Chemical and stable isotopic compositions (δD, δ18O, and δ34S) of non-volcanic hot spring waters around the Miocene Kofu granitic complex surrounding the Kofu basin in the South Fossa Magna region of central Honshu, Japan, were analyzed in order to investigate water-rock interactions and to determine the origin and sulfur isotopic characteristics of their trace amounts of SO42- ion. All water samples from the granitic rocks were classified as Na-Alkalinity (Alk) type, whereas water samples from the volcanic rocks were classified as Na-Alk, Na-SO4, Na-SO4·Cl·Alk, and Ca-SO4 types. The water in the samples originated from meteoric water, and the average recharge altitude of the samples ranged from 947 m to 1397 m based on the altitude effect of δ18O. The Na-Alk type waters from the granitic rocks were likely formed by the montmorillonization of plagioclase, cation exchange reaction of Na-montmorillonite, and calcite precipitation. Trace amounts of SO42- ion of this type of water were derived from the oxidation of sulfide such as pyrite in granitic rocks or the roof sedimentary rocks of the Shimanto group, where H+ caused by the sulfide oxidation was consumed in the process of plagioclase weathering. SO42- ion content in the Na-Alk type water from the granitic rocks reflected the δ34S values of granitic and sedimentary rocks of the Shimanto group. Water samples from the ilmenite series area have negative values ranging from -15.1 to -4.6‰, whereas waters from the magnetite series area have positive δ34S values ranging from +1.7 to +8.0‰. The hot spring water quality of the Na-Alk, Na-SO4, Na-SO4·Cl·Alk, and Ca-SO4 types from the volcanic rocks area were estimated to be controlled by anhydrite dissolution, plagioclase weathering, cation exchange reaction of Na-montmorillonite, and precipitation of calcite during the fluid flow and mixing process. Different concentrations of SO42- ions determined for these waters have a wide range of δ34S values ranging from -4.1 to +13.6‰, which is likely attributed to the dissolution of 34S-rich and 34S-poor anhydrite. The 34S-rich SO42- ions were interpreted to be derived from sulfate in sulfuric acid, which arose from the disproportionation reaction of volcanic sulfur dioxide, whereas the 34S-poor SO42- ions were derived from the oxidation of ascending hydrogen sulfide in shallow ground waters during the active stage of past volcanism.
The TAG hydrothermal mound on the Mid‐Atlantic Ridge (26°08′N, 44°50′W) was revisited in August 1994 with the submersible Shinkai 6500 in order to characterize time‐series fluid chemistry prior to the ODP drilling. Fluid samples were taken from both black smokers and white smokers. Si, pH, alkalinity, H 2 S, major cations (Na + , K + , Ca 2+ , Mg 2+ ), major anions (Cl − , SO 4 2− ), and minor elements (Li, Sr, B, Fe, Mn, Cu, Zn, Br) as well as Sr isotope ratios were measured. We report the first Br/Cl ratios for the TAG hydrothermal fluids, showing no fractionation between Br and Cl during the fluid‐rock interaction. This study shows small changes in composition of the black smoker fluids from the 1990 data (Edmond et al., 1995). Changes of pH, alkalinity, Fe, K, and 87 Sr/ 86 Sr values are suggestive of subsurface FeS precipitation and a decrease of water/rock ratio at a deeper reaction zone. Differences in chemical characteristics between the black and white smoker fluids were similarly observed as in 1990.
Abstract We provide an estimation of the heat output necessary to generate the neutrally buoyant plume above the TAG hydrothermal mound, Mid-Atlantic Ridge, located at 26°N, using a model of plume rise in a density-stratified environment with crossflow. The estimated heat output is 460 ± 40 MW. Previous studies have estimated that the heat outputs from high-temperature hydrothermal discharge and low-temperature diffuse flow at the TAG hydrothermal mound are 90 ± 20 MW and at least 780 MW, respectively. Consequently, the contribution of diffuse flow to make the neutrally buoyant plume is 370 ± 60 MW, which accounts for approximately 80% of the heat output to the neutrally buoyant plume. As this contribution is less than 50% of the total heat output from the diffuse flow, it is likely that more than 50% of the heat output from the diffuse flow dissipates in the ambient current.
The geothermal fluids in seven Japanese geothermal systems are tested for attainment of aqueous and gaseous equilibrium. The pH of fluids in the geothermal reservoir is approximately buffered by the assemblage K-feldspar–K-mica–quartz. (Na+)/(K+) and (Na+)/√(Ca2+) activity ratios are thermodynamically approximated by reactions between albite and K-feldspar, and between albite and anorthite (or Ca-zeolites), respectively. The (Mg2+)/(K+)2 activity ratio of high temperature geothermal fluids of Japan can be, represented by the reaction involving Mg-chlorite and K-bearing silicate minerals, though at lower temperatures other reactions may be responsible. The geothermal fluids are also commonly saturated with respect to anhydrite and calcite. A small amount of steam loss in the reservoir does not significantly affect the aqueous composition of the fluids. The partial pressure of CO2 is controlled by the reaction involving calcite, K-bearing silicate minerals, and albite or Ca-zeolite in geothermal systems which are not affected by steam loss and dilution. Equilibrium between CH4, CO2 and H2 is attained at high temperatures but not maintained to lower temperatures in most Japanese geothermal systems. The H2/H2S ratio is probably equilibrated with Fe-bearing minerals. Gaseous compositions are very good indicators to identify processes in the geothermal reservoir, such as boiling and dilution. Lastly, the major aqueous composition and pH of Japanese neutral Na-Cl type geothermal fluid are predictable if two variables (e.g., temperature and one of the cation activities) are provided.
