Implications of discordant U–Pb ages on Hf isotope studies of detrital zircons

Abstract

Zircon is the ideal tool for unraveling Earth's history because of its refractory nature, being readily datable by the U–Pb isotope system, and owing to the fact that its Hf isotope composition can be precisely determined. However, most analyzed zircons have experienced Pb-loss to various degrees, resulting in present-day measured 207Pb/206Pb ages being younger than that of the time of crystallization, if the loss is ancient. This is of particular importance for ancient zircons from the Archean and the Hadean, notably the Jack Hills zircons. A zircon Lu–Hf and U–Pb isotope evolution model has been developed and shows that Pb-loss at 3700Ma affecting a simple zircon population that crystallized at 4350Ma can reproduce most of the broad εHf versus 207Pb/206Pb age trend observed for Jack Hills Hadean zircons. In addition, the model demonstrates that crystals having experienced widely different degrees of Pb-loss (2–80%) appear only slightly discordant (0.1–2.2%), while their apparent 207Pb/206Pb ages are different from that of crystallization by up to 466My. This is insignificant for calculating initial Hf isotope compositions but results in major shifts in initial εHf values by up to 11.3 units. This effect is particularly relevant to the global Hadean/Archean detrital zircon record for which several apparent εHf versus age trends, similar to that defined by the Jack Hills Hadean zircons, stand out and could be fully or partly artificial. To overcome, or at least significantly reduce, these issues, multiple U–Pb analyses should be undertaken for each detrital zircon. Simultaneous measurement of Hf and U–Pb isotopes by the split-stream technique is the ideal approach because it can provide information about the cause of discordance (Pb-loss or concurrent analysis of two unrelated growth zones) and thereby allow for the most robust and accurate Hf isotope data to be obtained.