THE MICROSTRUCTURE OF ZIRCON AND ITS INFLUENCE ON THE AGE-DETERMINATION FROM PB U ISOTOPIC-RATIOS MEASURED BY ION MICROPROBE/

TEM observations of a large, high-U, Sri Lankan, gem-gravel zircon sug gest that certain microstructural features are associated with discord ant Pb/U ages measured in this crystal with the SHRIMP I ion microprob e. Some of the growth bands in the rim of the crystal are optically is otropic and the microstructure of one such band (ca 100 mu m wide; des ignated I) has been compared with the microstructure of the adjacent o ptically anisotropic regions (designated A). The I-band has a signific antly higher U and Pb content, and its electron diffraction pattern of very diffuse rings indicates the absence of periodic atomic arrangeme nts. A-regions consist of parallel-oriented zircon crystallites (ca IO 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 cor relates (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 degrees C has little effect on the microstructure s, but at T = 900 degrees C the diffraction rings become relatively sh arp and can be indexed unequivocally as zirconia which is present as r andomly oriented crystallites (ca 10 nm in size), presumably in associ ation with silica glass, in both the I-band and A-regions. However, th ere is no significant growth of the zircon crystallites in A-regions. After annealing at 1250 degrees 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-region s have recrystallized to a zircon single crystal with precipitates of baddeleyite (ca 40 nm in size) and, presumably, some silica glass. The Pb-207/Pb-206 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 Pb-206/U-238 divided by the value expected at 552 Ma) is about 1.1 in most specimens annealed up to 900 degrees C, but as high as 3.5 in the I-band after annealing at 1250 degrees C. It is suggested that Pb tends to concentrate in th e silica glass phase which then sputters preferentially during ion bom bardment in the ion microprobe, leading to an apparent excess of radio genic Pb and hence to reverse discordance.