THE HYDROTHERMAL STABILITY OF ZIRCON - PRELIMINARY EXPERIMENTAL AND ISOTOPIC STUDIES

Experimental investigations of the stability of the U-Pb isotopic system in nonmetamict zircons show that appreciable losses of Pb and U can be induced at amphibolite-grade conditions (400-degrees-C to 600-degrees-C, 4 to 6 kb) in 2 M NaCl and 2% HNO3 solutions. The severity of U loss and, to a lesser extent Pb loss, varies with solution composition: in this case the 2 M NaCl solution induced more Pb and U loss than the 2% HNO3 solution at the same P-T conditions. Scanning electron microscopy of the run products also revealed a range of corrosion-related surface features, which suggests that some of the observed trends in Pb and U loss must be attributed to zircon dissolution. Backscattered electron (BSE) imaging of the run products further suggests that partial homogenization of chemical zoning patterns occurred during the experiments. Microprobe analyses of treated and untreated grains show that both populations have a similar range of Hf contents. Thus, the apparent loss of sharp, well-defined zoning features is most likely due to small-scale "smearing out" of formerly sharp chemical gradients and is perhaps related to the annealing of lattice defects caused by alpha-recoil damage. Thus, experimentally induced U-Pb isotopic discordance in zircon is a complex function of zircon stability and annealing effects.