Metamictization and U-PB isotopic discordance in single zircons: a combined Raman microprobe and SHRIMP ion probe study

Abstract

We report results of a combined SHRIMP ion microprobe and Raman microprobe study of the correlation between metamictization and U-Pb isotopic discordance of zircon. The spatial resolution of the SHRIMP and Raman probe used are about 200 μm3 and 80 μm3, respectively. This allows a direct comparison of U-Pb isotopic discordance and metamictization of small areas within zircon crystals. We show that the impact of the oxygen ion beam on the zircon during the SHRIMP analysis does not cause significant amorphization or recrystallization in the remaining zircon on a scale of 1 μm. Consequently, it is possible to determine the initial degree of metamictization of zircon within and adjacent to a SHRIMP analytical spot by Raman microprobe measurements after performing SHRIMP analyses. A combination of the two microprobe techniques gives information on the concentration and distribution of radionuclides as well as the degree of metamictization and its heterogeneity and the lateral “age distribution” within the grain. We found that the degree of U-Pb isotopic discordance correlates closely with the degree of metamictization within single zircon grains, on a scale of 15 μm, which is consistent with previous results on the scale of single and multiple zircon grains showing that metamictization enhances the relative potential for secondary loss of radiogenic lead in zircon.