Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments

• Two new Na/Li thermometric relationships are presented for geothermal exploration.
• One concerns the fluids derived from seawater–basalt interaction processes.
• The other can be applied on HT dilute geothermal waters in contact with basalts.
• These relationships are controlled by chemical reactions involving illite and micas.
• No thermometric relationship using Li isotopes could be determined.

This work has made it possible to obtain two new Na/Li geothermometric relationships in addition to the three already known (Fouillac and Michard, 1981; Kharaka et al., 1982) and confirms that the Na/Li geothermometer, unlike the Na/K, Na/K/Ca, K/Mg and silica geothermometers, or the isotope δ18O (H2O–SO4) geothermometer, also depends on the fluid salinity and the nature of the reservoir rocks reacting with the geothermal water. One of the relationships concerns the fluids derived from seawater–basalt interaction processes existing in emerged rifts such as those of Iceland (Reykjanes, Svartsengi, and Seltjarnarnes geothermal fields) and Djibouti (Asal-Ghoubbet and Obock geothermal areas), or in numerous oceanic ridges and rises (Mid-Atlantic and Middle-Valley ridges, East Pacific rise, etc.). The best adapted Na/Li relationship for geothermal fluids discharged from emerged rifts between 0 and 365 °C is:TK=920/⁢log⁡Na/Li−1.105r2=0.994,n=27where Na and Li are the aqueous concentrations of these elements given in mol/L.The other Na/Li relationship was determined using dilute waters collected from wells located in different high-temperature (200–325 °C) volcanic geothermal areas of Iceland (Krafla, Námafjall, Nesjavellir and Hveragerdi). This relationship can be expressed as follows:T(K) = 2002/ [log(Na/Li) + 1.322] (r2 = 0.967, n = 17).These two relationships give estimations of temperature with an uncertainty close to ± 20 °C. The second Na/Li relationship was also successfully applied to HT dilute geothermal waters from the East African Rift (Ethiopia, Kenya).Some case studies in the literature and thermodynamic considerations suggest that the Na/Li ratios for this type of fluids could be controlled by full equilibrium reactions involving a mineral assemblage constituting at least albite, K-feldspar, quartz and clay minerals such as kaolinite, illite (or muscovite) and Li-micas. Unlike the Na/Li ratios, no thermometric relationship using Li isotopes could be determined for this type of water. However, it was noticed that δ7Li values higher than 16‰ are always associated with low- to medium-temperature waters.