| Abstract |
Fluid inclusion and epidote isotopic studies have been carried out on core-samples drilled in the liquid-dominated Berlin geothermal field (El Salvador) in order to obtain information on the thermo-chemical evolution of the geothermal fluids. Fluid inclusions microthermometry was performed on inclusions hosted in hydrothermal minerals (quartz, albite, adularia, calcite, anhydrite and prehnite) and igneous quartz. Homogenization temperatures of liquid-rich inclusions are between 191 and 344°C. Final ice melting temperatures of fluid inclusions are mostly from -3.7 to -0.1°C, corresponding to maximum salinities between 0.2 and 6.0 wt% NaCl eq. Few liquid-rich inclusions show lower final ice melting temperatures between -13.1 and -26.3°C, with calculated maximum salinities from 17.0 to 21.2 wt% NaCl eq.. Vapor-rich inclusions also occur in the some of the examined samples, in one case they coexist with liquid-rich inclusions suggesting boiling processes. A single three-phase (liquid-vapor-halite) hypersaline (38.5 wt% NalCl eq.) inclusion was also found. Isotopic analyses on hydrothermal epidote showed D values from about -79 to -90‰. The D of water (DH2O) in equilibrium with epidote, computed using temperatures derived from fluid inclusions, are between -42 and -56‰. Comparison between homogenization temperatures and boiling point curves suggests that two stages of hydrothermal flow characterized the study area: a) the first stage was restricted at depths <1100 m below the ground level (b.g.l.); and b) the second stage occurred also in the deepest part of the present-day system. During this stage, fluid inclusions with the highest homogenization temperatures documented the upflow of a hot boiling fluid. Later, a significant temperature decrease down to present-day value, testified by homogenization temperature variation, affected the system. Salinity and DH2O variation can be explained by the introduction and mixing of magmatic derived fluid and meteoric waters. In particular, relatively high-salinity values displayed by some liquid-rich fluid inclusions, the occurrence of a hypersaline inclusion and the highest DH2O value indicates a contribution of magmatic derived fluid to geothermal circulation. Whereas, the very low-salinity values displayed by other fluid inclusions, combined with the lowest DH2O values, testify the infiltration of meteoric fluids. |