the zircon and there is a negative relationship between partition coefficient of actinide elements and zircon crystallization temperatures. The calibrated equation can be directly applied to the 230 Th correction for an improvement in the accuracy of Thcorrected 206 Pb/ 238 U dates using estimates of the zircon crystallization temperatures.