|Title:||Decoupling of U‐Pb dates from chemical and crystallographic domains in granulite facies zircon|
|Authors:||R.M. Flowers, A.K. Schmitt, and M. Grove|
|Publication:||Chemical Geology, v. 270, p. 20‐30.|
High spatial resolution (12 µm spot size) and high density (up to 22 spots per crystal) U‐Pb ion probe data were acquired for zircon crystals from a mafic granulite in the Canadian shield. The zircon grains are metamorphic in origin and were subjected to a subsequent high‐pressure granulite facies event. 207Pb/206Pb dates span ≥ 650 Ma, and can be used to define discrete intracrystalline domains of dates by contouring the age results. The boundaries of these domains cut at high angles across compositional zonation revealed by cathodoluminescence, backscatter, Hf and U chemical mapping, and electron backscatter diffraction misorientation data. The results indicate decoupling of radiogenic Pb behavior from other trace elements in the zircon crystals. Previously published U‐Pb ID‐TIMS data define a highly linear discordia array with an upper intercept at 2554.5 ± 4.3 Ma associated with initial zircon growth during granulite facies metamorphism, and a lower intercept at 1896.0 ± 18.0 Ma associated with a second granulite facies metamorphic event that caused U‐Pb discordance. The intracrystalline patterns of ion probe dates are incompatible with multiple zircon growth events, and are inconsistent with reported examples of zircon recrystallization in which resetting of the U‐Pb systematics was accompanied by modification of zircon chemistry and CL patterns. Grains lack discernible core‐rim variations expected for diffusion profiles, and diagnostic indicators of low temperature radiation damage‐enhanced Pb loss are absent. Rather, the data appear to require heterogeneous Pb loss from non‐metamict metamorphic zircon during high‐temperature metamorphism, implying that under certain circumstances other intracrystalline fast pathways facilitate Pb loss in metamorphic zircon crystals. This result is incompatible with conventional thinking regarding Pb mobility in zircon, and indicates that important aspects of Pb behavior in zircon are not yet fully understood.