HYDROCHEMISTRY AND NOBLE GAS GEOCHEMISTRY OF HOT SPRINGS IN THE CHUNGCHEONG AREA, SOUTH KOREA
We investigated the chemical composition, stable and noble gas isotopic compositions of hot-spring samples from the Daejeon and its near areas in Korea. The chemical composition of hot waters varies according to their locations with electrical conductance ranging from 224 to 495 mS/cm (except CO2-rich hot water with 2,780 mS/cm). The pH values of hot water sample show the alkaline range of 7.6 to 9.8. The temperature of hot waters ranges from 21.4°ÆC to 47°ÆC. The hot waters can be grouped as three chemical types on the Piper diagram: Ca-HCO3, Ca(Na)-HCO3 and Na-HCO3 water types. The 18O and 2H values of the hot spring water show a range of from -10.4 to -7.8 and from -77.9 to -58.8, respectively. All 18O and 2H data are plotted near meteoric water line (MWL) or shifted from MWL.
3He/4He ratios are in a wide range from 0.04 to 2.3 in the unit of 10-6, though most of the hot-spring waters and gases have 3He/4He ratios lower than the atmospheric value of 1.4x10-6, suggesting dominated radiogenic 4He contribution from crustal rocks to the ground waters. 40Ar/36Ar ratios are close to or slightly higher than the atmospheric value of 296. Most samples gave the evidence that the He of mantle-origin is supplied into hot-spring waters in Korea. Calculation of mixing ratios of He between three end members (air, mantle and crust) shows that at least 90% of 3He comes from mantle for more than half of the samples. While more than 90% of 4He comes from crust. The 3He/4He and 4He/20Ne ratios show some relationships with temperature and stable isotopes. From a broad geographical view point, the decreased 3He/4He ratios would be closely related with the increasing depth of subducting oceanic plate beneath the Korean peninsula. However, the observed mantle He does not show close relationships with basement rock types and localities of hot-springs, the discharge rate might be controlled by invisible underground structure such as deep-seated faults. This work was supported by a grant (R01-2004-000-10759-0) from the Basic Research Program of the Korea Science and Engineering Foundation.