Fluid inclusion study of the Kirishima geothermal system, Japan

Gases from fluid inclusions in quartz and anhydrite were analyzed with a quadrupole mass spectrometer and a capacitance manometer. The quartz and anhydrite occur in hydrothermal veins in volcanic and pelitic rocks collected from geothermal wells in the Kirishima area, southwest Japan. The geothermal wells are located in a graben made up of Quaternary volcanic rocks underlain by sedimentary rocks of the Shimanto Group. Results of individual fluid inclusion analyses show that the fluid inclusions comprise mainly H20 and a variable but small amount of CO2. CH4 and other hydrocarbons are also detected in inclusions in a hydrothermal sample from the pelitic Shimanto Group. Peak ratios of CO2/H20 in individual fluid inclusions are variable in some samples. This indicates that there is a difference in gas compositions of the fluid inclusions, and suggests that the inclusions were formed in multistages or trapped heterogeneous boiling fluids. Results of bulk analyses show that the inclusions are mainly composed of H20 (98-99 mol%) with small amounts of non-condensable gases, mainly CO2 and N2, CH4 and Ar. The proportion of N2 is about one order of magnitude lower than CO2, CH4 is generally two orders of magnitude lower than CO2 and Ar is just above the detection limit of the mass spectrometer. The gas concentration in the fluid inclusions is much higher than that in the present-day discharge fluids in this area. CO2/N2 and CO2/CH 4 ratios of the fluid inclusions from the volcanic rocks are lower than those of the present-day discharge fluids. CO:ON 2 and CO2/CH 4 ratios in residual fluids increase with progressive degassing, because N 2 and cn 4 are released from the residual fluids more easily than CO2. Thus, the difference in the CO2/N2 and CO2/CH4 ratios between the fluid inclusions and the present-day discharge fluids in the Kirishima area may be ascribed to the degree of degassing, and the fluid inclusions in the area were probably formed by trapping fluids that were weakly influenced by degassing. Pc02 values calculated from the gas compositions of the fluid inclusions are higher than that of buffer systems involving alteration -tPresent address: New Energy and Industrial Technology Development Organization, Higashi-lkebukuro 3-1-1, Toshima-ku, Tokyo 170, Japan. 305 306 T. Sawaki et al. minerals in the area. This suggests that the fluid inclusions might be trapped fluids which were not in equilibrium with the alteration mineral assemblages, that is, fluids prior to considerable degassing and alteration. © 1997 CNR. Published by Elsevier Science Ltd.