Microstructure of Neoarchean zircon from the Acasta gneiss complex, Northwest Territories; Radiogenic age and isotopic studies: Report 13

Abstract

The microstructure of zircon from a late Archean granite dyke from the Acasta gneiss complex was investigated using a number of microprobe techniques. Zircons from the study sample are composed of low-Ca, unaltered and high-Ca, altered domains. Transmission electron microscopy indicates that the unaltered areas have a distorted crystalline microstructure. The detailed microstructure of altered zircon is heterogeneous, composed of a mixture of low-Ca, crystalline domains and amorphous, high-Ca domains. Calculations show that radiation damage accumulated from the decay of U and Th over the entire life of the sample would have rendered the sample metamict. We conclude that the observed crystalline structure is the result of recrystallization late in the geological history of the sample. Sensitive high-resolution ion microprobe U-Pb ages from the low Ca, crystalline zircon are concordant, suggesting that Pb loss did not accompany the recrystallization process.