COMPARISON OF THERMAL-STABILITY BETWEEN INTERNAL AND EXTERNAL SURFACES OF ZIRCON

F-T ages were measured for both internal and external surfaces of zircon crystals in quartz-diorite samples obtained from central Japan (Koshimizu, 1990). These data suggested that ages tightly cluster around 3.8 Ma for internal surfaces and around 7.4 Ma for external surfaces. This age discrepancy can be experimentally elucidated by thermal stability difference in internal and external surfaces of zircon. To establish a comparison of thermal stability between internal and external surfaces of zircon, 2 h annealing experiments for spontaneous and induced fission tracks on both surfaces of zircon were undertaken in this study. At temperatures between 200-degrees-C and 800-degrees-C, the density of spontaneous and induced fission tracks is reduced on both internal and external surfaces. This reduction, however, takes place at lower temperatures on internal surfaces of zircon. This suggests that external surface is more stable with respect to thermal influences and that fission track closure temperatures vary from internal to external surfaces of zircon.