抄録
Gases of fluid inclusions in quartz and anhydrite from the Kakkonda geothermal system, , northeast Japan, were analyzed with a quadrupole mass spectrometer and a capacitance manometer. The quartz and anhydrite were collected from an outcrop and geothermal wells drilled in the shallow geothermal reservoir. Results of individual fluid inclusion analyses show that the fluid inclusions comprise mainly H2O and a small amount of CO2. The gases of N2, CH4 and Ar were not detected. Results of bulk analyses show that fluid inclusions from the outcrop and upper levels of the shallow reservoir are mainly composed of H2O(99.8 -100mol% )with very small amounts of CO2, N2 and CH4. Ar is just above the detection limit of the analytical system. The gas contents of the fluid inclusions are very low, compared with those from other geothermal systems which the authors have analyzed. CO2/N2 and CO2/CH4 ratios of fluid inclusions are relatively low, compared to those of the present-day discharge fluids of the Kakkonda geothermal system. These ratios of residual fluids increase with progressive degassing, and the difference in the CO2/N2 and CO2/CH4 ratios between the fluid inclusions and the present-day discharge fluids in the area may be ascribed to the degree of degassing. Individual inclusion analyses show that gas-rich inclusions occur at deeper levels. This suggests that fluid inclusions at deeper levels were formed from gas-rich fluids which were not undergone by dilution of meteoric water.
