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Reconstitution chimique des paléofluides par spectrométrie d'émission optique couplée à l'ablation laser. Applications aux fluides alpins et aux fluides de bassins

Abstract : Chemical composition of geological fluids can be described in the H20-gas-salt system. Previous studies at CREGU and in the G2R laboratory have permitted to reconstruct the palaeofluid composition in the H2O-CO2-CH4-N2-H2S-NaC1 system. The aim of this work consists in the addition of the alkaline and alkaline-earth species (NaC1, KC1, ; LiC1, MgC12, CaC12), which is a necessary condition to characterise the chemical equilibrium between fluid and mineral. An analytical technique of single fluid inclusions, based on optical emission spectrometry coupled to laser ablation (LIBS) has been developed, using a prototype. This work is the result of collaboration with the LSLA (Laboratory of Analytical Laser Spectroscopy) at Saclay (CEA, France) and the Dilor firm. Using different standards (minerais, synthetic glasses and synthetic fluid inclusions): different calibration curves were established. Direct quantification of element concentrations for analysis on solids and estimations of cationic ratios (Na/K, Na/Li, Na/Ca, Ca/Mg, Ba/Sr .. . ) for the applications on fluid inclusions has been carried out. The integration of additional values obtained on the same samples, using microthermometry (salinity) and Raman spectroscopy (gas and chlorinity) allowed to determine the global composition offluid inclusions.First applications were performed on fluid inclusions from quartz samples, collected in alpine clefts in the Mont- Blanc massif. Significant data conceming the palaeo-circulation of fluids have been obtained. The variations of the chemical compositions of the alpine fluids demonstrate mixing between different end-members (Li-rich, Ca- rich and Na-rich), thus the existence of an homogenous fluid reservoir seems improbable. Fluid mixing is intermittent and local. Pressure- temperature conditions of trapping are estimated in the range of 2800 - 3600 bars and 330° and 420°C respectively, which correspond to estimated depths of 10.4 and l3.3 km, considering a lithostatic pressure. Studies of fluid inclusions in sedimentary basin have permitted the reconstitution 0 chemical composition of the palaeofluid (Caramal, Australia and Oklo, Gabon). The existence of mixing and/or dilution processes between several end-members for Caramal zone has been demonstrated. The presence of different generations of fluids, with contrasted compositions for the Oklo area, has been also described. This new analytical tool has been tested on solids. First analyses demonstrate that it is possible to quantify lithium in different matrixes (for example, quartz, glass and mica).
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Cécile Fabre. Reconstitution chimique des paléofluides par spectrométrie d'émission optique couplée à l'ablation laser. Applications aux fluides alpins et aux fluides de bassins. Sciences de la Terre. Université Henri Poincaré - Nancy 1, 2000. Français. ⟨NNT : 2000NAN10129⟩. ⟨tel-01746481⟩

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