Concentrations and isotopic ratios of Sr were determined for waters and rocks in the Sumikawa and Ohnuma geothermal fields to provide constraints on modeling fluid movement and understanding water-rock interaction. The results are summarized as follows: 1) 87Sr/ 87Sr ratios of Quaternary volcanic rocks in the Sumikawa section (< 0.7041) are significantly lower than those of Tertiary marine sediments (≦ 0.7075). As a result, 87Sr/86Sr ratios of rocks increase northward where marine sediments distribute predominantly. 2) The Sr isotopic ratios of the geothermal fluids and hot spring waters are affected significantly by : their surrounding rocks, Particularly, geothermal fluids discharged from the Sumikawa wells have similar 87Sr/86Sr ratios to rocks around the feed zones, indicating an attainment of water-rock equilibrium in the vicinity of the wells. 3) A meaningful relationship between the concentration of Sr and Ca, and temperature in the reservoir fluid indicates that the fluid is saturated with respect to anhydrite : this conclusion is supported by a chemical equilibrium model. 4) 87Sr/86Sr ratios of geothermal fluids discharged from the Sumikawa production well, SC-1, have been changing temporally over the past four years. This may be due to a boiling of fluid in reservoir, return of reinjection water, and change of flow path of fluid discharging into reservoir.
|Number of pages||12|
|Journal||Journal of the Geothermal Research Society of Japan|
|Publication status||Published - Jan 1999|
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