Strontium isotope geochemistry and its geochemical implication from hot spring waters in South Korea

摘要

Despite being a non-volcanic area, the southern Korean Peninsula has a number of hot water springs with temperatures more than 40 ℃, most of which are commercially used for spa. The hot springs are located at the fringes of the granite body rather than the center of the Mesozoic granite. Here we report on the geochemical characteristics of the hot water springs. The hot springs in the Jurassic granite area are mainly of Na( - Ca) - HCO_3 type, whereas those in the Cretaceous granite area are of Na - Cl (- SO_4) type. The stable isotope composition of 0 and H suggests a meteoric origin of the hot spring water. Further, the chemical components of the hot spring waters indicate that they were all derived from hot spring water-rock interaction rather than through anthropogenic input. The ~(87)Sr/~(86)Sr ratio of the hot spring waters in the Jurassic granite area ranges from 0.711088 to 0.729034 and that in the Cretaceous granite area ranges from 0.705652 to 0.707912. Such ~(87)Sr/~(86)Sr ratio distribution of hot spring waters corresponds with the distribution of the initial ~(87)Sr/~(86)Sr ratios for the emplacement age of the Jurassic and Cretaceous granites in the hot spring occurrence area. The ~(87)Sr/~(86)Sr ratios in the hot spring waters were also divided into three groups according to their geographical location: eastern, western, and southern groups. The constant ~(87)Sr/~(86)Sr ratio of hot spring water from one hot spring area during the last 6 years strongly suggests that the hot-spring-bearing aquifer system in South Korea should be separated from the upper cold shallow groundwater system. This indicates that the ~(87)Sr/~(86)Sr ratio of the hot spring waters in the granite area of South Korea originates from an equilibrium because of long-term water-rock interaction rather than short-term water-rock interaction due to the input of recent meteoric water. Hence, it may be concluded that hot spring water in the granite area may be a limited deep groundwater resource that might deplete with overuse.