Elemental distribution in zircon: Alteration and radiation-damage effects

Abstract

Isotopic analyses of U, Pb and REE in 144 zircon grains from the Bidoudouma stream, the Republic of Gabon, were performed to study correlation of elemental distribution with geological alteration effects. The U–Pb isotopic results reveal that the zircons formed around 2.8–2.1 Ga ago and altered around 500 Ma ago by igneous activity in association with dolerite dyke intrusion. The REE abundance patterns of the U–Pb discordant zircons are characterized by high contents of REE (977–98,154 ppm), small LREE–HREE fractionations and distinctly positive Eu anomalies. The discordant zircons also contain significant amounts of non-formula elements such as Ca, Mn, Al and Fe; whereas, their contents of Zr and Si are 1–15% lower than those of concordant zircon. Non-formula elements and REE, especially LREE, were incorporated into metamict zircon in association with Pb-loss, which indicates that the non-formula elements and REE substituted for Zr and Si. Judging from this correlation among the contents of non-formula elements, when zircon was initially altered, Fe substituted for major elements of zircon. After the saturation of Fe, Ca and Mn were incorporated into zircon together with REE and Al followed by Ca. The metamict zircons contain large amount of U (∼8215 ppm), which indicates that U had not migrated from zircon grain in association with the Pb-loss.