Fission Track Thermal History Analysis in Geothermal Field Using Heavy Ion Irradiation Method

Abstract

Geothermal manifestations such as hydrothermal alteration haloes have been reported to occur in the Goto-Fukue Island, SW Japan. In order to clarify the timing and magnitude of paleo-geothermal activities in the island, a fission track (FT) thermo-chronologic study has been carried out for graniteporphyry drilling core samples collected at several depths. For the purpose of FT length analysis, a technique to detect confined uranium-238 FTs in samples with low spontaneous track density (<<106/cm2) has been investigated by using heavy ion irradiation. Five zircon FT ages obtained in this study become progressively young from 11.3 Ma to 6.6 Ma with increasing depth. Based on track length (TL) distribution for the shallowest sample (115 m deep), FT age of 11.3 Ma is interpreted as the age of original cooling of the granite-porphry intrusion. TL distributions for three samples from deeper portions (279-1134 m) indicate that samples have experienced a thermal event that caused track annealing at a certain stage of track accumulation in zircon. Preexisting tracks have completely annealed in the deepest sample (1481 m deep) and no short tracks are found. The timing of the thermal event can be interpreted to be around 6.6 Ma. These results allow a chronological reconstruction of geothermal activity related with rhyolite dyke intrusion.