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

该提案要求支持对矿物锆石中Pb和U同位素的新认识行为的研究。锆石中的U-Pb系统被广泛用于确定地质事件的年龄，而充分了解该系统的行为是准确解释测得的年龄所必需的。从锆石的铅损失产生的“不一致”的年龄，可以准确地外推到“真正的”年龄，只有当铅的损失机制是完全理解。最近的实验表明，自然辐射损伤的锆石的低温流体浸出铅的损失可以伴随着铅和U同位素的同位素分馏。铅的同位素分馏与以前公认的锆石铅损失的观点相反，并对解释U-Pb年龄数据时使用的一些重要假设提出了质疑。了解这种分馏和铅损失行为是重要的，在获得准确的年龄从不一致的锆石U-Pb数据。拟议的研究将结合联合收割机锆石分析的一系列部分溶解步骤，退火的锆石在溶解前的温度范围。退火部分到完全修复了自然辐射损伤，从而详细了解了辐射损伤在系统中Pb损失中的确切作用。详细的研究（电子探针和扫描电子显微镜）的化学分区内的个别锆石晶粒，和可能的途径，使流体获得进入内部的锆石晶粒，将有助于我们更好地了解相互作用的流体与锆石的晶粒内的水平，和这些流体的作用，在U-Pb不一致。

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.