Conditions réductrices associées à la serpentinisation : suivi magnétique de l'hydratation de l'olivine de San Carlos, étude de cas naturels et application à la production industrielle d'H2

Abstract : Slow-spreading ridges are fed by a magmatic input and the exhumation of mantle peridotites. Part of the magmatic heat is evacuated through the hydrothermal circulation which can transport seawater up to the mantle rocks. A RedOx reaction between seawater and the olivine and pyroxene from the peridotite can then take place, producing serpentine, magnetite and hydrogen. This reaction, so-called serpentinization, controls the physical and chemical properties of the oceanic lithosphere and is, thus, considered in geophysical models even if its kinetics are poorly constrained. Therefore, this study focuses on the kinetics of San Carlos olivine hydratation, a simple serpentinization reaction. An experimental method has been developed which allows monitoring magnetically the kinetics of the reaction of interest through the amount of magnetite that it produces. This method provides also an indirect estimate of the hydrogen production when coupled to thermochemical modeling. The influence of initial grain size and temperature on the hydration kinetics has also been investigated. An increase in the serpentinization rate at the beginning of the reaction is explained by the formation of microtextures also observed in natural samples (fractures and etch pits) which contribute to the generation of new reactive surface area. The inferred kinetics of serpentinization are by one to two orders of magnitude lower than the one commonly input in geophysical models. In addition to this experimental study, a natural case of RedOx reaction involving serpentinites has been studied which deals with a decimetre wide reaction zone between serpentinites and marbles from Alpine Corsica. The presence of wollastonite, CaSiO3, is explained on a petrological and thermodynamical basis as related to highly reducing conditions characterized by the presence of H2-rich fluids in this subduction zone environment. These fluids are interpreted as resulting from the conservation of the reducing potential of serpentinites throughout the subduction process.
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Benjamin Malvoisin. Conditions réductrices associées à la serpentinisation : suivi magnétique de l'hydratation de l'olivine de San Carlos, étude de cas naturels et application à la production industrielle d'H2. Sciences de la Terre. Université de Grenoble, 2013. Français. ⟨NNT : 2013GRENU002⟩. ⟨tel-00934238⟩

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