A refined calibration of the Na-Li geothermometer for pelitic lithologies before and after orogenic metamorphism

Orogenic belts, especially those with recent tectonic and magmatic activities, can host geothermal waters with energy potential, however, exploration remains limited. In such terrains, geothermometers provide a cost-effective method of estimating reservoir temperatures based on water chemistry. In general, silica and Na-K geothermometers are used; however, both have limitations in metapelitic lithologies. Silica methods are biased by dilution and evaporation, while Na-K methods are skewed by K-feldspar metamorphic transformation to illite. Hence, in this context, the low reactivity of Li in geothermal waters makes the Na-Li geothermometer a more robust proxy for estimating reservoir temperatures. We compiled global geothermal well data on hydrogeochemistry and temperature to assess how lithology and water sources influence the Na/Li ratios. A consistent temperature-Na/Li ratios relationship were noticed across metamorphic, sedimentary and non-basaltic igneous rocks, suggesting metamorphism has limited effect on altering water Na/Li ratios. This implies the Na-Li geothermometer is equally applicable in pelitic and metapelitic settings, and a refined calibration for (meta)pelitic reservoirs recharged by meteoric or saline waters is as follows: 1000/T = 0.51±0.02 × log(Na/Li) + 1.22±0.05 Geothermal waters from the Chingshui metapelitic geothermal field validate this calibration, showing deviations of < 5.6%. Applying this calibration to the pelitic reservoirs in southwestern Taiwan yields temperatures consistent with those estimated by the Na-K geothermometer, highlighting the robustness of the Na-Li geothermometer for pelitic settings. Moreover, the Na-Li geothermometer demonstrates consistent accuracy before and after orogenic metamorphism, rendering it more reliable for metamorphic lithologies than the Na-K approach.