Traditional Water Geothermometers and $$f_{{{\text{CO}}_{2} }}$$-Indicators

Traditional water geothermometers and \(f_{{{\text{CO}}_{2} }}\)-indicators have been thoroughly reviewed in this chapter. The solubility of different silica minerals has been recalled, but the main focus has been placed on quartz solubility, as it plays a pivotal role in water geothermometry, underscoring the effects of grain size, pH, and salinity. The different Na-K geothermometric functions proposed by different authors have been examined. Since they are all plausible, there is not a unique Na-K geothermometer, but an infinite number of Na-K geothermometers which are controlled by the exchange reaction between low-albite and variably ordered adularia, from fully ordered maximum-microcline to completely disordered high-sanidine. Consequently, there is also an infinite number of the other cation geothermometers which are controlled by exchange reactions involving adularia. In particular, the Na-K-Ca geothermometer has been split in the three separate Na-K, Na-Ca, and K-Ca functions, following the suggestions of Tonani (1980). It turns out that the Na-Ca and K-Ca geothermometers work for waters equilibrated with an unspecified Ca-Al-silicate at relatively low \(f_{{{\text{CO}}_{2} }}\) values, whereas they do not work for waters equilibrated with calcite under comparatively high \(f_{{{\text{CO}}_{2} }}\) values. This implies that the Na2/Ca and K2/Ca ratios represent the basis not only for geothermometers but also for \(f_{{{\text{CO}}_{2} }}\)-indicators, which is somewhat different from the point of view of previous authors. Although Mg concentration decreases considerably with increasing temperature, the K-Mg and Na-Mg geothermometers suffer several limitations due to different reasons, including the variable composition of chlorites and illites and the varying order-disorder of adularia, impacting the K-Mg geothermometer much more than the Na-Mg geothermometer. Hence, some words of caution are needed on the use of the very popular Na-K-Mg1/2 triangular plot.