Geochemical study of fluid inclusions in anhydrite from the Kakkonda geothermal system, northeast Japan

Anhydrite occurs as a representative hydrothermal mineral in the upflow zon e of the Kakkonda geothermal field, northeast Japan. Gas analysis and micro thermometry of fluid inclusions and sulfur isotope measurements were perfor med for these anhydrites in order to discuss origin of the reservoir fluids and precipitation mechanism of the anhydrite. Fluid inclusions are classified into two-phase, vapor-rich and liquid-rich inclusions, and polyphase inclusions comprising liquid, vapor and solids. T he vapor-rich inclusions coexist with the liquid-rich inclusions in most sa mples, indicating that the fluid inclusions were trapped under boiling cond itions. Bulk gas analyses of the liquid-rich inclusions show that the main non-condensable gas component is CO2 (0.14-2.0 mol %) with subordinate amou nts of N-2, CH4 and Ar. The two-phase liquid-rich inclusions homogenize to liquid phase at temperatures between 222 and 380 degrees C, and have salini ties mostly from 0 to 29 wt.% NaCl + CaCl, equivalent. The polyphase inclus ions homogenize to liquid phase at temperatures between 302 and >480 degree s C after dissolving halite between 104 and 220 degrees C on heating, and h ave salinities from 28 to 35 wt.% NaCl + CaCl2 equivalent. In the deep rese rvoir, the salinities vary over a large range at homogenization temperature s of approximately 320 to 360 degrees C. The delta(34)S values of anhydrite s ranging mostly from +21.6 to +24.2 parts per thousand suggest that sulfur in the anhydrite from the Kakkonda field is derived from marine anhydrite. These data indicate that the hypersaline fluid was produced by exsolution f rom a residual magma at the time of final crystallization of the Kakkonda g ranite, and moderate to low salinity fluids formed by dilution of the hyper saline fluid with the heated meteoric water and boiling of the dilute fluid s. The precipitation of anhydrite might be explained by three mechanisms of a simple cooling of the hypersaline fluid, boiling of the reservoir fluid and pressure drop of the non-boiling reservoir fluid.