Éditeur(s) : HAL CCSD
Résumé : East African Rift (EAR) is the divergent plate boundary. EAR exposes different stages of extension, from early stage rifting in Tanzania to oceanic accretion in Afar (Ethiopia). Manyara basin is the southernmost rift system of the east branch of EAR with recent volcanism (< 1.5 Ma) and a seismic swarm in the lower crust (20 – 40 km). Due to its location and tectonic setting, the Manyara basin offers the opportunity to study the earliest stage of rift initiation. Manyara volcanism is composed of several types of hyper-alkaline lavas as Mg-nephelinites (Mg# > 55) (Labait, Kwaraha), calciocarbonatite (Kwaraha) and evolved nephelinites (Mg# < 35) (Hanang).Mg-nephelinites (Labait and Kwaraha) are primary lavas mainly composed of olivine and clinopyroxene (cpx). Geochemical modelling from trace elements suggests that these primary magmas result from a degree of partial melting < 1 % from a CO2-garnet-phlogopite-bearing peridotite. These magmas have an asthenospheric source at depth > 120 km (lava carries xenoliths with equilibrium conditions > 4 GPa). The minerals were crystallized from a magma with a low H2O content (0.1 and 0.5 wt% H2O). The calciocarbonatite and evolved nephelinites are derived from Mg-nephelinites by fractional crystallization and immiscibility processes. Hanang nephelinites are silica- and alkaline-rich lavas (44.2 – 46.7 wt % SiO2, 9.5 –12.1 wt % Na2O+K2O, respectively) composed by cpx, Ti-garnet, nepheline, apatite and titanite. Complex zonation of cpx (e.g. abrupt change of Mg#, Nb/Ta, and H2O) and trace element patterns of nephelinites record magmatic differentiation involving open system with carbonate-silicate immiscibility and primary melt replenishment. The low H2O content of cpx (3 – 25 ppm wt. H2O) indicates that at least 0.3 wt % H2O was present at depth during carbonate-rich nephelinite crystallization at 340 – 640 MPa and 1050 – 1100 °C. The study of hosted-nepheline melt inclusions from Hanang allows constraining the late magmatic evolution of nephelinites during storage and magma ascent. Melt inclusions are composed by a silicate trachytic glass, a carbonate phase and a shrinkage bubble. Trachytic glass contains high content in CO2 (0.43 wt %, SIMS analyses), sulfur (0.21 – 0.92 wt % S), chlorine (0.28 –0.84 wt % Cl) and H2O low content (< 0.1 wt %, Raman analyses). Immiscibility process leading to the formation of carbonate occurs in a closed system during rapid magma ascent between 200 – 500 MPa. The carbonate phase is a Ca-Na-K-S-rich and anhydrous carbonate (33 wt % CaO, 20 wt % Na2O, 3 wt % K2O, and 3 wt % S). The pre-immiscible liquid has a phonolitic composition with 6 ± 1.5 wt % CO2 at 700 MPa. A preliminary study of melt inclusions by XANES spectroscopy and whole rocks by Mössbauer spectroscopy was used to determine these Manyara lavas were formed at oxidizing conditions (~ ΔFMQ +1.5).The early stage rifting volcanism (Manyara Basin) is characterized by CO2-rich and H2O-poor magmas from at least 120 km below the rift escarpment. The presence of CO2-rich magmas and the small amount of volcanic rocks erupted at the surface may indicate that the storage and percolation of these magmas at depth is a potential trigger for deep seismic swarms.
Le rift Est africain (REA) est une frontière de plaque en extension. Ce rift présente plusieurs stades d’extension, de l’initiation du rift en Tanzanie jusqu’à l’accrétion océanique en Afar. Le bassin de Manyara se situe le plus au sud de branche Est du REA. Il est caractérisé par la présence de volcanisme récent (< 1,5 Ma) et d’un essaim sismique dans la croûte inférieure (20 – 40 km). De par sa localisation et son contexte tectonique, le bassin de Manyara offre l’opportunité d’étudier le stade le plus précoce de l’initiation du rift. Le bassin de Manyara est composé de plusieurs types de laves hyperalcalines, les néphélinites magnésiennes (Mg# > 55) (Labait, Kwaraha), de calciocarbonatite (Kwaraha) et des néphélinites différenciées (Mg# < 35) (Hanang).Les néphélinites magnésiennes (Labait et Kwaraha) sont des laves primaires composées d’olivines et de clinopyroxènes (cpx). La modélisation géochimique des éléments en trace suggère que ces magmas primaires résultent d'un degré de fusion partielle ≤ 1 % à partir d'une péridotite à grenat et phlogopite. Ces magmas proviennent d’une profondeur > 120 km (présence de xénolites avec des conditions d’équilibre > 4 GPa). Les minéraux ont cristallisés à partir d’un magma pauvre en eau (0,1 et 0,5 pds % H2O). La calciocarbonatite et les néphélinites différenciés sont issues des néphélinites magnésiennes par cristallisation fractionnée et processus d’immiscibilité. Les néphélinites du Hanang sont riches en éléments alcalins (9,5 – 12,1 pds % Na2O+K2O) et en silice (44,2 – 46,7 pds% SiO2) et sont composés de cpx, grenat, néphéline, titanite et apatite. La zonation complexe dans les cpx (par exemple, changement brusque de Mg#, Nb/Ta, et H2O) implique une différenciation magmatique en système ouvert avec immiscibilité de liquide carbonaté et silicaté ainsi qu’un remplissage de la chambre magmatique avec des liquides primaires. La faible teneur en eau des cpx (3 – 25 ppm H2O) indique la présence d’un magma pauvre en eau (0,3 pds % H2O) lors de la cristallisation des cpx à des conditions crustales (340 – 640 MPa et 1050 – 1100 °C). L’étude des inclusions vitreuses dans les néphélines de Hanang permet de contraindre l'évolution magmatique tardive des néphélinites et le comportement des éléments volatils (CO2, H2O, S, F, Cl) lors du stockage et de la remontée du magma. Les inclusions vitreuses sont composées d’un verre trachytique, d’une phase carbonatée et d’une bulle de rétraction. Le verre trachytique contient du CO2 (0,43 pds % CO2, analyses SIMS), du soufre (0,21 à 0,92 pds% S), du chlore (0,28 – 0,84 pds % Cl) et très peu d’H2O (< 0,1 pds % H2O, analyses Raman). Le processus d’immiscibilité conduisant à la formation du carbonate se produit dans un système fermé pendant l'ascension rapide du magma, entre 200 – 500 MPa. La phase carbonatée est un carbonate anhydre et riche en Ca-Na-K-S (33 pds % CaO, 20 pds % Na2O, 3 pds % K2O, et 3 pds % S). Le liquide pré-immiscible a une composition phonolitique avec 6 ± 1,5 pds % CO2 à une pression de 700 MPa. Une étude préliminaire des inclusions par spectroscopie XANES et des roches par spectroscopie Mössbauer a permis de déterminer que les laves de Manyara se sont formées à conditions oxydantes (~ ∆FMQ +1,5).À l’initiation du rift, le volcanisme dans le bassin de Manyara est caractérisé par des magmas riches en CO2 et pauvres en H2O issus d’au moins 120 km de profondeur sous l'escarpement du rift. La présence de ces magmas riches en CO2 et la faible quantité de roches volcaniques émises à la surface peuvent indiquer que le piégeage et la percolation de ces magmas en profondeur est un déclencheur potentiel des essaims sismiques profonds.
https://tel.archives-ouvertes.fr/tel-01563231

NNT : 2016MONTT114
tel-01563231
https://tel.archives-ouvertes.fr/tel-01563231
https://tel.archives-ouvertes.fr/tel-01563231v2/document
https://tel.archives-ouvertes.fr/tel-01563231/file/2016_BAUDOUIN_archivage.pdf

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Phase equilibrium data pertaining to melting of simplified carbonated peridotite in the systems CaO-MgO-SiO2-CO2 and CaO-MgO-Al2O3-SiO2-CO2 at pressures of 10-26 GPa, corresponding to similar to 300-750 km depths in the Earth, are presented. In both the studied systems, liquid compositions, with changing crystalline phase assemblage, are carbonatitic throughout the studied pressure range. In the system CMS-CO2, melting phase relations are isobarically invariant; liquid is in equilibrium with forsterite + clinoenstatite + clinopyroxene + magnesite, forsterite + majorite + clinopyroxene + magnesite, wadsleyite + majorite + clinopyroxene + magnesite, ringwoodite + majorite + calcium-silicate perovskite + magnesite, magnesium-silicate perovskite + periclase + calcium-silicate perovskite + magnesite at 12, 14, 16, 20, and 26 GPa, respectively. In the system CMAS-CO2, a phase assemblage consisting of forsterite + orthopyroxene + clinopyroxene + magnesite + garnet + melt from 10 to 14 GPa is isobarically invariant. However, owing to the disappearance of orthopyroxene at pressures greater than 14 GPa, from 16 and up to at least 26 GPa, the solidus of simplified carbonated peridotite spans a divariant surface in pressure-temperature space. The liquid coexists with wadsleyite + clinopyroxene + garnet + magnesite, ringwoodite + calcium-silicate perovskite + garnet + magnesite, and magnesium-silicate perovskite + periclase + calcium-silicate perovskite + magnesite at 16, 20, and 26 GPa, respectively. A curious, and as yet unexplained, feature of our study is an abrupt drop in the solidus temperature between 14 and 16 GPa that causes a small amount of melting of carbonated mantle in the Transition Zone of the Earth. In the systems CMS-CO2 and CMAS-CO2 liquid compositions at 16 and 20 GPa are highly calcic bona fide carbonatites; however, these liquids revert to being magnesiocarbonatites at 10-14 and 26 GPa. In the system CMS-CO2, at 16 GPa we locate an isobaric invariant point consisting of wadsleyite + clinopyroxene + anhydrous B + magnesite + melt. The presence of anhydrous B at 16 GPa and 1475 degrees C is interesting, as it lies outside the composition space of the mantle peridotite analog we have studied. However, despite the presence of two highly magnesian silicate crystalline phases, wadsleyite and anhydrous B, at 16 GPa and 1475 degrees C, the liquid composition remains calcic with molar Ca-number [Ca/(Ca + Mg) x 100] of about 63. The melting reactions at 16 and 20 GPa (with or without anhydrous B) show that lime-bearing crystalline silicates play a fairly large part in generating and controlling the composition of the liquids. At 16 GPa, in the system CMS-CO2, we also report an experimental run at 1575 degrees C, in which liquid coexists with only wadsleyite and majorite. The liquid composition is less calcic (Ca-number 54) than that for other runs at lower temperatures, but is still more calcic than liquids at 10-14 and 26 GPa in both the studied systems. At present, the likely cause for these changes in the reported phase relations is not known. For normally assumed mantle temperatures, melting in the Transition Zone of the Earth, owing to the presence of carbonate, is probably unavoidable. The depth range of the drop in the carbonated peridotite solidus closely matches that of commonly observed low seismic velocities at similar to 400-600 km depth in the Earth.
ISSN: 0022-3530

hal-00669945
https://hal.archives-ouvertes.fr/hal-00669945
DOI : 10.1093/petrology/egr048

Éditeur(s) : HAL CCSD Elsevier
Résumé : Carbon emission at volcanic centers requires a constant balance between output (mostly by volcanism, either at plate boundaries or intraplate) and input (mostly at trench settings) of carbon from and to the Earth's mantle. The form of carbon that resides in the mantle is controlled by depth (pressure) and oxygen fugacity, the latter in turn depending on the depth and the concentration of iron in the mantle. In the shallow, lithospheric mantle, carbon is likely to be present in the oxidized form of CO2 (except under cratons where carbon is reduced to graphite or diamond). Below approximately 90 km, in the asthenosphere, the oxidized form of carbon is carbonate, either mineral or melt, depending on the thermal regime. At depths greater than approximately 150 km, the asthenospheric mantle is too reducing for carbon to stay in its oxidized form and only diamond is present, unless there is sufficient hydrogen to form reduced C–H fluids. Hence, the region located in the depth range of 90 to 150 km deep is where carbonatitic melts can most likely be produced and impregnate the surrounding mantle through metasomatism. The upper bound of this region is called the carbonate ledge. This limit prevents carbonate (either solid or molten) from ascending because of degassing and CO2 liberation. The lower bound is a redox front where redox melting (that is, melting caused by oxidation) may take place in an ascending portion of carbon-containing mantle. Carbonatite eruptions and presence of carbonate mineral inclusions in deep-seated diamonds provide evidence that these boundaries can be trespassed in some cases.An analysis of the experimental data that has bearing on silicate melting in the presence of carbon further shows that the carbonate ledge is a melting curve with a negative or flat Clapeyron (dP/dT) slope. In the carbonated ultrabasic (peridotite) systems, the carbonate ledge is located between ~ 2–3 GPa. The ledge divides the pressure–temperature space into a region of low-pressure silicate melt production, and a high-pressure region where carbonatites can be produced. Carbonatitic melts in equilibrium with mantle peridotite have compositions close to dolomitic (approximately equal amounts of Ca and Mg) with a general trend of becoming markedly more magnesian with increasing pressure. Calcic carbonatites may be stable at pressures < 2 GPa if clinopyroxene is absent. At mantle transition zone pressure range, there seems to be a melting temperature decrease (negative fusion slope), which may be caused by the stabilization of majoritic garnet.The carbonated basic (broadly eclogitic) system is more complex than the peridotitic one, because of the strong control of bulk silicate composition on melting temperatures, and hence, on melt composition. In carbonated eclogite systems, we propose that the effect of bulk composition upon all observed features can perhaps be related to silica super-saturation, or lack thereof. In some cases, high calcium (> 80 mol% CaCO3) melts can be produced, making the melting of carbonated eclogites an appealing scenario for the genesis of calcio-carbonatites in the Earth's mantle. Comparison with modeled pressure–temperature paths of subducted oceanic lithosphere shows that fusion of carbonated eclogite at depths shallower than 200 km should be expected for hot (Cascadian-type) subduction thermal regimes. On the other hand, in the case of cooler thermal regimes (Honshu-type, for instance), subducted carbonates may be stable to greater depths in Earth at trench settings, depending on the bulk composition of the system.Furthermore, high-pressure experiments show evidence of a continuum among carbonatitic, kimberlitic, melilititic, and basaltic liquids, for increasing melting degree of carbonated peridotite. This continuum has not been documented in the case of fusion of carbonated eclogite. It may be present, however, when certain sediments are fused, although the silicate melts are granitic to rhyodacitic instead of being kimberlitic in composition. Additional high-pressure work on phase relations in the simple binary system CaCO3–MgCO3 and specific focus on oxide solubility in the vapor phase have the potential to further clarify phase relations on complex silicate–carbonate systems at mantle conditions.
ISSN: 0009-2541

hal-01277770
https://hal.archives-ouvertes.fr/hal-01277770
DOI : 10.1016/j.chemgeo.2015.05.018

Éditeur(s) : HAL CCSD
Résumé : International audience
The geology of the North Pyrenean Zone in the central Pyrenees allows for the observation in the field of theentire section of the Pyrenean rift, from the mantle to the crust and the Mesozoic cover (pre, syn and post rift).The good knowledge we have of the pre-Alpine history of the Pyrenees allows us to properly constrain the Alpinegeological evolution of the pre-Triassic rocks which record both Variscan and Alpine orogenic cycles.The mantle outcrop as kilometric to centimetric fragments of peridotite dispersed within a carbonate metamorphicbreccia. The study of peridotite serpentinisation shows several events of low-temperature serpentinisation, incontact with seawater. In some locallities, we can observe a mixture of fragments of variously serpentinizedperidotites. This suggests a tectonic context where fragments of peridotites from different structural levels weresampled more or less synchronously.The granulitic basement is characterized by a Variscan syndeformational HT event (300-280 Ma). So farwe have not found any trace of a Cretaceous HT event (> 500C). On the other hand, the basement is affectedby a regional metasomatism that began during the Jurassic and became more spatially focused with time until itwas restricted to the Pyrenean rift during the Aptien, Albian and Cenomanian. The talc-chlorite metasomatism(120-95 Ma) shows an evolution from a static toward a syn-deformation hydrothermal event, under a more or lessnormal geothermal gradient. Extensional deformation is recorded by the reworking of several inherited low-angleVariscan tectonic contacts, but also by dispersed high-angle extensional shear zones formed under greenshistconditions.The metamorphic Mesozoic cover of the basement massifs, which constitute the so-called Internal MetamorphicZone, is an allochtonous unit made of lenses of Mesozoic rocks enclosed into the breccia, whichlocally contains peridotite and basement clasts. The Mesozoic metamorphic carbonates show a first phase ofsyn-metamorphic (450-600C, P < 2 kb) ductile deformation, and subsequent phases of folding and fracturing.The datation of neoformed minerals give a 108-85 Ma time span for the metamorphism. We interpret this brecciaas an abandonment breccia which marks the emergence of the main detachment. The basal contact of the Mesozoiccover has a complex 3D geometry traced by Triassic evaporites. It corresponds to a major pre- and synorogenicpolyphased tectonic contact.All these data show a geometrically complex hyper-extended rift where the crust was not stretched under ahigh geothermal gradient but thinned by the tectonic extraction of relatively thin lenses and perhaps cut by highangle low-grade shear zones. The 3D geometry, as well as the strain records and the breccia lithologies stronglysuggest a non-cylindricity for the exhumation process, probably within a transtentional system
Geophysical Research Abstracts
Vienne, Austria

insu-01510371
https://hal-insu.archives-ouvertes.fr/insu-01510371

Éditeur(s) : Neidine Editeurs
Résumé : The object of this article is to offer a synthesis of the knowledge clarifying the originality and the diversity of the submarine geological formations of the New Caledonian EEZ. A reminder is given onto the geologic history of the region, based upon the dislocation of the Gondwana Continent, the opening of the Tasman Sea, the presence of submarine volcanism, the setting up of heavy peridotitic rocks, the building up of the Fairway and Lord Howe Ridges and of sedimentary basins, the formation of volcanic domes and sea-mounts or guyots, the subduction along the Vanuatu Trench, the uplift of the Loyalty Islands, the development of carbonated ptatforms around the islands etc.... Of this complex history ensues an exceptional submarine geo-diversity. After this presentation the article clarifies the methods and the tools used in marine geosciences to map, explore, observe, analyze, sample, that they are embarked or are autonomous, teleoperated or inhabited systems. A chapter describes the programs which were set up since about forty years and the results obtained as well as the scientific teams concerned with. In conclusion, based on the unpublished character of this geo-diversity, the article introduces the second text to come in a next issue of theTai Kona magazine, text which will focus on the scientific, economic (mineral resources), environmental (deep biodiversity) and risks (seismicity) challenges that are worn by one of the most original submarine geologic context in the world.
L’objet de cet article est de proposer une synthèse des connaissances explicitant l’originalité et la diversité des formations géologiques sous-marines de la ZEE calédonienne. Un rappel est donné sur l’histoire géologique de la région basée sur la dislocation du continent Gondwana, l’ouverture de la Mer de Tasman, la présence de volcanisme sous-marin, la mise en place des péridotites, la formation des rides de Fairway et Lord Howe et de bassins sédimentaires, la formation de dômes et monts sous-marins ou guyots, la subduction le long de la fosse de Vanuatu, la surrection des Loyautés, la mise en place de plateformes carbonatées etc... De cette histoire complexe découle une géo-diversité sous-marine exceptionnelle. Après cette présentation l’article explicite les méthodes et les outils utilisés en géosciences marines pour cartographier, explorer, observer, analyser, prélever, qu’ils soient embarqués ou soient des systèmes autonomes, télé-opérés ou habités. Un chapitre décrit les programmes qui ont été mis en place depuis une quarantaine d’années et les résultats obtenus ainsi que les équipes scientifiques en place. En conclusion sur le constat du caractère inédit de cette géo-diversité, l’article introduit un deuxième texte à venir dans un prochain numéro de Tai Kona, texte qui se focalisera sur les enjeux scientifiques, économiques (ressources minérales), environnementaux (biodiversité profonde) et de risques (sismicité) portés par un contexte géologique sous-marin des plus originaux.
Tai Kona (2269-7535) (Neidine Editeurs), 2014 , N. 8 , P. 48-57

Droits : 2014 Tai Kona
http://archimer.ifremer.fr/doc/00194/30574/29098.pdf
http://archimer.ifremer.fr/doc/00194/30574/

Éditeur(s) : HAL CCSD
Résumé : Volatile-rich, silica-undersaturated alkaline lavas record the important role of fluids during fluid-rock and magma-rock interactions in the mantle, which are key processes to understand the dynamics of the convective mantle and lithosphere-asthenosphere interactions in intracontinental settings. The aim of this thesis is to bring new constraints on the genesis of alkaline magmas by characterizing the crystallization conditions, the source and the partial melting processes taking part in the genesis of olivine nephelinites, pyroxene nephelinites and basanites from the Jbel Saghro volcanic field in the Moroccan Anti Atlas.The petrological and geochemical study of rocks and minerals coupled with the analysis of fluid inclusions constrains the pre-eruptive conditions of Saghro nephelinites to 1.7–2.2 GPa and ~1350 °C. Minerals show that nephelinitic magmas are rich in volatile elements (Cl, F, S), and fluid inclusions indicate that magmas were saturated with a CO2-rich fluid at pressures > 590 MPa. The various mineralogical assemblages and the presence of peridotite xenoliths suggest a rapid ascent for olivine nephelinites and more complex processes at depth for pyroxene nephelinites.Fractional crystallization and partial melting modelling of Saghro mafic lavas indicate that they are low-degree melts (0.6–2.5 %) of an amphibole-bearing carbonated peridotite enriched in incompatible elements, at the garnet-spinel transition (~80–85 km). Saghro nephelinites display a temporal evolution with a slight increase of the degree of melting and a decrease of the amount of residual amphibole from the oldest (olivine nephelinites, 9.6 Ma) to the most recent (pyroxene nephelinites, 2.9 Ma). Basanites form a system that is independent from nephelinites and are slightly higher-degree melts. Important variations in their chemical composition suggest variable amounts of fractional crystallization during ascent.The peculiar characteristics of Saghro nephelinites and basanites (enrichment in incompatible elements, negative anomalies in K, Zr, Hf and Ti, high Ca/Al and Zr/Hf ratios) indicate that their source was affected by carbonatitic metasomatism. The influence of this metasomatism is stronger for pyroxene nephelinites than for olivine nephelinites. These results suggest fluid-rock interactions beneath the Northwest African Craton, leading to the formation of a metasomatized mantle by CO2-rich carbonatitic components at the lithosphere-asthenosphere transition. The origin of the metasomatism inducing source enrichment and the formation of amphibole veins could be attributed to the melting of relict subducted oceanic lithosphere. The relatively low melting temperatures (< 1350 °C) suggest the absence of a thermal anomaly beneath the Jbel Saghro, and thus support a lithosphere delamination model as precursor of Saghro volca0,3nism. However, the increasing degree of partial melting over time, also observed in the Middle Atlas, together with the isotopic and geochemical similarities with Canary Islands alkaline lavas does not allow us to discard the influence of a deviation of the Canary mantle plume beneath northwest Africa.
Les laves alcalines sous-saturées riches en éléments volatils sont les marqueurs du rôle important des fluides dans le manteau et des interactions fluide-roche et magma-roche, processus clés pour comprendre la dynamique du manteau convectif et les interactions asthénosphère-lithosphère en domaine intracontinental. L’objectif de cette thèse est d’apporter de nouvelles contraintes sur la genèse des magmas alcalins en caractérisant les conditions de cristallisation, la source et les processus de fusion partielle à l’origine des néphélinites à olivine, des néphélinites à pyroxène et des basanites du champ volcanique du Jbel Saghro dans l’Anti-Atlas marocain.L’étude pétrologique et géochimique des roches et des minéraux, couplée à l’analyse des inclusions fluides a permis de contraindre les conditions pré-éruptives des néphélinites de Saghro à 1.7–2.2 GPa et ~1350 °C. Les minéraux montrent que les magmas néphélinitiques sont riches en éléments volatils (Cl, F, S), et les inclusions fluides indiquent que les magmas étaient saturés en fluide riche en CO2 à des pressions > 590 MPa. Les différents assemblages minéralogiques des néphélinites et la présence de xénolites péridotitiques suggèrent une ascension rapide des néphélinites à olivine et des processus plus complexes en profondeur pour les néphélinites à pyroxène.La modélisation des processus de cristallisation fractionnée et de fusion partielle des laves mafiques de Saghro a permis de déterminer qu’elles sont issues de faibles taux de fusion partielle (0.6–2.5 %) d’une péridotite carbonatée enrichie en éléments incompatibles, au niveau de la transition grenat–spinelle (~80–85 km) et en présence d’amphibole. Les néphélinites de Saghro montrent une évolution temporelle avec une légère augmentation du taux de fusion et une diminution de la quantité d’amphibole au résidu des plus anciennes (néphélinites à olivine, 9.6 Ma) aux plus récentes (néphélinites à pyroxène, 2.9 Ma). Les basanites forment un système indépendant des néphélinites et sont issues de taux de fusion plus élevés. Les fortes variations dans leur composition chimique suggèrent qu’elles ont subi de la cristallisation fractionnée lors de leur ascension.Les caractéristiques particulières des néphélinites et basanites de Saghro (enrichissement en éléments incompatibles, anomalies négatives en K, Zr, Hf et Ti, rapports Ca/Al et Zr/Hf élevés) indiquent que leur source a subi un métasomatisme principalement carbonatitique. L’influence de ce métasomatisme est plus forte pour les néphélinites à pyroxène que pour les néphélinites à olivine, impliquant une évolution temporelle de l’intensité du métasomatisme. Ces résultats suggèrent des interactions fluide-roche sous le craton Nord-Ouest Africain, entraînant la formation d'un manteau métasomatisé par des composants carbonatitiques riches en CO2 au niveau de la transition lithosphère-asthénosphère. L’origine du métasomatisme provoquant l’enrichissement de la source et la formation de veines d’amphibole pourrait être liée à la fusion de reliquats de croûte océanique subductée. Les températures de fusion relativement faibles (< 1350 °C) suggèrent l’absence d’anomalie thermique sous le Jbel Saghro, et favorisent donc un modèle de délamination de la lithosphère comme initiateur du volcanisme. Cependant, l’augmentation du taux de fusion partielle au cours du temps, également observée dans le Moyen Atlas, et les similitudes isotopiques et géochimiques avec les laves alcalines des îles Canaries ne permettent pas d’exclure une influence du panache des Canaries sur la source du volcanisme alcalin du Jbel Saghro.
https://tel.archives-ouvertes.fr/tel-01615888

NNT : 2015MONTS070
tel-01615888
https://tel.archives-ouvertes.fr/tel-01615888
https://tel.archives-ouvertes.fr/tel-01615888/document
https://tel.archives-ouvertes.fr/tel-01615888/file/53752_CHAMBOREDON_2015_archivage_cor.pdf

Éditeur(s) : HAL CCSD Copernicus Publications
Résumé : This workshop report describes plans for scientific drilling in the Samail ophiolite in Oman in the context of past, current, and future research. Long-standing plans to study formation and evolution of the Samail crust and upper mantle, involving igneous and metamorphic processes at an oceanic spreading center, have been augmented by recent interest in ongoing, low temperature processes. These include alteration and weathering, and the associated sub-surface biosphere supported by chemical potential energy due to disequilibrium between mantle peridotite and water near the surface. This interest is motivated in part by the possibility of geological carbon capture and storage via engineered, accelerated mineral carbonation in Oman.
ISSN: 1816-8957

hal-00858243
https://hal.archives-ouvertes.fr/hal-00858243
DOI : 10.2204/iodp.sd.15.10.2013

Éditeur(s) : HAL CCSD
Résumé : Based on fieldwork and modern analytical technics, this work presents an overview of the Mid-Cretaceous hyper-extended rift system exposed in the northern part of the Pyrenean mountain range. Its inversion during the Pyrenean orogeny allows for precious observations of the deep-seated processes occurring at the foot of distal margins. Some peculiar aspects characterizing the pre-Alpine hyper-extended domain are examined throughout the 400km long suture that is fossilized in the North Pyrenean Zone (NPZ). The High Temperature/Low Pressure (HT/LP) thermal imprint of the extensional event is studied thanks to a dataset of more than hundred peak temperature measurement by Raman spectroscopy of the carbonaceous material (RSCM). The pre- and syn-rift metasediments are characterized by an intense ductile deformation. Field and EBSD investigations of the mechanisms of deformation reveal ductile and syn-metamorphic processes. The emplacement conditions of the peridotite into crustal levels and localized exhumation is examined in the key-localities of Urdach, Saraillé-Tos de la Coustette, Moncaup, Lherz, Caussou-Bestiac and Salvezines, The results indicate an early boudinage of the crust during the rifting process, resulting in the direct superposition of pre-rift sediments on top of mantle peridotites. Field, textural and isotopic characterization of the various ophicalcite and breccias found in the vicinity of the peridotite outcrops allow to decipher their exhumation history. New Ar-Ar geochronological results allow to precise the temporal evolution of the NPZ.
Cette thèse présente une analyse structurale et thermique des marges passives crétacées inversées dans la Zone Nord Pyrénéenne (ZNP). L'application du paléothermomètre RSCM (spectrométrie Raman de la matière carbonée) sur plus de cent échantillons répartis à travers la ZNP permet de préciser l'évolution thermique de la déchirure continentale. Un métamorphisme de haute température (450 à >600°C) se développe particulièrement au droit de zones ou la croûte continentale est extrêmement amincie voire éliminée. Les sédiments pré- et syn-rift sont marqués par une intense déformation ductile. L'analyse à l'EBSD des fabriques minérales indique une déformation de Haute-température contemporaine du métamorphisme HT-BP. Les conditions de mise en place crustale des péridotites et leur exhumation localisée sont examinées dans les localités d'Urdach, Saraillé-Tos de la Coustette, Moncaup, Lherz, Caussou-Bestiac et Salvezines. Les résultats indiquent un boudinage de la croûte continentale au cours du processus de rifting. Des décollements de couverture, principalement localisés dans les évaporites triasiques conduisent à la superposition directe des sédiments pré-rift sur le manteau exhumé. La notion nouvelle de soutirage crustal est introduite pour désigner ce processus dans lequel l'écoulement gravitaire ne joue pas un rôle principal. La caractérisation sédimentologique, pétrographique et isotopique (O et C) des différentes formations de brèches et ophicalcites observées à proximité des corps de péridotites permet de reconstituer les étapes de l'exhumation du manteau en pied de marge. Enfin de nouvelles datations permettent de mieux contraindre le calendrier des évènements successifs.
https://tel.archives-ouvertes.fr/tel-00787952

tel-00787952
https://tel.archives-ouvertes.fr/tel-00787952
https://tel.archives-ouvertes.fr/tel-00787952/document
https://tel.archives-ouvertes.fr/tel-00787952/file/TheI_se-Clerc-2012.pdf

Éditeur(s) : HAL CCSD Mineralogical Society of America
Résumé : International audience
Pressure-temperature divariant melting phase relations of model carbonated peridotite in the system CaO-MgO-Al2O3-SiO2-CO2 from 8 to 12 GPa are reported. From 8 to 12 GPa, melting temperatures on the studied pressure-temperature divariant surface rise quite rapidly. Liquids, in equilibrium with forsterite, orthopyroxene, clinopyroxene, and garnet, on the low-temperature side of the divariant surface are magnesiocarbonatitic in composition. With increasing temperature along an isobar on the pressure-temperature divariant surface, and with the same crystalline phase assemblage, liquids gradually become kimberlitic in their composition. Given the model system data reported here, Group IB kimberlites and perhaps some kimberlites from Greenland, Canada, and South Africa could be generated from direct melting of carbonated peridotite in the pressure range of approximately 6-8 GPa. Some kimberlites from Canada and Russia could have formed by partial melting of carbonated mantle peridotite at pressures of about 10-12 GPa. From 8 to 12 GPa, liquid compositions on the studied divariant surface show a limited compositional range, which implies that the divariant surface is essentially flat. The implied flatness of the divariant surface, and so long as the liquid is in equilibrium with the crystalline assemblage of forsterite + orthopyroxene + clinopyroxene + garnet on the divariant surface, indicate that kimberlites belonging to Group IA and those more magnesian than Group IA cannot have their origin in only being a melting product of carbonated mantle peridotite. The absence of topography of the studied pressure-temperature divariant surface in all likelihood limits the generation of kimberlites in the Earth's upper mantle only.
ISSN: 0003-004X

hal-01053915
https://hal.archives-ouvertes.fr/hal-01053915
DOI : 10.2138/am.2014.4826

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Carbonation of peridotite is a widespread process in nature, with emerging societal and environmental implications through geological storage of CO2. We studied the micro- and nano-scale structures associated with the carbonation of peridotite pervaded by a CO2-rich water and the induced changes of its hydrodynamic properties. Focused ion beam (FIB) nanotomography was coupled with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. An experimental sample of dunite core percolated by a CO2-rich fluid (pCO2 = 11 MPa) at 160 °C was compared with a naturally carbonated sample from the Atlantic Ridge (Exp. IODP 304). Results of the 8-days percolation experiment show continuous alteration of olivine into magnesite locally associated with a poorly crystalline serpentine-type mineral referred to as proto-serpentine. The experiment is also characterized by an overall decrease in rock porosity (from 7.7 to 6.4%) and permeability (from 8.5 × 10− 16 to 1.7 × 10− 16 m2). The primary inter-granular porosity (1 to 10 μm) is progressively filled with magnesite, leading to the overall decrease of permeability. At the same time, a secondary small-scale porosity (10 nm to 1 μm) is created at the tip of the dissolution features along the (010) cleavage planes of olivine grains. Carbonate crystals grow within these etch-pit structures behind the dissolution front. Dissolution and precipitation occur at a relative rate that maintains the secondary porosity over the timescale of the experiment (i.e. carbonate growth has to be the slowest one). This process is interpreted as an interface coupled dissolution-precipitation mechanism of olivine carbonation. Proto-serpentine is only formed in highly reduced flow areas, such as small olivine etch-pits (10 to 500 nm wide), which are more isolated from CO2 input. SEM-FIB analyses show the 3D preferential orientation of the dissolution-precipitation planes within a given olivine grain. It suggests the formation of lateral connections between the etch-pits (infra-micrometric “channels”) providing access to reactants and removing reaction products at the reaction sites. The experimental results show that olivine dissolution and magnesite precipitation converge to a quasi-steady state resulting in a constant carbonation rate controlled by reaction kinetics and not by mass transfer for this flow regime. The system self-regulates, independently of the porosity change at least during this incipient stage of olivine carbonation. Similar textures are observed in natural samples collected at 170 m-depth in the vicinity of veinlets driving late fluids down into the cooling oceanic lithosphere. This suggests that similar mechanisms are active in nature and can play an important role in the late, low temperature alteration stages of fractured peridotite. These results also highlight the critical role of nm-scale spatial arrangement of dissolution and precipitation features on mass transfers and on the sustainability of olivine carbonation through coupled dissolution-precipitation processes.
ISSN: 0009-2541

hal-01684733
https://hal.archives-ouvertes.fr/hal-01684733
DOI : 10.1016/j.chemgeo.2017.11.013

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Mantle-derived xenoliths hosted by melilitite lavas from In Teria (Ahaggar, SE Algeria) include garnet and spinel peridotites, pyroxenite and phlogopite megacrysts. The spinel and garnet peridotites record an early deformation event, which formed porphyroclastic microstructures and olivine crystal preferred orientations, followed by static infiltration of hydrous alkaline melts. This metasomatic stage (stage 1) is characterized by the crystallization of phlogopite in the garnet and spinel peridotites, amphibole in the spinel peridotites and clinopyroxene in the garnet peridotite, which record chemical equilibration with an alkaline silicate melt. These early events were largely overprinted by carbonatitic metasomatism (stage 2), which is observed only in the spinel peridotites. Spinel peridotite major and trace element compositions, as well as the compositions of newly formed minerals, are characteristic of interaction with carbonate melt, associated with strong enrichment in incompatible trace elements in clinopyroxene. This second stage was followed by crystallization of pyroxenites (stage 3) in vein conduits, probably segregated from alkaline melts. We propose a scenario in which the different metasomatic imprints record successive stages of interaction between lithospheric mantle and sublithospheric melts throughout the Cenozoic. In Sr–Nd isotope space, the host melilitites and several xenoliths are clustered and plot close to the HIMU mantle end-member. However, some peridotite xenoliths are shifted towards more radiogenic 87Sr/86Sr values. In 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–06Pb/204Pb space the In Teria samples define a relatively large domain characterized by high 206Pb/204Pb and 208Pb/204Pb, consistent with a contribution of an HIMU component, considered to represent a sublithospheric signature. The highest 87Sr/86Sr values are comparable with those ascribed to the EM1 mantle end-member, representing the signature of the lower continental lithosphere, and are probably inherited from the pre-metasomatic lithospheric mantle beneath In Teria. Numerical modelling of porous percolation of melt of sublithospheric origin through an EM1-like lithospheric mantle protolith reproduces the In Teria peridotite compositions, using moderately sub-chondritic Sr/Nd values for the peridotite (e.g. In Teria garnet peridotite) and moderately super-chondritic Sr/Nd values in the melt (approximately ocean island basalt values). A few spinel peridotites require a component characterized by a 143Nd/144Nd signature higher than both the EM1 end-member and the local Ahaggar basalts; the 208Pb/204Pb compositions of several samples point to a component with a depleted mid-ocean ridge basalt (MORB) mantle (DMM) signature. Thus the lithospheric mantle beneath In Teria probably did not have a uniform EM1 signature before the onset of metasomatism; it included a DMM peridotite component as well as some peridotites with elevated 143Nd/144Nd values recording long-term LREE depletion.
ISSN: 0022-3530

hal-01356781
https://hal.archives-ouvertes.fr/hal-01356781
DOI : 10.1093/petrology/egw009

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : In the eastern Pyrenees, the lherzolites always outcrop as restricted bodies scattered across Mesozoic sedimentary units along the North Pyrenean fault. In this article, we focus on the sedimentary processes associated with the tectonic exhumation of the mantle rocks during the Lower Cretaceous and we provide a new detailed analysis of the ultramafic-bearing sedimentary sequences associated with the Lherz body. The Lherz body, 1.5 km long only, belongs to a series of ultramafic bodies of restricted size (a few m to some hundred of m), occurring within sedimentary formations composed mostly of carbonate breccias originating from the reworking of Mesozoic platform limestones and dolomites. The carbonate clasts include marbles displaying different evolutions in relation with the Cretaceous thermal metamorphic event. The clastic formations also include numerous layers of polymictic breccias reworking ultramafic clasts. These layers are found far from any peridotitic body, implying that ultramafic clasts have been transported far away from their sources by sedimentary processes after their disaggregation on the seafloor. In addition, clasts of ultramafic-rich breccias and sandstones are also observed which indicates rapid sedimentation and diagenesis within a tectonically active environment. We discuss the significance of these deposits and their bearing on our understanding of the tectono-sedimentary processes associated with mantle exhumation in a context of development of narrow intracontinental basins suffering extreme crustal thinning.
EISSN: 1777-5817

hal-00766884
https://hal.archives-ouvertes.fr/hal-00766884
DOI : 10.2113/gssgfbull.183.5.443

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : Solidus phase relations of carbon dioxide-saturated (CO2 vapor) model peridotite in the system CaO-MgO-Al2O3-SiO2-CO2 in the 1.1-2.1 GPa pressure range are reported. The solidus has a positive slope in pressure-temperature (PT) space from 1.1 to 2 GPa. Between 2 and 2.1 GPa, the melting curve changes to a negative slope. From 1.1 to 1.9 GPa, the liquid, best described as CO2-bearing silicate liquid, is in equilibrium with forsterite, orthopyroxene, clinopyroxene, spinel, and vapor. At 2 GPa, the same crystalline phase assemblage plus vapor is in equilibrium with two liquids, which are silicate and carbonatitic in composition, making the solidus at 2 GPa PT invariant. The presence of two liquids is interpreted as being due to liquid immiscibility. Melting reactions written over 1.1-1.9 GPa are peritectic, with forsterite being produced upon melting, and the liquid is silicate in composition. Upon melting at 2.1 GPa, orthopyroxene is produced, and the liquid is carbonatitic in composition. Hence, the invariance between 1.9 and 2.1 GPa is not only the reason for the dramatic change in the liquid composition over an interval of 0.2 GPa, but the carbonated peridotite solidus ledge itself most likely appears because of this PT invariance. It is suggested that because carbonatitic liquid is produced at the highest solidus temperature at 2 GPa in PT space in the system studied, such liquids, in principle, can erupt through liquid immiscibility, as near-primary magmas from depths of approximately 60 km.
ISSN: 0148-0227

hal-00939406
https://hal.archives-ouvertes.fr/hal-00939406
DOI : 10.1002/jgrb.50249

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Melting phase relations of model carbonated peridotite in the system CaO-MgO-Al2O3-SiO2-CO2 from 2 to 3 GPa are reported. Experimentally produced melts, which are model carbonatites, with approximately 36-40 wt % CaO, 12-17 wt % MgO, 0.2-1.5 wt % Al2O3, 1-4 wt % SiO2, and 40-42 wt % CO2 (carbon dioxide) are present at all pressures investigated. At 2.8 and 3 GPa, carbonatitic melts are seen experimentally at temperatures that are very close to the vapor-free (CO2) peridotite solidus and are found in equilibrium with forsterite, orthopyroxene, clinopyroxene, and garnet. Solidus phase relations with isobaric and pressure-temperature invariant points, defining the so-called carbonated peridotite solidus ledge, are also reported from 2.1 to 3 GPa. A divariant region exists from 2 to 2.6 GPa wherein two, compositionally different melts are present. In this region, these two melts, carbonatitic and silicate in composition, coexist with crystalline phase assemblage and free vapor. The silicate liquid has approximately 30-48 wt % SiO2 and approximately 6 to 20 wt % of dissolved CO2. The presence of carbonatitic and silicate liquids is interpreted to be due to liquid immiscibility. On the basis of melting phase relations reported here, we conclude that (a) the ledge is a feature along which model carbonatitic liquids are produced by reaction of silicates and CO2 vapor and (b) alkali-free carbonatites and silicate melts can form through melt unmixing at depths of ~60-80 km in the Earth's mantle.
ISSN: 0148-0227

hal-01053913
https://hal.archives-ouvertes.fr/hal-01053913
DOI : 10.1002/2013JB010913

Éditeur(s) : HAL CCSD
Résumé : National audience
Les lherzolites de l’étang de Lherz, de Freychinède et de Berqué sontassociées à des métasédiments détritiques contenant des fragments dominantsde manteau et de marbres issus du démantèlement de la couverturemétamorphique mésozoïque de la Zone Nord-Pyrénéenne(ZNP).Les relations originelles entre le socle ultrabasique et ces brèches etmicrobèches ne sont pas toujours accessibles, mais on peut observeren quelques endroits des caractères de contacts sédimentaires (surfaceirrégulière, anfractuosités emplies de microbrèches, remaniement dela lherzolite serpentinisée). La matrice des brèches est invariablementconstituée de grains issus du démantèlement du manteau serpentiniséou non (Ol, CPX, OPX, spinelles, serpentines). Ce cortège ultrabasiqueest associé à des fragments de marbres souvent foliés portant déjàl’empreinte du métamorphisme HT pyrénéen. On trouve également engrande quantité de petites phyllites dont l’origine est soit détritique, soitmétamorphique, par cuisson de la brèche lors d’un stade tardif métamorphismeHT crétacé. Sur le corps de Lherz, la composition des clastes desbrèches évolue entre un pôle 100% carbonaté et un pôle 100% ultrabasique.Par endroit, les carbonates envahissent la péridotite serpentiniséequi prend alors l’aspect typique des ophicalcites. En plusieurs points,à la périphérie immédiate des corps de Lherz et de Freychinède et dansles fentes au sein des lherzolites de Berqué, affleurent les roches à saphirine(voir Monchoux, 1973) faites de lits de minéraux souvent bien triéset de microbrèches dont la composition, surprenante, associe un cortègeultrabasique à un cortège singulier de débris de métaévaporites etde métaophites (phlogopites, anthophyllites, enstatites, hornblendes,...).La mise à nu du manteau dans la ZNP au Crétacé s’accompagne doncdu démantèlement de toute la couverture métamorphique prérift, depuisle Trias jusqu’au Crétacé. Cette contrainte est intégrée dans le modèled’exhumation proposé pour cette partie de la ZNP. Ce modèle tente derendre compte également de la présence au sein du corps principal desbrèches de Lherz (vallon de Fontête Rouge) de rares fragments d’uncroûte continentale à affinité granulitique
24 ème Réunion des sciences de la Terre 2014
Pau, France

insu-01080835
https://hal-insu.archives-ouvertes.fr/insu-01080835

Éditeur(s) : HAL CCSD Elsevier Masson
Résumé : International audience
Although they are famous among Earth scientists, the Lherz peridotites are exposed within geological formations of the North Pyrenean Zone (NPZ) still lacking detailed investigations. Our study focuses on the metasediments of the Aulus basin hosting the Lherz peridotite body and associated ultramafic fragments of smaller size. The new data set comprises of structural analysis and detailed geological mapping of the massive Mesozoic marbles that form the prerift sequence typical of the NPZ and of the ultramafic-rich clastic breccia formations surrounding the peridotite bodies. The massive marbles display an evolution from hot and ductile to cold and brittle deformation, indicative of an exhumation process ending with the sedimentary reworking of both the deformed Mesozoic metasediments and the exhumed ultramafic rocks. Crystal Preferred Orientations (CPO) measured in the marbles support a deformation mechanism by dislocation creep of calcite, which is dominant between 400 8C and 600 8C; these deformation temperatures are within the range determined earlier by Clerc et al. (2015), using RSCM (Raman Spectroscopy of Carbonaceous Material) geothermometry. As a consequence, we better describe the transition from ductile to brittle deformation in the prerift marbles and clarify the origin of the syn-rift breccias. Due to continuous exhumation along detachments' faults, the brecciated metamorphic carbonates of the prerift NPZ sedimentary cover were passively uplifted towards shallower levels and progressively unroofed, while transported passively on the back of the exhumed ultramafic footwall. These results are consistent with the recent interpretations of the North Pyrenean peridotites as remnants of subcontinental mantle rocks exhumed within the pre-Pyrenean rift system. We emphasize the importance of tectonic decoupling between the Mesozoic sedimentary cover and the Palaeozoic basement, which leads to the juxtaposition of metamorphosed and deformed Mesozoic Please cite this article in press as: Lagabrielle Y, et al. Very high geothermal gradient during mantle exhumation recorded in mylonitic marbles and carbonate breccias from a Mesozoic Pyrenean palaeomargin (Lherz area, North Pyrenean Zone, France). C. R. Geoscience (2016), http://dx.
ISSN: 1631-0713

Droits : http://creativecommons.org/licenses/by-nc-nd/
insu-01285079
https://hal-insu.archives-ouvertes.fr/insu-01285079
https://hal-insu.archives-ouvertes.fr/insu-01285079/document
https://hal-insu.archives-ouvertes.fr/insu-01285079/file/1-s2.0-S1631071315002060-main.pdf
DOI : 10.1016/j.crte.2015.11.004

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The N-S Languedoc volcanic province between the French Massif Central and the Mediterranean Sea is characterized by sporadic scattered low volume (similar to 2 km(3)) and geochemically homogeneous alkali basaltic activity spanning from 161 to 05 Ma The existence of magmatic activity of such a long duration within such a small area (similar to 140 km long and similar to 60 km wide) in spite of the extensive shift to the East of the European plate (about 2500 km during the last 160 Ma) is problematic Trace-element abundances in lavas suggest low degrees of melting (1-5%) in the spinet-garnet transition zone of an enriched Iherzolitic source The lavas display rather large ranges in Sr isotopic ratios (0 70307-0 70436) The Nd-143/Nd-144 ratio variations are smaller (0 51268-0 51300) and these of Pb-206/Pb-204 Pb-208/Pb-204 and Pb-207/Pb-204 are 18 745-19 515 38 53239 228 and 15 567-15 680 respectively The Languedoc lithospheric mantle as sampled by xenoliths is globally similar to the Pyrenees lithosphere The xenoliths show also rather large Sr Nd and Pb isotopic variations (Sr-87/Sr-86 0 70287-0 70578 Nd-143/Nd-144 0 51256-0 51414 Pb-208/Pb-204 37772-39 041 Pb-206/Pb-204 17 901-19 353) except for Pb-207/Pb-204 (15 570-15 620) The Pb-206/Pb-204 and La/Sm ratios are positively correlated both in xenoliths and lavas The increase of the Pb-206/Pb-204 (which could be interpreted as participation of the European Asthenospheric Reservoir EAR) is probably related to volatile-rich (carbonated?) fluid percolation This is corroborated by LILE and HFSE patterns observed in several xenoliths Therefore our data on lavas and xenoliths suggest a lithospheric origin for this long lived magmatism We propose (1) that the role of the asthenosphere in the Languedoc volcanism was restricted to volatile-rich fluid supplying and (2) that the fluid injection within the lithosphere may be related to the arrival of the Central Atlantic Plume head beneath Western Europe about 70 Ma ago In this model the isotopic signature of the oldest lavas (>70 Ma) would be that of the mantle lithosphere inherited from Hercynian processes The signatures of the subsequent lavas would be driven by the metasomatic component stored within the lithosphere and preferentially mobilized during incipient melting This metasomatised lower lithosphere was close to its solidus and small changes in P (or T) triggered incipient melting leading to basaltic volcanism Successive local re adjustments of the lithospheric blocks which accompanied the Meso-Cenozoic evolution of the Thetys Ligurian margin towards the present Mediterranean margin are the probable cause of these changes and so the sporadic volcanic activity in Languedoc is unrelated to deep asthenospheric processes
ISSN: 0024-4937

hal-00558884
https://hal.archives-ouvertes.fr/hal-00558884
DOI : 10.1016/j.lithos.2010.04.009

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 : International audience
We present an overview of the Mid-Cretaceous hyper-extended rift systemexposed in the northern part of the Pyrenean mountain range. Itsinversion during the Pyrenean orogeny allows for precious observationsof the deep-seated processes occurring at the foot of distal margins.Some peculiar aspects characterizing the pre-Alpine hyper-extended domainare examined throughout the 400 km long suture that is fossilizedin the North Pyrenean Zone (NPZ). The high temperature/low pressure(HT/LP) thermal imprint of the extensional event is studied thanksto a dataset of more than hundred peak temperature measurementsby Raman spectroscopy of the carbonaceous material (RSCM). Thepre- and syn-rift metasediments are characterized by an intense, synmetamorphicductile deformation. Focusing on several key-localitiesof the NPZ, we examine the emplacement conditions of the peridotitesin response to this extreme crustal thinning. Geological evidencessuch as the occurrence of peridotite bodies directly underlying metamorphicpre-rift sediments indicate an early attenuation of the rifted continentalcrust. Moreover, syn-rift, Albian-Cenomanian flysch sequenceswere deposited synchronously with the syn-metamorphic ductile deformationof the pre-rift sequences. All along the Internal MetamorphicZone (IMZ), the base of the flysch deposits also recorded the HT tectonicevent. Such a synchronicity between geological events, which aregenerally separated in time, is not common in mountain belts. We examinehow tectonics, metamorphism and sedimentation may be activein a single basin during extension. We propose an original mechanismfor the evolution of the basins involving continuous basal extraction ofthe pre-rift metamorphic sediments. This early HT deformation eventrelates to the « phase anté-Cénomanienne » described by Pyrenean geologistssince 1930.
24 ème Réunion des sciences de la Terre 2014
Pau, France

insu-01080795
https://hal-insu.archives-ouvertes.fr/insu-01080795

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Timétrine massif exposed west of the Pan-African suture zone in northeastern Mali belongs to the passive margin of the West African craton facing to the east intra-oceanic arc assemblages and 730 Ma old pre-collisional calc-alkaline plutons. The Timétrine lithologic succession includes from the base to the top Mesoproterozoic cratonic to passive margin formations overlain by deep-sea Fe-Mg schists. Submarine metabasalts and two ultramafic massifs of serpentinized mantle peridotites are inserted as olistoliths towards the top whereas turbidites of continental origin represent the younger unit. Field and petrological data have revealed a distinct metasedimentary sequence attached to the serpentinized peridotites. It essentially consists of impure carbonates, Fe jaspers and polymictic breccias containing altered blocks of mantle peridotites, most rocks being enriched in detrital chromite. This association is interpreted as reworked chemical and detrital sediments derived from the alteration of mafic-ultramafic rocks. It is argued that mantle exhumation above sea floor took place during the Neoproterozoic rifting and crustal thinning period under possible tropical conditions, as suggested by the large volume of silicified serpentinites. In spite of greenschist facies metamorphic overprint characterized by widespread Fe-rich blue amphiboles that are not diagnostic of high-pressure conditions, it is possible to reconstruct a former ocean-continent transition similar to that evidenced for the Mesozoic period, followed by the deposition of syn-to post rift terrigeneous turbidites roughly coeval with ocean spreading some time before 800 Ma. It is concluded that the serpentinite massifs were tectonically emplaced first in an extensional setting, then incorporated within deep-sea sediments as olistoliths and finally transported westward during late Neoproterozoic collisional tectonics onto the West African craton.
ISSN: 0899-5362

hal-00985586
https://hal.archives-ouvertes.fr/hal-00985586
DOI : 10.1016/j.jafrearsci.2013.11.004