HR: 0800h
AN: V51C-0587    [Abstracts]
TI: Oxygen Isotopic Composition and U-Pb Discordance in Zircon
AU: * Booth, A L
EM: mbooth@pangea.stanford.edu
AF: Stanford University, Department of Geological & Environmental Sciences, Stanford, CA 94305
AU: Kolodny, Y
EM: kolodny@vms.huji.ac.il kolodny@vms.hujl.ac.il
AF: Hebrew University, Institute of Earth Sciences, Jerusalem, 91904 Israel
AB: Zircon discordance is a common phenomenon in U-Pb geochronology, leading to difficulty in age assignment and uncertainties regarding geologic interpretation. Commonly invoked explanations for discordant zircon analyses include episodic lead loss and continuous lead diffusion. A likely mechanism for lead loss is via a fluid phase. In this study, we attempt to document fluid interaction with zircon through the use of high-spatial resolution oxygen isotopic measurements performed by ion microprobe. $\delta$$^{18}$O was analyzed in both concordant and discordant zircon grains, thus providing an indication of the relationship between discordance and $\delta$$^{18}$O. Results indicate that three characteristics of zircon appear to be interrelated: (1) U-Pb systematics and the associated age discordance; (2) $\delta$$^{18}$O and water-rock interactions that are implied therein; and (3) zircon texture as revealed by cathodoluminescence and BSE imaging. The key observation is that U-Pb disturbed zircons are often also depleted to various degrees in $^{18}$O. However, the relationship between discordance and $\delta$$^{18}$O is not systematic, as $\delta$$^{18}$O values of discordant zircons are lower, but irregular in their distribution. Textural differences between zircon grains also correlate with both U-Pb discordance and $\delta$$^{18}$O. Discordant grains exhibit either a recrystallized, fractured or strongly-zoned CL texture, possibly resulting from metamictization, and are characteristic of a lowered $\delta$$^{18}$O value. Concordant grains, in contrast, have less-expressed zoning and a smoother CL texture, and exhibit higher $\delta$$^{18}$O values. We interpret this to mean that various stages of water rock interaction, as evidenced by $\delta$$^{18}$O analyses, have the ability to leave their imprint on both the texture and U-Pb systematics of a zircon.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting