Production d'hydrogène par la serpentinisation des roches mantelliques (apport de la modélisation à la compréhension des processus physiques et géochimiques)

La circulation hydrothermale océanique est un élément clé dans le transfert de chaleur et de matière des profondeurs de la Terre vers sa surface. Certains sites hydrothermaux produisent de grandes quantités d hydrogène et de méthane. C est le cas du site de Rainbow (36N), situé sur la dorsale médio-atlantique, auquel on s intéresse ici. Les fortes concentrations d hydrogène ([H2]=16 mM kg-1) et de méthane ([CH4]=2,5 mM kg-1) des fluides chauds (~365C) émis par ce site ont été interprétées comme étant issues de l hydratation des péridotites du diapir mantellique qui constitue son socle géologique du site, mais l altération d assemblages gabbroïques pourrait également être à l origine de sa production d hydrogène. Ces processus géochimiques dépendent fortement des caractéristiques de l écoulement des fluides au sein de la matrice poreuse et fracturée du site. Cette étude a donc consisté au développement et à l application de modèle numériques, thermo-hydrogéologique et géochimique et de leur chaînage, dans le but de caractériser la circulation hydrothermale, la production d hydrogène et son transport au sein du site de Rainbow. Plusieurs résultats découlent de ces modélisations : la circulation hydrothermale est tridimensionnelle, de faible extension horizontale (Ø~2-8km), doit très probablement être canalisée par un chemin préférentiel de forte perméabilité, et peut être stable sur plusieurs milliers d années. Nous avons pu montrer que l hydrogène est produit par serpentinisation en amont de la zone de décharge et estimer les coefficients cinétiques in situ, la quantité d hydrogène produite et montrer la possibilité d une production d une durée supérieure à 25 000 ans.Hydrothermal activity along the axis of mid-ocean ridges is a key driver for energy and matter transfer from the Earth s interior to the ocean floor. Numerous hydrothermal sites have been discovered, and some of them emit high quantities of hydrogen and methane. In this manuscript, the presented studies focus on the Rainbow site (36N), situated on the Mid-Atlantic Ridge crest. The high hydrogen and methane concentrations ([H2]=16 mM kg-1and [CH4]=2,5 mM kg-1) of the hot fluids at this site have been interpreted as indicators of ongoing serpentinization processes. However, such high hydrogen and methane concentrations might be also produced by gabbroic alteration processes which could lead to a mineral assemblage such as chlorite + talc + magnetite + tremolite. These alteration processes are strongly dependent on the hydrothermal circulation characteristics and behaviours. In this study, we developed and used thermo-hydrogeological and geochemical numerical models, and their coupling, for characterization of hydrothermal circulation, and hydrogen production and transport, of the Rainbow vent site. We showed that hydrothermal circulation is conditioned by its 3D spatial configuration. This circulation is of small dimension (Ø > 2km), and should probably be organized by a preferential pathway of high permeability values. It has remained stable for several thousand years. We also showed that hydrogen was a product of serpentinization process that occurs far from the discharge zone. We estimated in situ kinetic coefficients, and the hydrogen mass currently produced each year. We showed the possibility of a long lifetime hydrogen production (>25000 yrs).BREST-SCD-Bib. electronique (290199901) / SudocSudocFranceF

Caractérisation physique et géochimique d'hydrates de gaz d'environnements géologiques différents (apport des techniques de Diffraction X Synchrotron et de Spectroscopie Raman)

L amélioration des connaissances concernant la distribution des hydrates de gaz dans les sédiments, leurs caractéristiques physiques, chimiques ou thermodynamiques est essentielle afin de prévenir les risques auxquels ils peuvent être associés, d anticiper leur rôle dan l évolution du climat ou de développer les technologies nécessaires à leur exploitation. Dans cette optique, ce travail de thèse apporte des données supplémentaires indispensables à une meilleure compréhension de leur origine, formation et stabilité. Le travail a porté su l étude de quatre hydrates de gaz naturels issus de la marge Africaine, la marge Norvégienne et la mer de Marmara. Les hydrates des marges Africaine et Norvégienne cristallisent dans une structure de type I, ce qui est en accord avec leur origine biogénique. En revanche, ceux provenant de la mer de Marmara, composés de gaz d origine thermogénique, ont une structure de type II. L utilisation de la spectroscopie Raman a également permis de suivre la dissociation des hydrates naturels et de montrer qu il n existe pas de dissociation préférentielle entre les petites et les grandes cages formant les hydrates naturels. En outre, la grande variabilité de la zone de stabilité des hydrates sur le volcan de boue Hakon Mosby a pu être évaluée par modélisation thermodynamique. Une étude identique a montré que seuls des hydrates d origine thermogénique peuvent se former en mer de Marmara. Enfin, l ensemble des données géochimiques des eaux interstitielles, des gaz et des hydrates de gaz obtenus au cours de la campagne Vicking (2006) a permis d étudier la circulation de fluide et de gaz formant les hydrates de gaz sur le volcan de boue Hakon Mosby.The knowledge of the occurrence of gas hydrates in the natural environment, their physical, chemical or thermodynamical properties is essential to prevent geohazards, to anticipate their role in climate change or to develop technologies to take advantage of this energy resource. To mis purpose, this work reports a physical and chemical characterization of four natural gas hydrates from African and Norwegian margins, and from the Sea of Marmara, for documenting their origin, formation and stability. Samples from African and Norwegian margins crystallize in type I structure. This observation is in agreement with their biogenic origin. On the other hand, hydrate samples from the sea of Marmara, characterized by a thermogenic origin, exhibit a type II structure. Raman spectroscopy was also used to investigate the dissociation processes of natural gas hydrates. These results indicate that there is no preferential dissociation of large small cages. Thermodynamical modeling let us evaluate the highly variable gas hydrate stability fields in sediments from the Hakon Most Mud Volcano, whereas it let us assert that only thermogenic gas hydrates can crystallize in the sea of Marmara. In a last chapter, geochemical data obtained from porewaters, gases, and gas hydrates collected during the Vicking cruise (2006) - HERMES Program - permitted to characterize the processes controlling the fluid circulation in the Hakon Mosby Mud Volcano where gas hydrates are present in great quantity.BREST-BU Droit-Sciences-Sports (290192103) / SudocSudocFranceF

Origine et circulation des fluides dans les sédiments des marges (contribution de l'hélium et du méthane dans la compréhension des processus)

La migration des fluides est très active dans la couverture sédimentaire des marges. L'étude de ces fluides est fondamentale pour mieux appréhender le potentiel énergétique des marges. Le méthane, gaz prédominant des sédiments marins, est formé essentiellement par dégradation de la matière organique. Cependant du méthane pourrait aussi être généré par serpentinisation des roches du manteau présentes sous la couche sédimentaire dans certains contextes géodynamiques.Pour tester cette hypothèse, les isotopes de l'hélium sont des traceurs de choix de l'interaction fluide-manteau. Une technique innovante a donc été mise au point pour l'échantillonnage et l'extraction quantitative de l'hélium dissous dans les fluides interstitiels des sédiments. Durant les missions océanographiques Zairov2 et Vicking, des échantillons ont été collectés dans deux zones de suintements froids : le Pochmark Regab sur la marge congo-angolaise et le volcan de boue Hakon Mosby sur la marge norvégienne. Les signatures isotopiques en hélium ne révèlent pas de composante mantellique, ce qui n'implique pas le processus de serpentinisation pour la genèse de méthane dans ces zones. Néanmoins, couplés à l'analyse du méthane et des ions majeurs des fluides interstitiels, les profils d'hélium apportent des résultats significatifs. Sur le Pockmark Regab, l'hélium, traceur bien plus sensible que la température à l'advection des eaux interstitielles, permet de déterminer des vitesses d'advection et de quantifier les débits d'eau expulsée, sur le volcan de boue Hakon Mosby. Les résultats confirment une zonation concentrique de l'advection et une origine profonde des fluides du volcan.Fluid migration is a very important process in marine sediments on margins. Studying fluids is fundamental to constrain the energy potential of margins. Methane is the predominant gas in marine sediments, deriving primarily from the degradation of organic matter. However methane could be also generated through the serpentinization of mantle rocks underlaying the sedimentary cover in some specific geodynamical settings. One way to test this hypothesis is to use helium isotopes, which are known to be powerful tracers of the fluid-mantle interaction. To this purpose, a new method has been developed for the sampling and the quantitative extraction of dissolved helium from sediment pore-waters. During the Zairov2 and Vicking cruises, samples were collected in two cold seep areas : the Regab Pochmark on the Congo-Angola margin, and the Hakon Mosby mud volcano on the Norwegian margin. Helium isotopes in pore-fluids reveal no mantle signature, which indicates the absence of any methane generated by serpentinization in these areas. Nevertheless, taken together with methane and major ion contents in pore-waters, helium isotope profiles obtained with this new analytical method give significant new results.At the Regab Pochmark, helium appears to be a more sensitive tracer of water advection than temperature, allowing the determination of advection velocities and the quantification of expelled water rate. At the Hakon Mosby mud volcano, the results confirm a concentric zonation of advection and a deep origin for the volcano fluids.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Nankai Trough, Japan Trench and Kuril Trench: geochemistry of Fluids sampled by submersible "Nautile"

Deep-water samples collected during the Kaiko project are often associated with biological communities located on geological structures favorable to fluid venting. The evidence of fluid venting are the temperature anomalies, the decrease in sulfate concentrations, the content in methane and the low C1 (C2 + C3) ratio of light hydrocarbons. Because of large dilution by ambiant seawater during sampling it is difficult to compute the composition of the advected end-member pore fluid. Part of this fluid should originate in the "petroleum window", i.e. at temperature about 60°C. Modeling the upward flow of water, taking into account the anomalies of temperature measured on the seafloor and the geochemical anomalies, leads to non-steady-state advection of the pore fluid. The occurrence of a deep component in the fluid has implications for the geological and tectonic models of the subduction zones off Japan. © 1987.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Coupled modeling of thermics and hydrogeology with the Cast3M code: application to the Rainbow hydrothermal field (Mid-Atlantic Ridge, 36°14′N)

International audienc

Helium isotopes at the Rainbow hydrothermal site (Mid-Atlantic Ridge 36,14'N)

The 3He/4He ratio and helium concentration have been measured in the vent fluids and the dispersing plume of the Rainbow hydrothermal site, on the Mid-Atlantic Ridge (MAR). The 3He/4He ratio (7.51 Ra) and 3He end-member concentration (25 pmol/kg) are in the range of observed values elsewhere on mid-ocean ridges, pointing to the relative homogeneity of the upper mantle with respect to helium isotope geochemistry. 3He is linearly correlated with methane and manganese throughout the plume, with CH4/3He and Mn/3He ratios identical to those measured in the hot fluids. The bulk residence time of the plume in the rift valley estimated from the plume 3He budget is V20 days. The 3He flux transported by the plume, calculated from current-meter data, is 12.3: 3 nmol/s, requiring a flux of 490 kg/s of high-temperature fluid. The scaling of the heat flux emitted by the Rainbow site to that of 3He, using the 3He/heat ratio measured in the hydrothermal fluids (9.3 :2U10318 mol/J), indicates a heat output of 1320 MW. With a regional spreading rate of V25 mm/year, we calculate that the annual 3He flux for this section of the MAR is of the order of 0.5:0.2 mmol per kilometre of ridge per millimetre of newly formed crust. This figure compares well with the flux calculated for the neighbouring Lucky Strike segment. Although the uncertainties remain large, both fluxes are V40-50% above the world average (0.33 mmol/km/mm), thus supporting earlier suggestions that the intensity of the hydrothermal activity south of the Azores is higher than expected from the regional spreading rate

Distribution and behavior of dissolved hydrogen sulfide in hydrothermal plumes

Through the deep ocean, hydrothermal plumes disperse high concentrations of key chemical tracers including He-3, CH4, Mn, Fe, H2S, etc. This paper focuses on the distribution and behavior of total dissolved sulfide (sulfide hereafter) in hydrothermal plumes to show that its plume concentration decreases to subnanomolar a few kilometers from the vents. We also report on sulfide removal rates determined at in situ conditions; we observe that they are two orders of magnitude greater than for open ocean seawater, consistent with sulfide being detected only in the vicinity of hydrothermal vents. From our observations, we infer that the sole presence of sulfide in hydrothermal plumes locates active venting at the kilometer scale

Characterisation of dissolved organic compounds in hydrothermal fluids by stir bar sorptive extraction - gas chomatography - mass spectrometry. Case study: the Rainbow field (36°N, Mid-Atlantic Ridge)

Abstract The analysis of the dissolved organic fraction of hydrothermal fluids has been considered a real challenge due to sampling difficulties, complexity of the matrix, numerous interferences and the assumed ppb concentration levels. The present study shows, in a qualitative approach, that Stir Bar Sorptive Extraction (SBSE) followed by Thermal Desorption – Gas Chromatography – Mass Spectrometry (TD-GC-MS) is suitable for extraction of small sample volumes and detection of a wide range of volatile and semivolatile organic compounds dissolved in hydrothermal fluids. In a case study, the technique was successfully applied to fluids from the Rainbow ultramafic-hosted hydrothermal field located at 36°14’N on the Mid-Atlantic Ridge (MAR). We show that n-alkanes, mono- and poly- aromatic hydrocarbons as well as fatty acids can be easily identified and their retention times determined. Our results demonstrate the excellent repeatability of the method as well as the possibility of storing stir bars for at least three years without significant changes in the composition of the recovered organic matter. A preliminary comparative investigation of the organic composition of the Rainbow fluids showed the great potential of the method to be used for assessing intrafield variations and carrying out time series studies. All together our results demonstrate that SBSE-TD-GC-MS analyses of hydrothermal fluids will make important contributions to the understanding of geochemical processes, geomicrobiological interactions and formation of mineral deposits.</p