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 0016-7037/05 $30.00 .00