Abstract
Thermoluminescence (TL) of quartz in the Miocene dacitic pyroclastics in Tsunagi hot springs located in western part of Morioka, Northeast Japan, was investigated. Quartz was picked up from the host rock, and then TL glow curve of quartz was measured in a temperature range from 50 to 400℃ with a heating rate of 1℃/sec. TL intensity of quartz (integrated intensity between 200 and 390℃ in glow curve) in the same geological formation showed heterogeneous variation, which means that TL intensity does not simply indicate geological age of the formation. Low TL intensity zone was recognized in the central part of this area where old natural hot springs and hot spring wells were located.
According to comparison between TL intensity and one-meter depth temperature profile, the central area with natural discharge of hot spring can be prospected as preference area for strong activity of hot spring where is less than half intensity of TL compared with the maximum value obtained in this area.
High radiation level (gamma ray intensity) appeared in the same area, but there was no clear relation to increase TL intensity due to irradiation dose. Those facts indicate that TL change can be affected by accumulation and emission processes of paleodose by geothermal activity.
Although the low TL intensity area was almost overlapped with hydrothermal alteration zones characterized by zeolite group (heulandite or stilbite) and clay minerals (sericite, smectite, etc.), the TL intensity widely changed in hydrothermal alteration zone. TL intensity indicates accumulation and emission process after the latest hydrothermal events.
In summary, coupled with 1 m depth temperature profile, TL intensity and hydrothermal alteration, it is possible to realize activities of geothermal and hot spring. In particular, TL intensity can show preference area of hot spring without data of 1 m depth temperature profiles for non and weak surface geothermal manifestation areas, although it is not necessarily due to a current activity.
