Assessing the suitability of lithium-related geothermometers for estimating the temperature of felsic rock reservoirs

Lithium-related geothermometers are relatively rarely used because they are based on empirical relations derived from poorly known water-mineral equilibrium reactions and do not always give valid estimations. In this study, we evaluated the applicability of Na-Li and Li-Mg geothermometers based on hydrochemical data for geothermal waters from nine geothermal systems in felsic reservoirs in China. The results indicate that the Na-Li geothermometer provided by Michard (1990), which is similar to those given by Fouillac and Michard (1981) and Verma and Santoyo (1997) with Cl <0.3 mol/L, is suitable for fully equilibrated waters from felsic reservoirs and their estimates better represent the temperatures of the deep rather than the shallow reservoirs. The Na-Li ionexchange equilibrium in thermal waters cannot be reached without a prior Na-K equilibrium, which ensures that the Na concentration remains constant. Also, variations in the Na-Li ratios may be significantly unpredictable such that the Michard (1990) Na-Li geothermometer may coincidentally give valid temperature estimates for thermal waters from the low-temperature geothermal systems. Furthermore, the Li in the thermal waters may originate from water-mineral reactions or mixing with magmatic water or both. In reservoirs consisting of basalt and felsic rocks, variations in hydrothermal alteration may be the key factor that explains the large differences in Na-Li equilibria in the thermal waters. Additionally, small measurement errors could bring about large deviations in the temperature estimations due to the large difference between Na and Li concentrations in the thermal waters from the felsic reservoirs. Unlike the Li concentration, which remains constant once the thermal waters leave the deep reservoirs, the concentration of Mg in the thermal waters is sensitive to the temperature in the shallow reservoirs. Hence, we discourage the use of the Li-Mg geothermometer for estimating felsic reservoir temperatures.

Automated summary

The study evaluated the applicability of Na-Li and Li-Mg geothermometers in felsic reservoirs in China, finding that Na-Li geothermometer is suitable for fully equilibrated waters from felsic reservoirs and represents temperatures of deep reservoirs better. On the other hand, Li-Mg geothermometer is not recommended for estimating temperatures of felsic reservoirs.