High Magnesian Dippsodote Dykes in the Oman Ophiolite : Evidence for High Temperature Hydrothermal Circulation in the Oceanic Mantle

金沢大学大学院自然科学研究科Observatoire Midi-PyreneesPromoting Environmental Pesearch in Pan-Japan Sea Area : Young Researchers\u27 Network, Schedule: March 8-10,2006,Kanazawa Excel Hotel Tokyu, Japan, Organized by: Kanazawa University 21st-Century COE Program, Environmental Monitoring and Prediction of Long- & Short- Term Dynamics of Pan-Japan Sea Area ; IICRC(Ishikawa International Cooperation Research Centre), Sponsors : Japan Sea Research ; UNU-IAS(United Nations University Institute of Advanced Studies)+Ishikawa Prefecture Government ; City of Kanazaw

Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids

International audienc

Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite: Implications for melt/fluid-rock reactions at Moho level beneath oceanic spreading centers

International audience15 The Maqsad area in the Oman ophiolite exposes a >300 m thick dunitic mantle-crust 16 transition zone (DTZ) that developed above a mantle diapir. The Maqsad DTZ is primarily 17 f "p " w h scattered chromite and chromite seams) and 18 " p g " which exhibit a significant lithological variability, including various 19 kinds of clinopyroxene-, plagioclase-, orthopyroxene-, amphibole (hornblende/pargasite)-20 bearing dunites. These minerals are interstitial between olivine grains and their variable 21 abundance and distribution suggest that they crystallized from a percolating melt. Generally 22 studied through in-situ mineral characterization, the whole rock composition of dunites is 23 poorly documented. This study reports on whole rock and minerals major and trace element 24 *Manuscript contents on 79 pure to variably impregnated dunites collected systematically along cross 2

Origin of Moho: dunite is the Rosetta stone between seismologists and geologists?

Oral session abstract EGU2018-11069-2, European Geosciences Union (EGU) General Assembly 2018 (8-13 April, 2018, Vienna, Austria

Seismic structure of an oceanic core complex at the Mid-Atlantic Ridge, 22°19′N

We present results from a seismic refraction and wide-angle experiment surveying an oceanic core complex on the Mid-Atlantic Ridge at 22°19′N. Oceanic core complexes are settings where petrological sampling found exposed lower crustal and upper mantle rocks, exhumed by asymmetric crustal accretion involving detachment faulting at magmatically starved ridge sections. Tomographic inversion of our seismic data yielded lateral variations of P wave velocity within the upper 3 to 4 km of the lithosphere across the median valley. A joint modeling procedure of seismic P wave travel times and marine gravity field data was used to constrain crustal thickness variations and the structure of the uppermost mantle. A gradual increase of seismic velocities from the median valley to the east is connected to aging of the oceanic crust, while a rapid change of seismic velocities at the western ridge flank indicates profound differences in lithology between conjugated ridge flanks, caused by un-roofing lower crust rocks. Under the core complex crust is approximately 40% thinner than in the median valley and under the conjugated eastern flank. Clear PmP reflections turning under the western ridge flank suggest the creation of a Moho boundary and hence continuous magmatic accretion during core complex formation

モホ反射面のつくりかた

日本地質学会第125年学術大会（つくば特別大会 / 2018年12月1～2日 産業技術総合研究所

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks-in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas-provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt

Structures des ophiolites d'Oman : flux mantellaire sous un centre d'expansion d'expansion oceanique et charriage a la dorsale

The Oman ophiolite is a piece of the tethysian oceanic lithosphere obducted on the arabian continental margin during late cretaceous times. Fortunately, it has not yet been crusnec by continental collision and has preserved the structures imprinted during its formation and emplacement. It crops out nearly continuously on a length of 475 km along the stike of the paleo-ridge axis and down to 9 km below the paleo-Moho. The structural data (Special Issue " Oman Ophiolite ", journal of Geophysical Research, 86, 84, 1981) favour the actualistic comparison of the Oman ophiolite with the present-day fast spreading centers. The mantle peridotites of Oman have recorded two successive plastic deformation events, the first one related to the accretion of the lithosphere (asthenospheric shear flow), the second one imprinted during the first step of its emplacement (intra-oceanic chrusting). The asthenospheric flow pattern and the composition of the mantle section show strong variations along the strike of the paleo-ridge axis. Small mantle diapirs spaced along the ridge axis by a few tens to more than one hundred kilometers are involved in the accretion processes. These diapirs display an exceptional magmatic activity : The modal composition of the peridotites is unusually depleted, chromite ore bodies and mafic dikes and impregnations features are particularly abundant. Most of the magma feeding the crustal magma chamber is probably channeled through them. Such a diapir, frozen and sampled during the oceanic thrusting, has been mapped in details (Maqsad area). The asthenospheric flow is vertical in a pipe 10 to 20 km in diameter and rotates abruptly into an horizontal attitude in a narrow transition zone lying at shallow depth below the paleo-Moho (a few hundred of meters at most). At the top and in the vicinity of the diapir, asthenospheric flow diverges in every directionsbut is preferentially channeled along the ridge axis. The zone of longitudinal flow extends at least 30 km away from the center of the diapir. The narrowness of the transition zone imolies a rheologic discontinuity at the top of the diapir attributed to an abrupt increase of the magma/rock ratio, consistent with field observations. Such a pattern is explained by a feed-back effect between the magma percolation, the viscosity drop of the asthenosphere and the steed rotation of the mantle flow below the Moho. Far from the diapirs, the asthenospheric flow can display a regular pattern at the scale of one hundred kilometers along strike. In such area, the flow plane is slightly oblique to the Moho and the flow lines are perpendicular to the ridge axis, suggesting steady-state accretion of the lithosphere on one flank of the ridge. The obliquity of about 10° between the Moho and the frozen flow plane reflects probably the slope of the base of the lithosphere equated with an isothermal surface in the zone of accretion. The size and the spacing of the mantle diapirs recognized in Oman are compatible with the structural, geophysical and petrological observations along present-day fast spreading ridges. During the intra-oceanic thrusting, the mantle peridotites were plastically deformed in high stress conditions in a thickness of a few hundred meters above the thrust plane. This deformation grades upward into mylonitic vertical shear zones. They can reach a thickness of a few kilometers and can be tracked on several tens of kilometers along strike. They are synchronous with important injections of hydrous magma thought to originate in the partial melting of the metamorphic sole. Major shear zones have recorded the same kinematics as the basal peridotites and the high-grade metamorphic sole. The early thrusting of the Oman ophiolite has involved considerable disolacement parallel to the ridge axis. These observations are best explained if the intra-oceanic thrusting was initiated along the ocean ridge itself. The thrusting at the ridge implies a rapid shift (less than 1 Ma duration) from expansion to compression as the thermal structure of the ridge has to be preserved. It could be the consequence of a short braking action in the tethysian subduction below Eurasia caused by collisions of microcontinents or island arcs with the eurasian active margin. Such collisions, contemporaneous with the oceanic thrusting of the south-tethysian ophiolites (Upper Albian times) occured along several segments of the north-tethysian active margin. Tectonic events recorded by the Oman ophiolite and by the associated metamorphic and sedimentary formations are usually well correlated with the kinematic and geological history of the tethysian area.L'ophiolite d'Oman est un fragment de la lithosphère océanique téthysienne obducté sur la marge arabe au crétacé supérieur. Elle occupe un domaine de la chaîne alpine où la convergence entre l'Arabie et l'Eurasie n'a pas encore atteint le stade de la collision continentale. Affleurant de façon, presque continue sur une longueur de 475 Kilomètres parallèlement à l'axe de la paléo- dorsale, c'est le plus grand segment de lithosphère océanique accessible à l'étude directe. La section mantellaire constitue 60% de la surface d'affleurement de l'ophiolite (30.000 Kilomètres carrées). Cette thèse est consacrée à la cartographie des structures internes de cette unité. Les structures crustales permettant d'établir une référentielle paléo-tectonique (paléo-horizontale, azimut et flanc de la paléo-dorsale) furent également relevées. Divers arguments pétrologiques et structuraux permettent d'apparenter l'ophiolite d'Oman aux dorsales rapides actuelles. Les péridotites mantellaires, de composition harzburgitique à dunitique, gardent l'empreinte de deux déformations plastiques successives, la première associée à la formation de la lithosphère (flux asthénosphérique), la seconde au charriage intra-océanique qui préluda à son obduction. La géométrie de l'écoulement asthénosphérique et la composition de la section mantellaire présentent de fortes variations longitudinales. La formation de la lithosphère océanique, au droit des dorsales rapides, implique l'ascension de diapirs asthénosphériques espacés de quelques dizaines à plus de cent Kilomètres les uns des autres. Siège d'une activité magmatique exceptionnelle, ces diapirs semblent également jouer le rôle de centres d'alimentation privilégiés de la chambre magmatique sus-jacente. Un de ces diapirs, figé et échantillonné lors du charriage à la dorsale, a pu être cartographié en détail (région de Maqsad) : le lux asthénosphérique, vertical dans un conduit de 10 à 20 Kilomètres de diamètre, se brise sous le plancher de la chambre magmatique dans une zone de transition épaisse seulement de quelques centaines de mètres et est ensuite canalisé parallèlement à l'axe de la dorsale sur une distance d'au moins 30 Kilomètres depuis le centre du conduit. Cette géométrie implique une modification brutale de la rhéologie mantellaire dans la zone de transition attribuée à une augmentation catastrophique du rapport magma/roche. Un modèle physique de circulation, asthénosphérique a été construit en introduisant une discontinuité de viscosité de plusieurs ordres de grandeur au sommet du diapir. Une telle condition permet, en effet, de canaliser un pourcentage important du flux dans un étroit créneau superficiel. La pression dans le diapir est discontinue sur une épaisseur d'une centaine de mètres sous l'interface pour pouvoir vaincre la surpression due au fluage plastique et continuer son ascension vers la surface. Loin des diapirs, le flux mantellaire peut être régulier à l'échelle de la centaine de kilomètres ; il est alors sub-parallèle au Moho et perpendiculaire à l'axe de la dorsale, évoquant l'accrétion de la lithosphère en régime d'expansion stationnaire. L'angle d'une dizaine de degrés entre le Moho et le plan de fluage reflète probablement la pente moyenne des isothermes au niveau de la zone d'accrétion (flanc de la dorsale). La déformation associée au charriage intra-océanique (CIO) affecte les périodiques sur une épaisseur de quelques centaines de mètres au-dessus du plan de charriage basal, lui-même situé à une profondeur maximale de neuf kilomètres sous le paléo-Moho. Elle peut affecter également des niveaux plus élevés de la section mantellaire et la section crustale sous forme de bandes de cisaillement mylonitiques verticales pouvant atteindre 2 kilomètres d'épaisseur. Ces cisaillements sont contemporains de l'intrusion de magmas hydratés au sein de la section mantellaire, peut-être à mettre en relation avec le volcanisme différencié (" volcanisme 2 ") coiffant l'Ophiolite. Le CIO s'accompagne localement de la fusion de la semelle. Lors de l'initiation du CIO, la lithosphère présentait un fort gradient thermique vertical. D'un point de vue cinématique, la déformation enregistrée par la semelle, les péridotites basales et les bandes de cisaillement sont en concordance parfaite. Le CIO s'accompagne de déplacements considérables de la lithosphère charriante parallèlement à l'axe de la dorsale (de l'ordre de la centaine de kilomètres). L'initiation du CIO à la dorsale elle-même rend le mieux compte de ces observations. Le charriage à la dorsale implique l'inversion rapide (1 à 2 millions d'années) du régime d'expansion en régime compressif. On l'explique par un blocage momentané de la subduction de la Téthys sous l'Eurasie causée par des collisions entre des microcontinents, des arcs insulaires et la marge active eurasienne survenues à cette époque (Albien supérieur). De manière générale, les événements enregistrés par l'Ophiolite d'Oman s'intègrent bien dans l'évolution cinématique et géologique du domaine téthysien

Nature et répartition des filons basiques dans la section mantellaire de l'ophiolite d'Oman (implications pour la genèse des MORBs)

TOULOUSE3-BU Sciences (315552104) / SudocTOULOUSE-Observ. Midi Pyréné (315552299) / SudocSudocFranceF