Mafic magma is the most common magma erupted at the surface of the earth. It is generated from partial melting of the mantle, which has been subdivided into end-members based on unique geochemical signatures. One reason these end members, or heterogeneities, exist is subduction of lithospheric plates back into the mantle. The amount of elements, such as Cl and K, removed during subduction and recycled into the deep mantle, is poorly constrained. Additionally, the amount of volatiles, such as Cl, that are recycled into the deep mantle will strongly affect the behavior of the system. I have looked at Cl and K in HIMU source melts to see how it varies. Cl/Nb and K/Nb suggest that elevated Cl/K ratios are the result of depletion of K rather than increased Cl recycled into the deep mantle. After the mantle has partially melted and mafic melt has migrated to the surface, it usually erupts effusively or with low explosivity because of its low viscosity, but it is possible for larger eruptions to occur. These larger, Plinian eruptions, are not well understood in mafic systems. It is generally thought that basalt has a viscosity that is too low to allow for such an eruption to occur. Plinian eruptions require fragmentation to occur, which means the melt must undergo brittle failure. This may occur if the melt ascends rapidly enough to allow pressure to build in bubbles without the bubbles expanding. To test this, I have done decompression experiments to try to bracket the ascent rate for two Plinian eruptions. One eruption has a fast ascent, faster than those seen in more silicic melts, whereas the other eruption is unable to be reproduced in the lab, however it began with a increased viscosity in the partly crystallized magma. After fragmentation and eruption, it is generally thought that tephra do not continue to crystallize. We have found that crystallinity increases from rim to core in two basaltic pumice. Textural data along with a cooling model has allowed us to estimate growth rates in a natural system, which are similar to experimental data. / text

Les profils de sismique réflexion de Marges Passives Volcaniques imagent une croûte inférieure réflective formée de deux domaines distincts. Ces réflecteurs sont interprétés comme des intrusions mafiques associées à la formation de la marge. Les observations géophysiques ne permettent pas de comprendre les conséquences associées à la mise en place de ces magmas sur la rhéologie et la déformation effective de la croûte inférieure des marges volcaniques. Cette thèse explore deux exemples géologiques de croûte inférieure à l’affleurement qui ont été affectés par d’importants volumes magmatiques pendant leur histoire extensive : la Zone d’Ivrée (Alpes italiennes) et la province de Seiland (Calédonides finmarkiennes). Une étude approfondie de terrain, associée à des analyses géochimiques et isotopiques ainsi qu’à des expériences de déformation en Presse Paterson permet, (1) de définir la structure synthétique finie d’une croûte inférieure extensive affectée par un ou plusieurs épisodes magmatiques intenses et d'identifier dans la Zone d’Ivrée une croûte moyenne ductile, dans laquelle s’enracinent les failles de la croûte supérieure, (2) de montrer l’évolution de la rhéologie et de la déformation ductile de la croûte inférieure relativement à la mise en place des magmas (3) de caractériser à partir de Seiland l’interface croûte manteau dans un contexte d’extension synmagmatisme mafique. L’ensemble des résultats montre la complexité des relations, à la fois structurales, spatiales et temporelles, entre les magmas et roches mafiques et la déformation en extension dans la croûte inférieure. / Seismic reflection profiles of Volcanic Passive Margins show a two-domain reflective lower crust. The particular reflectors are interpreted as mafic intrusions associated to the margin evolution.From those geophysical observations it is not possible to understand the consequences of mafic magma emplacement on the rheology and effective deformation of volcanic margins lower crust. This thesis explores two geologic examples of outcropping lower crusts affected by important volumes of magma during their extensional story: the Ivrea Zone (Italian Alps) and Seiland (Finmarkian Caledonides). An important field study associated to geochemical and isotopic analyses, and to Patersonapparatus deformation experiences led to (1) define a synthetic structure of an extensional lower crust affected by one or more major magmatic events and to indentify a ductile middle crust in the Ivrea Zone where roots the upper crust faults, (2) show the rheologic and deformation evolution of the lower crust in relation to magma emplacement timing, (3) characterize, from Seiland observations, the crustmantle interface in an extensional System. The whole study shows the complexity of the temporal, spatial and structural relationships between mafic rocks (as magma and as crystallised rock) and extensional deformation in the lower crust.