Pb and U Elemental and Isotopic Fractionation in Radiation-Damaged Zircon: Implications for Low-Temperature Discordance Mechanisms

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EAR-0073602MattinsonThis proposal requests support for investigation of a newly recognized behavior of Pb and U isotopes in the mineral zircon. The U-Pb system in zircon is widely used to determine the ages of geologic events, and fully understanding the system's behavior is required for accurate interpretation of measured ages. Loss of Pb from zircons produces "discordant" ages which can be accurately extrapolated to the "true" age only if the Pb loss mechanism is fully understood. Recent experiments reveal that loss of Pb related to low-temperature fluid leaching of naturally radiation-damaged zircon can be accompanied by isotopic fractionation of the Pb and also U isotopes. The isotopic fractionation of Pb is contrary to previously accepted ideas about Pb loss from zircons, and calls into question some important assumptions used in interpreting U-Pb age data. Understanding of this fractionation and Pb loss behavior is important in obtaining accurate ages from discordant zircon U-Pb data. The proposed research will combine zircon analysis by a series of partial dissolution steps, with annealing of the zircons at a range of temperatures prior to dissolution. The annealing partially to totally repairs the natural radiation damage, allowing detailed understanding of the exact role of the radiation damage in the loss of Pb from the system. Detailed study (electron microprobe and scanning electron microscope) of the chemical zoning within individual zircon grains, and of the possible pathways by which fluids gain access to the interiors of zircon grains, will help us better understand interactions of fluids with zircons on the intra-grain level, and the role of these fluids in U-Pb discordance.