Thermal History of the Onikobe Geothermal System, Based on Microthermometry of Fluid Inclusions

Abstract

The thermal history of the Onikobe geothermal system, northeast Japan, was studied, based on microthermometry of fluid inclusions in hydrothermal veins from the KR-series wells which were drilled by the New Energy Development Organization (NEDO). The hydrothermal veins are composed mainly of quartz, anhydrite and calcite with subordinate amounts of wairakite, epidote, chlorite, native sulfur and pyrite. Fluid inclusions occur in quartz, anhydrite, clacite and wairakite. Fluid inclusions of hydrothermal quartz and anhydrite in granitic rocks around the bottom of well KR-l, were formed by trapping boiling fluids. Their final melting points of ice (Tm) and homogenization temperatures (Th) range from -0.3 to -3.0°C and from 270 to 320°C, respectively. The Th's are plotted on the boiling point curve of water adjusted to the present water table (BPC). On the other hand, Th's of fluid inclusions in volcanic rocks shallower than 1200m in the borehole are plotted between the BPC and the present well temperature profile (WT). Fluid inclusions from well KR-2 are divided into two groups, based on their Th's; one's Th's plot on or above the BPC, and the other's on the WT. Th's of fluid inclusions from well KR-3 deeper than 1200m plot near the BPC, where the WT is also drawn near the BPC. Th's of fluid inclusions from well KR-4 have ranges of 40 to 80°C at each level, and the highest Th's correspond to the WT. Fluid inclusions from well KR-5 are divided into three groups, based on their Th's. Th's of group 1 are plotted above the BPC at very shallow levels, and their host rock are remarkably silicified. Those of group 2 are on the BPC, those of group 3 are between the BPC and the WT, or on the WT. The microthermometrical data of fluid inclusions and petrography of hydrothermal veins suggest that geothermal activity is still vigorous at deeper levels of the wells KR-1 and KR-3. Fluid inclusions whose Th's are different occur in the wells KR-2 and KR-5. These variations are probably reflected by a cooling process around the wells, and suggest that the geothermal activity around the wells KR-2 and KR-5 has declined. The higher WT than Th's around the well KR-4 suggests that fluid inclusions of lower Th's than the WT were formed in a past stage of lower temperatures, and that geothermal activity around the well is possibly high.