Oxygen Isotopic Composition and U-Pb Discordance in Zircon

We have investigated U-Pb discordance and oxygen isotopic composition of zircon using high– spatial resolution !O measurement by ion microprobe. !O in both concordant and discordant zircon grains provides an indication of the relationship between fluid interaction and discordance. Our results suggest that three characteristics of zircon are interrelated: (1) U-Pb systematics and concomitant age discordance, (2) !O and the water-rock interactions implied therein, and (3) zircon texture, as revealed by cathodoluminescence and BSE imaging. A key observation is that U-Pb–disturbed zircons are often also variably depleted in O, but the relationship between discordance and !O is not systematic. !O values of discordant zircons are generally lighter but irregular in their distribution. Textural differences between zircon grains can be correlated with both U-Pb discordance and !O. Discordant grains exhibit either a recrystallized, fractured, or strongly zoned CL texture, and are characteristic of O depletion. We interpret this to be a result of metamictization, leading to destruction of the zircon lattice and an increased susceptibility to lead loss. Conversely, grains that are concordant have less-expressed zoning and a smoother CL texture and are enriched in O. From this it is apparent that various stages of water-rock interaction, as evidenced by systematic variations in !O, leave their imprint on both the texture and U-Pb systematics of zircon. Copyright © 2005 Elsevier Ltd