Lithium-rich basement brines: Activity versus concentration geothermometry

Lithium-rich brine plays a crucial role in the energy transition. Specifically, geothermal Upper Rhine Graben brines and the fluid inclusions in the adjacent southwesternmost Black Forest (Germany) are lithium-rich due to their interactions with Variscan basement rocks (granite and gneiss). Given their high salinity, however, conventional empirical geothermometers may prove inadequate, as these rely on elemental ratios in terms of concentration (molar or weight basis) rather than the elements’ activity. This short communication compares several classical geothermometers, such as Na/K, Na/Li, Mg/Li, K2/Mg, Mg/K, with innovative activity geothermometers that use the ion activity products of Ca2+, K+, Li+, Na+ and H2O based on the equilibrium between a generic lithium-mica, quartz, and oligoclase. Activity is calculated using the PhreeqcI code and Pitzer database optimized for LiCl-H2O, CaCl-H2O and NaF-H2O systems. The lithium minerals thermodynamic database, used as a reference for calculating the solubility product, has been further supplemented with Li-tosudite and Li-donbassite. The results of the inferred temperatures highlight the new activity geothermometer's versatility, even when compared with the to activity Na/K geothermometer based on the albite-microcline equilibrium.