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To better understand serpentinization of mantle rocks at slow-spreading ridges, several series of experiments have been conducted at 50 MPa and 250, 300 et 350 ℃ to quantify the H₂ production associated with the serpentinization process, and to clarify the relations between the H₂ generation, the nature of product mineral phases and the redox properties of H₂ bearing hydrothermal systems. The main results of this work, which represents a significant effort toward the consideration of redox parameters and processes during serpentinisation, are the following: – A new experimental method of in situ monitoring of the \fH has been set up at 250 and 300 ℃, 50 MPa. The two variables expressing the H₂ production, \molalHydAq; aq and \fH , have been related quantitatively. Results enable the \fO of hydrothermal oceanic systems to be computed from the dissolved hydrogen concentration. H₂ production starts early, increases rapidly and is continuous in our serpentinization experiments. – Harzburgite-seawater interaction experiments show that serpentinisation is an irreversible dissolution reaction of olivine and orthopyroxene and that it consists of a sequence of discrete steps involving metastable and disequilibrium phase assemblages. – Serpentinisation mechanisms and phase assemblages (especially Fe oxides) depend on \fH, the latter being closely controlled by processes of fluid circulation in the oceanic crust. – Thermodynamic modelling of H₂ rich hydrothermal fluids stresses the need to know precisely \yHydAq, the activity coefficient of \hydAq.; Afin de mieux comprendre la serpentinisation des roches mantelliques au niveau des dorsales océaniques lentes, diverses séries d’expériences ont été réalisées à 50 MPa et 250, 300 et 350 ℃ pour quantifier l’H₂ produit et clarifier les liens entre la production d’ H₂, les phases minéralogiques produites et les propriétés rédox dues à la présence d’ H₂. Les résultats, qui constituent un effort cohérent de prise en compte des paramètres et conditions rédox lors de la serpentinisation, sont les suivants : – Une nouvelle méthode expérimentale de mesure in situ de la \fH a été mise au point à 250 et 300 ℃, 50 MPa. Les deux variables exprimant la concentration d’H2 dans le fluide, \molalHydAq et \fH, ont été reliées quantitativement. Ces résultats permettent le calcul de la \fO dans les systèmes hydrothermaux océaniques à partir de la concentration en hydrogène dissout. La production d’ H₂ commence précocement, augmente rapidement et est continue au cours de la serpentinisation. – Les expériences d’interaction harzburgite-eau de mer montrent que la serpentinisation est une dissolution irréversible de l’olivine et de l’orthopyroxene et qu’elle se décompose en une succession d’étapes impliquant des assemblages de phases hors d’équilibre et métastables. – Les mécanismes de la serpentinisation et les assemblages de phases produits (en particulier les oxydes de fer) dépendent de la \fH laquelle est étroitement contrôlée par les régimes de circulation des fluides dans la croûte océanique. – La modélisation thermodynamique des fluides hydrothermaux riches en H₂ souligne le besoin de connaître précisément \yHydAq , le coefficient d’activité de \hydAq.

International audience; To quantify the H2 production associated with serpentinization at slow spreading mid-oceanic ridges, hydrothermal alteration experiments have been performed. Starting materials are a fine-grained harzburgite from Pindos ophiolite and artificial seawater (3.2 wt.% NaCl). Two grain sizes (P1 and P2), both 60% and they contain variable amounts of magnetite and brucite, in addition to serpentine minerals. To link ƒH2 and [H2], experiments coupling the two types of measurement have been performed in the H2O-H2 system. For [H2]

International audience; H2-rich hydrothermal fluids generated by serpentinisation of mantle rocks at slow-spreading ridges have been revealed by recent studies [1, 2]. Fluxes and the future of the H2 produced by this process are poorly constrained [1, 3]. In this study, we aim to quantitatively evaluate the H2 production fluxes associated with these hydrothermal systems and to document the kinetics of the hydrogen-producing reaction. For this matter, hydrothermal serpentinisation experiments are being undertaken on mixtures composed of a natural peridotite from the Pindus ophiolite and olivine crystals from San Carlos. The experiments are conducted at a temperature of ~ 300° C and a pressure of 450-500 bars in large-volume Dickson-Seyfried bombs for periods of × 1 month. Starting materials are powders between 1 - 100 μm for the peridotites and individual grains ranging from 1 - 2 mm for the San Carlos olivine. They are reacted with a homemade artificial seawater in such proportion that water-rock ratio = 1.8. The reactants are loaded in a modified Ti cell fitted with a semi-permeable Au-Pd membrane simultaneously allowing direct sampling of the hydrothermal fluid and in situ monitoring of the pH2 during the advancement of the reaction. The gas fraction of the fluid sampled is then analyzed by gas chromatography (GC). The pH2 readings show traces of H2 to be present from the second day of experiment. The increase of the pH2 reaches a maximum after ~ 6 days and the pH2 finally stabilizes after ~ 16 days at ~ 12.5 bars, which corresponds to a local fO2 of about NNO-4. The GC measurements, performed after 30, 43, 51 and 65 days, yield respectively, H2 concentrations of 82.4, 89.7, 90.3 and 101 mmol.kg-1 of water, in reasonable agreement with results from previous studies [4-6]. Further experiments are being undertaken in order to: duplicate observations, especially the pH2 readings, more closely link the GC measurements and the in situ pH2 readings, especially during the first 15 days of experiment, and relate H2 production with the mineralogical composition of products of the serpentinisation reaction. The possible influence of the oxidation of the Ti cell on the H2 production will be also checked by using a Au bag instead of a Ti cell. However, from our results, it appears that H2 generation via serpentinisation is surprisingly rapid. [1] ] J.-L. Charlou et al., Chem. Geol., 191, 2002. [2] C. Mével, C.R. Geosc., 335, 2003. [3] M. Cannat et al., Geophys. Mono. Series, 188, 2010. [4] D.G. Allen, and W.E. Jr Seyfried, Geochim. Cosmochim. Acta 67 (8), 2003. [5] M.E. Berndt, et al., Geology 24 (4), 1996. [6] W.E. Seyfried,et al., Geochim. Cosmochim. Acta 71, 2007.