| Abstract |
Like their liquid counterparts, gas geothermometers are based upon either empirical or thermodynamic equations for chemical reactions thought to equilibrate in geothermal reservoirs. Presentations of geothermometer results take three different general forms: equations, y-T grids and gas ratio grids. Among the thermodynamics-based geothermometers, the H2, CO, HSH, H2S, CO2 and CO2-H2 geothermometer reactions appear to equilibrate at usual reservoir conditions, but the Fischer-Tropsch and NAH geothermometer reactions are likely too slow. Empirical geothermometers have been proposed for a number of gas combinations, but they either do not provide an advantage over their thermodynamic counterparts or, in the case of the DAP geothermometer, do not perform as expected. The y-T gas grid geothermometers employ a combination of two geothermometer reactions to allow the simultaneous determination of y (the fraction of a hypothetical equilibrated reservoir steam) and temperature. Processes that modify fumarole gas concentrations, such as reservoir boiling and near surface condensation adversely affect y-T grid results, limiting their application. They also suffer from uncertainty as to the appropriateness of the y model to a particular steam source. Gas ratio grids overcome some of these difficulties by fixing hydrogen fugacities through the assumption of a single redox state for the system, and by using argon concentration as a proxy for PH2O. They suffer from ambiguities with respect to the source of argon, but most of these can be adequately accounted for. Argon is sensitive to air contamination and volcanic input, and there are indications that its concentration in hydrothermal water of meteoric origin may deviate from that of air saturated ground water. While there usually are ambiguities with respect to the application any gas geothermometer, they can often be addressed satisfactorily by an interpretative approach which looks for consistency among a variety of y-T and gas ratio grids, and integrates information from other sources, such as stable isotope and noble gas chemistry. |