Éditeur(s) : Gauthier-villars
Résumé : 69 new hydrothermal sites were discovered during the Naudur diving cruise. Dives were conducted between 17 degrees 5 and 18 degrees 40'5 on four segments showing marked contrast in morphology, volcanic, tectonic and hydrothermal activity. At 17 degrees 10'5, 17 degrees 25'5 and 18 degrees 37'5 the ridge has a dome shaped cross-section and is dominated by very active volcanic activity. Early widespread low temperature (<50 degrees C) diffuse hydrothermal discharge is followed by focused high temperature black smokers. At 18 degrees 15'S tectonic activity is dominant, no recent lava was observed, and only two of the 20 hydrothermal sites are active. At 18 degrees 32'5 recent lava representing a new volcanic episode partly covers the bottom of the graben. Deep hydrothermal convection is reactivated with new black smokers along the graben wall. Also, there is low temperature shimmering water from cooling lava flows. These three segments can be considered as successive volcanic/tectonic episodes typical of a fast spreading ridge. Further, the observations support a new model for the temporal evolution of episodic hydrothermal activity. Hydrothermal convection is unstable and superficial during the volcanic stage (dykes injection). At the beginning of the teaonic stage (graben formation) faults allow deep circulation and hot fluid to reach the surface. As the graben widens hydrothermal activity is less important and may cease. The heat of a new volcanic episode reactivates the deep hot water circulation along the graben faults completing a cycle.
69 nouveaux sites hydrothermaux ont été découverts entre 17'5 et 18'40'5 sur quatre segments présentant des activités volcaniques et tectoniques contrastées. A 17°10'S,17025'S et 18'37'5, la ride forme un dôme et l'activité volcanique est dominante. Les émissions hydrothermales sont d'abord diffuses et de basse température, puis focalisées et de haute température. A 18°15'S la majorité des sites hydrothermaux sont inactifs dans un graben axial dépourvu de laves récentes. A 18'32'5, des laves récentes s'épanchent au fond du graben. Les circuits hydrothermaux réactivés par le nouvel épisode volcanique se traduisent par des diffusions sur les laves récentes et par une reprise des émissions de haute température le long des murs du graben. Ces quatre segments traduisent des épisodes volcanoltectoniques successifs, caractéristiques de I'évolution temporelle des dorsales rapides. Les observations permettent de proposer un nouveau modèle de l'activité hydrothermale. Les circuits hydrothermaux sont diffus, peu structurés, instables et superficiels durant les épisodes volcaniques. Au début des épisodes tectoniques, les failles du graben drainent les fluides chauds en profondeur, puis lorsque le graben s'élargit, l'activité s'interrompt. La reprise de l'activité volcanique réamorce les circulations profondes le long des failles du graben.
Comptes Rendus de l'Academie des Sciences Serie II (1251-8069) (Gauthier-villars), 1994-12 , Vol. 319 , N. 11 , P. 1399-1406

Droits : Académie des Sciences
http://archimer.ifremer.fr/doc/00183/29455/27889.pdf
http://archimer.ifremer.fr/doc/00183/29455/

Éditeur(s) : HAL CCSD
Résumé : Geothermal energy is considered as a green and infinite energy source at human scale. Currently, the yield of geothermal power plants is limited to temperatures of the operating fluid which 350 °C. From tectonic and volcanic activity at mid-ocean ridges, Iceland is a location where supercritical fluid extraction (T> 375 °C) can considered for the near future. Exploiting such fluids could theoretically multiply by a factor of ten the electrical power delivered by geothermal wells. Can such fluids circulate at the base of brittle oceanic crust? This work investigates the petrophysical properties of basalts in order to constrain geophysical observations in Iceland and predict the behavior of very high temperature geo-hydrothermal reservoirs. The first approach consisted in studying the physical properties of rocks that have hosted deep hydrothermal circulations at oceanic ridges. The study of these rocks at ODP Site 1256 shows that the porosity measured both in the field and in the lab is associated with amphibolite facies alteration minerals (T> 500 ° C). The second approach was to recreate in the laboratory the conditions of pressure, temperature and pore fluid pressure of high temperature to supercritical hydrothermal systems to predict the mechanical and electrical properties of basalts under these conditions. The mechanical results indicate that the brittle/ductile transition occurs at a temperature of about 550° C, where a strong permeability decrease is expected. The implementation and calibration of a new cell for measuring electrical conductivity at high temperature provide the first results for the interpretation of geophysical data. When applied to basaltic crustal conditions in Iceland, these results indicate that hydrothermal fluids could circulate, at least temporarily, in a supercritical state up to 5 km depth.
La géothermie est considérée comme une source d'énergie propre et inépuisable à échelle humaine. Actuellement, le rendement des centrales géothermiques est limité à l'exploitation de fluides de températures inférieures à 350 °C. L'association de l'activité tectonique et volcanique aux dorsales océaniques fait de l'Islande un lieu où l'extraction de fluides supercritiques (T> 375 °C) peut être envisagée. Cette exploitation pourrait multiplier par dix la puissance électrique délivrée par le système géothermal. Ces fluides peuvent-ils circuler dans la croûte océanique ? Ce travail propose de contraindre les observations géophysiques et de prédire le fonctionnement des réservoirs géo-hydrothermaux de très haute température par l'étude des propriétés physiques des basaltes. La première approche est focalisée sur l'étude de roches ayant accueilli une circulation hydrothermale par le passé. L'étude de ces roches au site ODP 1256, montre que leur porosité est associée à la présence de minéraux d'altération hydrothermale du facies amphibolite (T> 500 °C). La seconde approche a consisté à recréer, en laboratoire, les conditions des systèmes hydrothermaux, à très haute température, afin de prédire les propriétés mécaniques et électriques des basaltes dans ces conditions. Les résultats mécaniques indiquent que la transition fragile/ductile, souvent associée à une forte décroissance de perméabilité, intervient à une température d'environ 550 °C. La mise en place d'une cellule de mesure de la conductivité électrique de haute température a fourni les premiers résultats utiles à l'analyse des données géophysiques. Appliqués aux conditions de la croûte basaltique Islandaise, ces résultats indiquent que des fluides hydrothermaux pourraient circuler au moins transitoirement à l'état supercritique jusqu'à ~ 5 km de profondeur.
https://tel.archives-ouvertes.fr/tel-00591798

tel-00591798
https://tel.archives-ouvertes.fr/tel-00591798
https://tel.archives-ouvertes.fr/tel-00591798/document
https://tel.archives-ouvertes.fr/tel-00591798/file/THESE_MV_pro.pdf

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The brittle to ductile transition (BDT) in rocks may strongly influence their transport properties (i.e., permeability, porosity topology ...) and the maximum depth and temperature where hydrothermal fluids may circulate. To examine this transition in the context of Icelandic crust, we conducted deformation experiments on a glassy basalt (GB) and a glass-free basalt (GFB) under oceanic crust conditions. Mechanical and micro-structural observations at a constant strain rate of 10(-5) s(-1) and at confining pressure of 100-300 MPa indicate that the rocks are brittle and dilatant up to 700-800 degrees C. At higher temperatures and effective pressures the deformation mode becomes macroscopically ductile, i.e., deformation is distributed throughout the sample and no localized shear rupture plane develops. The presence of glass is a key component reducing the sample strength and lowering the pressure of the BDT. In the brittle field, strength is consistent with a Mohr-Coulomb failure criterion with an internal coefficient of friction of 0.42 for both samples. In the ductile field, strength is strain rate-and temperature-dependent and both samples were characterized by the same stress exponent in the range 3 < n < 4.2 but by very different activation energy Q(GB) = 59 +/- 15 KJ/mol and Q(GFB) = 456 +/- 4 KJ/mol. Extrapolation of these results to the Iceland oceanic crust conditions predicts a BDT at similar to 100 degrees C for a glassy basalt, whereas the BDT might occur in non-glassy basalts at deeper conditions, i.e., temperatures higher than 550 +/- 100 degrees C, in agreement with the Icelandic seismogenic zone.
ISSN: 0148-0227

hal-00760905
https://hal.archives-ouvertes.fr/hal-00760905
DOI : 10.1029/2011JB008884

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Rainbow is a dome-shaped massif at the 36°14′N nontransform offset along the Mid-Atlantic Ridge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high temperature; Clamstone and Ghost City are fossil and low temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W dipping detachment fault, now inactive, uplifted the massif and exhumed these deep-seated rocks. Present-day deformation is accommodated by SSW-NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatiotemporal progression. Markers of high-temperature hydrothermal activity (∼350°C) are restricted to some samples from the active field while a more diffuse, lower temperature hydrothermal activity (<220°C) is inferred at various locations through anomalously high As, Sb, and Pb contents, attributed to element incorporation in serpentines or microscale-sulfide precipitation. Petrographic and geochemical analyses show that the dominant basement alteration is pervasive peridotite serpentinization at ∼160–260°C, attributed to fluids chemically similar to those venting at Rainbow, and controlled by concomitant alteration of mafic-ultramafic units at depth. Rainbow provides a model for fluid circulation, possibly applicable to hydrothermalism at oceanic detachments elsewhere, where both low-temperature serpentinization and magmatic-driven high-temperature outflow develop contemporaneously, channeled by faults in the footwall and not along the detachment fault.
ISSN: 1525-2027

hal-01115281
https://hal.archives-ouvertes.fr/hal-01115281
https://hal.archives-ouvertes.fr/hal-01115281/document
https://hal.archives-ouvertes.fr/hal-01115281/file/Andreani_et_al-2014-Geochemistry%2C_Geophysics%2C_Geosystems.pdf
DOI : 10.1002/2014GC005269

Éditeur(s) : HAL CCSD
Résumé : Ocean Drilling Program Hole 1256D reached for the first time the transition zone between the sheeted dike complex and the uppermost gabbros. The recovered crustal section offers a unique opportunity to study the deepest part of the hydrothermal system in present-day oceanic crust. We present a structural analysis of electrical borehole wall images. We identified, and measured the orientations of four categories of structures: major faults, minor fractures, possibly hydrothermal veins, and dikes. All structures tend to strike parallel to the paleo-ridge axis. Three major fault zones (meter thick) and dikes are steeply dipping (~ 75° on average) outward the ridge. Centimeter-thick moderately conductive planar features are interpreted as hydrothermal veins, are organized in arrays of consistent spacing, thickness, and orientation, and are dipping about 15-20° toward the ridge. This structural pattern is interpreted as an on-axis paleohydrothermal circulation system, with vertical, dike-parallel fractures, and sub-horizontal high-temperature hydrothermal veins at the base of the sheeted dike, which was subsequently rotated ~ 15° westward around a ridge-parallel, sub-horizontal axis. This rotation can be caused by upper-crustal block rotation along a listric normal fault, and/or subsidence at the ridge axis.
Ofioliti

hal-00757592
https://hal.archives-ouvertes.fr/hal-00757592
DOI : 10.4454/ofioliti.v37i1.402

Éditeur(s) : HAL CCSD Oxford University Press (OUP): Policy B - Oxford Open Option A Oxford University Press (OUP)
Résumé : International audience
Peridotites exhumed in the footwall of axial detachment faults at slow-spreading ridges are highly serpentinized. Most mid-ocean ridge detachment settings are magmatically active and hydrous fluid circulation in and near the fault has been shown to be influenced by the presence of melt or magmatic lithologies. Our working area along the Southwest Indian Ridge (62–65°E) is nearly amagmatic and represents an end-member to study the hydrous alteration of exhumed peridotites without these magmatic influences. We use an integrated petrological approach combining microstructural, mineralogical and chemical observations to unravel the sequence of serpentinization in 272 dredged samples of variably serpentinized peridotites and to document the circulation of serpentinizing fluids in and near the exhumation faults. We find that serpentine recrystallization and veins overprint the initial serpentinite mesh texture in ∼25% of the samples. Oxygen isotope data suggest that this sequence developed at relatively high temperatures (271–336°C) and under increasing fluid–rock ratios, from near stoichiometry for mesh texture formation to >10 during recrystallization. Increasing fluid supersaturation relative to serpentine favors the replacement of mesh texture lizardite by chrysotile and polygonal or polyhedral serpentine. We attribute local recrystallization into antigorite to moderate Si-metasomatism, possibly following pyroxene serpentinization. We do not observe the more pronounced Si-metasomatism leading to talc replacing serpentine that is reported for the more magmatically active Mid-Atlantic Ridge detachment settings and is attributed to prior leaching of magmatic rocks. Scales of preferential fluid pathways in our samples evolved from pervasive and close-spaced (<500 µm) microfractures during the formation of the initial serpentine mesh texture, to centimeter-thick planar domains of enhanced fluid flux, spaced at ∼10 cm intervals and probably grouped in corridors that may be up to ∼100 m across. Serpentine minerals are enriched in some fluid-mobile elements (Cl, B, U) relative to the peridotite protolith, and several elements (Al, Fe, Si, Cu, As, Sb, REE) are redistributed at the millimeter to decimeter scale. Serpentinizing fluids were seawater-derived, probably mildly alkaline (small to no europium anomalies), reducing and H2-enriched (formation of magnetite). These fluids may have been similar to, though warmer than, those venting at the ultramafic-hosted Lost City hydrothermal fluid (30°N, Mid-Atlantic Ridge).
ISSN: 0022-3530

hal-01174047
https://hal.archives-ouvertes.fr/hal-01174047
DOI : 10.1093/petrology/egv014

Éditeur(s) : HAL CCSD Society of Economic Geologists
Résumé : International audience
The ~8-Moz Sadiola Hill gold deposit is located in the Kédougou-Kénieba inlier, a window of deformed ca. 2200 to 2050 Ma rocks that crop out in eastern Senegal and western Mali. The geology of the inlier differs from other Paleoproterozoic granite-greenstone belts and sedimentary basins by the abundance of carbonate rocks. The Sadiola Hill gold deposit occurs within 3 km of the Senegal-Mali shear zone, and country rocks in the region have undergone polycyclic deformation. The lithostratigraphy of the open pit consists of impure limestones overlain by younger sequences of detrital sedimentary rocks, which include arenite, wacke, and siltstone. The sedimentary rocks have been subjected to regional greenschist-facies metamorphism and have been intruded by multiple generations of synkinematic, calc-alkaline stocks and dikes. The bulk of the ore is hosted within a N-S- to NNW-trending, 10- to 50-m-wide, brittle-ductile dilational shear zone defined as the Sadiola shear zone. Gold lodes also occur along an array of steep NNE-trending shear zones. Geometric and kinematic analyses indicate that the ore-hosting structures were undergoing sinistral displacement at the time of mineralization, locally defined as the D3s NNW-SSE-shortening event. Hydrothermal alteration is polyphase and includes an early high-temperature calc-silicate phase (i.e., porphyroblastic growth of actinolite-tremolite) followed by a potassic phase (i.e., biotite-calcite-quartz ± K-feldspar-tourmaline-actinolite) that was synchronous with ore mineral deposition. Paragenetic studies reveal a multistage ore development that includes an As-rich sulfide stage, followed by an Au-Sb stage. The ore is associated with a metal enrichment suite of Au-As-Sb ± Cu-Fe-W-Mo-Ag-Bi-Zn-Pb-Te. The Sadiola Hill deposit shares a similar relative timing and structural setting to that of other world-class orogenic gold systems in the West African craton (e.g., Ashanti, Loulo), with gold deposition occurring during a period of transcurrent tectonics, after the cessation of region-wide compressional deformation. The high-temperature paragenesis at Sadiola Hill is atypical of gold mineralization in the craton and indicates that the late Eburnean tectono-magmatic activity between ca. 2080 and 2060 Ma played an important role in the dynamics of hydrothermal fluid circulation along the Senegal-Mali shear zone.
ISSN: 0361-0128

hal-01583358
https://hal.archives-ouvertes.fr/hal-01583358
DOI : 10.2113/econgeo.112.1.23

Éditeur(s) : HAL CCSD
Résumé : A section of ancient fast-spreading ocean crust was sampled at Wadi Gideah, located in the Wadi Tayin Massif of the Oman ophiolite. This sample suite was used for generating a coherent data set combining various petro-geochemical and structural investigations, with the aim of building a reference section for advancing our understanding of crustal accretion processes at fast-spreading mid-ocean ridges. Additionally, this study focuses on one outcrop that represents part of the fossil axial melt lens (AML).Major and trace element analyses reveal a trend of chemical evolution up section in the lower crust, with a marked difference between the layered gabbros and the foliated gabbros above. Petrological modeling shows that the chemical evolution up section can be produced by hydrous fractionation crystallization, with melt H2O contents of ~0.8 wt% and 0.8 to 1.2 wt% for the lower and upper crust, respectively. Together with the observed very steep bulk Zr/Hf vs. Zr gradient, high F and Cl content of magmatic amphibole, general Nb-Ta depletion of melts compared with NMORB, and high Sr87/Sr86 ratio compared with modern fast-spread crust, this suggests that Wadi Gideah layered gabbros were accreted by crystallization of ascending melts in sills, in a context of subduction initiation. The distinct changes in chemical trends and average grain size at the layered to foliated gabbro transition are tentatively explained by enhanced hydrothermal cooling deeper than the AML. The gabbro/dike transition displays complex structural and lithological relationships that, together with fractional crystallization modeling, point to episodic vertical movements of the AML. Plagioclase crystallographic preferred orientations (CPO) measured over the whole gabbro section are always consistent with magmatic flow. The CPO strength generally increases downward, with more scattering in the layered gabbro section. From the top to the bottom of the layered gabbros, CPO become progressively more prolate, possibly reflecting increasing shear induced by active mantle flow underneath.Wadi Gideah lower crust recorded a formation history consistent with a hybrid accretion model, combing several processes such as downward magmatic flow and/or upward melt migration in the upper foliated gabbros, sill intrusions in the layered gabbros, and deep hydrothermal circulation. The latter was presumably focused in channels, preserved today as several, up to 100 m wide zones of extensively altered former layered gabbro, cross-cutting the magmatic layering. These metagabbros display significantly higher Sr87/Sr86 ratio, late stage magmatic phases, and evidence for high temperature partial melting.
Une coupe d'ancienne croute océanique rapide a été échantillonnée dans le Wadi Gideah, situé dans le Massif de Wadi Tayin de l'Ophiolite d'Oman. Cette série d'échantillons a permis de générer un jeu de données cohérent, combinant des études pétro-géochimiques et structurales, dans le but de construire une coupe de référence pour mieux contraindre notre compréhension des processus d'accrétion crustale aux dorsales océaniques rapides. Ce travail intègre aussi l'étude détaillée d'un affleurement représentant en partie la lentille magmatique axiale (LMA) fossile.Les analyses d'éléments majeurs et traces révèlent une évolution chimique dans la croûte inférieure, avec une nette différence entre les gabbros lités et les gabbros foliés sus-jacents. La modélisation pétrologique montre que l'évolution chimique vers le haut peut être produite par cristallisation fractionnée hydratée, avec des teneurs en H2O des magmas de ~0.8 wt% dans la croute inférieure et 0.8 to 1.2 wt% dans la croute supérieure. Combiné aux forts gradients de Zr/Hf vs. Zr, aux teneurs élevées de F et Cl dans les amphiboles magmatiques, à l'appauvrissement général en Nb-Ta des magmas comparé aux NMORB, et aux rapports Sr87/Sr86 élevés comparés à la croûte rapide moderne, ceci suggère que les gabbros lités du Wadi Gideah ont été formés par la cristallisation des magmas ascendants dans des sills, dans un contexte d'initiation de subduction. Les changements de signature géochimique et de taille de grain à la transition entre gabbros lités et gabbros foliés témoignent possiblement d'un fort refroidissement hydrothermal plus profond que la LMA. La transition complexe filonien/gabbro montre des relations structurale et lithologiques complexes, qui, combinées o la modélisation de la cristallisation fractionnée, traduisent les mouvements verticaux épisodiques de la LMA. Les orientations préférentielles cristallographiques (OPC) des plagioclases mesurées sur l'ensemble de la coupe dans les gabbros témoignent toujours d'une déformation magmatique. L'intensité des OPC a tendance à augmenter vers le bas, avec une plus forte variabilité dans les gabbros lités. Dans les gabbros lités, les OPC sont de plus en plus linéaires vers le bas, résultant possiblement d'un cisaillement plus fort induit par l'écoulement actif du manteau sous-jacent.La croûte inférieure dans le Wadi Gideah a enregistré une histoire compatible avec un modèle d'accrétion hydride, combinant plusieurs processus tels que l'écoulement magmatique vers le bas et/ou la migration de liquides magmatiques vers le haut dans les gabbros foliés supérieurs, l'intrusion de sills dans les gabbros lités, et une circulation hydrothermale profonde. Cette dernière était probablement concentrée dans des chenaux, préservés aujourd'hui sous la forme de plusieurs zones, dont l'épaisseur atteint 100 m, de gabbros lités fortement altérés, recoupant le litage magmatique. Ces métagabbros ont des rapports Sr87/Sr86 élevés, comportent des phases magmatiques tardives et montrent des évidences de fusion partielle.
https://hal.archives-ouvertes.fr/tel-01302675

tel-01302675
https://hal.archives-ouvertes.fr/tel-01302675
https://hal.archives-ouvertes.fr/tel-01302675/document
https://hal.archives-ouvertes.fr/tel-01302675/file/Th%C3%A8se_Tim-Mueller_2015.pdf

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Hydrothermal circulation at ultramafic-hosted sites supports a large variety of high-and low-temperature hydrothermal vents and associated ecosystems. The discovery of abundant fossil vesicomyid and thyasirid shell accumulations at the ridge crest, approximately 2.5 km east of the active Rainbow vent field on the Mid-Atlantic Ridge (MAR, 36 degrees 13'N), increased our knowledge regarding the diversity of vent communities at slow spreading ridges. Bivalve molluscs of the family Vesicomyidae were represented by the genus Phreagena. Here we present the first record of this genus in the Atlantic Ocean. This second vesicomyid species known from the MAR, Phreagena sp., was found to be associated with a Thyasira species that is affiliated with T. southwardae (at the Logatchev vent field on the MAR) and with T. vulcolutre (in the Gulf of Cadiz). These two clams have close relationships with seep taxa along the continental margin, and were likely associated with sedimented vent fields. delta O-18 and delta C-13 analyses of the shells suggested that the burrowing bivalve Thyasira could incorporate isotopically light carbon, derived from the oxidation of methane in the sediment, while the signature of Phreagena sp. shells denoted a different carbonate source. C-14 dating of the shells denoted that the hydrothermal activity in the Rainbow area began at least similar to 25.5 kyr BP, which is similar to the model of the hydrothermal vent field distribution that was proposed for the Logatchev hydrothermal site. The results provide new insight regarding the diversity of chemosynthetic fauna on the MAR over geologic time. Ultramafic-hosted, on-axis sedimented vent fields extend the range of habitats for chemosynthetic communities, underlying the need to further explore the geology of these types of environments on slow-spreading ridges and to determine their role in the ecology of deep-sea vent communities.
ISSN: 1525-2027

hal-00523467
https://hal.archives-ouvertes.fr/hal-00523467
https://hal.archives-ouvertes.fr/hal-00523467/document
https://hal.archives-ouvertes.fr/hal-00523467/file/G3-Lartaud-_-al.pdf
DOI : 10.1029/2010GC003079

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
ODP (Ocean Drilling Program)/IODP (Integrated Ocean Drilling Program) Site 1256 is located on the Cocos Plate in the Eastern Equatorial Pacific Ocean, in a 15 Ma old oceanic lithosphere formed at the EPR during a period of superfast spreading (>200 mm/yr). ODP/IODP Hole 1256D reached for the first time the contact between sheeted dikes and underlying gabbros. It consequently offers a unique opportunity to study in situ, in present-day oceanic crust, the root zone of the sheeted dike complex. This root zone is a thin, 100 m thick boundary layer between the magmatic system (i.e., the axial melt lens, similar to 1100 degrees C), and the overlying high temperature hydrothermal system (<= 450 degrees C). The understanding of interactions within this boundary layer is critical to that of crustal processes along mid-ocean ridges.;This work focuses on the petrophysical characterization of the root zone of the sheeted dike complex in order to further constrain the hydrothermal circulation system in the vicinity of the axial melt lens, as recorded in non-granoblastic dikes, granoblastic dikes, and varitextured gabbros. The petrophysical properties were determined from sample measurements in the laboratory and were compared to in situ downhole geophysical probing. The porosity structure is bipolar, depending on lithology, resulting in a layered system. Non-granoblastic dikes are generally altered in the greenschist fades (similar to>250 degrees C) with relatively high and interconnected (cementation index m similar to 1.72, electrical tortuosity tau similar to 28.3) porosity (1.5%). In contrast, gabbros are retrogressively metamorphosed in the amphibolite (similar to>450 degrees C) and greenschist facies, with lower porosity (1.3%) that involves numerous fissures and cracks, resulting in a more connected medium (m similar to 1.58, tau similar to 11.8) than non-granoblastic dikes. These cracks are more abundant but also tend to close with increasing depth as indicated in downhole geophysical data. Porosity and alteration, as viewed from surface electrical conductivity, appear to be directly correlated.
ISSN: 0040-1951

hal-00544556
https://hal.archives-ouvertes.fr/hal-00544556
DOI : 10.1016/j.tecto.2010.07.013

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Brecciated and fractured peridotites with a carbonate matrix, referred to as ophicalcites, are common features of mantle rocks exhumed in passive margins and mid-oceanic ridges. Ophicalcites have been found in close association with massive peridotites, which form the numerous ultramafic bodies scattered along the North Pyrenean Zone (NPZ), on the northern flank of the Pyrenean belt. We present the first field, textural and stable isotopic characterization of these rocks. Our observations show that Pyrenean ophicalcites belong to three main types: (1) a wide variety of breccias composed of sorted or unsorted millimeter- to meter-sized clasts of fresh or oxidized ultramafic material, in a fine-grained calcitic matrix; (2) calcitic veins penetrating into fractured serpentine and fresh peridotite; and (3) pervasive substitution of serpentine minerals by calcite. Stable isotopic analyses (O, C) have been conducted on the carbonate matrix, veins and clasts of samples from 12 Pyrenean ultramafic bodies. We show that the Pyrenean ophicalcites are the product of three distinct genetic processes: (1) pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity; (2) ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity; and (3) polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid-Cretaceous. We highlight a major difference between the eastern and western Pyrenean ophicalcites belonging, respectively, to the sedimentary and to the hydrothermal types. Our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins, but a post-depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out. Finally, we discuss the significance of such discrepancy in the characteristics of the NPZ ophicalcites in the frame of the variable exhumation history of the peridotites all along the Pyrenean realm.
ISSN: 1437-3254

insu-00944405
https://hal-insu.archives-ouvertes.fr/insu-00944405
https://hal-insu.archives-ouvertes.fr/insu-00944405/document
https://hal-insu.archives-ouvertes.fr/insu-00944405/file/clerc-etal-IJES-Review.pdf
DOI : 10.1007/s00531-013-0927-z

Éditeur(s) : HAL CCSD
Résumé : 38 pages
Rapport
The Chicxulub impact crater, México, is unique. It is the only known terrestrial impact structure that has been directly linked to a mass extinction event and the only terrestrial impact with a global ejecta layer. Of the three largest impact structures on Earth, Chicxulub is the best preserved. Chicxulub is also the only known terrestrial impact structure with an intact, unequivocal topographic peak ring. Chicxulub’s role in the Cretaceous/Paleogene (K-Pg) mass extinction and its exceptional state of preservation make it an important natural laboratory for the study of both large impact crater formation on Earth and other planets and the effects of large impacts on the Earth’s environment and ecology. Our understanding of the impact process is far from complete, and despite more than 30 years of intense debate, we are still striving to answer the question as to why this impact was so catastrophic.During International Ocean Discovery Program (IODP) Expedition 364, Paleogene sediments and lithologies that make up the Chicxulub peak ring were cored to investigate (1) the nature and formational mechanism of peak rings, (2) how rocks are weakened during large impacts, (3) the nature and extent of post-impact hydrothermal circulation, (4) the deep biosphere and habitability of the peak ring, and (5) the recovery of life in a sterile zone. Other key targets included sampling the transition through a rare midlatitude section that might include Eocene and Paleocene hyperthermals and/or the Paleocene/Eocene Thermal Maximum (PETM); the composition and character of the impact breccias, melt rocks, and peak-ring rocks; the sedimentology and stratigraphy of the Paleocene–Eocene Chicxulub impact basin infill; the chronology of the peak-ring rocks; and any observations from the core that may help us constrain the volume of dust and climatically active gases released into the stratosphere by this impact. Petrophysical property measurements on the core and wireline logs acquired during Expedition 364 will be used to calibrate geophysical models, including seismic reflection and potential field data, and the integration of all the data will calibrate impact crater models for crater formation and environmental effects. The proposed drilling directly contributes to IODP Science Plan goals:Climate and Ocean Change: How resilient is the ocean to chemical perturbations? The Chicxulub impact represents an external forcing event that caused a 75% level mass extinction. The impact basin may also record key hyperthermals within the Paleogene.Biosphere Frontiers: What are the origin, composition, and global significance of subseafloor communities? What are the limits of life in the subseafloor? How sensitive are ecosystems and biodiversity to environmental change? Impact craters can create habitats for subsurface life, and Chicxulub may provide information on potential habitats for life, including extremophiles, on the early Earth and other planetary bodies. Paleontological and geochemical studies at ground zero will document how large impacts affect ecosystems and effects on biodiversity.Earth Connections/Earth in Motion: What are the composition, structure and dynamics of Earth’s upper mantle? What mechanisms control the occurrence of destructive earthquakes, landslides, and tsunami? Mantle uplift in response to impacts provides insight into dynamics that differ between Earth and other rocky planets. Impacts generate earthquakes, landslides, and tsunami, and scales that generally exceed plate tectonic processes yield insight into effects, the geologic record, and potential hazards.IODP Expedition 364 was a Mission Specific Platform expedition to obtain subseabed samples and downhole logging measurements from the sedimentary cover sequence and peak ring of the Chicxulub impact crater. A single borehole was drilled into the Chicxulub impact crater on the Yucatán continental shelf, recovering core from 505.7 to 1334.73 m below seafloor with ~99% core recovery and acquiring downhole logs for the entire depth.
International Ocean Discovery Program

hal-01639520
https://hal.archives-ouvertes.fr/hal-01639520
DOI : 10.14379/iodp.pr.364.2017

Éditeur(s) : HAL CCSD IODP
Résumé : The Superfast Spreading Crust campaign, echoing long-standing ocean lithosphere community endeavors, was designed to help us understand the formation, architecture, and evolution of ocean crust formed at fast spreading rates. Integrated Ocean Drilling Program (IODP) Expedition 335, "Superfast Spreading Rate Crust 4" (13 April-3 June 2011), was the fourth scientific drilling cruise of the Superfast Spreading Crust campaign to Ocean Drilling Program (ODP) Hole 1256D. The expedition aimed to deepen this basement reference site several hundred meters into the gabbroic rocks of intact lower oceanic crust to address the following fundamental scientific questions: Does the lower crust form by subsidence of a crystal mush from a high-level magma chamber (gabbro glacier), by intrusion of sills throughout the lower crust, or by some other mechanism? How does melt percolate through the lower crust, and what are the reactions and chemical evolution of magmas during migration? Is the plutonic crust cooled by conduction or hydrothermal circulation? What are the role and extent of deeply penetrating seawater-derived hydrothermal fluids in cooling the lower crust and the chemical exchanges between the ocean crust and the oceans? What are the relationships among the geological, geochemical, and geophysical structure of the crust and, in particular, the nature of the seismic Layer 2-3 transition? What is the magnetic contribution of the lower crust to marine magnetic anomalies?
https://hal.archives-ouvertes.fr/hal-00688990

hal-00688990
https://hal.archives-ouvertes.fr/hal-00688990
DOI : 10.2204/iodp.pr.335.2011

Éditeur(s) : HAL CCSD
Résumé : International audience
Tin mineralization in Achmmach deposit (NE of Moroccan Central Massif) is associated with tourmaline-rich alteration halos and faults, within N70 shear zones, hosted in sandstones and metapelites of the Upper Visean-Namurian. Such deposits were reported as late-Variscan in age and related to the emplacement of late orogenic granite not outcropping in the studied area.From field constraints, it appears that the late Variscan phase is marked by a transition from transpression to extension with conditions of deformation evolving from ductile to brittle environments. During the transpressive phase (horizontal shortening direction roughly trending E-W), pervasive tourmaline is deposited under the form of metasomatic halos around fractures in the upper levels relative to the deep batholith. This hydrothermal activity is coeval with ductile shear zones within a large corridors trending N70. Tourmaline-rich alteration appears as a hardening stage contributing to the modification of the tectonic style of rocks. Thereafter, a strong brecciation occurs in response of re-using and re-opening of the strike-slip structure. The result is (1) the formation of dextral (N070, N020) and sinistral (N120) shears, and (2) the circulation of fluids within early tourmalinites and formation of tourmaline-rich breccia. Subsequently, during the late orogenic extensional phase, tourmalinite breccias reopened as normal faults acting as overpressure fluid pathways for developing other breccias and veins. Such event is marked by deposition of tin mineralization i.e., cassiterite followed by sulphides, and then fluorite and carbonates at the end.From analysis of the shear-zone structure and superimposition of tourmalines and tin mineralization in the same structures, it is possible to suggest a structural control continuum. Such control is well expressed during metasomatism and deposition of hydrothermal tourmaline under the transpressive phase. Thereafter, under the extensive phase allowing open-space, tin-mineralization were trapped.This study has been supported by AI MA/09/210
24ème RST
Pau, France

insu-01546927
https://hal-insu.archives-ouvertes.fr/insu-01546927

Éditeur(s) : HAL CCSD National Academy of Sciences
Résumé : International audience
Among the deep-sea hydrothermal vent sites discovered in thepast 30 years, Lost City on the Mid-Atlantic Ridge (MAR) isremarkable both for its alkaline fluids derived from mantle rockserpentinization and the spectacular seafloor carbonate chimneysprecipitated from these fluids. Despite high concentrations ofreduced chemicals in the fluids, this unique example of a serpentinite-hosted hydrothermal system currently lacks chemosyntheticassemblages dominated by large animals typical of high-temperaturevent sites. Here we report abundant specimens of chemosymbioticmussels, associated with gastropods and chemosymbioticclams, in approximately 100 kyr old Lost City-like carbonates fromthe MAR close to the Rainbow site (36 °N). Our finding shows thatserpentinization-related fluids, unaffected by high-temperaturehydrothermal circulation, can occur on-axis and are able to sustainhigh-biomass communities. The widespread occurrence of seafloorultramafic rocks linked to likely long-range dispersion of ventspecies therefore offers considerably more ecospace for chemosyntheticfauna in the oceans than previously supposed
ISSN: 0027-8424

insu-01352929
https://hal-insu.archives-ouvertes.fr/insu-01352929
https://hal-insu.archives-ouvertes.fr/insu-01352929/document
https://hal-insu.archives-ouvertes.fr/insu-01352929/file/PNAS-2011-Lartaud-7698-703.pdf
DOI : 10.1073/pnas.1009383108

Éditeur(s) : HAL CCSD IODP
Résumé : Integrated Ocean Drilling Program (IODP) Expedition 335 (14 April–4 June 2011) will be the fourth ocean cruise of the "Superfast" campaign to drill a deep hole into intact oceanic basement and will return to Ocean Drilling Program (ODP) Hole 1256D to deepen this scientific reference penetration a significant distance into cumulate gabbros. Cores and data recovered during the Superfast 4 expedition will provide hitherto unavailable observations that will test models of the accretion and evolution of the oceanic crust. Site 1256 was specifically located on oceanic crust that formed at a superfast spreading rate (>200 mm/y) to exploit the observed relationship between spreading rate and depth to axial low velocity zones, thought to be magma chambers, seismically imaged at active mid-ocean ridges. This was a deliberate strategy to reduce the drilling distance to gabbroic rocks because thick sequences of lavas and dikes have proved difficult to penetrate in past. ODP Leg 206 (2002) initiated operations at Site 1256, including the installation in Hole 1256D of a reentry cone with 16 inch casing inserted through the 250 m thick sedimentary cover and cemented into basement to facilitate deep drilling. The hole was then cored ~500 m into basement. IODP Expeditions 309 and 312 (2005) successfully completed the first sampling of an intact section of upper oceanic crust from lavas, through the sheeted dikes, and into the upper gabbros. Hole 1256D now penetrates >1500 meters below seafloor (mbsf) and >1250 m subbasement and currently resides in the dike–gabbro transition zone. The first gabbroic rocks were encountered at 1407 mbsf. Below this lies a ~100 m complex zone of fractionated gabbros intruded into contact metamorphosed dikes. Although previous cruises achieved the benchmark objective of reaching gabbro in intact ocean crust, critical scientific questions remain. These include the following: 1. Does the lower crust form by the recrystallization and subsidence of a high-level magma chamber (gabbro glacier), crustal accretion by intrusion of sills throughout the lower crust, or some other mechanism? 2. Is the plutonic crust cooled by conduction or hydrothermal circulation? 3. What is the geological nature of Layer 3 and the Layer 2/3 boundary at Site 1256? 4. What is the magnetic contribution of the lower crust to marine magnetic anomalies? Hole 1256D is poised at a depth where samples that should conclusively address these questions can be obtained, possibly with only a few hundred meters of drilling. Importantly, as of the end of Expedition 312, the hole was clear of debris and open to its full depth. Increased rates of penetration (1.2 m/h) and enhanced core recovery (>35%) in the gabbros indicate that this return to Hole 1256D could deepen the hole >300 m into plutonic rocks, past the transition from dikes to gabbro, and into a region of solely cumulate gabbroic rocks.
https://hal.archives-ouvertes.fr/hal-00564953

hal-00564953
https://hal.archives-ouvertes.fr/hal-00564953
DOI : 10.2204/iodp.sp.335.2010

Éditeur(s) : HAL CCSD American Geophysical Union (AGU) American Geophysical Union
Résumé : International audience
The brittle to ductile transition may strongly influence hydraulic properties of rocks at the depth and temperature ranges that hydrothermal fluids circulate. To examine this transition in the context of the oceanic crust, we conducted a series of deformation experiments on a natural basalt sample at in situ oceanic crust conditions. Dilatancy was measured during deformation. The method consisted in monitoring the volume of pore fluid that flows into or out of the sample at constant pore pressure. Mechanical and microstructural observations at experimental constant strain rate of 10−5 s−1 indicated that the basalt was brittle and dilatant up to 800°C. At higher temperature, the deformation mode became macroscopically ductile and samples compacted. These observations have important implications on heat transfer and fluid migration in oceanic crust.
ISSN: 0094-8276

hal-01172352
https://hal.archives-ouvertes.fr/hal-01172352
DOI : 10.1002/2015GL063437

Éditeur(s) : Elsevier
Résumé : Deep-sea bivalves of the subfamily Bathymodiolinae (family Mytilidae) are very widespread and form dense beds in reduced environments such as hydrothermal vents and cold seeps. Bathymodiolus mussels recently discovered on African cold seeps strangely resemble Gulf of Mexico and Barbados seep species. This raises intriguing questions regarding their taxonomic relationships and their dispersal capabilities across the Atlantic equatorial belt. The morphological study of the shell and soft parts of mussels from either sites of the Atlantic shows that they form two distinct groups: the Bathymodiolus boomerang group (also including Bathymodiolus heckerae and a species from Africa), and the Bathymodiolus childressi group (also including Bathymodiolus mauritanicus and one species from Barbados). Phylogenetic relationships inferred from the nucleotide sequences of the ribosomal DNA internal transcribed spacer (ITS-2) and mitochondrial cytochrome c oxidase subunit I (COI) confirmed morphological analyses and the existence of two amphi-Atlantic complexes of species. Both ITS2 and COI phylogenies indicate almost no difference between the two eastern Atlantic seep mussels (Bathymodiolus sp. A and B. mauritanicus) and their western Atlantic counterparts (B. boomerang and Bathymodiolus sp. B; Barbados Prism cold seeps). In the B. boomerang complex, B. heckerae seems to differ from the Barbados and the African species, whereas these latter two are not distinguishable. In the B. childressi complex, relationships are less clear and do not support the description of new species from the Barbados. Past and present-day connections across the Atlantic are discussed in the light of both larval dispersal capabilities of the mussels and the equatorial Atlantic circulation to appreciate whether these species could represent true amphi-Atlantic species.
Deep Sea Research Part I: Oceanographic Research Papers (0967-0637) (Elsevier), 2007-11 , Vol. 54 , N. 11 , P. 1890-1911

Droits : 2007 Elsevier Ltd All rights reserved.
http://archimer.ifremer.fr/doc/2007/publication-4064.pdf
DOI:10.1016/j.dsr.2007.07.004
http://archimer.ifremer.fr/doc/00000/4064/