Sr Concentration and its Isotopic Ratio in the Sumikawa and Ohnuma Geothermal Fields

Abstract

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/⁸⁶Sr 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.