In this dissertation, I use seismic imaging and waveform modeling methods to investigate melt migration processes and the structure of the magma plumbing system beneath the East Pacific Rise (EPR) and Endeavour segment of the Juan de Fuca Ridge, respectively. This work begins by studying shallow mantle reflections beneath the EPR. I find the amplitude versus offset and waveform characteristics of the reflections to be consistent with a sub-horizontal dunite channels located up to 20 km off-axis. The depth of the dunite channels correlate with patterns of mantle melt delivery and the predicted base of the thermal lithosphere, suggesting the channels are thermally controlled and may have formed in situ via dissolution by focused flow at the base of the lithosphere. This interpretation is consistent with field observations in ophiolites and numerical modeling of melt-focusing channels. The three-dimensional velocity structure of the Endeavour segment is then investigated to identify how patterns of mantle melt delivery influence the segment-scale distribution of crustal melt and crustal accretion. The results from this study indicate that the mantle magmatic system is skewed relative to the ridge-tracking crustal magmatic system and that this skew exerts primary control on magmatic, tectonic, and hydrothermal activity at the Endeavour segment. In regions where mantle melt delivery is axis-centered, mantle-derived melts are efficiently transported from the mantle to the crust, resulting in frequent crustal melt replenishment, associated seismogenic cracking, and enhanced crustal melt content that drives vigorous hydrothermal activity. Conversely, sites of off-axis melt delivery are characterized by less efficient vertical melt transport, resulting in infrequent crustal melt injection and hence, reduced crustal melt content and hydrothermal activity. Next, I focus on how along-axis variations in magma replenishment modulate crustal permeability and the intensity of hydrothermal circulation. Using full-waveform inversion, I show that sites of localized magma replenishment to the axial magma lens, along with induced seismogenic cracking, coincide with enhanced permeability. I conclude that the frequency of magma injection governs hydrothermal circulation patterns and heat flux at mid-ocean ridges. This dissertation includes previously published and unpublished coauthored material.

Over the last fifty years, our knowledge of the Solar System has increased exponentially. Many planetary surfaces were seen for the first time through spacecraft observations. Yet the interiors of most planetary bodies remain poorly studied. This dissertation focuses on two main topics: the formation of central pit craters and what this reveals about the subsurface volatile content of the target material, and the mantle dynamics of Io and how they relate to the extensive volcanism on its surface. Central pit craters are seen on icy satellites, Mars, the Moon, and Mercury. They have terraced rims, flat floors, and a pit at or near their center. Several formation mechanisms have been suggested. This dissertation assesses the feasibility of central pit crater formation via drainage of impact melt through impact-generated fractures. For impacts on Ganymede, the expected volume of melt and volume of fracture space generated during the impact and the volume of melt able to drain before fractures freeze shut all exceed the observed central pit volumes on Ganymede. This suggests that drainage of impact melt could contribute to central pit crater formation on Ganymede. Molten rock draining through solid rock fractures will freeze shut more rapidly, so this work suggests that impact melt drainage is unlikely to be a significant factor in the formation of central pit craters on rocky bodies unless a significant amount of volatiles are present in the target. Io is the most volcanically active body in the Solar System. While volcanoes are most often associated with plate tectonics on Earth, Io shows no signs of plate tectonics. Previous work has suggested that Io could lose a significant fraction of its internal heat through volcanic eruptions. In this dissertation, I investigate the relationship between mantle convection and magma generation, migration by porous flow, and eruptions on Io. I couple convective scaling laws to a model solving the two-phase flow equations applied to a rising column of mantle. I show that Io has a partially molten upper mantle and loses the majority of its internal heat through volcanic eruption. Next, I present two-dimensional numerical simulations that self-consistently solve the two-phase flow equations including mantle convection and magma generation, migration by porous flow, and eruption. These simulations produce a high heat flux due to volcanic eruption, a thick lithosphere, a partially molten upper mantle, and a high eruption rate—all consistent with observations of Io. This model also reveals the eruption rate oscillates around the statistical steady state average eruption rate suggesting that the eruption rate and total heat flux measurements from the past 35 years may not be representative of Io's long term behavior.

Décrypter les mécanismes et cinétiques de croissance et dissolution des minéraux dans les liquides silicatés est indispensable à la compréhension des processus magmatiques fondamentaux. La migration transcristalline des inclusions magmatiques sous l'effet d'un gradient thermique permet de quantifier une loi cinétique de croissance et dissolution du minéral hôte dans des conditions proches de celles qui prévalent le plus souvent dans la nature. L'objectif principal de ce travail de thèse était : i) d'étudier l'effet de la composition du liquide piégé sur le processus de migration dans l'olivine et ii) d'exploiter systématiquement le processus de migration afin de quantifier les lois cinétiques pour d'autres couples minéral-liquide. Lorsqu'elles sont soumises à un gradient thermique, les inclusions siliceuses (SiO2 ≥ 60pds%) piégées dans les olivines mantelliques et les inclusions basaltiques piégées dans les clinopyroxènes volcaniques migrent à travers leur hôte en direction du point chaud de la zone de travail. La migration s'effectue à une vitesse constante et, dans les olivines, sans modification de la composition du liquide piégé ; les inclusions des clinopyroxènes subissent en revanche une rééquilibration chimique transitoire en début de migration. Les cavités, subsphériques avant la migration, évoluent vers la forme en cristal-négatif du minéral hôte en cours d'expérience. L'achèvement de l'évolution morphologique nécessite un temps caractéristique gouverné par la diffusion chimique dans le liquide. La bulle de gaz exsolvé dans les inclusions n'est pas entraînée dans la migration. Elle se sépare du liquide magmatique et donne naissance à une inclusion fluide isolée au sein du cristal hôte. Les résultats expérimentaux indiquent que la migration procède par dissolution du minéral hôte à l'avant et recristallisation à l'arrière de l'inclusion. La vitesse de migration est limitée par les mécanismes à l'interface cristal-liquide, non par la diffusion chimique. Les taux de croissance et dissolution de l'olivine et du clinopyroxène que nous obtenons sont respectivement trente et quinze fois inférieurs à ceux déterminés dans une étude antérieure à partir d'expériences de migration d'inclusions basaltiques dans des olivines volcaniques. Ils obéissent cependant à la même forme de loi cinétique, qui peut être aisément transposée à des environnements de cristallisation ou de fusion naturels, similaires ou de plus faible déséquilibre. Le taux de croissance et dissolution de l'olivine lors des migrations n'a pas de lien simple avec la composition du liquide piégé ; il est peut-être aussi en grande partie contrôlé par la densité de dislocations du cristal hôte. Le phénomène de migration n'a pas été observé dans le quartz et le plagioclase pour les durées d'expériences réalisées. Il est néanmoins probable que l'absence de migration lors des expériences ne soit qu'apparente. Notamment, la prédominance de liaisons de forte énergie dans la structure du quartz et du plagioclase est susceptible de rendre les processus interfaciaux, et donc aussi la dissolution, particulièrement lente. Au cours de ce travail de thèse, l'opportunité s'est présentée d'étudier les inclusions magmatiques piégées dans les cristaux d'olivine de la pallasite Brahin. En particulier, deux familles d'inclusions ont été identifiées. La première consiste en des plans d'inclusions secondaires contenant de nombreuses chromites et des assemblages à métal-sulfure et olivine phosphorée ; la seconde correspond à des inclusions isolées renfermant pour la plupart de la stanfieldite, une bulle de gaz et de l'olivine phosphorée. Les inclusions secondaires se seraient formées suite à un choc ayant eu lieu alors que l'assemblage minéralogique actuel de Brahin était déjà formé, ou en cours de formation. En revanche, les inclusions de stanfieldite témoigneraient d'un choc prépallasitique. / Deciphering the mechanisms and kinetics of crystal growth and dissolution in silicate melts is essential for understanding the fundamental magmatic processes. When remelted and subjected to an imposed thermal gradient, melt inclusions migrate through their host, which provides a direct access to the host crystal growth / dissolution kinetics and allows to quantify the kinetic laws at very low undercoolings or overheatings, i.e. in conditions appropriate to many natural systems. The main goal of the present study was: i) to study the effect of the trapped melt composition on the migration process in olivine and ii) to extend the transcrystalline melt migration approach to quantify the kinetic law for other mineral-melt pairs. When subjected to a thermal gradient, Si-rich melt inclusions (SiO2 ≥ 60 wt%) in mantle olivines and basaltic melt inclusions in volcanic clinopyroxenes migrate through their host toward the host spot of the working zone. Migration proceeds at a constant rate and in olivine, without modification of the trapped melt composition ; melt inclusions in clinopyroxene, however, undergo a transient chemical reequilibration while they begin to migrate. While moving, the melt inclusions gradually change from subspherical to a faceted, negative-crystal shape. Completion ofthe morphological evolution requires a characteristic time that is governed by chemical diffusion. When a gas bubble is initially present, it responds to elastic forces by quickly shifting toward the cold end of the inclusion, where it soon becomes engulfed as an isolated fluid inclusion in the reprecipitated crystal. The experimental results indicate that the migration operates by progressive dissolution and recrystallistion of the host, governed by interface kinetics with no interference of chemical diffusion.The growth / dissolution rates we obtained for olivine and clinopyroxene are respectively thirty and fifteen times lower than those determined in a previous study from melt migration experiments on basaltic inclusions in volcanic olivines. Nevertheless, they obey the same form of kinetic law, which can be transposed to equally or more sluggish melting or crystallisation events in nature. Dependence of the growth / dissolution rate of olivine on trapped melt composition is not straightforward ; rates may be largely controlled by the density of dislocations in the host crystal. The melt migration phenomenon was not observed in quartz and plagioclase for the investigated experiment durations. Nonetheless, the lack of migration during experiments may only be apparent. Notably, the dominance of high-strength bonds in the quartz and plagioclase structure may render interfacial reactions, and so dissolution, particularly slow. During this research work, we had the opportunity to study the melt inclusions trapped in olivine crystals from the Brahin pallasite. In particular, two contrasted sets of melt inclusions were evidenced. The first set consists of plans of secondary inclusions containing abundant chromite and assemblies of metal, sulfide, and phosphoran olivine ; the second set corresponds to isolated inclusions consisting for the most part of stanfieldite, a gas bubble, and phosphoran olivine. Secondary inclusions may have formed during a shock event that took place while the current stony-iron assembly of the Brahin pallasite was already formed, or was created by this shock. However, stanfieldite inclusions may originate from a pre-pallasitic shock event.

This thesis presents the results of a combined magnetic fabric and palaeomagnetic analysis of lower crustal rocks exposed in the Oman (Semail) ophiolite. This has long been an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution, but magnetic investigations in the ophiolite have been limited. Analyses presented here involve using: (i) magnetic anisotropies as a proxy for magmatic petrofabrics in lower crustal rocks in order to contribute to outstanding questions regarding the mode of accretion of fast-spread oceanic crust; and (ii) classical palaeomagnetic analyses to determine the nature of magnetization in these rocks and gain further insights into the regional-scale pattern of tectonic rotations that have affected the ophiolite. The extensive layered gabbro sequences exposed in the Semail ophiolite have been sampled at a number of key localities. These are shown to have AMS fabrics that are layer-parallel but also have a regional-scale consistency of the orientation of maximum anisotropy axes. This consistency across sites separated by up to 100 km indicates large-scale controls on fabric development and may be due to consistent magmatic flow associated with the spreading system or the influence of plate-scale motions on deformation of crystal mushes emplaced in the lower crust. Detailed analysis of fabrics in a single layer and across the sampled sections are consistent with either magmatic flow during emplacement of a melt layer into a lower crustal sill complex, or traction/drag of such layers in response to regional-scale stresses (e.g. mantle drag). Together, results support formation of the layered gabbros by injection of melt into sill complexes in the lower crust. New anisotropy data from the overlying foliated gabbros sampled at two key localities also provide insights into the style of melt migration at this crustal level. Fabrics are consistent with either focused or anastomosing magmatic upwards flow through this layer, reflecting melt migration beneath a fossil axial melt lens. Previous palaeomagnetic research in lavas of the northern ophiolitic blocks has demonstrated substantial clockwise intraoceanic tectonic rotations. Palaeomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization. Systematic sampling resolves for the first time a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used previously to infer lack of significant rotation of the southern blocks. In contrast, E/ENE-directed remanences in the uppermost gabbros imply a large, clockwise rotation of the southern blocks, of a sense and magnitude consistent with that inferred from extrusive sections in the northern blocks. Hence, without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times may lead to erroneous tectonic interpretation.

Melting and melt segregation are key processes in the geochemical evolution of the Earth. However, mechanism and time scale of melt transport from the source to the surface are still not well understood and are dependent on the grain-scale distribution of melt. A related question is the retention of melt in partially molten regions of the Earths upper mantle. Seismic observations from mid-ocean ridges (MOR) and subduction zones are interpreted to show in-situ melt contents up to 3%, while geochemical observations from MOR basalts are inferred to indicate very efficient extraction of melt (porosities of order 0.1%). Earlier theoretical models of the melt distribution were based on the balance of surface tension between melt and uniform crystalline grains, predicting a simple net- work of melt along three-grain edges. Analyses of experimentally produced samples of olivine and basaltic melt show that the melt geometry is much more complex, and includes wetted two-grain boundaries. I reconstructed the melt geometry of two experimentally produced samples by serial sectioning and 3-D rendering of the pore geometry which demonstrates for the first time that melt exists in thin layers on two-grain boundaries. This confirms the inferences from previous 2-D observations and has significant implications for physical properties of partially molten regions, for example seismic velocities and attenuation. The wetted two-grain boundaries are inferred to be a consequence of continuous grain growth. Due to the complexity of the 3-D melt geometry the perme- ability of partially molten rocks can not be predicted from simple models. I therefore investigated the permeability as a function of porosity for both synthetic and ex- perimentally determined pore geometries using a lattice-Boltzmann method. The calculated permeability is not a simple function of porosity, but increases rapidly at a critical fraction of wetted two-grain boundaries. In order to extrapolate the experimentally based findings to grain sizes expected in natural rocks I examined the geometry of secondary phases inferred to represent relict melt in mantle peridotites from the Krivaja massif in Bosnia. These findings corroborate the experimental observations of wetted two-grain boundaries.