| Abstract |
Thermoluminescence (TL) of mineral is widely recognized as a dating techniques for relatively young geomaterials which are younger than 1 Ma. However, TL phenomena of minerals was affected by natural annealing which was caused by geothermal activity and hydrothermal alteration. We applied TL of mineral to geothermal exploration. Quartz is usually used as a mineral for TL dating because of stability of crystal structure and impurity in crystal, and TL of quartz is indicator to evaluate thermal effect of geothermal activity. We already applied TL of quartz in siliceous pyroclastics and intrusive rock (Tsuchiya et al., 2000, Geothermics) for evaluation of geothermal activities, however andesite and basaltic rocks are widely distributed in geothermal field, and sometimes it is very hard to separate quartz and feldspar in hydrothermal altered rocks even though they have both of quartz and feldspar. We measured TL phenomena of bulk rock sample without separating quartz, feldspar and other minerals. In the central zone of geothermal area (Hachimantai geothermal field, Northeast Japan), we could not observe any TL of rocks, which indicates geothermal activity is strong enough to reset of natural TL (NTL) of quartz and feldspar. However, after irradiation of cobalt-60, we could obtain TL of quartz and feldspar (ATL: Artificial TL), and integrated intensity of ATL is relevant indicator to evaluate geothermal activities. NTL could not been observed in strong manifestation area in geothermal field, however ATL is alternate indicator to evaluate geothermal activity in central zone of geothermal area. TL of minerals (quartz and feldspar) could be effective method to evaluate geothermal activity such as surface manifestations and subterranean heat flow. |