Oxygen isotopic composition and U-Pb discordance in zircon

Abstract

We have investigated U-Pb discordance and oxygen isotopic composition of zircon using high–spatial resolution δ 18O measurement by ion microprobe. δ 18O 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) δ 18O 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 18O, but the relationship between discordance and δ 18O is not systematic. δ 18O 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 δ 18O. Discordant grains exhibit either a recrystallized, fractured, or strongly zoned CL texture, and are characteristic of 18O 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 18O. From this it is apparent that various stages of water-rock interaction, as evidenced by systematic variations in δ 18O, leave their imprint on both the texture and U-Pb systematics of zircon.