Conduit Dynamics in Transitional Rhyolitic Activity Recorded by Tuffisite Vein Textures from the 2008–2009 Chaitén Eruption

The mechanisms of hazardous silicic eruptions are controlled by complex, poorly-understood conduit processes. Observations of recent Chilean rhyolite eruptions have revealed the importance of hybrid activity, involving simultaneous explosive and effusive emissions from a common vent. Such behavior hinges upon the ability of gas to decouple from magma in the shallow conduit. Tuffisite veins are increasingly suspected to be a key facilitator of outgassing, as they repeatedly provide a transient permeable escape route for volcanic gases. Intersection of foam domains by tuffisite veins appears critical to efficient outgassing. However, knowledge is currently lacking into textural heterogeneities within shallow conduits, their relationship with tuffisite vein propagation, and the implications for fragmentation and degassing processes. Similarly, the magmatic vesiculation response to upper conduit pressure perturbations, such as those related to the slip of dense magma plugs, remains largely undefined. Here we provide a detailed characterization of an exceptionally large tuffisite vein within a rhyolitic obsidian bomb ejected during transitional explosive-effusive activity at Chaitén, Chile in May 2008. Vein textures and chemistry provide a time-integrated record of the invasion of a dense upper conduit plug by deeper fragmented magma. Quantitative textural analysis reveals diverse vesiculation histories of various juvenile clast types. Using vesicle size distributions, bubble number densities, zones of diffusive water depletion, and glass H2O concentrations, we propose a multi-step degassing/fragmentation history, spanning deep degassing to explosive bomb ejection. Rapid decompression events of ~3–4 MPa are associated with fragmentation of foam and dense magma at ~200–360 m depth in the conduit, permitting vertical gas and pyroclast mobility over hundreds of meters. Permeable pathway occlusion in the dense conduit plug by pyroclast accumulation and sintering preceded ultimate bomb ejection, which then triggered a final bubble nucleation event. Our results highlight how the vesiculation response of magma to decompression events is highly sensitive to the local melt volatile concentration, which is strongly spatially heterogeneous. Repeated opening of pervasive tuffisite vein networks promotes this heterogeneity, allowing juxtaposition of variably volatile-rich magma fragments that are derived from a wide range of depths in the conduit. This process enables efficient but explosive removal of gas from rhyolitic magma and creates a complex textural collage within dense rhyolitic lava, in which neighboring fused clasts may have experienced vastly different degassing histories

Spatially heterogeneous argon-isotope systematics and apparent ⁴⁰Ar/³⁹Ar ages in perlitised obsidian

In situ laser ablation Ar-isotope analyses of variably hydrated and devitrified obsidian from the ~ 27 Ma Cochetopa Dome, San Juan, USA, reveal complex interplay between degassing of initial Ar and absorption of atmospheric Ar. These processes have locally modified the Ar-isotope composition of the obsidian and led to spurious, spatially-heterogeneous Ar-isotope and 40Ar/39Ar age data. Small perlite beads exhibit older apparent Ar-ages at the rims than the cores. This is interpreted as an apparent excess of 40Ar at the rims, produced either by a) diffusion of excess 40Ar into the bead during flushing of the lava with excess 40Ar-bearing volcanic gas, or by b) isotopic fractionation during degassing of initial Ar, causing preferential loss of 36Ar over 40Ar at the bead rims. The second interpretation is favoured by a relative enrichment of 36Ar in the core of a perlite bead along a microlite-free (poorly degassed) flow band, and by a lack of age variation in a larger, fresh, well-degassed perlite bead. These isotopic gradients were later overprinted during glass hydration by absorption of Ar with near-atmospheric composition, resulting in elevated 36Ar and reduced radiogenic 40Ar* yields at the rims of perlite beads. These complex interactions essentially represent the mixing of three distinct Ar reservoirs: initial trapped Ar that may or may not be fractionated, an isotopically atmospheric Ar component introduced during hydration, and radiogenic 40Ar*. Such reservoir mixing is the underlying reason for poor correlations on isotope correlation diagrams and the difficulties in validating the composition of the non-radiogenic Ar component. We thus suggest that high 36Ar yields are a combination of the incomplete degassing of initial (possibly magmatic) Ar and the gain of Ar during interaction between the obsidian and meteoric/atmospheric fluids. Our analyses emphasise the challenging nature of 40Ar/39Ar dating obsidian samples, but also point to possible solutions by careful sample characterisation and selection of highly degassed samples

Comparative field study of shallow rhyolite intrusions in Iceland:emplacement mechanisms and impact on country rocks

Shallow silicic intrusions are known to exist in many active volcanoes and can fuel both eruptions and hydrothermal fields. However, our knowledge of magma intrusions remains far from complete, and processes occurring at intrusion margins are poorly understood. In this field-based study, we characterise four shallows, dissected rhyolitic intrusions at three sites in Iceland (Njarðvík-Dyrfjöll, Krafla and Húsafell central volcanoes). We focus on the relationship between intrusion emplacement mechanisms and country rock response, employing scanline mapping of fractures and in-situ rock property measurements (hardness and permeability) along transects from the intrusion margins to damaged and undamaged country rocks.We identify various scenarios of shallow intrusion emplacement style, based upon their diverse geometry and lithofacies architecture. Additional information from rock properties and characteristics of fractures and vesicles, indicates that initial country rock properties strongly influence the emplacement style. We identify two discrete types of country rock response to magma injection. The matrix permeability of weak, porous and permeable lithologies (conglomerate and hyaloclastite) is reduced by >1 order of magnitude adjacent to intrusions due to pore occlusion. Stronger and denser, low-permeability lithologies (basalt and welded ignimbrite) undergo a decrease in hardness by a factor >2 related to an up to fivefold increase in fracture density, with no significant change in matrix permeability.Our observations highlight the importance of robust characterisation of the mechanical properties of caldera-filling or geothermal reservoir formations, for appropriate forecasting of magma mobility, geophysical data interpretation, and geothermal resources characterisation

Insights into the shallow intrusion of rhyolite from field, textures, and drilling the IDDP-1 magma well, Krafla, Iceland.

This thesis focusses on shallow intrusions of rhyolite, in the context of heat resources for geothermal development. Rhyolite is the dominant sub-surface magma type at the roots of some of the most vigorous geothermal systems worldwide, including Krafla, in Iceland, and the Taupo Volcanic Zone, in New Zealand. Supercritical fluids located in deeper and hotter environments than those conventionally extracted provide more efficient energy production and greater power output. As potential reservoir, the margins of magma chambers constitute an enticing new target. The Iceland Deep Drilling Project consortium accidentally drilled magma in 2009 at Krafla, at 2 km depth. This IDDP-1 well was the hottest geothermal well worldwide, with high productive capability (35 MWe). Based on this experience, a new consortium, the Krafla Magma Testbed (KMT), aims to drill back into this magma body for geothermal exploration, creation of research opportunities and monitoring. Improved scientific knowledge of the targeted reservoir is critical to the success of the project, and is the topic of this thesis. The main objectives of this work are (1) to characterize rock properties at the margins of shallow rhyolite intrusions, specifically around the active Krafla rhyolite magma body, (2) to constrain the size and structure of this intrusion, and (3) to better understand magma response to drilling. Results are obtained via four complementary approaches: Firstly, fossil rhyolite intrusions in Iceland are used as a macro-scale analogy of rock properties around active intrusions. Four field case studies are compared to investigate the host rock response to intrusion, with field measurement of rock strength, permeability and fracture density, as well as textural and lithological information on the intrusions. Secondly, chilled margins thickness and the extent of impacted host rocks of a further 14 intrusions that emplaced at <1 km depth and are <1 km thick are compared to a numerical model of heat transfer by thermal diffusion. Deviation from the heat diffusion model is discussed to determine the dominant processes of heat exchange at the magma/host rocks interface. Thirdly, insights specific to the active rhyolite body at Krafla are gained through micro-scale characterisation of magmatic glass and host felsite drilling particles. These were retrieved from the IDDP-1 well as a time-series, after the drilling bit encountered magma. Analyses of texture and geochemistry are combined with previous results from the literature to examine possible scenarios of magma genesis, storage and reaction to drilling. Fourthly, drilling logs recorded during the IDDP-1 drilling are examined to interpret lithology and rock mass conditions in the blind zone at the chamber margin, from which no cuttings were returned to the surface. Monitored drilling parameters are used to identify rock masses in terms of relative strength, fracture properties and permeability. The field results reveal that the impact of shallow rhyolite intrusions on surrounding host rocks is dependent on the initial host rocks properties, which in turn influence the style of magma propagation. Initially weak, porous and highly-permeable conglomerate and hyaloclastite respond by pore occlusion, with permeability decreasing accordingly. Their ductile compaction accommodates magma propagation, allowing irregular intrusive geometries to form. In contrast, initially strong, low-permeability basaltic lava and welded ignimbrite respond by brittle deformation, with intense fracturing associated with weakening. Rhyolitic intrusions in Iceland have variably sized and fractured chilled margins, with a maximum possible thickness of ~7 m. The aureole width in host rocks increases with increasing intrusion size, but aureole formation around < 10 m wide intrusions requires more rapid heat transfer than provided by conduction alone. For these small intrusions, intense fracture development in chilled margins could allow fluid advection and convection to significantly boost the heat transfer. At Krafla, isotopic compositions and thermodynamic analysis suggest that the rhyolite melts formed by partial melting of felsite possibly triggered by the heat released from a shallow magma intrusion, and promoted by hydrothermal circulation. The presence of lenses of distinct magma composition and texture does not support intense magma convection. The main brown melt generated by high degree felsite partial melting is overlain by a clear melt formed by low- degree partial melting, and which is richer in silica, crystals, and vesicles. As the drill bit approached the magma, it encountered a ~14.5 m thick weak zone, where permeability have been enhanced by thermal cracking of hot rocks in contact with cold drilling fluids. Below this, a stronger ~8 m thick lithology, possibly damaged by the intrusion, separated the magma from the overlying hydrothermal system. Magma then responded to the encounters by limited bubble growth, deformation and nucleation, and caused systematic and consistent reaction of the drilling parameters, revealing it was approached nine times over three drilling attempts

La réécriture filmique des contes de fées : Jean Cocteau <i>La Belle et la Bête</i>, Jacques Demy <i>Peau d'âne</i>, Tim Burton <i>Edward aux mains d'argent</i>

Ce mémoire de recherche de Master Lettres et Arts s'attache à décrire les liens entretenus entre trois films de la deuxième moitié du XXe siècle et leurs sources littéraires apparentes, soit des contes de fées du XVIIe et XVIIIe siècles. Il s'interroge sur ce qui motive de telles réécritures, alors que l'hypertexte est déjà daté de plusieurs siècles et que l'adaptation des contes de fées a déjà fait l'objet des premiers temps du cinéma. Il s'agit alors de voir comment les réalisateurs échappent, contre toute apparence, à l'écueil de la redondance et d'une esthétique passéiste pour créer un style parfaitement original. Une étude des autres influences des réécritures, issues de l'art plus globalement comme du contexte socio-historique, sert de point d'ancrage à la démonstration de la résolue modernité des films. Cette démonstration s'appuie également sur de nombreuses analyses esthétiques des trois films, croisées parfois avec des commentaires précis sur l'œuvre source. Cette comparaison permet alors de souligner les limites de la fidélité de l'adaptation et la façon dont les réalisateurs réutilisent le genre du conte afin de servir une esthétique personnelle

Effets des variations démographiques sur la structure génétique de populations, dans le cadre d'une maladie émergente

The demography and genetic structure of a population are closely linked. The study of this interplay is crucial, especially for organisms with frequent demographic fluctuations such as pathogen species responsible for emerging diseases. Classical population genetics models have been used to explore this link for simplified demographic processes. In this thesis, I further investigate the interplay between demography and genetic evolution in pathogen species that display complex life cycles. For this purpose, I focus on two ecological systems that strongly deviate from demographic equilibrium, each of which allows for realistic modelling assumptions. The first ecological system focuses on a major selection event with the temporal dimension being of prime importance. The second system reports recurrent colonisation events in which the spatial aspect is decisive.In the first part, I study an event of resistance overcoming by a pathogen population. Firstly, a modelling approach is used to establish the conditions for observing resistance overcoming and to identify the determinants of resistance durability. The results highlight the antagonistic effect of the proportion of resistant hosts deployed in the agricultural landscape, which decreases the probability of overcoming but increases the speed of overcoming when it occurs. Secondly, this model is implemented to account for genetic evolution at neutral loci. The results identify three demographic scenarios associated with distinct genetic signatures during resistance overcoming: 1) small variations in population sizes and small changes in genetic structures, 2) a strong founder event on the resistant host that in turn creates a genetic structure on the susceptible host, and 3) an evolutionary rescue event that results in a strong founder event on the resistant host, preceded by a bottleneck on the susceptible host. Finally, this theoretical framework of demogenetic analysis is applied to empirical data to infer the parameters underlying the overcoming of resistant RMlp7 poplars by the pathogen Melampsora larici-populina. Two parameters are particularly well estimated and the inferred values are in agreement with our biological knowledge: a high proportion of resistant hosts in the landscape (more than 80%) and an initial frequency of virulent alleles in the pathogen population between 5 and 10%.In the second part, I study colonisation and its genetic consequences. These analyses focus on the recurrent invasion of the Durance River valley by Melampsora larici-populina. Firstly, a mechanistic-statistical model is coupled to epidemiological data to infer the parameters underlying the pathogen's expansion dynamics. This approach shows that colonisation results from frequent long-distance dispersal events, with an average dispersal distance of more than two kilometres. Secondly, the characterisation of several annual colonisation events highlights a similar genetic structure which amplitude however varies greatly between years. Two extremes are identified: in 2011, strong conservation of the initial genetic diversity along the colonised domain; in 2004, rapid erosion of genetic diversity. The inter-annual variations in these structures can be explained by variations in the frequency of long-distance dispersal events.This work highlights the importance of contextualised models that take into account demogenetic variations for a better understanding of biological systems. The information obtained is then crucial for developing relevant control strategies against pathogen populations responsible for emerging diseases.La démographie et la structure génétique d'une population sont étroitement liées. L’étude de ce lien est cruciale, ce d’autant plus pour les organismes présentant des fluctuations démographiques fréquentes tels que les agents pathogènes responsables de maladies émergentes. Les modèles classiques de génétique des populations ont permis d’explorer ce lien pour des processus démographiques simplifiés. Dans cette thèse, j'examine de manière plus approfondie l'interaction entre la démographie et l'évolution génétique de populations d'agents pathogènes aux cycles de vie complexes. Pour ce faire, deux situations écologiques hors équilibre démographique sont étudiées, chacune permettant de poser des hypothèses de modélisation réalistes. L’une présente un évènement majeur de sélection où l’aspect temporel est primordial. L’autre présente des évènements récurrents de colonisation où l’aspect spatial est déterminant.Dans une première partie, je m'intéresse à l'étude d'un contournement de résistance par une population d'agents pathogènes. En premier lieu, une approche par modélisation permet d’établir les conditions d’observation d’un contournement et d'identifier les déterminants de la durabilité d’une résistance. Les résultats mettent en évidence l'effet antagoniste de la proportion d'hôtes résistants déployés dans le paysage agricole, qui diminue la probabilité de contournement mais augmente la vitesse du contournement lorsqu'il a lieu. Ce modèle est ensuite complété par la prise en compte de l'évolution génétique aux loci neutres. Les résultats permettent d'identifier trois scénarios démographiques associés à des signatures génétiques distinctes lors du contournement : 1) de faibles variations de tailles de populations et peu d'évolution de la structure génétique, 2) un événement fondateur fort sur l’hôte résistant qui crée en retour une structure sur l’hôte sensible et 3) un événement de sauvetage évolutif qui se traduit par un fort événement de fondation sur l’hôte résistant, précédé par un goulot d'étranglement sur l’hôte sensible. Enfin, ce cadre théorique d'analyse démogénétique est appliqué à des données empiriques pour inférer les paramètres régissant le contournement des peupliers résistants RMlp7 par l'agent pathogène Melampsora larici-populina. Deux paramètres sont particulièrement bien estimés et en accord avec nos connaissances biologiques : une forte proportion d'hôtes résistants dans le paysage (plus de 80 %) et une fréquence initiale d'allèles virulents dans la population d'agents pathogènes entre 5 et 10 %.Dans une seconde partie, je m'intéresse à l'étude de la colonisation et ses conséquences génétiques. Ces analyses portent sur un système écologique particulier, l’invasion récurrente de la vallée de la Durance par Melampsora larici-populina. Premièrement, un modèle mécanico-statistique est couplé aux données de suivi épidémiologique afin d'inférer les paramètres régissant la dynamique d'expansion de l’agent pathogène. Cette approche montre que la colonisation résulte de fréquents évènements de dispersion à longue distance, pour une distance moyenne de dispersion de plus de 2 km. Deuxièmement, la caractérisation de plusieurs évènements annuels de colonisation montre une structure génétique similaire mais qui varie fortement en amplitude d'une année sur l'autre. Deux extrêmes sont identifiés : en 2011, une forte conservation de la diversité génétique initiale le long du domaine colonisé ; en 2004, une érosion rapide de la diversité génétique. Les variations inter-annuelles de ces structures peuvent être expliquées par des variations dans la fréquence des évènements de dispersion à longue distance.Ce travail permet de discuter l’importance de modèles contextualisés et prenant en compte les variations démogénétiques dans la compréhension des systèmes biologiques. Les informations obtenues sont déterminantes pour le renforcement des stratégies de lutte contre les agents pathogènes responsables de maladies émergentes

Sainte Élisabeth de Hongrie

SignaturizadoAntepPort. con anagrama da orde dos Jesuita

Influence of demographic variations on the genetic structure of populations in the context of emerging disease

La démographie et la structure génétique d'une population sont étroitement liées. L’étude de ce lien est cruciale, ce d’autant plus pour les organismes présentant des fluctuations démographiques fréquentes tels que les agents pathogènes responsables de maladies émergentes. Les modèles classiques de génétique des populations ont permis d’explorer ce lien pour des processus démographiques simplifiés. Dans cette thèse, j'examine de manière plus approfondie l'interaction entre la démographie et l'évolution génétique de populations d'agents pathogènes aux cycles de vie complexes. Pour ce faire, deux situations écologiques hors équilibre démographique sont étudiées, chacune permettant de poser des hypothèses de modélisation réalistes. L’une présente un évènement majeur de sélection où l’aspect temporel est primordial. L’autre présente des évènements récurrents de colonisation où l’aspect spatial est déterminant.Dans une première partie, je m'intéresse à l'étude d'un contournement de résistance par une population d'agents pathogènes. En premier lieu, une approche par modélisation permet d’établir les conditions d’observation d’un contournement et d'identifier les déterminants de la durabilité d’une résistance. Les résultats mettent en évidence l'effet antagoniste de la proportion d'hôtes résistants déployés dans le paysage agricole, qui diminue la probabilité de contournement mais augmente la vitesse du contournement lorsqu'il a lieu. Ce modèle est ensuite complété par la prise en compte de l'évolution génétique aux loci neutres. Les résultats permettent d'identifier trois scénarios démographiques associés à des signatures génétiques distinctes lors du contournement : 1) de faibles variations de tailles de populations et peu d'évolution de la structure génétique, 2) un événement fondateur fort sur l’hôte résistant qui crée en retour une structure sur l’hôte sensible et 3) un événement de sauvetage évolutif qui se traduit par un fort événement de fondation sur l’hôte résistant, précédé par un goulot d'étranglement sur l’hôte sensible. Enfin, ce cadre théorique d'analyse démogénétique est appliqué à des données empiriques pour inférer les paramètres régissant le contournement des peupliers résistants RMlp7 par l'agent pathogène Melampsora larici-populina. Deux paramètres sont particulièrement bien estimés et en accord avec nos connaissances biologiques : une forte proportion d'hôtes résistants dans le paysage (plus de 80 %) et une fréquence initiale d'allèles virulents dans la population d'agents pathogènes entre 5 et 10 %.Dans une seconde partie, je m'intéresse à l'étude de la colonisation et ses conséquences génétiques. Ces analyses portent sur un système écologique particulier, l’invasion récurrente de la vallée de la Durance par Melampsora larici-populina. Premièrement, un modèle mécanico-statistique est couplé aux données de suivi épidémiologique afin d'inférer les paramètres régissant la dynamique d'expansion de l’agent pathogène. Cette approche montre que la colonisation résulte de fréquents évènements de dispersion à longue distance, pour une distance moyenne de dispersion de plus de 2 km. Deuxièmement, la caractérisation de plusieurs évènements annuels de colonisation montre une structure génétique similaire mais qui varie fortement en amplitude d'une année sur l'autre. Deux extrêmes sont identifiés : en 2011, une forte conservation de la diversité génétique initiale le long du domaine colonisé ; en 2004, une érosion rapide de la diversité génétique. Les variations inter-annuelles de ces structures peuvent être expliquées par des variations dans la fréquence des évènements de dispersion à longue distance.Ce travail permet de discuter l’importance de modèles contextualisés et prenant en compte les variations démogénétiques dans la compréhension des systèmes biologiques. Les informations obtenues sont déterminantes pour le renforcement des stratégies de lutte contre les agents pathogènes responsables de maladies émergentes.The demography and genetic structure of a population are closely linked. The study of this interplay is crucial, especially for organisms with frequent demographic fluctuations such as pathogen species responsible for emerging diseases. Classical population genetics models have been used to explore this link for simplified demographic processes. In this thesis, I further investigate the interplay between demography and genetic evolution in pathogen species that display complex life cycles. For this purpose, I focus on two ecological systems that strongly deviate from demographic equilibrium, each of which allows for realistic modelling assumptions. The first ecological system focuses on a major selection event with the temporal dimension being of prime importance. The second system reports recurrent colonisation events in which the spatial aspect is decisive.In the first part, I study an event of resistance overcoming by a pathogen population. Firstly, a modelling approach is used to establish the conditions for observing resistance overcoming and to identify the determinants of resistance durability. The results highlight the antagonistic effect of the proportion of resistant hosts deployed in the agricultural landscape, which decreases the probability of overcoming but increases the speed of overcoming when it occurs. Secondly, this model is implemented to account for genetic evolution at neutral loci. The results identify three demographic scenarios associated with distinct genetic signatures during resistance overcoming: 1) small variations in population sizes and small changes in genetic structures, 2) a strong founder event on the resistant host that in turn creates a genetic structure on the susceptible host, and 3) an evolutionary rescue event that results in a strong founder event on the resistant host, preceded by a bottleneck on the susceptible host. Finally, this theoretical framework of demogenetic analysis is applied to empirical data to infer the parameters underlying the overcoming of resistant RMlp7 poplars by the pathogen Melampsora larici-populina. Two parameters are particularly well estimated and the inferred values are in agreement with our biological knowledge: a high proportion of resistant hosts in the landscape (more than 80%) and an initial frequency of virulent alleles in the pathogen population between 5 and 10%.In the second part, I study colonisation and its genetic consequences. These analyses focus on the recurrent invasion of the Durance River valley by Melampsora larici-populina. Firstly, a mechanistic-statistical model is coupled to epidemiological data to infer the parameters underlying the pathogen's expansion dynamics. This approach shows that colonisation results from frequent long-distance dispersal events, with an average dispersal distance of more than two kilometres. Secondly, the characterisation of several annual colonisation events highlights a similar genetic structure which amplitude however varies greatly between years. Two extremes are identified: in 2011, strong conservation of the initial genetic diversity along the colonised domain; in 2004, rapid erosion of genetic diversity. The inter-annual variations in these structures can be explained by variations in the frequency of long-distance dispersal events.This work highlights the importance of contextualised models that take into account demogenetic variations for a better understanding of biological systems. The information obtained is then crucial for developing relevant control strategies against pathogen populations responsible for emerging diseases