The Activity-Based Theoretical Na–K Geoindicators

Accepting that a single universally-working Na-K geothermometer does not exist and the Na-K geothermometers are controlled by the exchange reaction involving fully-ordered low-albite and variably-ordered adularia, from fully ordered maximum-microcline to completely disordered high-sanidine, the Na/K activity ratio of the aqueous solution is expected to be a function of both temperature and the ordering parameter of adularia, Z. However, the two limiting Na-K geothermometers involving low-albite and either high-sanidine (with Z = 0) or maximum-microcline (with Z = 1) give equilibrium temperatures differing by 125 ± 3 °C on average, in the range 100–350 °C, and have little usefulness. We used Multiple Regression Analysis to try to predict the ordering parameter of adularia based on the chemical characteristics of the 1013 selected reservoir liquids, but it turned out that Z is chiefly controlled by Na, K, and SiO2. This is a disappointing result being affected by a circular argument. Therefore, for the moment, it is advisable to utilize the Na/K log activity ratio to estimate the ordering parameter Z of the hydrothermal adularia in hypothetical equilibrium with the considered aqueous solutions and to use this information in other geothermometers representing the subject of the following chapters.