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

• Main Authors: Sanjuan, Bernard, Asmundsson, Ragnar, Romain, Millot, Michel, Brach
• Other Authors: Bureau de Recherches Géologiques et Minières (BRGM) (BRGM), Tiger Energy Services, Financement européen, European Project:
• Format: Conference Object
• Language: English
• Published: HAL CCSD 2012
• Subjects: Géothermomètre; sodium; lithium; fluide géothermal; Islande; Djibouti; ride; rift; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; [SDE.MCG]Environmental Sciences/Global Changes; Canada; Reykjanes; Seltjarnarnes; Iceland
• Online Access: https://hal-brgm.archives-ouvertes.fr/hal-00662009; https://hal-brgm.archives-ouvertes.fr/hal-00662009/document; https://hal-brgm.archives-ouvertes.fr/hal-00662009/file/Goldschmidt2012_Sanjuan.pdf

Thermometers such as Silica, Na/K, Na/K/Ca, Na/K/Ca/Mg or delta18O (H2O-SO4), based on empirical or semi-empirical laws derived from chemical equilibrium reactions between water and minerals in the deep reservoirs, are commonly used in geothermal exploration in order to estimate the reservoir temperatures. Unfortunately, these estimations are not always concordant because of processes which can perturb the chemical composition of the fluids during their ascent up to the surface (water mixing, fluid cooling, etc.). Given these discordances, auxiliary thermometers such as Na/Li, based on statistical relationships, were also developed. As Li is rather lowly reactive, the use of this thermometer can give more reliable temperature estimations. Presently, three different Na/Li relationships ([1], [2]) are mainly available according to the fluid salinity and the geological environment (volcanic/granitic and sedimentary rocks). This study carried out in the framework of the European HITI project (HIgh Temperature Instruments for supercritical geothermal reservoir characterization and exploitation) with the collaboration of ISOR Iceland Geosurvey proposes two new Na/Li thermometric relationships. The first concerns the fluids derived from high-temperature seawater-basalt interaction processes existing in the oceanic ridges and rises as well as in the emerged rifts such as those of Iceland (Reykjanes, Svartsengi and Seltjarnarnes geothermal fields) and Djibouti (Asal-Ghoubbet and Obock geothermal areas). It can be expressed as follows: Log (Na/Li in mol/l) = 920/(T°K) + 1.105 (r2 = 0.994). The second relationship, developed using dilute fluids collected only from Icelandic geothermal wells in the 100-325°C range and surprisingly close to that determined by Fouillac and Michard (1981) for volcanic saline fluids at temperatures higher than 200°C, is: Log (Na/Li in mol/l) = 1786/(T°K) - 0.936 (r2 = 0.976). The uncertainty on temperature estimation is ± 25°C for both relationships. These results confirm that the Na/Li ratios ...