L’éruption d’avril 2007 du Piton de la Fournaise (Île de la Réunion, Océan Indien) a été marquée par les plus gros volumes de lave émis de ces deux derniers siècles, ainsi que par l’effondrement du cratère sommital (le Dolomieu) sur plus de 300 mètres de haut. Des données d’interférométrie radar (InSAR) montrent que les déplacements associés à cette éruption sont inhabituels pour deux raisons : (1) ils ont affecté l’ensemble de l’enclos Fouqué durant l’éruption, (2) deux motifs de déformation ont persisté plus d’un an après la fin de l’éruption. Le premier résulte d’une subsidence centripète du cône central et le second d’un glissement vers l’est du flanc est du volcan. Une méthode, appelée tomographie de déplacements, a été développée pour modéliser les déplacements, basée sur une discrétisation du sous-sol en sources unitaires, et sur la minimisation de deux fonctions coût. Elle permet de trouver une répartition compacte des variations de volume des sources unitaires permettant de reproduire au mieux les déplacements observés. En parallèle, la procédure NA-MBEM, une méthode de modélisation basée sur la combinaison d’un modèle numérique (MBEM) et d’une inversion de type Monte Carlo (NA), a été modifiée pour diminuer le temps de calcul nécessaire à l’obtention d’un bon modèle, et nous montrons que l’utilisation de données temporellement interpolées permet d’améliorer les résultats d’inversion. L’application des deux méthodes de modélisation aux données de déplacements de l’éruption d’avril 2007, montre que durant la période post-éruptive, la subsidence du cône central est provoquée par une source localisée de manière superficielle sous le cône central. Cette source est interprétée comme un système hydrothermal en déflation, dont le drainage aurait été amorcé par l’effondrement du Dolomieu. Les déplacements du flanc est seraient dus à deux sources différentes, toutes les deux superficielles et parallèles à la topographie. L’une est interprétée comme un réservoir temporaire en cours de vidange durant la fin de l’ éruption, et l’autre comme un niveau de glissement sur lequel glisserait le flanc est. Enfin, nous proposons un modèle préliminaire des déplacements ayant eu lieu durant l’éruption, ainsi qu’un scénario de la succession de tous les événements à l’origine des déformations enregistrées entre le 30 mars 2007 et juin 2008. / The April 2007 eruption of Piton de la Fournaise (Réunion Island, Indian Ocean) was characterised by the largest lava emission in the past two centuries, and by a 300 m deep caldera collapse at the summit craters (the Dolomieu). Synthetic aperture radar interferometry (InSAR) data show complex displacements associated with this eruption, which are unusual for two reasons : (i) the whole Enclos Fouqué was affected during the eruption, (ii) two deformation patterns persist more than one year after the end of the eruption. The first signal results from subsidence of the summit area, and the second from a sliding of the volcano’s eastern flank towards the East. A method, called displacement tomography, was developed to model these displacements. It is based on discretization of the volcano’s interior into unitary sources, and on a minimisation of two cost functions to find a compact repartition of the volumetric variations of these sources, which best reproduce the observed displacements. On the other hand, the NA-MBEM procedure (a modelling procedure based on a combination of a fully 3D boundary element method and a Monte Carlo inversion procedure), was improved to reduce the necessary computational time to obtain satisfying results. Furthermore, we show that temporal interpolation of the data improves the inversion results. The application of both modelling methods on the April 2007 displacement data, shows that during the post-eruptive period, the subsidence of the central cone is caused by a shallow source, located under the cone. This source is interpreted as a deflating hydrothermal system, whose drainage could have begun during the Dolomieu collapse. The eastern flank displacements could be the consequence of two sources, both shallow and parallel to the topography. The first one is interpreted as a temporary magma chamber, which is emptying during the end of the eruption. The second one is interpreted as a layer on which the eastern flank was sliding. Finally, we propose a preliminary model of the displacements that occurred during the eruption, and a scenario of the successive events causing the displacements recorded between March 30, 2007, and June 2008.

O presente trabalho investiga processos de interação e mistura parcial de magmas monzodioríticos e sieníticos s.l. no Plúton Piracaia,(~ 580 Ma), que pertence à Província Granítica Itu, de caráter pós-orogênico. O Plúton Piracaia é uma intrusão alongada com cerca de 30 km2, formada por cinco unidades: Monzodioritos grossos (Mdr); Monzodioritos finos (Mdf); Monzonitos heterogêneos (Mh); Sienitos (Sie); quartzo sienitos e quartzo monzonitos (Qsie). Como base para a caracterização dos processos de interação de magmas, foram estimados os parâmetros físico-químicos dos magmas primários e híbridos identificados no plúton. As temperaturas liquidus foram estimadas a partir da saturação em apatita, e são da ordem de 950-1050°C para os magmas mais primitivos (Mdf) e 850-900°C para os mais diferenciados (quartzo sienitos). As temperaturas solidus, estimadas através das relações de equilíbrio entre hornblenda e plagioclásio, são da ordem de 750°C para Mdf, e 650-700°C para Sienitos e quartzo sienitos (Qsie). A profundidade de alojamento do plúton é estimada em ~13-15 km com base nas pressões estimadas para Mdf com base no conteúdo de Al na hornblenda. A concentração de H2O nos magmas foi estimada com base no teor de An do plagioclásio, conhecida a temperatura de cristalização, que indicou valores da ordem de 2,5-3,3% para Mdf, alcançando até 5% nos quartzo sienitos. Estimativas da fO2 a partir da composição química de cristais de magnetita e ilmenita não puderam ser obtidas, devido a reequilíbrio pós-magmático. Deste modo, foram obtidos valores aproximados com base no conteúdo de ulvoespinélio de cristais de magnetita reconstituídos e no conteúdo da molécula ilmenita dos cristais de ilmenita; em paralelo, foram também utilizadas as razões Fe/(Fe/Mg) de anfibólio e biotita. Os resultados revelaram um importante contraste entre as unidades Mdf e Mdr, caracteristicamente oxidadas, com valores próximos aos do buffer NNO, e as demais unidades, onde quartzo sienitos (Qsie) e especialmente Sie mostram-se mais reduzidas. Os baixos valores de susceptibilidade magnética medidos em campo para as unidade Sie e Mh, que a ela se associa, devem ser reflexos do seu caráter mais reduzido. Ao longo da história de construção da câmara, que foi alimentada intermitentemente por magmas de composição variada, dois eventos principais de interação de magmas foram identificados, com características distintas: (i) interação de magmas monzodioríticos (Mdf) e sieníticos (Sie), gerando a unidade de Monzonitos heterogêneos (Mh), na qual a intensa interdigitação de porções monzodioríticas e sieníticas sugere uma forte atuação mecânica, favorecendo a hipótese de que os magmas tenham se misturado previamente ao alojamento final; e (ii) interação de magmas monzodioríticos (Mdf) e quartzo sieníticos (Qsie), gerando diversas estruturas de coexistência, mistura localizada e possivelmente as rochas quartzo monzoníticas da unidade ( Qsie); neste caso, a interação entre os magmas possivelmente ocorreu na câmara magmática, como sugerido pelas características das estruturas indicativas de coexistência (enclaves e pillows de composição monzodiorítica). / The present study investigates the interaction and partial mixing of monzodiorite and syenite s.l. magmas in the Piracaia Pluton (~580 My), which is part of the post-orogenic Itu Granite Province. The Piracaia Pluton is an elongated intrusion with approximately 30 km², composed by five units: Coarse-grained monzodiorites (Mdr); Fine-grained monzodiorites (Mdf); Heterogeneous monzonites (Mh); Syenites (Sie); quartz syenites and quartz monzonites (Qsie). The physico-chemical crystallization parameters of the primary and hybrid magmas identified in the pluton were determined as references for the characterization of the magma interaction processes. The liquidus temperatures were estimated based on apatite saturation, and are in the range of 950-1050º C for the most primitive magmas (Mdf) and 850-900º C for the more differentiated ones (quartz syenites). The solidus temperatures, estimated on the basis of the equilibrium between hornblende and plagioclase, are about 750º C for Mdf, and 650-700º C for and quartz syenites (Qsie). The depth of emplacement is estimated at 13-15 km, as indicated by pressure estimates for Mdf, based on the Al-in-hornblende content. The H2O concentration of the magmas was estimated based on the An content of plagioclase, with results are in the range of 2.5-3.3% for Mdf, reaching up to 5% in quartz syenites. fO2 estimates based on the composition of coexisting magnetite and ilmenite could not be performed, due to post-magmatic re-equilibration. However, approximate values were obtained based on the ulvospinel content of reconstructed titanomagnetite crystals, and on the content of ilmenite molecule in ilmenite crystals; in parallel, the Fe/(Fe/Mg) ratios of amphibole and biotite were also used. The results show an important difference between the Mdf and Mdr units, which are distinctively oxidized, with values near the NNO buffer, and the other units which have lower ?O2, the quartz syenites and especially the syenites being the more reduced units. The lower magnetic susceptibility values measured in the field for the Sie and Mh units must reflect their more reduced nature. During the construction of the magma chamber, which was intermittently recharged by magmas of varied composition, two main events of magma interaction with distinct characteristics were identified: (i) the interaction of monzodiorites (Mdf) and syenites (Sie), generating the Heterogeneous monzonite unit, in which the interfingering of monzodiorite and syenite portions suggest a strong mechanical interaction, favoring the hypothesis that the magmas were already mixing previously to the final emplacement; (ii) interaction of monzodiorite (Mdf) and quartz syenite (Qsie), generating diverse mingling structures, local hybridization and possibly the quartz monzonite rocks of the unit Qsie; in this case, the magma interaction must have occurred at the magma chamber, as suggested by the type of structures indicative of coexistence (enclaves and monzodiorite pillows).

O presente trabalho investiga processos de interação e mistura parcial de magmas monzodioríticos e sieníticos s.l. no Plúton Piracaia,(~ 580 Ma), que pertence à Província Granítica Itu, de caráter pós-orogênico. O Plúton Piracaia é uma intrusão alongada com cerca de 30 km2, formada por cinco unidades: Monzodioritos grossos (Mdr); Monzodioritos finos (Mdf); Monzonitos heterogêneos (Mh); Sienitos (Sie); quartzo sienitos e quartzo monzonitos (Qsie). Como base para a caracterização dos processos de interação de magmas, foram estimados os parâmetros físico-químicos dos magmas primários e híbridos identificados no plúton. As temperaturas liquidus foram estimadas a partir da saturação em apatita, e são da ordem de 950-1050°C para os magmas mais primitivos (Mdf) e 850-900°C para os mais diferenciados (quartzo sienitos). As temperaturas solidus, estimadas através das relações de equilíbrio entre hornblenda e plagioclásio, são da ordem de 750°C para Mdf, e 650-700°C para Sienitos e quartzo sienitos (Qsie). A profundidade de alojamento do plúton é estimada em ~13-15 km com base nas pressões estimadas para Mdf com base no conteúdo de Al na hornblenda. A concentração de H2O nos magmas foi estimada com base no teor de An do plagioclásio, conhecida a temperatura de cristalização, que indicou valores da ordem de 2,5-3,3% para Mdf, alcançando até 5% nos quartzo sienitos. Estimativas da fO2 a partir da composição química de cristais de magnetita e ilmenita não puderam ser obtidas, devido a reequilíbrio pós-magmático. Deste modo, foram obtidos valores aproximados com base no conteúdo de ulvoespinélio de cristais de magnetita reconstituídos e no conteúdo da molécula ilmenita dos cristais de ilmenita; em paralelo, foram também utilizadas as razões Fe/(Fe/Mg) de anfibólio e biotita. Os resultados revelaram um importante contraste entre as unidades Mdf e Mdr, caracteristicamente oxidadas, com valores próximos aos do buffer NNO, e as demais unidades, onde quartzo sienitos (Qsie) e especialmente Sie mostram-se mais reduzidas. Os baixos valores de susceptibilidade magnética medidos em campo para as unidade Sie e Mh, que a ela se associa, devem ser reflexos do seu caráter mais reduzido. Ao longo da história de construção da câmara, que foi alimentada intermitentemente por magmas de composição variada, dois eventos principais de interação de magmas foram identificados, com características distintas: (i) interação de magmas monzodioríticos (Mdf) e sieníticos (Sie), gerando a unidade de Monzonitos heterogêneos (Mh), na qual a intensa interdigitação de porções monzodioríticas e sieníticas sugere uma forte atuação mecânica, favorecendo a hipótese de que os magmas tenham se misturado previamente ao alojamento final; e (ii) interação de magmas monzodioríticos (Mdf) e quartzo sieníticos (Qsie), gerando diversas estruturas de coexistência, mistura localizada e possivelmente as rochas quartzo monzoníticas da unidade ( Qsie); neste caso, a interação entre os magmas possivelmente ocorreu na câmara magmática, como sugerido pelas características das estruturas indicativas de coexistência (enclaves e pillows de composição monzodiorítica). / The present study investigates the interaction and partial mixing of monzodiorite and syenite s.l. magmas in the Piracaia Pluton (~580 My), which is part of the post-orogenic Itu Granite Province. The Piracaia Pluton is an elongated intrusion with approximately 30 km², composed by five units: Coarse-grained monzodiorites (Mdr); Fine-grained monzodiorites (Mdf); Heterogeneous monzonites (Mh); Syenites (Sie); quartz syenites and quartz monzonites (Qsie). The physico-chemical crystallization parameters of the primary and hybrid magmas identified in the pluton were determined as references for the characterization of the magma interaction processes. The liquidus temperatures were estimated based on apatite saturation, and are in the range of 950-1050º C for the most primitive magmas (Mdf) and 850-900º C for the more differentiated ones (quartz syenites). The solidus temperatures, estimated on the basis of the equilibrium between hornblende and plagioclase, are about 750º C for Mdf, and 650-700º C for and quartz syenites (Qsie). The depth of emplacement is estimated at 13-15 km, as indicated by pressure estimates for Mdf, based on the Al-in-hornblende content. The H2O concentration of the magmas was estimated based on the An content of plagioclase, with results are in the range of 2.5-3.3% for Mdf, reaching up to 5% in quartz syenites. fO2 estimates based on the composition of coexisting magnetite and ilmenite could not be performed, due to post-magmatic re-equilibration. However, approximate values were obtained based on the ulvospinel content of reconstructed titanomagnetite crystals, and on the content of ilmenite molecule in ilmenite crystals; in parallel, the Fe/(Fe/Mg) ratios of amphibole and biotite were also used. The results show an important difference between the Mdf and Mdr units, which are distinctively oxidized, with values near the NNO buffer, and the other units which have lower ?O2, the quartz syenites and especially the syenites being the more reduced units. The lower magnetic susceptibility values measured in the field for the Sie and Mh units must reflect their more reduced nature. During the construction of the magma chamber, which was intermittently recharged by magmas of varied composition, two main events of magma interaction with distinct characteristics were identified: (i) the interaction of monzodiorites (Mdf) and syenites (Sie), generating the Heterogeneous monzonite unit, in which the interfingering of monzodiorite and syenite portions suggest a strong mechanical interaction, favoring the hypothesis that the magmas were already mixing previously to the final emplacement; (ii) interaction of monzodiorite (Mdf) and quartz syenite (Qsie), generating diverse mingling structures, local hybridization and possibly the quartz monzonite rocks of the unit Qsie; in this case, the magma interaction must have occurred at the magma chamber, as suggested by the type of structures indicative of coexistence (enclaves and monzodiorite pillows).

Hydraulic fracturing is an important and abundant process in both industrial applications and natural environments. The current work is the first systematic quantitative study of the effect of interaction in and between complex hydraulic fractures at different spatial scales. A mathematical model, based on the boundary collocation method, has been developed. The model has been employed for a typical field case, a highly segmented vein. This vein is well-mapped, and therefore, represents a well constrained example. The computed apertures are compared to the measured apertures. By using the simplest constitutive model, based on an ideal elastic material, and including the effect of interaction between the segments, it was possible to obtain an excellent match at all considered scales. It was also shown that the concept of effective fracture, as currently accepted in the literature, is not always applicable and may lead to unbounded inaccuracy. Unfortunately, in most cases, very little (if any) directly measured data on fracture and material properties is available. An important example of such a weakly constrained case, involving hydraulic fracturing, is diking beneath the seafloor at mid-oceanic ridges. In this study, it is shown that the commonly accepted scenario of a dike propagating from the center of the pressurized magma chamber to the ocean floor is not consistent with conventional fracture mechanics due to the fact that the chamber has the shape of a thin lens. Even at such a large scale (i.e., a kilometer or more), the mechanical principles of elastic interaction appear to be applicable. Since diking is likely to generate a region of high permeability near its margin, in addition to heat, the ongoing hydrothermal activity becomes localized. Our modeling suggests the probable positions of the propagating dikes. Consequently, comparing the observed locations of hydrothermal sites with respect to that of the magma chamber could be useful for constraining the mechanisms of magma lens evolution.

Xenoliths preserve evidence of magma-crust interactions in magmatic reservoirs and conduits. They reveal processes of partial melting of country rock, and disintegration into magma. Widespread evidence for frothy xenoliths in volcanic deposits exists, and these evidently indicate processes of gas liberation, bubble nucleation and bubble growth. This report focuses on textural analysis of frothy sandstone xenoliths from Krakatau in Indonesia, Cerro Negro in Nicaragua, Cerro Quemado in El Salvador and from Gran Canaria, Canary Islands, and involves attempts to experimentally reproduce xenolith textures. To achieve this, magmatic conditions acting upon country rock in volcanoes are simulated by subjecting sandstones to elevated temperature and pressure in closed system-autoclaves. Subsequent decompression imitates magma ascent following xenolith entrainment, and is largely responsible for the formation of frothy xenolith textures. The experiments show a range of successive features, such as partial melting, gas-pressure build up, bubble nucleation, growth and development of bubble networks. The experiments closely reproduced textures of natural xenoliths and help to assess the controlling P-T parameters that encourage efficient bubble growth. Conditions proved ideal between 850˚C and 870˚C and pressure release from 1 kbar. Such conditions limit bubble overprinting by secondary crystallization and melt infilling. Country rock lithology proved vital regarding gas pressure build-up and resulting bubble nucleation during decompression. In particular, increased water content and relict crystals in the melt produced appear to ease and promote gas liberation by enabling early and effective bubble nucleation. Moreover, experiments confirm a decisive role for bubble coalescence. These results attest to the great potential of country rock to develop interconnected bubble networks upon magma contact, exsolving large amounts of crustal volatiles into the magma. Volatile input involves a change in magma viscosity and thus an accompanied change in disruptive behaviour, and may hence be responsible for increased potential to cause explosive volcanic eruptions. Moreover, H2O and CO2 vapour are severe greenhouse gases, which seems to be added to the atmosphere from crustal rocks via recycling by volcanic activity, and may have yet underappreciated effects on Earth’s climate.

La présence de magmas en profondeur permet de contraindre des processus géologiques passés et actuels. Ces magmas (i.e. liquides silicatés) participent aux cycles géochimiques des éléments volatils comme vecteur de matière.Nous étudions deux éléments volatils complémentaires : l'iode (I), un halogène, et le xénon (Xe), un gaz rare. Leur système radioactif éteint 129I/129Xe (T1/2 = 15.7Ma) est utilisé pour dater les processus hadéens et la formation de l'atmosphère, issu de l'évolution d'un océan magmatique. Or on connait peu le comportement de l'iode et du xénon dans les magmas en profondeur à haute pression et température.Notre protocole expérimental vise l'étude de l'incorporation de l'iode et du xénon et de leur solubilité dans les magmas. Pour étudier l'incorporation, la structure des silicates liquides a été caractérisée par diffraction de rayons X avec des expériences in situ réalisées dans des cellules à enclumes de diamant et dans des presses Paris-Édimbourg. Les teneurs de solubilité de l'iode et du xénon ainsi que l'eau ont été mesurés par les méthodes PIXE et ERDA.À hautes pressions, l'iode possède une forte solubilité (quelques %pds) dans les magmas. Les résultats préliminaires sur son incorporation dans du basalte montrent que l'iode ne formerait pas des liaisons covalentes. À haute pression et température (T>300°C - P>1GPa), le xénon forme une liaison covalente Xe-O avec les oxygènes des anneaux de 6 tétraèdres SiO44-. Le xénon a une solubilité élevée dans les magmas (4pds% - 1600°C - 3.5GPa).Les modèles de datation et des cycles géochimiques de l'iode et du xénon doivent être revus en tenant compte de leur comportement différentiel dans les magmas. / The presence of magmas at depth helps to constrain past and actual geological processes. Magmas (i.e. silicate melts) participate in geochemical cycles of volatile elements, as vectors of chemical transfers. We study two complementary volatile elements: iodine (I), a halogen, and xenon (Xe), a noble gas. Their extinct 129I/129Xe isotopic system (half-life of 15.7Ma) is used to date Hadean processes and Earth’s atmosphere formation since the atmosphere originated from the Magma Ocean’s evolution. However, little is known about the behavior of both iodine and xenon in silicate melts at depth, under HT and HP conditions. Our experimental protocol aims at elucidating the incorporation process of xenon and iodine in silicate melts, and their solubility. To understand the incorporation of iodine and xenon in magmas, the structure of silicate melts was investigated by in situ diamond anvil cells and Paris-Edinburgh press experiments coupled with X-ray diffraction characterization. Iodine and xenon’s solubility, along with water content are obtained by PIXE and ERDA methods using a nuclear microprobe. At high pressure, iodine has a high solubility (about few wt.%) in magmas. Preliminary results on iodine incorporation in basaltic melt show an absence of covalent bond. At high pressure and temperature conditions (T>300°C – P>1GPa), xenon forms a Xe-O covalent bond with the oxygens of the 6-membered-rings of the melt network. Its solubility in silicate melts is also high (about 4wt.% in haplogranite melts at 1600°C and 3.5GPa). Considering the xenon and iodine differential behavior in melts at depth, a revision of dating models in xenon and iodine cycles must be considered.

Contraindre l'architecture et la dynamique des systèmes magmatiques est d’une importance capitale dans la compréhension des phénomènes volcaniques. Les principaux objectifs de cette thèse sont (1) de contraindre l'architecture du système magmatique du Piton de la Fournaise dans son ensemble, et (2) d'étudier l’évolution des magmas et les transferts associés en son sein. En couplant l'étude pétro-géochimique des magmas et du dégazage de CO2 à travers le sol, nous développons une approche intégrée, focalisée sur le flanc ouest de l'édifice. Cette zone témoigne d'une activité éruptive récente très peu étudiée, traçant potentiellement des processus magmatiques profonds. Nos résultats permettent de valider l'hypothèse du caractère décentré de la plomberie magmatique profonde du Piton de la Fournaise sous le flanc ouest de l'édifice. Nous démontrons que les magmas stockés dans la partie profonde de la plomberie magmatique préservent une certaine variabilité géochimique, principalement liée à de légères hétérogénéités de source et à des processus polybariques de cristallisation et d'assimilation. Nous montrons également que les flux de CO2 à travers le sol du flanc ouest de l'édifice enregistrent le dégazage précoce des magmas au sein du manteau lithosphérique. Nous soulignons ici que les variations temporelles des flux de CO2 à travers le sol peuvent être utilisées pour détecter les recharges du système magmatique central via des transferts magmatiques profonds.Nos résultats ouvrent des perspectives prometteuses quant à la surveillance des processus magmatiques profonds sous les édifices volcaniques, même en conditions tropicales. / Constraining the architecture and dynamics of magma systems is fundamental in volcanology. The main objectives of this study are to (1) constrain the architecture of the whole plumbing system of Piton de la Fournaise, and (2) study magma evolution and transfers from the deepest roots of the plumbing system. Coupling magma petrogeochemistry and diffusive soil degassing, we develop an integrated approach, focused on the western flank of the volcano. This zone shows evidences of a recent eruptive activity, poorly documented, but potentially related to deep magma processes. Our results allow to confirm the offset of the deep part of the Piton de la Fournaise plumbing system beneath the western flank of the volcano. We demonstrate that magma stored in the deepest roots of the plumbing system display a geochemical variability, mainly related to minor mantle source heterogeneities and to polybaric crystallization and assimilation processes. We show that soil CO2 fluxes on the western flank of the volcano record early magma degassing in the lithospheric mantle. We highlight that temporal variations of soil CO2 flux may be used to detect magma replenishement of the central magmatic system by deep magma transfers, hardly detectable by the geophysical network.Our results opens exciting prospects to improve the monitoring of deep magma processes below volcanoes, even in tropical conditions.

Magma plumbing systems represent the physical framework of magma transport and storage from the source region in the mantle, through the crust, until reaching the surface in a volcanic eruption. Characterising the different aspects of magma plumbing, in particular the distribution of magma storage zones throughout the crust, is of key importance to better understand the behaviour of individual volcanoes. In particular, shallow crustal magma storage and associated magma-crust interaction processes could potentially explain some of the worlds most unpredictable and explosive volcanoes. This thesis studies magma plumbing architecture in the Sunda Arc (Indonesia), and the North Atlantic Igneous Province, based on elemental and isotope geochemistry, and derived petrological modelling. In this study, I have employed petrological models, so called geothermobarometers, to calculate pressures and temperatures (P-T) of crustal magma storage. Geothermobarometers are calibrated thermodynamic formulations based on the composition of magmatic minerals and their co-existing melt as a function of the P-T conditions of crystallisation. Using the calculated P-T estimates, I was able to derive the depth of magma storage, and thereby reconstruct the architecture of magma storage systems. A number of different geothermobarometers based on different mineral phases, including plagioclase, clinopyroxene and olivine, were used for this purpose, The geothermobarometric modelling was combined with additional elemental and isotope geochemical analyses, as well as collaborations with geophysical investigations. These additional approaches were used to corroborate the findings of the geothermobarometric modelling, and also to model and quantify magma-crust interaction processes that take place during crustal magma storage, such as assimilation of crustal lithologies into the magmatic system. The findings of this thesis build upon the growing body of evidence in support of the prevalence of shallow magma storage in different volcanic settings worldwide. This realisation is relevant to volcano monitoring and hazard mitigation worldwide.

La télédétection thermique en temps réel de systèmes volcaniques actifs est une technique cruciale pour comprendre le comportement et l'activité éruptive de corps magmatiques chauds. Cette technologie repose sur la détermination de l'émissivité thermique du magma, un paramètre permettant d'identifier la température du magma. Nous avons utilisé une méthode directe pour obtenir des spectres dans la plage de nombres d’onde allant de 400 à 13000 cm-1; d’échantillons naturels volcaniques (volcan Erebus et Teide) et synthétiques (avec différents éléments). Ces matériaux ont été chauffés de la température ambiante à 2000K avec un laser CO2 et les données ont été collectées pendant toute la phase de chauffage avec un spectromètre IRTF.Nos résultats indiquent que l’émissivité d’une roche magmatique est affectée par les changements de composition et l’histoire thermique. Les mesures d'émittance montrent le rôle important du fer et de la vitesse de refroidissement du magma sur la réponse spectrale des compositions de type phonolitique. Ces observations sont importantes, car une méconnaissance des valeurs d’émissivité engendre des erreurs sur la détermination de la température et, donc à une interprétation erronée de la rhéologie et de l'efficacité thermique du corps magmatique. / Real time thermal remote sensing of active volcanic systems is a crucial technique for understanding the behavior and eruptive activity of hot magmatic bodies. Such technology relies on determining the thermal emissivity of the magma, a parameter to identify the temperature of magma. We used a direct method to obtain a spectrum in the wavenumber range from 400 to 13000 cm-1; the natural (volcano Erebus and Teide) and synthetic samples were heated up from room temperature to 2000K with a CO2 laser and data were collected during all the heating stage with a FTIR spectrometer.Our results thus indicate that thermal emissivity of magmatic rock is affected by changes in composition and thermal history. The emissivity measurements show the important role of the iron and the cooling rate on the spectral response of the phonolite composition. These are important observations since different emissivity will lead to different temperature determinations and hence, an erroneous interpretation on the rheology and thermal efficiency of the magmatic body.

A travers deux champs d'étude, nous nous intéressons au couplage mécanique entre le magma et l'encaissant, utilisant des méthodes numériques. Tout d'abord, nous étudions l'influence de perturbations de contraintes sur les réservoirs magmatiques, avec comme exemple deux volcans sous-glaciaires (Islande). Au volcan Grímsvötn, notre modèle montre que les vidanges, du lac sous-glaciaire (jökulhlaup) présent dans la caldera, peuvent déclencher une éruption comme ce fut le cas en 2004, en favorisant la rupture du réservoir magmatique. L'effet est cependant faible, ce qui implique que le système magmatique doit déjà être proche des conditions de rupture avant que le jökulhlaup se produise. Au volcan Katla, notre modèle indique que les conditions de rupture sont favorisées en été durant la fonte du glacier Mýrdalsjökull. Les changements de contrainte de Coulomb montrent aussi une plus forte probabilité de séismes durant la même période, résultats en accord avec la sismicité enregistrée sous le Mýrdalsjökull. Il existe une modulation à la fois de l'activité volcanique et sismique au Katla, en relation avec la variation saisonnière de la charge glaciaire. Dans un second temps, nous travaillons sur les écoulements de magma dans les conduits andésitiques. Améliorer nos connaissances sur la dynamique du magma durant son ascension est nécessaire, car les processus dans le conduit volcanique semblent gouverner l'évolution de l'activité éruptive de ces volcans. Des précédents modèles ont montré que la viscosité du magma augmente dans la partie supérieure du conduit lors de l'écoulement, ce qui cause la formation d'un plug visqueux. Mais la relation entre la mise en place du plug et les signaux précurseurs, telles que la déformation ou la sismicité, n'est pas totalement établie. A partir de nos modèles de plug, nous trouvons que les déplacements de surface sont contrôlés par la géométrie du conduit et du plug ainsi que le contraste de viscosité entre le plug et la colonne de magma. Nous montrons que l'évolution de la taille du plug est une hypothèse possible pour expliquer les rapides transitions inflation/subsidence observées à la surface des volcans andésitiques. / Through two different applications, we focus on the mechanical coupling between magma and host rocks, using numerical method. First, we study the influence of stress perturbations on shallow magma chambers, with the exemple of two subglacial volcanoes (Iceland). A variation in the stress field acts to modify the magma pressure within the reservoir as well as the failure conditions to initiate an intrusion at the reservoir wall. At Grímsvötn volcano, subglacial lake discharges (so-called "jökulhlaup") often occur in relation to eruptions. Our models show that jökulhlaup promote the failure of the magma reservoir and thus trigger eruptions, as observed for 2004 eruption. The triggering effect is small, so magmatic system must be already pressurized and close to failure before the discharge of the lake occurs. At Katla volcano, our models indicate that reservoir failure is highest in summer period when the ice load at Mýrdalsjökull icecap is reduced. Coulomb stress calculation predicts also an increase of earthquake occurrence at the caldera rim at the same time. A seasonal modulation of volcanic and seismic activity at Katla related to the loading/unloading of the icecap can exist. Secondly, we focus on magma conduit flow for andesitic volcanoes. Improving our knowledge of magma dynamic during ascent is a challenge, because processes occurring in the conduit seem to govern the temporal evolution of eruptive activity. Previous models showed that magma viscosity increases during flow in the upper part of the conduit, which causes the emplacement of a viscous plug. But the relationship between the plug emplacement and precursors signals, such as ground deformation or seismicity, is not yet fully described. From our plug flow model, we find that surface deformation is mainly controlled by the geometry of the conduit, the ratio between the plug length and the total conduit length and the viscosity contrast between the plug and the magma column. We show that the evolution of the plug size is a possible explanation for rapid transition between ground inflation and ground subsidence observed on andesitic volcanoes before extrusion.