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Le phénomène de dégazage permanent observé sur le volcan Erebus s’accompagne d’une variation cyclique de la composition des gaz et du niveau de son lac de cratère que nous nous sommes proposé de modéliser en partant de l’hypothèse que ces fluctuations sont causées par l’arrivée de batch de magma naissant à faible profondeur, ascendant dans un conduit à travers duquel percole un flux continu de gaz dont l’origine est plus profonde. Nous avons tout d’abord montré par simulation numérique que la vigueur de la convection observée en surface ne pouvait être expliquée par la seule convection thermique d’un mélange liquide-cristaux. Si une alimentation continue en magma et gaz dans un système ouvert permet de simuler un comportement pulsatif de la surface, cet apport doit être suffisamment important pour que les changements de vitesse de surface ainsi générés puissent être appréciés. Le conduit doit avoir un diamètre suffisant large pour assurer la pérennité de la convection et maintenir le magma au-dessus de sa température de transition vitreuse. La présence de cristaux accélère la convection et améliore l’efficacité du transfert de chaleur entre les régions inférieures et supérieures du système magmatique ; ces cristaux se déposent dans le fond de la chambre pour former une couche de forte concentration d’une dizaine de mètres d’épaisseur. L’introduction de deux batch de magma d’origines différentes a permis de mettre en évidence combien leur composition et le comportement du lac de lave étaient sensibles à la température à laquelle ces batch sont générés. D’autre part, un batch ne contribuera au budget de dégazage dans des proportions consistantes avec les observations que si un seuil de flottabilité suffisant a été atteint. Dans le cas contraire, la migration d’une quantité de magma nécessaire à ce dégazage ne pourrait se faire que dans un conduit de très grand diamètre. Les paramètres physiques de la croute qui se forme lors du refroidissement de la surface du lac de lave, tels que son épaisseur et sa perméabilité, influent sur sa capacité à se déformer sous la pression qu’elle piège et à permettre un dégazage de type effusif. Cette pression conditionne à son tour la porosité du magma en surface et éventuellement le degré de dégazage du magma qui redescend vers la chambre magmatique. Cette étude nous a permis de mieux appréhender les mécanismes associés à une éruption de régime effusif et constitue une étape dans la compréhension de la transition à un régime explosif, préoccupation majeure des centres de surveillance volcaniques. / The observed phenomenon of permanent degassing of the volcano Erebus is accompanied by a cyclic variation of gas composition and level of its crater lake that we proposed to model in the assumption that these fluctuations are caused by the arrival of batch of magma originated at shallow depth, ascending in a conduit through which percolates a continuous flow of gas whose origin is deeper. We first showed that the observed effect of convection on the surface could not be explained solely by thermal convection of a liquid-crystals mixture. If a continuous supply of magma and gas in an open system simulates a pulsating behavior of the surface, this contribution must be large enough for the changes in surface velocity thus generated can be appreciated. The conduit diameter should be sufficient large to ensure the sustainability of convection and maintain the magma above its glass transition temperature. The presence of crystals convection accelerates and improves the efficiency of heat transfer between the upper and lower regions of the magmatic system; these crystals settle at the bottom of the chamber to form a layer of high concentration of about ten meters thickness. The introduction of two batches of magma from different depths of nucleation helped to highlight how their composition and the behavior of the lava lake were sensitive to the temperature at which they are generated. On the other hand, a batch will contribute to the degassing budget in proportions consistent with the observations only if a threshold of sufficient buoyancy was achieved. Otherwise, the migration of the amount of magma required for the degassing will only occur in a conduit of very large diameter. The physical parameters of the crust forming as the surface of the lava lake cools down, such as thickness and permeability, affect its ability to deform under pressure and to allow the trapped pressure to degas under an effusive regime. This pressure in turn influences the porosity of the magma at surface and possibly the degree of degassing of the magma being recirculated toward the magma chamber. This study allowed us to better understand the mechanisms associated with an effusive eruption regime and is a step in understanding the transition to an explosive regime, a major concern for the volcanic monitoring centers.

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Though continental magmatic arcs are factories for new continental crust, a significant proportion of continental arc magmas are recycled from supracrustal material. To evaluate the relative contributions of retroarc underthrusting and trench side partial sediment subduction for introducing supracrustal rocks to the middle and lower crust of continental magmatic arcs, we present results from the deeply exposed country rocks of the Coast Mountains batholith of western British Columbia. Prior work demonstrates that these rocks underwent widespread partial melting that contributed to the Coast Mountains batholith. We utilize U-Pb zircon geochronology, Sm-Nd thermochronology, and field-based studies to document the protoliths and early burial history of amphibolite and granulite-facies metasedimentary rocks in the Central Gneiss Complex. U-Pb detrital zircon data from the structurally highest sample localities yielded similar to 190Ma unimodal age peaks and suggest that retroarc rocks of the Stikine terrane constitute a substantial portion of the Central Gneiss Complex. These supracrustal rocks underwent thrust-related burial and metamorphism at >25km depths prior to similar to 80Ma. These rocks may also be underlain at the deepest exposed structural levels by Upper Cretaceous metasedimentary rocks, which may have been emplaced as a result of trench side underplating or intraarc burial. These results further our understanding of the mechanisms of material transport within the continental lithosphere along Cordilleran subduction margins.

<p> The Proterozoic' Coldwell Alkaline Complex is the southernmost intrusion of a number of N -S trending igneous bodies occurring in the Midcontinental Rift system exposed in the Lake Superior area. The Coldwell complex is host to several Ni-Cu-PGEbearing intrusions two of which; the Two-Duck Lake intrusion (Marathon deposit) and the Geordie Lake gabbro (MacRae occurrence) have been investigated in some detail with respect to PGE mineralisation. Both of these have been suggested to have experienced crustal contamination in conjunction with mineralisation. As a test of this possibility, a detailed Sm-Nd, oxygen isotope, and whole-rock geochemical study of these mineralised occurrences as well as of the Dunlop occurrence and the Middleton occurrence, together with unmineralised rocks of the complex was undertaken. The primary objectives are to determine whether crustal contamination is indicated in mineralised rocks and to try and ascertain the nature of the magma which formed the complex. </p> <p> The Coldwell complex is thought to have been formed by emplacement of magma at three intrusive centres. Sm-Nd data for rocks from these three centres reveal similar isotopic values, with slight variations; samples taken from the western gabbros exhibit eNd values averaging -0.9 ranging from -2.9 to 0.9, whereas rocks from the eastern margin and centre of the complex have eNd values of about an average of 0.5 ranging from -0.5 to 1.2 suggesting that the magma that formed these rocks has undergone a lesser degree of crustal contamination. This data, supported by oxygen isotope and wholerock geochemical information indicates that crustal contamination seems to play a small, and varied role in the genesis of the Coldwell magmas. The Nd isotope data all clusters at values for CHUR, which indicates that it has been enriched relative to the depleted mantle. It has been postulated that an enriched mantle plume resided under the rift and promoted rift-related magmatism. The data from this study would seem to support this supposition. </p> <p> Geochemical parameters utilised to define fields to geochemically delineate possible end member contributors to this primarily plume-derived magma indicate, that in addition to small, variable amounts of assimilation of upper and lower crust, the plume magmas also interacted with the lithospheric upper mantle to a small degree. </p> / Thesis / Master of Science (MSc)

Layered mafic intrusions represent an important aspect of magmatism on earth and have occurred from Archean to present times. Literature on the geochemistry and petrology of these intrusions abounds but their physical properties, which could provide significant constraints on their formation, have seldom been investigated. Classic petrological methods such as whole-rock geochemistry, textural analysis and mineral chemistry have been applied to several intrusions of various ages. Most of these methods are relatively expensive or time intensive which limits high resolution studies. On the contrary, magnetic methods are typically inexpensive and fast and have been successfully applied to various occurrences of mafic rocks. In this study, several magnetic methods have been applied to a 600 m-long continuous borehole core drilled through one of the world's largest layered mafic intrusion, the Great Dyke of Zimbabwe. The main goal of this study is to constrain the magmatic history of the intrusion. More specifically, it is important to determine if the intrusion functioned as an open system, characterized by multiple magma pulses, or as a closed system, undergoing differentiation after a single magmatic pulse. The magnetic methods have also been validated by other independent approaches including image analysis, and electron microprobe. This study demonstrates that magnetic methods can be used to rapidly obtain critical information on the internal structure of this type of intrusion before applying more costly chemical analyses. The main scientific result of this study is to document the closed system nature of the Great Dyke of Zimbabwe, at least throughout the sequence investigated.

Silicate melt inclusions are ubiquitous in quartz phenocrysts, yet there are few studies of such inclusions from porphyry copper systems. A melt inclusion forms when magma is trapped in a growing phenocryst. If a phenocryst is able to preserve the original parent magma, then accurate information can be obtained for ancient volcanic systems. In recent igneous systems, melt inclusions are commonly preserved as optically clear homogeneous glass representative of magma stored at depth before eruption. Melt inclusions are difficult to recognize in quartz phenocrysts from porphyry copper system because they are crystalline and hidden by exsolved magmatic volatiles. The inclusions range in size from less than 5 to over 150 μm. In order to evaluate the magmatic contribution to economic mineralization, we conducted three separate studies to determine whether or not crystallized melt inclusions preserve representative samples of magma. The first study modeled the phase relationships that occur during equilibrium crystallization and melting of haplogranite magma trapped in quartz. Results from the model are similar to observations made during the heating of crystallized melt inclusions from porphyry copper systems. It is necessary to re-melt the crystal and volatile phases before chemical analysis. Micro-explosions caused by heating resulted in the loss of important chemical components. Our second study evaluated several microthermometric heating procedures using synthetic melt inclusions trapped at conditions similar to those inferred for porphyry copper systems. A synthetic hydrous melt was saturated with saline hydrothermal solutions allowing both melt and aqueous fluids to be trapped in quartz. Based on microthermometric measurements from these coeval melt and aqueous fluid inclusions we were able to predict the known trapping temperature and pressure of formation. This technique can be applied to natural samples to constrain trapping pressures and temperatures. It was found that slower heating rates could be used to avoid overheating and that heating under a confining pressure greatly minimizes the decrepitation of inclusions. The third study examined the copper concentrations in melt inclusions from the Red Mountain, Arizona porphyry copper system. Older andesite magma contains pyroxene with melt inclusions of higher copper concentrations compared to melt inclusions in quartz from quartz latite. The higher water concentrations in crystallized melt inclusions in the quartz, and abundant aqueous fluid inclusions indicates that the exsolution of water from the magma occurred prior to the trapping of melt inclusions in quartz. The lower water concentrations and the absence of aqueous fluid inclusions indicates that the andesite never reached the stage of water exsolution. The results obtained here are consistent with models that suggest that copper is extracted from the melt by saline magmatic fluids, producing a metal-charged hydrothermal solution and leaving behind a metal-depleted melt and serves to identify the potential contribution of melt inclusion studies to constrain the origin of ore metals in porphyry copper deposits. / Ph. D.