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The pH determination of palaeofluids : experimental and thermodynamic approach

Abstract : The aim of this study is to develop the methodology to get the pH values of the palaeo-circulation in the system H2O-CO2-HCO3--NaCl in the P-T conditions of circulation. It combines the use of an experimental approach for calibrating the analytical method, Raman spectrometry, and numerical modelling for the quantitative interpretation of phase and chemical species equilibria. The routine analytical techniques for fluid inclusions are used to carry out the experimental analyses, such as microthermometry and micro-Raman spectroscopy. Experiments are based both on the use of synthetic fluid inclusions produced in quartz crystal and a new method to make synthetic fluids in fused-silica glass capillary. The synthetic fluid in silica glass capillary can be applied to a wide range of pressure (up to 2 to 3 kbar) at room temperature and temperature (as high as at least 673 K). The experimental protocol of capillary loading with different chemical species in the system H2O-CO2-HCO3--NaCl and the methods to quantify their amount are elaborated and validated. Numerical modelling is based on the use of the Pitzer model to describe the liquid aqueous phase, especially the activity coefficients of each chemical species in the liquid aqueous phase. The Duan equation of state is used for the P-V-T-X properties of the liquid and vapour phases in the simplified subsystem H2O-CO2-NaCl. The application pressure and temperature ranges of the model are respectively 273 to 523 K, and 0 to 2000 bar. Range of NaCl concentration is between 0 and 5 molal and corresponds to the range of ice melting temperature (Tmice). The determination of the concentration of aqueous species HCO3- and CO2 is performed by using micro-Raman spectrometry and is calibrated with using the standards of synthetic fluid inclusion and silica glass capillary. A software program based on the Pitzer and Duan models has been done for the interpretation of i) ice melting temperature (Tmice) and bicarbonate concentration in terms of NaCl concentration, and ii) total homogenisation temperature in terms of bulk molar volume and pressure at homogenisation temperature. Then speciation of the fluid is calculated along the isochore till the trapping temperature if lower than 250 °C. A first application to the Mokrsko gold deposit in Bohemian Massif is done and discussed
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Junying Ding. The pH determination of palaeofluids : experimental and thermodynamic approach. Earth Sciences. Université Henri Poincaré - Nancy 1, 2010. English. ⟨NNT : 2010NAN10106⟩. ⟨tel-01746356⟩

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