Through a glass, darkly: Trying to understand geothermal systems by means of geothermometers and fCO2-indicators

There are three classes of geothermometers and fCO2-indicators (collectively known as geoindicators): the traditional or classical ones, the theoretical activity-based geoindicators, and multicomponent chemical geothermometry. Most traditional geothermometers (e.g., SiO2, Na–K, Na-K-Ca, K–Mg) are linear functions linking the logarithm of the equilibrium constant of the controlling reaction to the absolute temperature reciprocal. Total concentrations are considered in most traditional geothermometers. Thanks to these two characteristics, the traditional geothermometers are simple to be used, which explains their widespread use. However, there are at least two issues which have been disregarded or not properly considered in the derivation of traditional geothermometers. One is that total concentrations of relevant solutes can deviate considerably from the activities of the aqueous species participating to the controlling mineral-solution reactions due to the formation of aqueous complexes. The other concerns the hydrothermal minerals, with some having variable composition and adularia being affected by Al–Si order-disorder on the tetrahedral sites. To overcome these two issues, theoretical activity-based geoindicators were recently proposed. To use these geoindicators is necessary to run a speciation program, but computed temperatures and CO2 fugacities are more reliable than those given by traditional geoindicators and additional information is obtained on the hydrothermal minerals present in the geothermal aquifer. Multicomponent chemical geothermometry consists in simulating the heating of the aqueous solution of interest from the P-T conditions present at the surface to those occurring in the geothermal aquifer. Both multicomponent chemical geothermometry and theoretical activity-based geoindicators call for the use of a suitable computer code and an internally consistent thermodynamic database. It would be desirable for the database to be shared, accepted and used by the entire scientific community.