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Use of a new Sodium/Lithium (Na/Li) geothermometric relationship for High-Temperature (HT) geothermal fluids derived from seawater/basalt interaction processes: Application to the Djibouti case

Abstract : This study has allowed improving the Na/Li geothermometric relationship for High-Temperature (HT) fluids derived from seawater and basalt interaction processes existing in emerged Rifts such as those of Djibouti (Asal-Ghoubbet and Obock geothermal areas) and of Iceland (Reykjanes, Svartsengi and Seltjarnarnes geothermal fields), or in numerous Oceanic Ridges and Rises (Middle-Atlantic Ridge, East Pacific Rise, etc.). The best adapted Na/Li relationship for this type of fluids seems to be: T (°K) = 920 / [log (Na/Li) - 1.105] (r2 = 0.994) where Na and Li are the aqueous concentrations of these elements given in mol/l. This relationship which can give estimations of temperature for a value range from 0 to 365°C, with an uncertainty of ± 25°C, is very different from the three main relationships known in the literature (Fouillac and Michard, 1981; Kharaka and Mariner, 1989) and from that recently found for the HT dilute geothermal fluids from Iceland (Krafla, Namafjall, Nesjavellir and Hveragerdi geothermal areas; Sanjuan et al., 2010) in the framework of the European HITI project (HIgh Temperature Instruments for supercritical geothermal reservoir characterization and exploitation). The existence of different Na/Li geothermometric relationships seems to indicate that the Na/Li ratios not only depend on the temperature but also on other parameters such as the fluid salinity and origin, the nature of the geothermal reservoir rocks in contact with the deep hot fluids or the equilibrium reactions between these fluids and the reservoir mineral assemblages. Some case studies in the literature and thermodynamic considerations suggest that the Na/Li ratios for the HT geothermal fluids derived from seawater and basalt interaction processes could be controlled by a full equilibrium reaction involving a mineral assemblage constituted, at least, of albite, K-feldspar, quartz and clay minerals such as kaolinite, illite (or muscovite) and Li-micas. Moreover, this study confirms the presence of a deep seawater-derived geothermal fluid indicating a temperature close to 210°C and salinity of about 35 g/l, which would supply the littoral hot springs located in the Obock area (Houssein et al., 1993). It also suggests that the fluid collected from the thermal spring located near the Ghoubbet Channel (“Passe du Ghoubbet”; Sanjuan et al., 1990) results from a mixing between a marine origin fluid which would interact with basalt rocks at a temperature value close to 160°C and cold seawater.
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Bernard Sanjuan. Use of a new Sodium/Lithium (Na/Li) geothermometric relationship for High-Temperature (HT) geothermal fluids derived from seawater/basalt interaction processes: Application to the Djibouti case. Third East African Rift Geothermal Conference ARGEO-C3-DJIBOUTI. Exploring and harnessing the renewable and promising geothermal energy., Nov 2010, Djibouti, Djibouti. pp.231-248. ⟨hal-00575990⟩

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