Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A reactive percolation experiment was conducted by injecting seawater into a permeable olivine aggregate at 190 °C and 19 MPa to explore the relationships between hydration reactions and hydrodynamic properties during the onset of serpentinization in the ultramafic lithosphere exposed at mid-ocean ridges. The experiment was stopped after 23 days when the sample became impermeable. The initial flow rate was 0.2 mL/h and then was decreased down to 0.06 mL/h after 8 days. Permeability decreased continuously throughout the experiment. The analyses of fluid chemistry time series and of the mineralogy and structure of the reacted sample showed olivine dissolution and precipitation of proto-serpentine, brucite and Fe-oxides. These reactions are controlled by coupled hydrodynamic and chemical processes interacting at different time and spatial scales. Their first characteristic is the production of silica rich outlet fluids in disequilibrium with the observed reacted mineral assemblages. These compositions are interpreted as resulting from a suite of coupled dissolution-precipitation reactions, controlled at the pore scale by surface kinetic processes, that rapidly reaches steady-state (constant fluid composition independent of fluid flux). Hence, the effective rate of serpentinization was controlled, to the first order, by the transport of reactants during the experiment. Mass balance calculations show that the rate of olivine conversion was fast (0.2-0.5 wt%/day), yet only ∼8 wt% of the olivine sample reacted, because the permeability drop limited fluid circulation. Porosity varied little during the experiment compared to permeability changes: the decrease of permeability was explained by the structure of the newly formed serpentine favouring the clogging of fluid paths. The rate at which permeability decreased was the fastest at low flux conditions. This suggests that permeability changes were not controlled simply by the kinetics of the serpentinization reactions, but mainly reflected the development of low Peclet pore scale zones forming micro-environments that control the serpentinization reaction-paths. These results outline the need to take into account the coupling of hydrodynamic and chemical processes when modelling the effects of reactive transport within the hydrating oceanic lithosphere.
ISSN: 0012-821X

hal-00855043
https://hal.archives-ouvertes.fr/hal-00855043
DOI : 10.1016/j.epsl.2013.03.052

Éditeur(s) : HAL CCSD
Résumé : This work provides new constraints on the interactions between deformation and percolation of melt or fluids, as well as on the implications of these processes for the seismic properties of the mantle wedge. It is based on the analysis of spinel peridotites from the massif of Ronda (Spain) and two xenolith suites from active subduction zones (Kamchatka, Papua New Guinea). A coupled structural, petrological and geochemical study of these samples shows that they underwent a reactive percolation of melts or Si-rich fluids, which was synchronous to a deformation event occurring under high temperature and low stress conditions, consistent with the PT conditions at the base of the lithosphere or in the asthenosphere. This reactive percolation is responsible for enrichment in pyroxenes, mainly orthopyroxene, which is often localized in bands parallel to the high-temperature foliation. This enrichment is associated with grain size reduction and dispersion of the crystallographic orientation of olivine. The dominant slip system in olivine is {0kl}[100], which results in fast S-wave polarization parallel to the flow direction in the mantle. The decrease in the intensity of the olivine crystal preferred orientations associated with the enrichment in pyroxene results in significant decrease of the anisotropy that may induce an underestimation of the thickness of the anisotropic layer by up to 33%. The observed orthopyroxene enrichment also lowers the Vp/Vs ratio, but cannot explain Vp/Vs < 1.7 mapped locally in the fore-arc mantle in Japan and the Andes. Such low Vp/Vs ratios may however be explained by considering the intrinsic anisotropy of the peridotites, which is generally ignored in large-scale Vp/Vs ratio mapping of the mantle wedge. Infrared analyses show that olivine from the both xenolith collections contains less water than the theoretical saturation calculated for their estimated equilibrium temperature in the spinel stability field. These low water content are similar to those observed in spinel peridotites from other subduction zones and probably record both the low solubility of water in olivine at relatively low pressure and dehydration during exhumation of the xenoliths. These measured water contents as well as theoretical saturation estimations are not sufficient to change the dominant slip direction of in olivine from [100] to [001].
Ce travail apporte de nouvelles contraintes sur les interactions entre déformation et processus d'hydratation et de percolation de magma ou de fluides dans le manteau à l'aplomb d'une zone de subduction et leurs implications sur les propriétés sismiques dans le coin mantellique. Il se base sur l'analyse de péridotites à spinelle provenant du massif de Ronda (Espagne) et deux séries de xénolites issues de zones de subduction actives (Kamchatka, Papouasie-Nouvelle-Guinée). L'étude structurale, pétrologique et géochimique de ces échantillons montrent qu'ils ont subit une percolation réactive de magma ou de fluide synchrone d'une déformation de haute température basse contrainte cohérente avec les condition PT de la base de la lithosphère ou de l'asthénosphère. Cette percolation réactive est responsable d'un enrichissement en pyroxènes, qui est souvent localisé dans des bandes parallèles à la foliation. Cet enrichissement est associé à une dispersion de l'orientation cristallographique de l'olivine. Les systèmes de glissement dominant dans l'olivine sont dans tous les cas {0kl}[100], ce qui implique que la direction de polarisation rapide des ondes S dans la partie supérieure du coin mantellique est parallèle à la direction d'écoulement dans le manteau. La décroissance de l'intensité des OPR de l'olivine associée à l'enrichissement en pyroxènes a pour conséquence une réduction non négligeable de l'anisotropie sismique qui peut induire jusqu'à 33% d'erreur sur l'interprétation de l'épaisseur de la couche anisotrope. Un enrichissement en orthopyroxène peut aussi entraîner une baisse du rapport Vp/Vs, mais ne peut expliquer les rapports Vp/Vs <1,7 cartés dans certains avant-arcs. Cependant de telles valeurs peuvent être expliquées si l'anisotropie des péridotites du coin mantellique est prise en compte. Les analyses par spectroscopie infra-rouge montrent que les olivines des deux séries de xénolites étudiées contiennent moins d'eau que la saturation théorique calculée pour leurs températures d'équilibre dans le domaine du spinelle. Ces faibles concentrations en eau sont similaires à celles observées dans les olivines des péridotites à spinelle des autres zones de subduction. Elles enregistrent probablement la faible solubilité de l'eau dans l'olivine à des pressions relativement basses et la déshydratation au cours de l'exhumation des xénolites. Ces teneurs en eau mesurées ainsi que les estimations de la saturation théorique ne sont pas suffisantes pour changer la direction de glissement dominante dans l'olivine de [100] à [001].
https://tel.archives-ouvertes.fr/tel-00689832

tel-00689832
https://tel.archives-ouvertes.fr/tel-00689832
https://tel.archives-ouvertes.fr/tel-00689832/document
https://tel.archives-ouvertes.fr/tel-00689832/file/These-Soustelle2010.pdf

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : Direct numerical simulations of flow through two millimeter-scale rock samples of limestone and sandstone are performed using three diverse fluid dynamic simulators. The resulting steady-state velocity fields are compared in terms of the associated empirical probability density functions (PDFs) and key statistics of the velocity fields. The pore space geometry of each sample is imaged at 5.06−μm voxel size resolution using X-ray microtomography. The samples offer contrasting characteristics in terms of total connected porosity (about 0.31 for the limestone and 0.07 for the sandstone) and are typical of several applications in hydrogeology and petroleum engineering. The three-dimensional fluid velocity fields within the explicit pore spaces are simulated using ANSYS® FLUENT® ANSYS Inc. (2009), EULAG Prusa et al. (Comput. Fluids 37, 1193–1207 2008), and SSTOKES Sarkar et al. (2002). These computational approaches are highly disperse in terms of algorithmic complexity, differ in terms of their governing equations, the adopted numerical methodologies, the enforcement of internal no-slip boundary conditions at the fluid-solid interface, and the computational mesh structure. As metrics of comparison to probe in a statistical sense the internal similarities/differences across sample populations of velocities obtained through the computational systems, we consider (i) integral quantities, such as the Darcy flux and (ii) main statistical moments of local velocity distributions including local correlations between velocity fields. Comparison of simulation results indicates that mutually consistent estimates of the state of flow are obtained in the analyzed samples of natural pore spaces despite the considerable differences associated with the three computational approaches. We note that in the higher porosity limestone sample, the structures of the velocity fields obtained using ANSYS FLUENT and EULAG are more alike than either compared against the results obtained using SSTOKES. In the low-porosity sample, the structures of the velocity fields obtained by EULAG and SSTOKES are more similar than either is to the fields obtained using ANSYS FLUENT. With respect to macroscopic quantities, ANSYS FLUENT and SSTOKES provide similar results in terms of the average vertical velocity for both of the complex microscale geometries considered, while EULAG tends to render the largest velocity values. The influence of the pore space structure on fluid velocity field characteristics is also discussed.
ISSN: 1420-0597

hal-01174152
https://hal.archives-ouvertes.fr/hal-01174152
DOI : 10.1007/s10596-015-9486-7

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Despite decades of petrological and geochemical studies, the nature and setting of obducted ophiolites remain controversial: the influence of supra-subduction zone environments on pre-existing oceanic lithosphere is yet to assess, and the processes leading to subduction/obduction initiation are still poorly constrained. Our study documents successive influx of slab-derived fluids and progressive strain localization within the upper mantle in a supra-subduction environment during the first few My of the subduction history. We focus on strongly sheared mafic amphibolites intruding peridotites near the mantle–crust transition of the New Caledonia obducted ophiolite and ~ 50 to 100 m above the basal thrust contact of the ophiolite. These m- to hm-long and several m-thick shear bands are interpreted as inherited small-scale intrusions of mafic melts, probably dikes or sills, which were derived from a moderately refractory mantle source refertilized by supra-subduction zone fluids. 40Ar/39Ar age constraints on pargasite at ca. 90 Ma suggest that they could be inherited from the former Pacific west-dipping subduction.Secondary deformation of these mafic intrusions is intimately associated to three major stages of fluid infiltration: (1) the first stage of deformation and metasomatism is marked by syn-kinematic growth of Ca-amphibole (at 700–800 °C and 3–5 kbar) with a distinctive supra-subduction zone signature, and controlled later channelization of aqueous fluids. 40Ar/39Ar dating on magnesio-hornblende indicates that this deformation episode occurred at ca. 55 Ma, coincident with east-dipping subduction initiation; (2) the main metasomatic stage, characterized by the development of a phlogopite-rich matrix wrapping peridotites and amphibolite boudins, points to the percolation of alkali-rich aqueous fluids at still high temperature (650–750 °C); (3) the last, low temperature (< 600 °C) metasomatic stage results in the formation of deformed veinlets containing talc, chlorite and serpentine.
ISSN: 0024-4937

hal-01306664
https://hal.archives-ouvertes.fr/hal-01306664
https://hal.archives-ouvertes.fr/hal-01306664/document
https://hal.archives-ouvertes.fr/hal-01306664/file/Soret_2016_Strain_localization.pdf
DOI : 10.1016/j.lithos.2015.11.022

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The 40Ar/39Ar geochronological technique has significantly contributed to the study of crustal deformation. However, the frequent occurrence of extraneous argon in high-pressure terrains can make the interpretation of 40Ar/39Ar ages problematic. This contribution attempts to: i) characterize the distribution and origin of extraneous argon in the Cycladic Blueschist Unit (CBU, Greece) by applying the 40Ar/39Ar dating technique on three Cycladic islands, Syros, Tinos and Ios, which span a complete transect of the CBU and ii) discuss regional implications for the tectonometamorphic evolution of the CBU. Our results, combined with ages available in the literature, show that periods of peak-burial conditions (55–49 Ma) and syn- (45–38 Ma) to post-orogenic exhumation (37–21 Ma) are well constrained in the CBU by the 40Ar/39Ar method. Moreover, Variscan apparent ages are preserved in the Cycladic basement of Ios, indicating that the 40Ar/39Ar system was not completely reset during the Eocene metamorphic episode. Additionally, our 40Ar/39Ar data show that the occurrence of extraneous argon is heterogeneously distributed in the CBU. While high-pressure rocks of Syros and Tinos are only locally affected, extraneous argon is clearly evidenced in the CBU of Ios, yielding geologically meaningless ages up to 900 Ma. On each island, the most affected ages were obtained in microstructures where fluid circulation is enhanced such as shear bands or strain shadows around garnets. We suggest that the Cycladic Basement, a polymetamorphic continental unit, behaved as an argon reservoir that has been mobilized by fluid circulation at the Eocene. These argon-enriched fluids have then heterogeneously contaminated metamorphic rocks when percolating through the CBU. Finally, we claim that a minimum closure temperature of 550 °C must be accepted for the phengites of the CBU and conclude that volume diffusion is not the main process resetting the argon isotopic system in high-pressure rocks compare to recrystallization enhanced by fluid circulation and deformation.
ISSN: 0024-4937

Droits : http://creativecommons.org/licenses/by-nc-nd/
insu-01424268
https://hal-insu.archives-ouvertes.fr/insu-01424268
https://hal-insu.archives-ouvertes.fr/insu-01424268/document
https://hal-insu.archives-ouvertes.fr/insu-01424268/file/Valentin-Lithos-Accepted-2016.pdf
DOI : 10.1016/j.lithos.2016.12.013

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We provide new insights into the geochemistry of serpentinites from the Alpine orogenic wedge representing a paleo-subduction zone. These serpentinites are derived from similar oceanic protoliths, but they have experienced different metamorphic conditions related to three different structural levels of the paleo-subduction zone ((1) obducted: Chenaillet ophiolite, (2) accretionary wedge: Queyras Schistes lustrés complex and (3) serpentinite channel: Monviso ophiolite). Metamorphism undergone by these three units is well defined, increasing eastward from sub-greenschist to eclogite facies conditions, and allows us to examine trace element behavior from the oceanic ridge environment to subduction. Serpentinites first record moderate trace element enrichment due to seawater interaction resulting in the replacement of olivine and pyroxene by chrysotile and lizardite below 300 °C. In the sediment-dominated accretionary wedge, serpentinites are strongly enriched in fluid-mobile-elements (B, Li, As, Sb, and Cs) and act as a trapping system following the metamorphic gradient (from 300 to 390 °C) up to total replacement of the lizardite/chrysotile assemblage by antigorite. Under higher temperature conditions (T > 390 °C), no enrichment was observed, and some fluid-mobile elements were released (B, Li, Cs, and Sr). Moreover, in the serpentinite channel (T > 460 °C), most of the fluid-mobile elements are absent due to the scarcity of metasediments which prevent geochemical exchange between metasediments and serpentinites. This is also due to the onset of antigorite breakdown and the release of fluid-mobile elements. Thus, we emphasize that the geochemistry of Alpine serpentinites is strongly dependent on (1) the grade of metamorphism and (2) the ability of metasediments to supply fluid-mobile elements. We conclude that serpentinites act as a trap-and-release system for fluid-mobile elements in a subduction context.
ISSN: 0009-2541

hal-00996839
https://hal.archives-ouvertes.fr/hal-00996839
DOI : 10.1016/j.chemgeo.2013.02.008

Éditeur(s) : Nature
Résumé : Mud volcanism is an important natural source of the greenhouse gas methane to the hydrosphere and atmosphere(1,2). Recent investigations show that the number of active submarine mud volcanoes might be much higher than anticipated ( for example, see refs 3 - 5), and that gas emitted from deep-sea seeps might reach the upper mixed ocean(6-8). Unfortunately, global methane emission from active submarine mud volcanoes cannot be quantified because their number and gas release are unknown(9). It is also unclear how efficiently methane-oxidizing microorganisms remove methane. Here we investigate the methane-emitting Haakon Mosby Mud Volcano (HMMV, Barents Sea, 72 degrees N, 14 degrees 44' E; 1,250 m water depth) to provide quantitative estimates of the in situ composition, distribution and activity of methanotrophs in relation to gas emission. The HMMV hosts three key communities: aerobic methanotrophic bacteria (Methylococcales), anaerobic methanotrophic archaea (ANME-2) thriving below siboglinid tubeworms, and a previously undescribed clade of archaea (ANME-3) associated with bacterial mats. We found that the upward flow of sulphate- and oxygen-free mud volcano fluids restricts the availability of these electron acceptors for methane oxidation, and hence the habitat range of methanotrophs. This mechanism limits the capacity of the microbial methane filter at active marine mud volcanoes to <40% of the total flux.
Nature (depuis 1997) (0028-0836) (Nature), 2006-10 , Vol. 443 , N. 7113 , P. 854-858

Droits : 2006 Nature Publishing Group
http://archimer.ifremer.fr/doc/2006/publication-4514.pdf
DOI:10.1038/nature05227
http://archimer.ifremer.fr/doc/00000/4514/

Éditeur(s) : Elsevier
Résumé : We report on a multidisciplinary study of cold seeps explored in the Central Nile deep-sea fan of the Egyptian margin. Our approach combines in situ seafloor observation, geophysics, sedimentological data, measurement of bottom-water methane anomalies, pore-water and sediment geochemistry, and Th-230/U dating of authigenic carbonates. Two areas were investigated, which correspond to different sedimentary provinces. The lower slope, at similar to 2100 m water depth, indicates deformation of sediments by gravitational processes, exhibiting slope-parallel elongated ridges and seafloor depressions. In contrast, the middle slope, at similar to 1650 m water depth, exhibits a series of debris-flow deposits not remobilized by post-depositional gravity processes. Significant differences exist between fluid-escape structures from the two studied areas. At the lower slope, methane anomalies were detected in bottom-waters above the depressions, whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities. Carbonate U/Th age dates (similar to 8 kyr BP), pore-water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past. In contrast, large (similar to 1 km(2)) carbonate-paved areas were discovered in the middle slope, with U/Th isotope evidence for ongoing carbonate precipitation during the Late Holocene (since similar to 5 kyr BP at least). Our results suggest that fluid venting is closely related to sediment deformation in the Central Nile margin. It is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of 'fossil' carbonate ridges, whereas pervasive diffuse flow prevails at the unfailed middle slope.
Marine Geology (0025-3227) (Elsevier), 2009-06 , Vol. 261 , N. 1-4 , P. 92-104

Droits : 2009 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6586.pdf
DOI:10.1016/j.margeo.2008.10.008
http://archimer.ifremer.fr/doc/00000/6586/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A set of four reactive flow-through experiments at temperature T = 100 degrees C and total pressure P=12 MPa was performed in limestone reservoir samples. By using various ranging from 0.7 to 10 MPa, these experiments mimic mass transfers occurring (1) near the injection well, where the brine is almost saturated with CO2 (i.e. P-CO2 approximate to P), and (2) at increasing distances from the injection well, where the fluid displays lower values and higher divalent cation concentrations due rock dissolution along the fluid pathway. Results for P-CO2 = 10 MPa show non-uniform dissolution features associated with transport-controlled mass transfer, while reaction-controlled uniform dissolution is observed for P-CO2 = 2.5 MPa. The experiment with P-CO2 = 6 MPa allows investigating in detail the transition from transport-to reaction-controlled dissolution. Conversely, the experiment reproducing conditions far from the injection well (P-CO2 = 0.7 MPa), shows a decrease of porosity triggered by the precipitation of Mg-rich calcite. For all the dissolution experiments, the time-resolved porosity phi(t) can be modeled by a simple non-linear equation including parameters that characterise the dissolution regime triggered by the reactivity of the inlet fluid (measured by the Damkohler number, Da). Furthermore, all dissolution experiments display power scaling between permeability (k) and porosity (phi) with distinctly different scaling exponents characterising the reactivity of the fluid percolating the sample, independently from the decrease with time of the reactive surface area. It is shown also that dissolution at moderate positive values of Da seems the most efficient to increase permeability and promote a rapid spreading of the reaction front, while inducing minimal modification of the porosity in the vicinity of the CO2 injection well. These results can be used to parameterize the k-phi function for modeling the earliest dissolution processes occurring in the vicinity of the reaction front.
ISSN: 0009-2541

hal-00445277
https://hal.archives-ouvertes.fr/hal-00445277
DOI : 10.1016/j.chemgeo.2009.03.028

Éditeur(s) : Ofioliti
Résumé : Serpentinisation of mantle rocks, leading to natural venting of hydrogen and methane, has been reported to occur at the global scale, wherever fluids percolate in ultramafic formations. Here we compare gas composition from two on-land, low-temperature, and hyper-alkaline springs hosted on ultramafic rocks in the ophiolite massifs of the Sultanate of Oman and the Ligurian Alps (Voltri Group, Genoa region, Northern Italy). These two settings exhibit similar chemical and mineralogical features but show diverse styles of gas venting. Commonly to all hyper-alkaline springs, gases are characterised by relatively high N-2, very low O-2 and CO2 concentrations, and a strong enrichment in H-2 and CH4. The comparison between Oman and Liguria highlights a high variability of the H-2/CH4 ratios whereby the gas phase of the Oman Ophiolite is enriched in H-2 whereas being CH4 enriched in the Voltri Ophiolite. These results combined with literature data define three groups that may reflect different stages of serpentinisation producing fluids from hydrogen-dominated to methane-dominated. The origin of these distinct groups might lie in the difference of the mineralogical composition of the rocks within which the fluids circulate, on the degree of alteration of the rocks and finally on the geological/metamorphic history of the ophiolite.
Ofioliti (0391-2612) (Ofioliti), 2013-12 , Vol. 38 , N. 2 , P. 143-156

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00175/28634/27059.pdf
DOI:10.4454/ofioliti.v38i2.423
http://archimer.ifremer.fr/doc/00175/28634/

Éditeur(s) : HAL CCSD
Résumé : International audience
In orogens, shortening is mainly accommodated by thrusts, which constitute preferential zones for fluid–rock interactions. Fluid flow, mass transfer, and mineralogical reactions taking place along thrusts have been intensely investigated, especially in sedimentary basins for petroleum and uranium research. This study combines petrological investigations, mineralogical quantifications, and geochemical characterizations with a wide range of analytical tools with the aim of defining the fluid properties (nature, origin, temperature, and redox) and fluid–host rock interactions (mass transfers, recrystallization mechanisms, and newly formed synkinematic mineralization) in the Pic-de-Port-Vieux thrust fault zone (Pyrenees, Spain). We demonstrate that two geochemically contrasted rocks have been transformed by fluid flow under low-grade metamorphism conditions during thrusting. The hanging-wall Triassic red pelite was locally bleached, while the footwall Cretaceous dolomitic limestone was mylonitized. The results suggest that thrusting was accompanied by a dynamic calcite recrystallization in the dolomitic limestone as well as by leaching of iron via destabilization of iron oxides and phyllosilicate crystallization in the pelite. Geochemical and physical changes highlighted in this study have strong implications on the understanding of the thrust behavior (tectonic and hydraulic), and improve our knowledge of fluid–rock interactions in open fluid systems in the crust.
Contributions to Mineralogy and Petrology

hal-01575548
https://hal-univ-fcomte.archives-ouvertes.fr/hal-01575548
DOI : 10.1007/s00410-017-1394-5

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750-850°C and pressures <3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at ∼800°C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth's mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface setting.
ISSN: 0022-3530

hal-01054319
https://hal.archives-ouvertes.fr/hal-01054319
DOI : 10.1093/petrology/egt068

Éditeur(s) : HAL CCSD Elsevier
Résumé : A synthesis of backscatter imagery coupled with a large 3D seismic dataset in the Lower Congo Basin (LCB) reveals a patchy distribution of features interpreted to be associated with fluid seepage from 300 m to 2500 m water depth. With the exception of one region of anomalous backscatter positive-relief mounds, all inferred seep sites occur in negative-relief pockmarks. The extensive 3D seismic dataset in the LCB offers a unique opportunity to study the plumbing system that is feeding surface cold seep systems, and in general, to reconstruct the relationship between tectonics and fluid flow in continental margins. The fluid seeps in the LCB are associated with morphologically, stratigraphically or tectonically controlled focused fluid flow. The integration of the geophysical datasets, backscatter imagery coupled to 3D seismic, clearly indicates that fluid seeps are not randomly distributed, but their seabed organization reflects 1) the location of the underlying structure (reservoir or trap) where the fluids are coming from, 2) the geometry and morphology of the reservoir/trap, and 3) the discontinuities in the sedimentary column along which fluids have migrated. In the LCB seafloor pockmarks are always associated with underlying tectonic structures (fault zones, salt diapirs, polygonal faults) or buried sedimentary bodies (turbiditic channels, erosional surfaces), whereas they never occur above sub-horizontal parallel-stratified fine-grained sediments. Even if triggering processes can not be clearly defined here, we propose a model of seafloor fluid seep organization, which represents a new tool for identifying the geometry of flow pathways and the underlying buried bodies where the fluids are originating from. This qualitative 3D model provides insight into the geohydrologic processes of continental margins.
ISSN: 0025-3227

hal-00406640
https://hal.archives-ouvertes.fr/hal-00406640
DOI : 10.1016/j.margeo.2007.06.003

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
Observations from deep boreholes at several locations worldwide, laboratory measurements of frictional strength on quartzo-feldspathic materials, and earthquake focal mechanisms indicate that crustal faults are strong (apparent friction μ ≥ 0.6). However, friction experiments on phyllosilicate-rich rocks and some geophysical data have demonstrated that some major faults are considerably weaker. This weakness is commonly considered to be characteristic of mature faults in which rocks are altered by prolonged deformation and fluid-rock interaction (i.e., San Andreas, Zuccale, and Nankai Faults). In contrast, in this study we document fault weakening occurring along a marly shear zone in its infancy (<30 m displacement). Geochemical mass balance calculation and microstructural data show that a massive calcite departure (up to 50 vol %) from the fault rocks facilitated the concentration and reorganization of weak phyllosilicate minerals along the shear surfaces. Friction experiments carried out on intact foliated samples of host marls and fault rocks demonstrated that this structural reorganization lead to a significant fault weakening and that the incipient structure has strength and slip behavior comparable to that of the major weak faults previously documented. These results indicate that some faults, especially those nucleating in lithologies rich of both clays and high-solubility minerals (such as calcite), might experience rapid mineralogical and structural alteration and become weak even in the early stages of their activity.
ISSN: 0278-7407

hal-01211481
https://hal.archives-ouvertes.fr/hal-01211481
DOI : 10.1002/2014TC003716

Éditeur(s) : HAL CCSD Elsevier
Résumé : Understanding chemical element mobility during high-pressure metamorphism is paramount to the knowledge of the transformations occurring during the course of eclogite-facies metamorphism. In particular, the role of deformation and fluid circulation appear essential. In order to better decipher this role, we investigated two eclogite-facies samples from the Lindas Nappe in the Bergen arcs (Norwegian Caledonides), which are related to a fluid/deformation metamorphic event. This study, based on detailed microtextural investigations, coupled with major and trace element analyses, either in situ EMPA and LA-ICP-MS or by conventional ICP-MS nebulisation on mineral fractions, illustrates the complexity of chemical reactions accompanying the eclogite- and amphibolite-facies metamorphism. A single eclogite-facies mineral species two eclogite-facies (i.e. phengite and epidote) depicts various chemical features depending on its textural location at thin-section scale. The correlation with the mineralogical reaction occurring in each textural site demonstrates that the compositional variations are inherited from the precursor minerals. Thus, the composition of eclogite-facies minerals is locally controlled by the composition of few hundred-micron domains. On the contrary, alteration phases of the amphibolite facies (symplectite and calcic amphibole) display clear enrichment in LILE, Pb, Sr and LREE compared to their precursor minerals (omphacite and garnet). This supports that the transport of elements at hand-sample scale was enhanced during the retrogression and elements were efficiently delocalised from one textural site to another. The contrasting behaviour of element mobility between peak eclogite-facies metamorphism and retrogression can be explained in terms of rate processes and fluid/deformation activity. During eclogitisation,deformation allows the formation of textural and mineralogical microdomains. Contemporaneously the fluid circulation favours the development of heterogeneities by transport of elements in excess towards areas protected from deformation where phengite-bearing quartz lenses crystallise. This, together with the very fast recrystallisation of eclogite-facies minerals, results in a heterogeneous redistribution of elements at sample scale and in a chemical disequilibrium between different microdomains of hundred-micron size. During retrogression in the amphibolite facies, coeval with long-term ductile deformation, the fluids act as an efficient vector to redistribute the elements from a given textural site to another and, thus, allow to reach, at hand-sample scale, a new chemical equilibrium between the different crystallising phases.
ISSN: 0024-4937

hal-00404602
https://hal.archives-ouvertes.fr/hal-00404602
DOI : 10.1016/j.lithos.2006.10.001

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : International audience
La Cabaña peridotite is part of a dismembered ophiolite complex located within the metamorphic basement of the Coastal Cordillera of south-central Chile, and is the only location in Chile were Cr-spinels have been described so far. The La Cabaña peridotite is part of the Western Series unit, which comprises meta-sedimentary rocks, metabasites, and serpentinized ultramafic rocks. This unit has been affected by greenschist-facies metamorphism with reported peak PT conditions of 7.0–9.3 kbar and 380°–420 °C. Within La Cabaña peridotite Cr-spinels are present in two localities: Lavanderos and Centinela Bajo. In Lavanderos, Cr-spinel occurs in small chromitite pods and as accessory/disseminated grains with a porous or spongy texture in serpentinite, whereas in Centinela Bajo Cr-spinel is present as accessory zoned grains in partly serpentinized dunites, and in chromitite blocks. All Cr-spinels display variable degrees of alteration to Fe2+-rich chromite with a variation trend of major elements from chromite to Fe2+-rich chromite similar to those observed in other locations, i.e., an increase in Fe2O3 and FeO, a decrease in Al2O3 and MgO. Cr2O3 content increases from chromite to Fe2+-rich chromite in chromitite pods from Lavanderos and chromitite blocks from Centinela Bajo, but decreases in ferrian chromite zones in accessory grains from Centinela Bajo. The minor element (Ti, V, Zn, Ni) content is mostly low and does not exceed 0.4 wt.%, with the exception of MnO (<0.9 wt.%), which shows a correspondence with increasing degree of alteration. Cr# (Cr/Cr + Al) versus Mg# (Mg/Mg + Fe2+) and Fe3+/Fe3++Fe2+ versus Mg# plots are used to illustrate the Cr-spinel alteration process. Overall, the Cr-spinels from Lavanderos (chromitite pods and disseminated grains) exhibit Cr# values ranging from 0.6 to 1.0, Mg# (Mg/Mg + Fe2+) below 0.5, and (Fe3+/Fe3++Fe2+) <0.4. Cr-spinels from chromitites in Centinela Bajo have Cr# and Mg# values that range from 0.65 to 1.0, and 0.7-0.3, respectively, and (Fe3+/Fe3++Fe2+) < 0.4. Accessory Cr-spinels from Centinela Bajo have Cr# and Mg# values that range from 0.55 to 1.0, and 0.6- < 0.1, respectively. The (Fe3+/Fe3++Fe2+) ratio is less than 0.4 in chromite cores and Fe2+-rich chromite, and >0.5 in ferrian chromite and Cr-magnetite. Interpretation of the data obtained and Cr-spinel textures indicate that the alteration of Cr-spinel is a progressive process that involves in its initial stages the reaction of chromite with olivine under water-saturated conditions to produce clinochlore and Fe2+-rich chromite. During this stage the chromite can also incorporate Ni, Mn, and/or Zn from the serpentinization fluids. As alteration progresses, Fe2+-rich chromite loses mass resulting in the development of a spongy texture. In a later stage and under more oxidizing conditions Fe3+ is incorporated in chromite/Fe2+-rich chromite shifting its composition to an Fe3+-rich chromite (i.e., ferrian chromite). Depending on the fluid/rock and Cr-spinel/silicate ratios, Cr-magnetite can also form over Fe2+-rich chromite and/or ferrian chromite as a secondary overgrowth. The compositional changes observed in Cr-spinels from La Cabaña reflect the initial stages of alteration under serpentinization conditions. Results from this study show that the alteration of Cr-spinels is dependent on temperature. The degree and extent of alteration (formation of Fe2+-rich and/or ferrian chromite) are controlled by the redox nature of the fluids, the Cr-spinel/silicate and the fluid/rock ratios.
ISSN: 0930-0708

hal-01172266
https://hal.archives-ouvertes.fr/hal-01172266
DOI : 10.1007/s00710-014-0335-5

Éditeur(s) : Gauthier-Villars
Résumé : We report here our findings on the chemistry of methane, manganese and chloride dissolved in the interstitial water of the Barbados accretionary complex. The decollement separating the subduction plate from the accretionary wedge is geochemically characterized by high methane and manganese and low chloride concentrations relative to buried seawater. Sandstone layers occurring below the decollement and the compressive structures recording some of the earliest effects of offscraping are also characterized by similar anomalies. A fluid with the same characteristics is also found six kilometres to the east of the deformation front within the oceanic plate. High manganese concentration and low chlorinity also occur in the pore water of the offscraped sediment packages, but without associated anomalous methane concentrations. These results suggest that: Dewatering processes occur below the decollement in association with thermogenic methane production; Fluid with low chlorinity and high methane concentrations circulates mainly along the decollement, its propagation in the oceanic domain and along the main fracture zones being associated with these detachment surfaces; Diagenetic processes essentially control the dissolved manganese concentration and probably affect the methane content of the interstitial waters; The observations made in this paper illustrate advective processes in the front of an accretionary prism.
Oceanologica Acta (0399-1784) (Gauthier-Villars), 1991 , Vol. 14 , N. 1 , P. 33-49

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00101/21262/18873.pdf
http://archimer.ifremer.fr/doc/00101/21262/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Tso Morari serpentinites in the Ladakh area, northwest Himalaya, originated from the forearc mantle overlying the northward subducting Neo-Tethys lithosphere and the margin of the Indian continent. The serpentinites are characterized by high concentration of fluid-mobile elements (FME: As, Sb, B, Li, and U) compared to ophiolitic or abyssal serpentinites. The Pb isotopic compositions of serpentinites show influence of the subducted Indian continental lithosphere. Trace element concentrations of antigorite determined in situ with Laser Ablation High Resolution Inductively Coupled Mass Spectrometer (LA-HR-ICP-MS) show high contents of FME including Pb, in contrast to the spatially associated iron oxides. Rare earth elements (REE) and compatible elements, such as Sc and Co, remained immobile during the hydration, allowing the identification of the primary minerals (olivine or orthopyroxene) from which serpentine formed. Serpentinized olivine displays higher Sb and As concentrations (up to 1000 x PM) than serpentinized orthopyroxenes that are enriched in Pb, Cs and Li (2 to up to 10 x PM).;We propose that the observed FME distribution in two types of serpentine reflect the differential incorporation of FME during the downward movement of the serpentinite along the subduction plane. At temperature lower than 400 degrees C, at shallow depths, olivine is preferentially serpentinized and incorporates elements that are fluid soluble at low temperatures, such as Sb and As. Above 400 degrees C, orthopyroxene is hydrated and incorporates Pb, Cs, Li and possibly Ba. Boron and U are incorporated in both types of serpentine suggesting that they are released from slabs at temperatures around 300-400 degrees C. The serpentine acts as a sink for water, but also for FME and transports them to deeper and hotter levels in the mantle, down to the isotherm 600-650 degrees C where dehydration occurs.
ISSN: 0009-2541

hal-00475565
https://hal.archives-ouvertes.fr/hal-00475565
DOI : 10.1016/j.chemgeo.2009.10.002

Éditeur(s) : HAL CCSD Mineralogical Society of America
Résumé : International audience
Accessory phases are important hosts of trace elements; allanite may contain >90% of the REE in a bulk rock. The mobility and redistribution of several trace elements, notably HREE, Th, U, and Y is thus controlled by reactions involving allanite and other REE phases, as well as several rock-forming minerals. As these elements are commonly concentrated in mature elastic sediments, a suite of impure quartzite was studied. Two eclogite facies samples from the Monometamorphic Cover Complex of the Sesia Zone (Western Italian Alps) are presented in some detail, as they reveal a remarkably rich spectrum of reaction relationships that involve REE phases.;Two allanite-forming reactions were inferred from textures and phase compositions;(1) monazite + Ca-silicate(?) + fluid -> allanite + apatite + thorite;;(2) monazite + thorite + Ca-silicate(?) + fluid -> Th-rich allanite + auerlite +/- apatite.;Petrographic observations and thermodynamic models suggest that allanite entered the HP assemblage at similar to 530 degrees C and 17-18 kbar during prograde metamorphism.;In one sample, allanite is rimmed by epidote rich in Y and HREE that grew at the expense of xenotime. Two net transfer reactions were derived;(3) xenotime + allanite + fluid -> Y-rich epidote + apatite + thorite;;(4) xenotime + allanite + fluid -> Y-rich epidote + aeschynite + thorite + (phosphate?).;Textural relationships and trace element analyses of coexisting allanite/monazite and xenotime/Y-rich epidote reveal systematic partitioning of the REE. Partition coefficients for the HREE are compatible with equilibrium fractionation, whereas those for the LREE show patterns that seem to be inherited from the precursor phases, in this case zircon with variable LREE composition.
ISSN: 0003-004X

hal-00745674
https://hal.archives-ouvertes.fr/hal-00745674
DOI : 10.2138/am.2012.3832

Éditeur(s) : HAL CCSD Springer-Verlag
Résumé : International audience
With hull-mounted multibeam echosounder data, wereport for the first time along the active Ecuadorian margin,acoustic signatures of water column fluid emissions and seep-relatedstructures on the seafloor. In total 17 flare-shaped acousticanomalies were detected from the upper slope (1250 m) to the shelfbreak (140 m). Nearly half of the flare-shaped acoustic anomaliesrise 200–500 m above the seafloor. The base of the flares is generallyassociated with high-reflectivity backscatter patchescontrasting with the neighboring seafloor. We interpret these flaresas caused by fluid escape in the water column, most likely gases.High-resolution seismic profiles show that most flares occur closeto the surface expression of active faults, deformed areas, slopeinstabilities or diapiric structures. In two areas tectonic deformationdisrupts a Bottom Simulating Reflector (BSR), suggesting thatburied frozen gas hydrates are destabilized, thus supplying free gasemissions and related flares. This discovery is important as it opensthe way to determine the nature and origin of the emitted fluids andtheir potential link with the hydrocarbon system of the forearcbasins along the Ecuadorian margin.
ISSN: 0033-4533

insu-01267238
https://hal-insu.archives-ouvertes.fr/insu-01267238
DOI : 10.1007/s00024-015-1230-7