Application of thermoluminescence dating to geothermal history of the Tamagawa-Kowase area, Hachimantai geothermal field, northeast Japan

Abstract

Alteration ages have been determined for 26 samples by the thermoluminescence dating of phenocrystic quartz separated from altered rhyo-dacitic rocks. The samples were collected mainly from the widely distributed green colored alteration halos. Obtained ages range from 0.057 to 1.80Ma with the accuracy of ±30%. The thermal manifestation areas correspond to the places with the alteration ages of younger than 0.5Ma. It is statistically considered that the hydrothermal activity periodically occurred four times in separate places during 1.78Ma to present with the ending times of 1.7, 1.2, 0.7 and 0.5Ma B. P., respectively. The duration of each hydrothermal activity is preliminary estimated as about 0.3 to 0.4Ma. A linear relation is observed for the alteration ages and subsurface temperature of 1500m depth. The temperatures of 1500m depth of the areas of 0.9Ma and 0.5Ma are 95°C and 145°C, respectively. However, the temperature of the area close to active volcano (Yakeyama) shift to higher than expected value from above relation. It may be attributed to the effect of the heat supply from Yakeyama volcano at present time.