| Keywords |
fluid inclusions, gas analysis, active geothermal system, granitic rock, magnetite-series, ilmenite-series, Itype, S-type, Japan |
| Abstract |
We have studied fluid inclusions in minerals from geothermal systems and granitic bodies in Japan, especially their gas compositions. Five types of fluid inclusions occur in the studied samples: (1) monophase liquid, (2) aqueous two-phase liquid-rich, (3) aqueous two-phase vapor-rich, (4) liquid CO2-bearing aqueous and (5) halite and/or other daughter mineral-bearing polyphase inclusions. In general, fluid inclusions of low salinity and gas content (types 1, 2 and 3) occur in active geothermal systems in Japan. Coexistence of fluid inclusions of types 2 and 3 (boiling of geothermal fluids) sometimes occur in very active geothermal systems. Highly saline fluid inclusions (type 5) occur in the magmarelated geothermal system (Kakkonda), and liquid CO2-bearing inclusions (type 4) occur in the geothermal reservoir is in limestone (Mori). As for granitic rocks in Japan, various granitic rocks mainly of Cretaceous-Paleogene to Neogene with small numbers of Paleozoic, Triassic-Jurassic and Quaternary are exposed. Probably, fluid inclusions of types 2, 3, 4 and 5 are trapped fluids under cooling processes of the crystallized granitic magmas. Aqueous two-phase liquidrich inclusions (type 2) are the most ubiquitous among the types. Daughter mineral-bearing polyphase (type 5) and vapor-rich inclusions (type 3) are more frequently in Neogene and Quaternary granitic rocks than in the Cretaceous ones, or are also common in granitic rocks intruding contemporaneous volcanic rocks. The Neogene granitic rocks are thought to be eroded less than the Cretaceous ones, and their exposed levels are shallow. That is, the shallower plutons have the more saline fluids in inclusions. Experimental data on the NaCl-H2O system and solidus of granitic rocks also shows that highly saline fluid in which halite is stable at room temperature are trapped in inclusions at shallower depths. Gas analyses by a quadrupole mass spectrometer show relatively high gas contents of the inclusions in granitic rocks, compared with the geothermal systems in Japan. XCO2/XCH4 ratios of the fluid inclusions in the granitic rocks have a wider range than that of XCO2/XN2 ratios, and the former ratios may reflect the difference of fO2 around the granitic rocks where the fluids flew. CO2-CH4-N2 ternary plots of the fluid inclusions show a different trend from those of the hydrothermal systems. XCO2/XCH4 ratios of fluid inclusions from S-type granites intruded the Paleogene- Miocene sediments are quite low. These results suggest that gas compositions of inclusion fluids are probably controlled by fluid-rock interaction in the granitic rocks and/or in sedimentary rocks around the granitic rocks, or, possibly, by source of granitic magma. |