METAMICTIZATION AND U-PB ISOTOPIC DISCORDANCE IN SINGLE ZIRCONS - A COMBINED RAMAN MICROPROBE AND SHRIMP ION PROBE STUDY

We report results of a combined SHRIMP ion microprobe and Raman microp robe study of the correlation between metamictization and U-Pb isotopi c discordance of zircon. The spatial resolution of the SHRIMP and Rama n probe used are about 200 mu m(3) and 80 mu m(3), respectively. This allows a direct comparison of U-Pb isotopic discordance and metamictiz ation of small areas within zircon crystals. We show that the impact o f the oxygen ion beam on the zircon during the SHRIMP analysis does no t cause significant amorphization or recrystallization in the remainin g zircon on a scale of 1 mu m. Consequently, it is possible to determi ne the initial degree of metamictization of zircon within and adjacent to a SHRIMP analytical spot by Raman microprobe measurements after pe rforming SHRIMP analyses. A combination of the two microprobe techniqu es gives information on the concentration and distribution of radionuc lides as well as the degree of metamictization and its heterogeneity a nd the lateral ''age distribution'' within the grain. We found that th e degree of U-Pb isotopic discordance correlates closely with the degr ee of metamictization within single zircon grains, on a scale of 15 mu m, which is consistent with previous results on the scale of single a nd multiple zircon grains showing that metamictization enhances the re lative potential for secondary loss of radiogenic lead in zircon.