Discordant distribution of U and Pb in zircon of Naegi granite

The distribution of U, Th and Pb is determined on an electron microprobe for zircon in the Naegi granite of the Late Cretaceous from Hirukawa, Gifu Prefecture, Japan. The UO2, ThO2 and PbO contents of zircon range from less than the detection limit (50 ppm) to 3.45 %, 2.46 % and 360 ppm, respectively. The Pb content of some portions in each grain is equivalent to what has been generated by the radioactive decay of co-existing U and Th since the granite crystallization, but other portions evidently show either deficient or excess Pb with respect to the U and Th contents. The granite has not undergone any thermal event after the intrusion. The Pb-deficiency and -excess are explained in terms of Pb-loss and -gain owing to the migration of gaseous daughters (Rn) through radiation damage. The Rn atoms that entered into a network of radiation damage by recoil at the birth can move at random: they are adsorbed for a moment on the walls of radiation damage, and then re-emitted in random directions. Their residence time on the walls depends on the heat of adsorption which increases with increasing number of irregular sites on the walls, so that Rn atoms tends to decay on irregular walls as compared with smooth walls. This process disturbs the distribution of radiogenic Pb, and brings forth the enrichment and depletion of Pb with respect to the U and Th contents of zircon. (author)