Geothermometry

Geothermometry is the use of a fluid’s (or, although not discussed here, a rock’s) chemical composition to estimate the temperature at which it equilibrated in the subsurface. The specialty is important, for example, in exploring for and exploiting geothermal fields, characterizing deep groundwater flow systems, and understanding the genesis of ore deposits. Several chemical geothermometers are in widespread use. The silica geothermometer (Fournier and Rowe, 1966) works because the solubilities of the various silica minerals (e.g., quartz and chalcedony, SiO2) increase monotonically with temperature. The concentration of dissolved silica, therefore, defines a unique equilibrium temperature for each silica mineral. The Na-K (White, 1970) and Na-K-Ca (Fournier and Truesdell, 1973) geothermometers take advantage of the fact that the equilibrium points of cation exchange reactions among various minerals (principally, the feldspars) vary with temperature. In applying these methods, it is necessary to make a number of assumptions or corrections (e.g., Fournier, 1977). First, the minerals with which the fluid reacted must be known. Applying the silica geothermometer assuming equilibrium with quartz, for example, would not give the correct result if the fluid’s silica content is controlled by reaction with chalcedony. Second, the fluid must have attained equilibrium with these minerals. Many studies have suggested that equilibrium is commonly approached in geothermal systems, especially for ancient waters at high temperature, but this may not be the case in young sedimentary basins like the Gulf of Mexico basin (Land and Macpherson, 1992). Third, the fluid’s composition must not have been altered by separation of a gas phase, mineral precipitation, or mixing with other fluids. Finally, corrections may be needed to account for the influence of certain dissolved components, including CO2 and Mg++, which affect the equilibrium composition (Paces, 1975; Fournier and Potter, 1979; Giggenbach, 1988). Using geochemical modeling, we can apply chemical geothermometry in a more generalized manner. By utilizing the entire chemical analysis rather than just a portion of it, we avoid some of the restricting assumptions mentioned in the preceding paragraph (see Michard et al., 1981; Michard and Roekens, 1983; and especially Reed and Spycher, 1984). Having constructed a theoretical model of the fluid in question, we can calculate the saturation state of each mineral in the database, noting the temperature at which each is in equilibrium with the fluid.