The microstructure of zircon and its influence on the age determination from Pb/U isotopic ratios measured by ion microprobe

Abstract

TEM observations of a large, high-U, Sri Lankan, gem-gravel zircon suggest that certain microstructural features are associated with discordant Pb U ages measured in this crystal with the SHRIMP I ion microprobe. Some of the growth bands in the rim of the crystal are optically isotropic and the microstructure of one such band (ca 100 μm wide; designated I) has been compared with the microstructure of the adjacent optically anisotropic regions (designated A). The I-band has a significantly higher U and Pb content, and its electron diffraction pattern of very diffuse rings indicates the absence of periodic atomic arrangements. A-regions consist of parallel-oriented zircon crystallites (ca 10 nm in size) in a matrix with the same characteristics as the I-band. The volume fraction of crystalline zircon in any given growth band correlates (inversely) with the measured birefringence and is taken to be an indication of the degree of radiation damage. Annealing in air at temperatures T < 900° C has little effect on the microstructures, but at T = 900° C the diffraction rings become relatively sharp and can be indexed unequivocally as zirconia which is present as randomly oriented crystallites (ca 10 nm in size), presumably in association with silica glass, in both the I-band and A-regions. However, there is no significant growth of the zircon crystallites in A-regions. After annealing at 1250°C, the zirconia crystallites in the I-band transform to baddeleyite. The crystallites are ca 100 nm in size and the silica glass phase is easily identified. However, the A-regions have recrystallized to a zircon single crystal with precipitates of baddeleyite (ca 40 nm in size) and, presumably, some silica glass. The 207Pb 206Pb ages determined with the ion microprobe are essentially independent of the microstructure and average 552 Ma. However, the Pb U ages are all reverse discordant (that is, greater than 552 Ma). The discordance (defined as the measured value of 206Pb 238U divided by the value expected at 552 Ma) is about 1.1 in most specimens annealed up to 900°C, but as high as 3.5 in the I-band after annealing at 1250°C. It is suggested that Pb tends to concentrate in the silica glass phase which then sputters preferentially during ion bombardment in the ion microprobe, leading to an apparent excess of radiogenic Pb and hence to reverse discordance.