Thermoluminescence behaviors of quartz in caldera fill deposit and geothermal exploration in Shirasawa caldera, Sendai, Northeast Japan

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. Thermoluminescence of quartz in the Shirasawa caldera located in western part of Sendai, NE Japan, was investigated. The Shirasawa caldera was formed during 10 - 8 Ma (late Miocene to Pliocene), and filled with felsic pyroclastics. Quartz was picked up from the host rock, and then TL glow curve of quartz was measured in temperature range from 50 to 400°C with a heating rate of 1°C/sec. TL intensity of quartz (integrated intensity between 200 and 390°C) in the same geological formation showed heterogeneous variation, which means that TL intensity does not indicate geological accurate age of the formation. Low TL intensity samples were corresponded to hydrothermal alteration which was identified by formation of sericite, and high TL intensity samples contained smectite and/or sericite/smectite mixed layer clay minerals. Those facts indicate that TL intensity of quartz in the Shirasawa caldera shows thermal effect and hydrothermal alteration by geothermal activity. The low TL intensity area is overlapped to negative gravity anomaly, and also drastical decrease of TL intensity is related to positive gravity anomaly around the intrusive rock. Those facts suggest that the low TL intensity area was corresponded to subterranean hydrothermal system (negative gravity anomaly) and fractured zone which was hydrothermal upwelling zone around the intrusive rock (positive gravity anomaly). Thermoluminescence of quartz is effective method to evaluate geothermal activity, and it is possible to apply to geothermal exploration techniques coupled with mineralogical investigation of hydrothermal alteration and geophysical survey of gravity measurement. TL of quartz could be applied to evaluate geothermal activity such as surface manifestations and subterranean heat flow.