Processos de fusão parcial e metassomatismo foram identificados em xenólitos mantélicos da Província Ígnea Alto Paranaíba (PIAP), na região de Coromandel (Minas Gerais), pela combinação de técnicas de análise textural (petrografia, microscopia eletrônica de varredura), análises químicas minerais pontuais de elementos maiores (por Microssonda Eletrônica), traços (por LA-Q-ICPMS) e isótopos (Sr em clinopiroxênio por LA-MC-ICPMS). Os xenólitos, oriundos dos kimberlitos Indaiá 1, Limeira 1 e Forca, foram divididos em três grupos exibindo texturas, mineralogia e química distintas: peridotitos com espinélio ou Cr- espinélio, interpretados como fragmentos de fácies espinélio no manto litosférico raso; xenólitos com flogopita e/ou clinopiroxênio secundário representando porções do manto litosférico que sofreram metassomatismo por interação com fluidos ou magmas; e dunitos e harzburgitos com textura mosaico porfiroclástica a fluidal compatíveis com porções que sofreram efeitos de um fluxo térmico maior, possivelmente associadas a astenosfera ou a zonas de maior permeabilidade na litosfera. De modo geral os xenólitos mantélicos da Província Ígnea do Alto Paranaíba foram afetados por intensa fusão parcial, indicada especialmente pelo alto valor de mg# em olivina e ortopiroxênio, e pelo empobrecimento de Al em clinopiroxênio e minerais do grupo do espinélio. Boa parte desses xenólitos foi modificada por metassomatismo modal e/ou críptico, que atingiu inclusive níveis rasos do manto litosférico (fácies espinélio). Os harzburgitos e dunitos com textura porfiroclástica com mosaico têm baixos valores de mg# em olivina e ortopiroxênio, e altas concentrações de de Ti, Ca e Na em relação aos demais grupos de amostras sugerindo um processo de metassomatismo críptico de alta temperatura (astenosfera, ou zonas permeáveis da litosfera). Estimativas com base em razões \'Y IND.N\' e \'Yb IND.N\' (normalizadas por Manto PrimitivoPM) em clinopiroxênio, indicam que os peridotitos com Cr-espinélio sofreram uma porcentagem maior de fusão parcial (15-20%), em relação aos peridotitos com espinélio (5-8%), e preferencialmente fracionada. Em algumas amostras nos peridotitos com Crespinélio ainda são encontrados padrões enriquecidos de \'ETRL IND.N\', indicando processos de metassomatismo críptico. Os cristais de clinopiroxênio das amostras de xenólitos metassomatisados têm padrão de distribuição de \'ETR.IND.N\' semelhantes entre si, com padrão de \'ETRL IND.N\' convexo e enriquecido em relação aos \'ETRP IND.N\' (\'La IND.N\' e \'Lu IND.N\', respectivamente 5-14xPM e 0,3-1xPM). Uma amostra de harzburgito com bolsões de clinopiroxênio (In1-12), tem o clinopiroxênio com as maiores razões de \'ETR IND.N\' do conjunto, mantendo o padrão de \'ETRL IND.N\' convexo e enriquecido em relação aos \'ETRP IND.N\' \'(La IND.N\' e \'Lu IND.N\', respectivamente 2543xPM e 1-2,5xPM) dos peridotitos metassomatisados. Razões \'ANTPOT.87 Sr\'/ANTPOT. 86 Sr\' de clinopiroxênio desta última amostra (0,706-0,714) e de flogopita ilmenita wehrlito (0,706-7,10) sugerem que o metassomatismo seria derivado de porção da astenosfera ou litosfera com material proveniente de subducção de crosta. Um valor mais baixo de razão \'ANTPOT. 87 Sr\'/\'ANTPOT.86 Sr\' para clinopiroxênio de peridotito com espinélio (0,703),indica, juntamente com aspectos texturais e de química de maiores e traços, pouca ou nenhuma, influencia do metassomatismo nesta amostra. / Partial melting processes and metasomatism were identified in mantle xenoliths of the Alto Paranaíba Igneous Province (APIP), next Coromandel (Minas Gerais), by combining textural analysis techniques (petrography, scanning electron microscopy) and specific mineral chemical analyzes of major elements (for Electronic Microprobe), trace elements (LA-Q-ICPMS) and isotopes (Sr in clinopyroxene by LA-MC-ICPMS). The xenoliths which are from Indaiá 1, Limeira 1 and Forca kimberlite were divided into three groups displaying different textures, mineralogy and chemistry: peridotites with spinel or Cr-spinel, were interpreted as fragments in the shallow lithospheric mantle; xenoliths with phlogopite and/or secondary clinopyroxene representing the lithospheric mantle portions that suffered metasomatism by interaction with fluids or magmas; and dunites and harzburgites with porphyroclastic texture with mosaic are compatible with portions that have suffered the effects of a higher heat flow, possibly associated with asthenosphere or zones of greater permeability in the lithosphere. Mantle xenoliths from APIP were affected by intense partial melting, suitable for high-value mg# of olivine and orthopyroxene, and the depletion of Al in clinopyroxene and spinel minerals from the group. Many of these xenoliths were modified by modal and/or cryptic metasomatism, which reached even shallow levels of the lithospheric mantle (facies spinel). Harzburgites and dunites with prophyroclastic texture with mosaic have low values of mg# and high concentrations of Ti, Ca and Na in olivine and orthopyroxene compared to other groups of samples suggesting a cryptic metasomatism process of high temperature. \'Y IND.N\' and \'Yb IND.N\' (normalized by Primitive Mantle-PM) ratios in clinopyroxene indicate that the peridotites with Cr-spinel suffered a higher percentage of partial melting (15-20%) compared to the peridotites with spinel (5-8%), and preferably fractional. Some samples in peridotites with Cr-spinel are still found \'LREE IND.N\' enriched ratios patterns, indicating cryptic metasomatism processes. The clinopyroxene crystals of metasomatized xenoliths samples have similar \'REE IND.N\' ratios pattern, with convex \'LREE IND.N\' pattern with higher ratios compared to \'HREE IND.N\' (LaN:5-14xPM and LuN 0,3-1xPM). A harzburgite with insterticial pockted clinopyroxene (In1-12), has the clinopyroxene with the highest \'REE IND.N\' of the set, the convex and enriched \'LREE IND.N\' pattern as well (\'La IND.N\': 25-43xPM and \'Lu IND.N\' 1-2.5XPM) of metasomatized peridotites. Clinopyroxene \'ANTPOT.87 Sr\'/\'ANTPOT.86 Sr\' ratios of the latter sample (0,706 to 0,714) and from phlogopite wehrlite ilmenite sample (0,706 to 0,710) suggest that metassomatism fluids from astenosphere or lithosphere were derivative from crust subduction processes. A lower value ratio \'ANTPOT.87 Sr\'/ANTPOT.86 Sr\' for clinopyroxene in peridotite with spinel (0,703), is in agreement with textural aspects, major and trace elements information, that this group of xenolith little, or no influence of metasomatism.

Melting processes in the mantle have a key role in plate tectonics and in the most colossal phenomena in the Earth, like large igneous provinces, mantle plume upwellings, and the constant growth of the planet's tectonic plates. In this study we use the geochemical and petrological evidence preserved in ophiolites, large igneous provinces, and intraplate seamounts to understand causes, timing and implications of melting in these different tectonic environments. We studied melting at extensional environments, in mid-ocean ridges and back-arc basins, preserved in ophiolites. The Santa Elena Ophiolite in Costa Rica comprises a well-preserved fragment of the lithospheric mantle that formed along a paleo-spreading center. Petrological models of fractional crystallization suggest deep pressures of crystallization of >0.4 GPa for most of the samples, in good agreement with similar calculations from slow/ultra-slow spreading ridges and require a relatively hydrated (~0.5 wt% H2O) MORB-like source composition. Our findings suggest a complex interplay between oceanic basin and back-arc extension environments during the Santa Elena Ophiolite formation. Secondly, we analyzed large igneous provinces and their mechanisms of formation. As the surface expression of deep mantle processes, it is essential to understand the time frames and geodynamics that trigger these massive lava outpourings and their impact to life in the planet. We analyze the record and timing of preserved fragments of the Pacific Ocean Large Igneous Provinces to reconstruct the history of mantle plume upwellings and their relation with a deep-rooted source like the Pacific Large Low Shear Velocity Province during the Mid-Jurassic to Upper Cretaceous. Lastly, we explore the occurrence of low-volume seamounts unrelated to mantle plume upwellings and their geochemical modifications as they become recycled inside the mantle, to answer questions related to the nature and origin of upper mantle heterogeneities. We present evidence that an enriched mantle reservoir composed of recycled seamount materials can be formed in a shorter time period than ancient subducted oceanic crust, thought to be the forming agent of the HIMU mantle reservoir endmember. A "fast-forming" enriched reservoir could explain some of the enriched signatures commonly present in intraplate magmas not related with an active mantle plume upwelling. / Ph. D.

Cette thèse vise à proposer de nouvelles méthodes permettant de reconstruire la circulation dans le manteau terrestre et l'évolution de la tectonique de surface pour les deux cents derniers millions d'années. Nous utilisons des modèles numériques de convection mantellique dans lesquels la dynamique de surface est comparable à la tectonique terrestre. En combinant ces modèles avec des reconstructions de la tectonique des plaques il est possible d'estimerla structure et l'évolution du champ de température dans le manteau. Jusqu'à présent, l'inclusion des reconstructions de la tectonique des plaques se faisait en imposant des conditions aux limites du modèle (équilibre des forces, vitesses imposées...). Ces techniques, bien que permettant de tester la validité de différents scénarios tectoniques alternatifs, n'autorisent pas de rétroaction dynamique de la convection mantellique sur la tectonique de surface.Dans ce travail, nous avons développé des techniques d'assimilation de données permettant d'intégrer les reconstructions de la tectonique des plaques dans un modèle numérique tout en laissant se développer de manière auto-cohérente cette rétroaction. Les techniques développées permettent également de prendre en compte les incertitudes associées aux reconstructions de la tectonique des plaques et de calculer les erreurs sur l'estimation finale de la circulationmantellique.Dans un premier temps, nous avons développé un filtre de Kalman suboptimal qui permet d'estimer la structure et l'évolution de la circulation mantellique la plus probable à partir d'un modèle numérique de convection et d'une sérietemporelle d'observations de surface, ainsi que de leurs incertitudes respectives.Ce filtre a été testé sur des expériences synthétiques. Celles-ci consistent à tenter de retrouver une évolution témoin à partir d'une série temporelle de données issues de cette évolution. Ces expériences ont montré qu'il était possible, enprincipe, de reconstruire la structure et l'évolution de l'ensemble du manteau à partir d'observations de vitesses et de flux de chaleur à la surface.Dans un second temps, nous avons développé un filtre de Kalman d'ensemble. Ce filtre permet non seulement d'estimer de manière plus précise la géométrie des structures mantelliques, mais aussi les incertitudes sur cette estimation. / This dissertation focuses on the developpement of data assimilation methods to reconstruct the circulation of the Earth's mantle and the evolution of its surface tectonics for the last 200~Myrs. We use numerical models of mantle convection in which the surface dynamics is similar to the Earth's. By combining these models with plate tectonics reconstructions, it is possible to estimate the structure and evolution of the temperature field of the mantle. So far, the assimilation of plate tectonics reconstructions was done by imposing specific boundary conditions in the model (force balance, imposed velocities...). These techniques, although insightful to test the likeliness of alternative tectonic scenarios, do not allow the full expression of the dynamical feedback between mantle convection and surface tectonics. We develop sequential data assimilation techniques able to assimilate plate tectonics reconstructions in a numerical model while simultaneously letting this dynamicalfeedback develop self-consistently. Moreover, these techniques take into account errors in plate tectonics reconstructions, and compute the error on the final estimation of mantle circulation.First, we develop a suboptimal Kalman filter. This filter estimates the most likely structure and evolution of mantle circulation from a numerical model of mantle convection, a time series of surface observations and the uncertainty on both. This filter was tested on synthetic experiments. The principle of a synthetic experiment is to apply the data assimilation algorithm to a set of synthetic observations obtained from a reference run, and to then compare the obtained estimation of the evolution with the reference evolution. The synthetic experiments we conducted showed that it was possible, in principle, to reconstruct the structure and evolution of the whole mantle from surface velocities and heat flux observations.Second, we develop an Ensemble Kalman Filter. Instead of estimating the most likely evolution, an ensemble of possible evolutions are computed. This technique leads to a better estimation of the geometry of mantle structures and a more complete estimation of the uncertainties associated.

Teleseismic data recorded by broad-band seismic stations in the Swedish National Seismic Network (SNSN) have been used in a suite of studies of seismic wave conversion in order to assess the structure of the crust and upper mantle beneath the Baltic Shield. Signals of seismic waves converted between P and S at seismic discontinuities within the Earth carry information on the velocity contrast at the converting interface, on the depth of conversion and on P and S velocities above this depth. The conversion from P to S at the crust-mantle boundary (the Moho) provides a robust tool to constrain crustal thicknesses. Results of such analysis for the Baltic Shield show considerable variation of Moho depths and significantly improve the Moho depth map. Analysis of waves converted from S to P in the upper mantle reveals a layered lithosphere with alternating high and low velocity bodies. It also detects clear signals of a sharp velocity contrast at the lithosphere-asthenosphere boundary at depths around 200 km. Delay times of P410s, the conversion from P to S at the upper mantle discontinuity at 410 km depth, were used in a tomographic inversion to simultaneously determine P and S velocities in the upper mantle. The polarisation of P410s was also used to study anisotropy of the upper mantle. Results of these analyses are found to be in close agreement with independently derived results from arrival time tomography and shear-wave splitting analysis of SKS. The results presented in this thesis demonstrate the ability of converted wave analysis as a tool to detect and image geological boundaries that involve sharp contrasts in seismic properties. The results also show that this analysis can provide means of studying aspects of Earth’s structure that are conventionally studied using other types of seismic data.

Mass dependent isotope fractionation of stable isotopes between meteorites and planetary materials has been used to assess processes that occurred during formation of Earth and its core. However, thus far little is known about the mass dependent isotope fractionation of Mo in the solar system, and at high temperatures in the Earth, in particular during mantle processes. Molybdenum is a refractory and moderately siderophile element. The processes that might have fractionated Mo in the early solar system include condensation and evaporation of dust grains, metal-silicate segregation, core crystallization, silicate and sulphide melting and aqueous alteration. In order to investigate the processes fractionating Mo isotopes, it is first necessary to assess how much fractionation takes place during mantle melting, estimate the isotopic composition of the bulk silicate Earth, and then make comparisons with primitive and differentiated meteorites. I present double spike Mo isotope data for forty-two mafic and seven ultramafic samples from diverse locations, and nineteen extra-terrestrial samples. The delta^98/95Mo values of all the terrestrial samples (normalized to NIST SRM 3134) exhibit a significant range from +0.53±0.21 to -0.56±0.09‰. The compositions of mid-ocean ridge basalts (MORBs) (+0.03±0.07‰, 2s.d.) and ultramafic rocks (+0.38±0.15‰, 2 s.d.) are relatively uniform and well resolved, providing evidence of fractionation associated with partial melting. In contrast intraplate and ocean island basalts (OIBs) display significant variability within a single locality from MORB-like to strongly negative (-0.56‰). The most extreme values measured are for nephelinites from the Cameroon Line and Trinidade, which also have anomalously high Ce/Pb and low Mo/Ce relative to normal oceanic basalts. The observed relationships between delta^98/95Mo and Ce/Pb, U/Pb and Mo/Ce provide evidence that sulphide plays a critical role in retaining Mo in the mantle and fractionating its isotopic composition in basaltic magmas. If residual sulphides are responsible the Mo isotopic composition, Mo budget of the bulk silicate Earth will be misrepresented by values estimated from basalts. On this basis a revised best estimate of the Mo content in the bulk silicate Earth (BSE) ranging between 251 to 268 ppb is derived, approximately 6 times higher than previously assumed, and similar to the levels of depletion in refractory siderophile elements such as W, Ni and Co. This significantly ameliorates the argument for Mo removal via late stage sulphide extraction to the core. The Mo isotopic composition of the BSE (0.35‰) is distinct from the delta^98/95Mo values found in primitive and iron meteorites. Although Mo isotopic fractionation varies between different phases within a single iron meteorite, and occurs during fractional crystallization in asteroidal cores, most iron meteorites have ddelta^98/95MoSRM3134 (-0.14 to -0.06‰) that are similar to ordinary and CI carbonaceous chondrite (-0.12 to -0.09‰). This range of delta^98/95Moo is not only significantly lighter than the BSE, but also enstatite chondrites, which have delta^98/95Mo values of 0.04 to 0.13‰. Several possible explanations are proposed. (A) Core-mantle differentiation fractionates Mo isotopes. The recently proposed Mo effect of sulphide liquid removal is likely to be minor because this should have generated a light Mo isotope composition for the BSE. However, isotopic fractionation associated with metal-silicate partitioning may be responsible for the heavy Mo in the BSE. (B) A distinct isotopic composition for the late material that contributed Mo to the BSE. Enstatite chondrites (or other putative groups of chondrites with a heavy Mo isotope composition) and sulphur-rich components form the cores of impacting bodies are the most likely candidates that could deliver heavy Mo to Earth. (C) The Mo isotopic composition of the Solar System is heterogeneous in a mass dependent fashion such that heavier Mo isotopes are enriched in the section of the disk from which Earth accreted. There are some difficulties behind each of these models and further work is needed to determine which is correct.

No description available.

Les processus de déformation contrôlant l'amincissement de la lithosphère continentale sont encoremal contraints. Nos connaissances sont principalement basées sur la modélisation thermomécaniqued'extension à l'échelle de la lithosphere—utilisant des lois rhéologiques derivées expérimentalement,l'imagerie géophysique et l'analyse de xénolithes provenant de rift continentaux actifs à ce jour, tels quele Rift Est-Africain. L'originalité de ce travail reside dans l'étude des deux plus grands massifs depéridotites sous-continentales ayant enregistrées des conditions primaires du facies à diamant: lesmassifs de Beni Bousera au nord du Maroc et de Ronda au sud de l'Espagne, respectivement. Lesstructures et la zonation petrologique et métamorphique —impliquant une évolution polybarique etpolythermique— préservéees dans ces massifs offrent une opportunité unique pour étudier l'évolutionthermo-mécanique du manteau sous-continental dans un contexte extensif.Dans ce travail, nous avons étudié les mécanismes de déformation des péridotites et despyroxénites afin de contraindre les modes d'exhumation du manteau lithosphérique sous-continental,depuis des conditions du facies des lherzolites à grenat, jusqu'au facies à spinelle et enfin à plagioclase.Nous avons combiné la cartographie des faciès tectono-métamorphiques et des structures ductiles dedéformation, l'analyse des microstructures, la mesure d'orientations préférentielles de réseau (OPR), etla géothermobarométrie conventionelle couplée à la modélisation thermodynamique (PerpleX) afin decontraindre les conditions de pression et température de la déformation. Nous avons montré quel'exhumation précoce du facies à grenat au facies à spinelle était accomodée par une faille transtensiveaffectant le manteau lithosphérique. Dans ce contexte, la zonation tectono-métamorphique et legradient thermique important (ca. 100ºC/km) préservés à Beni Bousera résultent de la juxtapositionmécanique de domaines lithosphériques initialement équilibrés à différentes pressions et températures,fossilisée à une profondeur de ca. 60 km durant l'Oligocène supérieur (ca. 25 Ma). L'exhumation finaledu facies de lherzolite à spinelle au facies à plagioclase et l'emplacement final dans la croûte, mieuxenregistrés dans Ronda, se sont produits par inversion et plissement de la section lithosphériquefortement amincie dans un contexte arrière-arc, probablement lors du retrait vers le sud de lalithosphère subduite et la collision de l'arc avec les paléo-marges maghrébines au Miocène inférieur(21-23 Ma). / The mantle deformation processes that control the thinning and break-up of continentallithosphere remain poorly understood. Our knowledge is restricted to either lithospheric scalethermo-mechanical models —that use experimentally derived flow laws—, geophysicalimaging and/or rare xenoliths from active continental rifts, such as the East African Rift System.The originality of this work relies on the study of the two largest outcrops of diamond faciessubcontinental lithospheric mantle in the world: the Beni Bousera and Ronda peridotite massifsin N Morocco and S Spain, respectively. The structures and petrologic and metamorphic zoningpreserved in these massifs —implying a polybaric and polythermal evolution— provide aunique opportunity to investigate the thermo-mechanical evolution of thick subcontinentallithospheric mantle in extensional settings.In this thesis we studied the deformation mechanisms in both peridotites andpyroxenites to constrain the modes of exhumation of subcontinental lithospheric mantle fromgarnet-, to spinel-, and finally, to plagioclase lherzolite facies conditions. We combined fieldmapping of tectono-metamorphic domains and structural mapping of ductile structures,microstructural analysis, crystal preferred orientations (CPO) measurements and conventionalthermobarometric calculations and thermodynamic modeling (Perple_X) to unravel the pressureand temperature conditions of deformation. We showed that exhumation from garnet- to spinellherzolite facies conditions was accommodated by fast shearing —in thermal disequilibrium—along a lithospheric scale transtensional shear zone. In this context, the petrological zoning andthe large temperature gradient (ca. 100ºC/km) preserved in the Beni Bousera massif representthe mechanical juxtaposition of progressively deeper and hotter lithospheric levels at depths ofca. 60 km in the latest Oligocene (ca. 25 Ma). Final exhumation from spinel- to plagioclasefacies lherzolite and emplacement into the crust is best recorded in the Ronda massif where itoccurred by inversion and lithospheric scale folding of the highly attenuated continentallithosphere in a back-arc region, probably in relation with southward slab rollback andsubsequent collision with the palaeo-Maghrebien passive margin in the early Miocene (21-23Ma).

. 21 ont des teneurs en Os > 1 ppb et des rapports 187Re/188Os < 3. Leurs TRD ERUPT varient de 0.7 à 2.3 Ga (moyenne 1.7 Ga). Les analyses isotopiques du Re/Os de 9 péridotites résiduelles (0.1-1.2% Al2O3) donnent des âges TRD ERUPT variant de 1.4 à 2.3 Ga (maximum de probabilité 2.1 Ga) Les cratons sont de vastes portions de lithosphère continentale formés d’une croûte continentale type TTG et d’un manteau lithosphérique épaissi (>200km), froid (40 mW/m²) et composé majoritairement par des péridotites fortement réfractaires (Mg#WR>0.92, Al2O3-CaO <1%), échantillonnées sous forme de xénolites lors des éruptions kimberlitiques. Il est admis que les péridotites cratoniques ont été formées par de hauts degrés de fusion partielle, cependant, les fortes teneurs en SiO2 de certaines xénolites impliquent des mécanismes de fusion très particuliers. De plus, les fortes teneurs en FeO, TiO2, REE, en cpx et en grenats d’un grand nombre d’échantillons ne sont pas cohérentes avec une origine résiduelle. En effet la majorité des péridotites cratoniques a subi des transformations qui masquent actuellement leur composition initiale. C’est pourquoi les conditions de la fusion et les processus post-formation qui affectent le manteau cratonique demeurent très mal contraints. Udachnaya est la seule kimberlite du craton Sibérien qui fournit des xénolites de péridotite de tailles suffisantes pour les études géochimiques et isotopiques (>>100g). Cette thèse se base sur une grande collection de xénolites de péridotites (>50) particulièrement fraîches i.e. les olivines et les autres minéraux sont préservés de l’altération et les pertes au feu des roches totales (LOI) sont <2%. Les péridotites sont des harzburgites et des lherzolites, généralement pauvres en cpx (<6%) du faciès à spinelle et à grenat, avec des microstructures aussi bien grenues que déformées. Cette collection est représentative de l’ensemble du profil lithosphérique (du Moho jusqu’à 220km). La majorité des péridotites à spinelle est pauvre en opx (<20%). Les péridotites riches en silice sont moins courantes dans le manteau cratonique sibérien que ce que l’étude de Boyd et al. (1997) a montré. Seul ¼ des péridotites à spinelle de notre collection a des compositions en opx >30%. Les péridotites à spinelle, pauvres en opx ont des compositions en éléments majeurs proches d’un résidu de fusion. Contrairement aux autres études, leurs compositions en Al2O3, FeO et Mg#WR définissent des trends qui, comparés aux études expérimentales, indiquent une origine par 38% d’une fusion fractionnée par décompression entre 7-4 GPa et ≤1–2 GPa ; ce qui est en accord avec les modélisations des éléments en trace. L’origine des péridotites riches en silice est incertaine, et l’hypothèse d’une interaction avec des liquides de subduction est peu probable : leurs compositions en SiO2 et FeO diffèrent des péridotites d’arc, et leur δ18OWR ~5.3 ± 0.2‰ est proche de la valeur référence pour le manteau (5.5‰). 1/3 des péridotites à grenat grenues a des compositions similaires aux péridotites à spinelle et indiquent donc des conditions de formation similaires. Le grenat, dans la majorité de ces péridotites, est d’origine résiduelle, contrairement au cpx qui lui est clairement d’origine métasomatique. La majorité des péridotites à grenat forme des séquences d’enrichissement en FeO, TiO2 et REE par rapport aux péridotites résiduelles. Cela traduit un métasomatisme modal (précipitation de cpx et de grenat), et ce par interaction entre le protolite des péridotites avec un magma riche en Si, Al, Fe (Ti, REE). Les cpx sont en équilibre avec un liquide de composition kimberlitique. Les kimberlites sont des magmas pauvres en Si, Al, Fe et ne peuvent pas être responsables du métasomatisme modal, mais un magma parent peut avoir interagi avec la roche peu de temps avant l’éruption. 28 péridotites ont été analysées pour obtenir leur composition isotopique Re/Os et leur composition en PGE / English abstract not supplied by the author. Résumé en anglais non fourni par l'auteur.

Déformation dans le manteau partiellement fondu: observations dans des systèmes naturelsNous avons analysé les interactions entre magmas et déformation dans le manteau supérieur par une étude microstructurale détaillée de deux sections de péridotites: un profil long de 100m dans le Moho – zone de transition croûte-manteau (MTZ) de l'Ophiolite d'Oman - et des lherzolites à plagioclase dans le massif de Lanzo (Alpes Occidentales). La section de la MTZ se caractérise par un litage à l'échelle cm à métrique ; les compositions varient de dunites à des gabbros à olivine. Le parallélisme entre le litage et la foliation, des limites de couche diffuses, et l'alignement de lentilles riches en plagioclase d'échelle mm impliquent un litage contrôlé par la déformation. Tous les minéraux majeurs ont des orientations préférentielles des cristaux (ou fabrique), mais la fabrique de l'olivine varie en fonction de la composition de la couche. Les lits avec <70% olivine montrent des fortes concentrations de l'axe [100] parallèle à la linéation (pattern axial-[100]). Les lits avec <50% olivine ont [100] dispersé dans la foliation et une concentration des axes [010] autours d'une direction normale à cette dernière (fabrique axiale-[010]). Ces changements ont lieu de façon répétée à l'échelle du mm, aux limites de couches, indiquant une déformation en présence de teneurs variables de magma. La fabrique dans les lits riches en olivine est cohérente avec déformation par fluage dislocation à haute température. Les fabriques d'olivine avec une symétrie axiale-[010] dans les lits pauvres en olivine impliquent une transpression ou un glissement le long de limites de grain (010) « lubrifiés » par du magma; elles sont accompagnées par une activité accrue du glissement [001]. Parce que le changement des fabriques d'olivine n'est pas associé à une dispersion de ces dernières, les teneurs instantanées de magma doivent être <30-40%. La variation continue de la symétrie de la fabrique d'olivine CPO avec la décroissance de la teneur en olivine implique que la fabrique dépend de la déformation finie cumulée en présence de magma, plutôt que de la teneur instantanée de magma.Les lherzolites à plagioclase de Lanzo se caractérisent par un réseau anastomosé de bandes riches en plagioclase, qui passe localement à un litage planaire avec des bords diffus. Olivine et orthopyroxene montrent des évidences de déformation par fluage dislocation à haute temperature: extinction onduleuse, sous-grains ou kinks, des limites de grain irréguliers. Le plagioclase forme des agrégats allongés composés par des cristaux avec des formes interstitielles. Tous les minéraux majeurs (à l'exception du clinopyroxene) ont une claire fabrique cristallographique, indépendamment de la composition de la couche. La fabrique de l'olivine CPO a une symétrie orthorhombique, avec [100] parallèle à la linéation et [010] normal à la foliation (et au litage), mais les maxima de [010] sont toujours plus forts que ceux de [100]. Les fabriques d'olivine sont donc intermédiaires entre les deux patterns observés en Oman, ce qui est cohérent avec le litage plus diffus observé à Lanzo. Ces données impliquent que dans un manteau soumis à une déformation cisaillante, les magmas tendent à se concentrer dans des bandes parallèles au plan de cisaillement, formant un réseau anastomosé qui peut évoluer dans un litage planaire. Les fractions instantanées de magma restent faibles (inférieures au seuil pour la disruption de la matrice solide) – la déformation est donc accommodée largement par fluage dislocation. La présence de magmas produit toutefois un changement dans la symétrie des fabriques d'olivine – produisant des patterns axial–[010] et donc dans une anisotropie sismique différente. La ségrégation du magma et le développement d'un litage compositionnel contrôlé par le cisaillement peut aussi créer une anisotropie mécanique, avec une réduction de la viscosité cisaillante parallèlement aux bandes. / Deformation in a shallow partially molten mantle: constraints from natural systemsWe analysed the interactions between melts and deformation in the upper mantle through detailed microstructural studies in two settings: a 100m section in the Moho Transition Zone (MTZ) of the Oman Ophiolite and plagioclase-rich lherzolites from the Lanzo massif (Alps). The MTZ section is characterized by a cm to m-scale layering; compositions range from dunites to gabbros à olivine. Parallelism between the layering and the foliation, diffuse layers limits, and alignment of mm-scale plagioclase-rich lenses in intermediate composition layers imply shear-controlled layering. All phases have crystal-preferred orientations (CPO), but olivine CPO varies depending on the layer composition. Layers with <70% olivine show strong [100] maxima parallel to the lineation (axial-[100] pattern). Layers with <50% olivine have [100] dispersed in the foliation and concentration of [010] normal to it (axial-[010] pattern). These changes occur repeatedly on the mm-scale, at the layers limits, providing strong evidence for deformation in presence of variable melt fractions. CPO in olivine-rich layers is consistent with high-temperature dislocation creep. Axial-[010] olivine CPO patterns imply transpression or sliding along preferentially-wetted (010) grain boundaries in melt-rich layers; they are accompanied by increased activity of [001] glide. Since the change in CPO symmetry is not associated with dispersion, instantaneous melt fractions must have remained <30-40%. The continuous variation in olivine CPO symmetry with decreasing olivine content implies therefore that the CPO depends on the strain cumulated in presence of melt rather than on the instantaneous melt fraction.Plagioclase-rich lherzolites in Lanzo are characterized by an anastomozed network of plagioclase-rich layers, which locally grades into a diffuse cm-scale planar compositional layering. Olivine and orthopyroxene show evidence for deformation by dislocation creep under high temperature conditions: undulose extinction, subgrains or kinks, sutured grain boundaries, but plagioclase forms elongated aggregates of crystals with interstitial shapes. All major phases (except clinopyroxene) show clear CPO, independent of the layer composition. The olivine CPO has an orthorhombic symmetry, with [100] parallel to the lineation and [010] normal to the foliation (and to the layering), but [010] maxima are stronger than [100] ones. These olivine CPO are intermediate between the two patterns observed in the Oman MTZ, consistently with the less developed segregation in Lanzo. Together these data imply that melts tend to concentrate in bands aligned in the shear plane, forming an anastomosed network that may grade with increasing shear into a planar layering. Instantaneous melt fractions remain below the solid matrix disruption threshold; the deformation is still essentially accommodated by dislocation creep in the solid matrix. Deformation in presence of melt results, however, in different olivine CPO with axial–[010] patterns and hence in a different seismic anisotropy. Shear-controlled compositional layering may also create a mechanical anisotropy, with a directional reduction of the shear viscosity parallel to the bands.

The Ethiopian Plateau is one of the few tectonically-active regions on Earth that is situated in continental rift zones. About 1.6 km deep gorge of the Nile was carved by the Blue Nile River on the Ethiopian Plateau, as the plateau has been experiencing continuous uplift and exhumation in the Cenozoic. Here, we used quantitative analysis of longitudinal rive-profile forms and parameters (knickpoint and normalized steepness-index ksn) of the Blue Nile tributaries to tease out regional tectonic signals. 244 knickpoints were examined in the tributaries, majority (>80%) of which are unassociated with lithology or geological structures. Knickpoint distribution throughout the plateau reveals three incision phases. The novel approach of correlation of ksn with mantle tomography suggests that higher and lower ksn values occur above low-velocity and high-velocity zones, respectively, indicating that thermal upwelling beneath the plateau linked to Afar mantle plume largely controls the uplift thus incision of the plateau.