Chemical equilibria in icelandic geothermal systems—Implications for chemical geothermometry investigations

Chemical geothermometry represents the most important tool for estimating reservoir temperatures in the exploration of geothermal resources. Chemical equilibria between alteration minerals and solution are generally attained in geothermal systems for all major components except chloride. For the interpretation of analyses of natural waters involving geothermometry major emphasis should be placed on assessing the overall water composition with respect to mineral equilibria, rather than attempting to distinguish geothermal waters from shallow waters by a classification involving the relative abundance of major anions and major cations. Generally, cold waters may be distinguished from geothermal waters by low chloride (< 10 ppm), in conjunction with relatively low pH (6–7) and low Na/K ratios (same as the associated rock), calcite undersaturation and low √Ca²⁺ H⁺ activity ratios.