In recent decades there has been an increased interest in the carbon content of Earth’s geochemical reservoirs due to the impact of atmospheric carbon on the habitability of our planet. Earth’s interior likely hosts a greater mass of carbon than that of the oceans, atmosphere and crust combined, which has buffered the carbon content of the atmosphere over geological time. Yet only a few direct measurements of carbon from the upper mantle, and none from the lower mantle, have been made. Undegassed basalts erupted at mid-ocean ridges have previously been used to estimate the carbon content of the upper mantle. However, due to the low solubility of carbon within silicate melt, these undegassed basalt suites are rare. The majority of basalts have lost their mantle carbon information en route to eruption through the crust. Various crustal processes act to modify the geochemistry of melts before eruption, therefore it is important to be able to characterise the effect of these processes to better interpret the volatile signals preserved in erupted products. Pressure, and therefore depth, is a key parameter controlling volatile solubility and can be estimated using a variety of igneous barometers. This thesis presents results from crys- tallisation experiments conducted on basaltic glass from the Miðfell eruption, Iceland. The experiments provide new data that has been used to test a variety of barometers and crystalli- sation models used by igneous petrologists, and could aid future barometer recalibration. A key part of this work was the development of an experimental method for stabilising 5 kbar conditions in a piston cylinder apparatus. The experiments have shown that clinopyroxene- liquid barometry is more reliable than multi-reaction barometry. However, knowledge of equilibrium clinopyroxene compositions is crucial for accurately determining pressure using the clinopyroxene-liquid barometer. More experiments conducted at mid-crustal pressures are required for a full recalibration of these barometers. The results of testing igneous barometers and crystallisation models have been applied to two suites of olivine-hosted melt inclusions from the Kistufell and Miðfell eruptions to help determine the melt evolution history of these basalts. These eruptions were targeted due to previously measured noble gas isotopic ratios that suggest a primordial mantle component present in their melting regions, and therefore evoking the possibility that they could hold information about deep mantle carbon. Barometry suggests that Miðfell phases equilibrated, and therefore crystallised, at mid-crustal pressures (5–7 kbar), which could allow for the entrapment of undegassed melt inclusions within olivine. The two melt inclusion suites were found to differ in trace element variability, with the observation that the more trace element enriched eruption, Kistufell, had lower relative trace element variability than the more depleted eruption, Miðfell. Several processes, both in the crust and the mantle, are likely responsible for the level of trace element enrichment and variability, including extent of mantle melting, source heterogeneity, and melt transport. The depleted nature of the Miðfell melt inclusions has allowed them to preserve some of the highest CO$_2$/Ba and CO$_2$/Nb ratios ever recorded in basaltic glass, with ratios over five times greater than undegassed mid-ocean ridge basalt values. This carbon enrichment is not due to any crustal melt modification process, but rather pertaining to lower mantle carbon-rich lithologies that have been tapped by the Icelandic mantle plume. The carbon reservoir beneath Miðfell is estimated to contain 744 $\pm$ 188 ppm carbon, 15 times greater than the depleted upper mantle. This value matches estimates of bulk mantle carbon from planetary mass balance calculations and provides evidence for carbon-rich domains within the Earth.

A região sul da placa Sul-Americana, hoje pertencente à região da Patagônia Argentina e Chilena, formou-se por consequência de acreções continentais desde o Proterozóico. Atualmente, a região é caracterizada por um complexo sistema de placas tectônicas, no qual as placas oceânicas de Nazca, Antártica e Scotia interagem diretamente com a placa continental Sul-Americana através dos processos de subducção e transcorrência. Entre as placas de Nazca e Antártica, ocorre a dorsal do Chile, e a subducção desta dorsal sob a placa Sul-Americana forma a Junção Tríplice do Chile, ocorrendo o soerguimento da astenosfera na região. O magmatismo Cenozóico de composição alcalina que ocorre na região da Patagônia Argentina e Chilena hospeda xenólitos mantélicos ultramáficos de classificação espinélio- e granada-peridotitos. Estes xenólitos são de extrema importância para a caracterização e identificação dos processos atuantes no manto superior abaixo dessa complexa região que hoje é a Patagônia. Estudos do sistema isotópico Re-Os nos xenólitos de Prahuaniyeu (41°20’09.4”S, 67°54’08.1”W), e Chenque (43°38’39.3”S, 68°56’22”W), na região norte da Patagônia Argentina, sugerem que a litosfera abaixo de Prahuaniyeu (TRD ~ 1.69 Ga) é mais antiga que Chenque (TRD ~ 0.71 Ga). Dados de Rb-Sr mostram que a litosfera da região norte da Patagônia possui altas razões 87Sr/86Sr (Prahuaniyeu: 0,7037 a 0,7041; Chenque: 0.7037 a 0.7086), devido fluidos relacionados a desidratação de uma placa de subducção. Através destes dados e dos dados geoquímicos, o manto litosférico subcontinental da região norte da Patagônia sofreu metassomatismo relacionado a slabs derivados de antigas placas de subducção e que proporcionou características de metassomatismo por líquidos/fluidos do tipo-OIB, e atualmente sofreu metassomatismo relacionado aos fluidos derivados da desidratação da placa de subducção atual (Nazca), caracterizados pelo enriquecimento em calcófilos. Todos os peridotitos de Laguna Timone (52°01’39” S, 70°12’53” W), no Campo Vulcânico de Pali Aike, região sul da Patagônia Chilena, também apresentam expressivo enriquecimento nos elementos calcófilos sugerindo que o manto litosférico subcontinental da região sul da Patagônia também foi metasomatisado pelos fluidos derivados da desidratação da placa de subducção atual (Antártica). Em Laguna Timone também há a ocorrência de um glimerito entre os xenólitos e a presença de flogopita e pargasita nos peridotitos classificados como gr-sp lherzolitos, sp-lherzolitos e gr-sp harzburgitos. A presença de um glimerito, de peridotitos com minerais hidratados (flogopita e pargasita) e as similaridades com peridotitos metassomatisados por líquidos astenosféricos (peridotitos do distrito de Manzaz, Argélia e do campo vulcânico Vitim, no lago de Baikal, Sibéria) com baixas razões Ba/Nb, Ba/La e U/Nb, indicam que a litosfera da região sul da Patagônia sofreu metassomatismo por fluidos astenosféricos, ocasionado devido o soerguimento da astensofera durante a passagem da Junção Tríplice do Chile pela região de Pali Aike. / The southern of the South-American plate, today is the Chile and Argentina Patagonia region, was formed as a result of continental accretions since the Proterozoic.Currently, this region is characterized for a complex tectonic plates system, in which Nazca, Antartica and Scotia oceanic plates interact directly to the South-American continental plate by subduction and transcorrent process. Between Nazca and Antartica plate occurs the Chile Ridge, and the Chile Ridge subduction under the South-American plate creates the Chile Triple Junction and the upwelling of underlying asthenospheric mantle in this region. The Cenozoic alkali magamtism that occurs in Patagonia Argentina and Chilena hosts ultramafic mantle xenoliths (spinel- and garnet-peridotites). These xenoliths are extremely important to characterization and identification of the processes that occurred in the upper mantle underneath the Patagonia region. The Re-Os isotopic studies in Prahuaniyeu (41°20’09.4”S, 67°54’08.1”W), and Chenque (43°38’39.3”S, 68°56’22”W) xenoliths, in north Patagonia Argentina, suggests the Prahuaniyeu lithosphere (TRD ~ 1.69 Ga) were formed previously to Chenque (TRD ~ 0.71 Ga). Rb-Sr data show high 87Sr/86Sr ratio (Prahuaniyeu: 0.7037 to 0.7041; Chenque: 0.7037 to 0.7086), suggesting interactions with subduction plate dehydration related fluids. Trough this data, and geochemistry data, the sucontinental lithospheric mantle underneath the north Patagonia region suffered two metasomatic events: one related to the OIB-like melt/fluids from slabs derived by ancient subductions; and another related to the fluids derived from the current subducted plate (Nazca) dehydration, characterized by the chalcophiles enrichment. Peridotites from Laguna Timone (52°01’39” S, 70°12’53” W), in the Pali Aike Volcanic Field, southern Patagonia Chilena region, also shows expressive enrichment in chalcophile elements suggesting metasomatism by fluids from currently subduction (Antartica plate). Another kind of metasomatism occurs in subcontinental lithospheric mantle underneath Pali Aike due the glimmerite occurrence, hydrated minerals (phlogopite and pargasite) in peridotites and similarities with peridotites that suffered metasomatism by asthenospheric melts (Manzaz, Argelia peridotites and Vitim Volcanic Field, Baikal, Siberia peridotites), with low Ba/Nb, Ba/La and U/Nb. All these carachteristics suggest that lithosphere suffered interactions between asthenosphere-lithosphere due upwelling of underlying asthenospheric mantle when the Chile Triple Junction was on the same latitude of Pali Aike.

abstract: Boron concentrations and isotopic composition of phlogopite mica, amphibole, and selected coexisting anhydrous phases in mantle-derived xenoliths from the Kaapvaal Craton were measured by secondary ion mass spectrometry in an effort to better understand the B isotope geochemistry of the subcontinental lithospheric mantle (SCLM) and its implications for the global geochemical cycle of B in the mantle. These samples display a wide, and previously unrecognized, range in their boron contents and isotopic compositions reflecting a complex history involving melt depletion and metasomatism by subduction- and plume-derived components, as well as late stage isotopic exchange related to kimberlite emplacements. Micas from ancient lithospheric harzburgite metasomatized by slab-derived fluids suggest extensive B-depletion during subduction, resulting in low-B, isotopically light compositions whereas kimberlite-related metasomatic products and a sample from the 2 Ga Palabora carbonatite have boron isotopic compositions similar to proposed primitive mantle. The results suggest that subduction of oceanic lithosphere plays a limited role in the B geochemistry of the convecting mantle. / Dissertation/Thesis / Masters Thesis Geological Sciences 2014

A região sul da placa Sul-Americana, hoje pertencente à região da Patagônia Argentina e Chilena, formou-se por consequência de acreções continentais desde o Proterozóico. Atualmente, a região é caracterizada por um complexo sistema de placas tectônicas, no qual as placas oceânicas de Nazca, Antártica e Scotia interagem diretamente com a placa continental Sul-Americana através dos processos de subducção e transcorrência. Entre as placas de Nazca e Antártica, ocorre a dorsal do Chile, e a subducção desta dorsal sob a placa Sul-Americana forma a Junção Tríplice do Chile, ocorrendo o soerguimento da astenosfera na região. O magmatismo Cenozóico de composição alcalina que ocorre na região da Patagônia Argentina e Chilena hospeda xenólitos mantélicos ultramáficos de classificação espinélio- e granada-peridotitos. Estes xenólitos são de extrema importância para a caracterização e identificação dos processos atuantes no manto superior abaixo dessa complexa região que hoje é a Patagônia. Estudos do sistema isotópico Re-Os nos xenólitos de Prahuaniyeu (41°20’09.4”S, 67°54’08.1”W), e Chenque (43°38’39.3”S, 68°56’22”W), na região norte da Patagônia Argentina, sugerem que a litosfera abaixo de Prahuaniyeu (TRD ~ 1.69 Ga) é mais antiga que Chenque (TRD ~ 0.71 Ga). Dados de Rb-Sr mostram que a litosfera da região norte da Patagônia possui altas razões 87Sr/86Sr (Prahuaniyeu: 0,7037 a 0,7041; Chenque: 0.7037 a 0.7086), devido fluidos relacionados a desidratação de uma placa de subducção. Através destes dados e dos dados geoquímicos, o manto litosférico subcontinental da região norte da Patagônia sofreu metassomatismo relacionado a slabs derivados de antigas placas de subducção e que proporcionou características de metassomatismo por líquidos/fluidos do tipo-OIB, e atualmente sofreu metassomatismo relacionado aos fluidos derivados da desidratação da placa de subducção atual (Nazca), caracterizados pelo enriquecimento em calcófilos. Todos os peridotitos de Laguna Timone (52°01’39” S, 70°12’53” W), no Campo Vulcânico de Pali Aike, região sul da Patagônia Chilena, também apresentam expressivo enriquecimento nos elementos calcófilos sugerindo que o manto litosférico subcontinental da região sul da Patagônia também foi metasomatisado pelos fluidos derivados da desidratação da placa de subducção atual (Antártica). Em Laguna Timone também há a ocorrência de um glimerito entre os xenólitos e a presença de flogopita e pargasita nos peridotitos classificados como gr-sp lherzolitos, sp-lherzolitos e gr-sp harzburgitos. A presença de um glimerito, de peridotitos com minerais hidratados (flogopita e pargasita) e as similaridades com peridotitos metassomatisados por líquidos astenosféricos (peridotitos do distrito de Manzaz, Argélia e do campo vulcânico Vitim, no lago de Baikal, Sibéria) com baixas razões Ba/Nb, Ba/La e U/Nb, indicam que a litosfera da região sul da Patagônia sofreu metassomatismo por fluidos astenosféricos, ocasionado devido o soerguimento da astensofera durante a passagem da Junção Tríplice do Chile pela região de Pali Aike. / The southern of the South-American plate, today is the Chile and Argentina Patagonia region, was formed as a result of continental accretions since the Proterozoic.Currently, this region is characterized for a complex tectonic plates system, in which Nazca, Antartica and Scotia oceanic plates interact directly to the South-American continental plate by subduction and transcorrent process. Between Nazca and Antartica plate occurs the Chile Ridge, and the Chile Ridge subduction under the South-American plate creates the Chile Triple Junction and the upwelling of underlying asthenospheric mantle in this region. The Cenozoic alkali magamtism that occurs in Patagonia Argentina and Chilena hosts ultramafic mantle xenoliths (spinel- and garnet-peridotites). These xenoliths are extremely important to characterization and identification of the processes that occurred in the upper mantle underneath the Patagonia region. The Re-Os isotopic studies in Prahuaniyeu (41°20’09.4”S, 67°54’08.1”W), and Chenque (43°38’39.3”S, 68°56’22”W) xenoliths, in north Patagonia Argentina, suggests the Prahuaniyeu lithosphere (TRD ~ 1.69 Ga) were formed previously to Chenque (TRD ~ 0.71 Ga). Rb-Sr data show high 87Sr/86Sr ratio (Prahuaniyeu: 0.7037 to 0.7041; Chenque: 0.7037 to 0.7086), suggesting interactions with subduction plate dehydration related fluids. Trough this data, and geochemistry data, the sucontinental lithospheric mantle underneath the north Patagonia region suffered two metasomatic events: one related to the OIB-like melt/fluids from slabs derived by ancient subductions; and another related to the fluids derived from the current subducted plate (Nazca) dehydration, characterized by the chalcophiles enrichment. Peridotites from Laguna Timone (52°01’39” S, 70°12’53” W), in the Pali Aike Volcanic Field, southern Patagonia Chilena region, also shows expressive enrichment in chalcophile elements suggesting metasomatism by fluids from currently subduction (Antartica plate). Another kind of metasomatism occurs in subcontinental lithospheric mantle underneath Pali Aike due the glimmerite occurrence, hydrated minerals (phlogopite and pargasite) in peridotites and similarities with peridotites that suffered metasomatism by asthenospheric melts (Manzaz, Argelia peridotites and Vitim Volcanic Field, Baikal, Siberia peridotites), with low Ba/Nb, Ba/La and U/Nb. All these carachteristics suggest that lithosphere suffered interactions between asthenosphere-lithosphere due upwelling of underlying asthenospheric mantle when the Chile Triple Junction was on the same latitude of Pali Aike.

La subduction est une des principales expressions superficielles de la convection mantellique et représente un ingrédient crucial de la géodynamique globale. Cela affecte différents processus de la Terre comme la génération des méga-tremblements de terre et des volcans explosifs sur la surface ou le recyclage des espèces volatiles dans l’intérieur profond. Malgré son importance, plusieurs aspects de la subduction restent à clarifier.Dans ce travail, nous avons étudié la mécanique et l’énergétique du phénomène en adoptant un modèle numérique 2-D de “subduction libre”, basé sur la méthode des éléments frontière. En interprétant systématiquement nos solutions numériques utilisant la théorie des couches minces visqueuses, nous avons déterminé diverses lois d’échelle décrivant les mécanismes physiques sous-jacents aux différents aspects du phénomène. Deux paramètres adimensionnels se distinguent par leur récurrence dans ces lois d’échelle: i) la résistance (adimensionelle) de l’interface de subduction, qui contrôle la contrainte de cisaillement agissant à l’interface entre les deux plaques et ii) la rigidité de la plaque en subduction, qui décrit la résistance mécanique opposée par cette plaque à la flexion. Ce dernier paramètre est particulièrement important, car il met en évidence l’échelle de longueur qui décrit correctement la déformation en flexion de la plaque en subduction (bending length).En ce qui concerne les aspects énergétiques de la subduction, nous avons également étudié l’effet de la dissipation de l’énergie produite dans les zones de subduction sur la convection du manteau à grande échelle. Nos résultats semblent suggérer que la loi d’échelle classique trouvée dans l’étude de la convection de Rayleigh-Bénard en régime permanent d’une couche de fluide isovisqueux reste généralement valable aussi pour la convection du manteau terrestre.Pour conclure, nous avons mis en place une expérience de convection basée sur le séchage d’une suspension colloidale de nanoparticules de silice. Comme les résultats préliminaires ont montré, grâce à sa rhéologie particulière, ce matériau semble être un candidat prometteur pour la modélisation de la convection mantellique en laboratoire. / Subduction is one of the principal surface expressions of mantle convection and it represents a key ingredient of global geodynamics. It affects Earth processes ranging from the generation of mega-earthquakes and explosive volcanoes at thesurface to the recycling of volatile species back into the deep interior. Yet despite its obvious importance, several aspects of subduction remain to be clarified.In this work we endeavored to shed light on the mechanics and the energetics of the phenomenon adopting of a 2-D numerical model of “free subduction” based on the Boundary-Element Method. Systematically interpreting our numerical solutions in the light of thin viscous-sheet theory, we determined various scaling laws describing the physical mechanisms underlying different aspects of the phenomenon. Two dimensionless parameters stand out for their recurrence in suchscaling laws: i) the (dimensionless) strength of the subduction interface, which controls the shear stress acting at the interface between the two plates and ii) the flexural stiffnes of the subducting plate, which describes the mechanical resistance opposed by such plate to bending. This latter parameter is particularly important as it highlights the length scale that properly describes the bending deformation of the subducting plate (bending length).For what concerns the energetics of subduction, we also investigated the effect of the dissipation of energy occurring at subduction zones on large-scale mantleconvection. Our results seem to suggest that the classical scaling law found in the study of the steady-state Rayleigh-Bénard convection of an isoviscous fluid layer remains generally valid also for Earth’s mantle convection.To conclude, we ran a convection experiment based on the drying of a colloidal suspension of silica nanoparticles. As preliminary results have shown, thanksto its particular rheology, this material seems to be a promising candidate for effective laboratory modeling of mantle convection.

The Saglek-Hebron Gneiss Complex (SHC) is located in Northern Labrador within the Nain Province. It has recorded multiple magmatic events over more than 1 billion years, making it ideal to study the evolution of mantle-derived rocks through time. Here we present a 147Sm-143Nd isotopic study focussing on the different generations of mantle-derived rocks in the SHC. A total of 83 samples have been analysed, including: 1) mafic metavolcanic rocks; 2) ultramafic rocks divided into two distinct groups (a Fe-rich group enriched in incompatible elements and more depleted ultramafic rocks with lower Fe contents); 3) mafic metamorphosed dikes called the Saglek dikes; and 4) undeformed mafic dikes. Some samples exhibit evidence of post-magmatic geochemical and isotopic disturbance but only the least disturbed samples have been considered to constrain the timing of formation of the different lithologies and the isotopic composition of their mantle source. The mafic metavolcanic rocks combined with the co-genetic low-Fe ultramafic rocks yield an isochron age of 3819 ± 190 Ma (MSWD=34, n=25) with an initial εNd value of +2.3 ± 0.6. The high-Fe enriched ultramafic rocks yield a younger age of 3433 ± 220 Ma (MSWD=10.4, n=10) with an initial εNd= +1.8 ± 0.5. The two generations of mafic dikes appear to have been emplaced in the Mesoarchean and the Neoarchean. The Saglek dikes yield an isochron age of 3565 ±120 Ma (MSWD=1.17, n=10) with an initial εNd value of +1.7 ± 0.1, while the Sm-Nd isochron age for the undeformed mafic dikes is 2694 ±79 Ma (MSWD=3.2, n=21) with an initial εNd value of +1.7 ± 0.1. All generations of mantle-derived rocks yield positive initial εNd values, where only the Eoarchean rocks display an initial Nd isotopic composition similar to the depleted mantle. The Mesoarchean ultramafic rocks, Saglek dikes and Neoarchean mafic dikes display almost identical initial εNd values, despite an age difference of ~800 Ma. This could suggest the contribution of distinct mantle sources or, if all generations of mantle-derived rocks in the SHC were produced from the same mantle source, it implies that this source evolved with a nearly chondritic Sm/Nd ratio for almost the whole Archean Eon. The fact that the initial isotopic compositions of the mantle-derived rocks appear to deviate from the depleted mantle with time, could also suggest an increasing interaction with older evolved crust.

This thesis focuses on two different topics in seismology: imaging the global structures of the mantle transition zone discontinuities and studying the site response effects in the Atlantic and Gulf Coastal Plain. Global structures of the mantle transition zone discontinuities provide important constraints on thermal structures and dynamic processes in the mid mantle. In this thesis, global topographic structures of the 410- and 660-km discontinuities are obtained from finite-frequency tomography of SS precursors. The finite-frequency sensitivities of SS waves and precursors are calculated based on a single-scattering (Born) approximation and can be used for data selection. The new global models show a number of smaller-scale features that were absent in back-projection models. Good correlation between the mantle transition zone thickness and wave speed variations suggests dominantly thermal origins for the lateral variations in the transition zone. The high-resolution global models of the 410- and 660-km discontinuities in this thesis show strong positive correlation beneath western North America and eastern Asia subduction zones with both discontinuities occurring at greater depths. Wavespeed and anisotropy models support vertical variations in thermal structure in the mid mantle, suggesting return flows from the lower mantle occur predominantly in the vicinity of stagnant slabs and the region overlying the stagnant slabs. In oceanic regions, the two discontinuities show a weak anti-correlation, indicating the existence of a secondary global far-field return flow. The Atlantic and Gulf Coastal Plain is covered by extensive Cretaceous and Cenozoic marine sediments. In this thesis, the site response effects of sediments in the Coastal Plain region relative to the reference condition outside that region are investigated using Lg and coda spectral ratios. The high-frequency attenuation factors (kappa) in the Coastal Plain are strongly correlated with the sediment thickness. At frequencies between 0.1-2.86 Hz, the Lg spectral ratio amplitudes are modeled as functions of frequency and thickness of the sediments in the Coastal Plain. Analysis of the residuals from the stochastic ground motion prediction method suggests that incorporating the site response effects as functions of sediment thickness may improve ground motion prediction models for the Coastal Plain region. / Doctor of Philosophy / The mantle transition zone is the region in the Earth’s interior between depths of ∼410 km and ∼660 km. The structure of the mantle transition zone plays an important role in understanding temperature variations and mass exchanges in the interior of the Earth. This dissertation aims at resolving depth variations of the top and bottom boundaries of the mantle transition zone at a global scale using underside reflected seismic waves. The advanced method used here resolved stronger small-scale depth variations of the boundaries than a conventional method using the same dataset. The two mantle transition zone boundaries both occur at depths greater than the global average beneath eastern Asia and western North America where cold oceanic lithosphere subducted under the continents. This positively correlated behaviors of the two boundaries agree with a scenario where cold subducted slabs have been horizontally deflected and stagnant above the bottom boundary of the mantle transition zone while hot materials beneath the mantle transition zone flow upwards due to the stagnant slabs penetrating the bottom boundary of the mantle transition zone. This dissertation also provides an examination of the differences between response of earthquake ground shaking in the Atlantic and Gulf Coastal Plain and that outside the Coastal Plain using seismic-wave spectral ratios. Ground shaking in the Coastal Plain is found to be amplified at low frequencies and de-amplified at high frequencies relative to that outside the Coastal Plain due to the extensive marine sediments in the Coastal Plain region. The amplification and attenuation factors can be estimated from spectral ratios and are found to be strongly correlated with the sediment thickness in the Coastal Plain. The spectral ratio functions derived in this dissertation may be adopted by studies on analyzing the seismic hazard in the Central and Eastern United States.

Cette étude vise caractériser le manteau lithosphérique du massif du Hoggar (Algérie) et son évolution, grâce à une étude multidisciplinaire (pétrologique, géochimique et pétrophysique) d'enclaves mantelliques échantillonnées par le volcanisme cénozoïque. L'échantillonnage provient de deux districts volcaniques (Tahalagha et Manzaz) situés respectivement en périphérie et au coeur du bombement du Hoggar. Le district de Tahalgha est par ailleurs situé à cheval sur un grand cisaillement pan-africain (le 4°35), séparant deux domaines structuraux majeurs du socle du Hoggar : le Hoggar Central Polycyclique à l'Est (domaine LATEA) et le Hoggar occidental à l'Ouest (bloc d'Iskel). Les xénolites étudiés apportent des informations sur l'évolution du manteau lithosphérique depuis l'orogenèse pan-africaine, au cours de laquelle s'est structuré le socle de cette région (le Bouclier Touareg), jusqu'aux événements cénozoïques responsables du bombement topographique et du volcanisme.L'héritage pan-africain est essentiellement préservé dans les échantillons du district périphérique de Tahalgha, sous la forme de lherzolites équilibrées à basse température (750 - 900°C), à clinopyroxènes appauvris en terres rares légères. Ces échantillons sont considérés comme représentant la lithosphère sous-continentale à l'issue des processus de réjuvénation qui ont marqué les derniers stades de l'orogenèse pan-africaine. Ils montrent des textures de déformation (porphyroclastiques à equigranulaires) bien préservées, attribuées à ces événements et caractérisées par des orientations cristallographiques préférentielles (OPRs) de l'olivine (axiales-[010]) compatibles avec un régime transpressif. Les événements cénozoïques sont marqués par un recuit partiel de ces textures, particulièrement prononcé à Manzaz et dans les échantillons de Tahalgha équilibrés à des températures moyennes à élevées (900-1150°C), et affectés par différents degrés de métasomatisme. Les xénolites de Tahalgha représentent un cas d'étude exemplaire du métasomatisme mantellique, couplant variations texturales, minéralogiques et chimiques le long de gradient locaux de température. Une modification des OPRs d'olivine est observée, qui résulterait à la fois de l'infiltration de liquides métasomatiques et d'une réactivation des accidents pan-africains en cisaillement pur.Des implications importantes de cette étude résident dans l'échelle des variations de premier ordre attribuées aux interactions lithosphère-asthénosphère au Cénozoïque. Celles-ci sont essentiellement à l'échelle du bombement du Hoggar (différences entre Manzaz et Tahalga, c'est-à-dire entre Hoggar central et périphérique) ou à celle de conduits magmatiques et de leurs épontes (variabilité locale des xénolites de Tahalgha). Par contre, les résultats obtenus montrent peu de variations significatives pour les échelles intermédiaires, notamment pour des localités de Tahlagha situées de part et d'autre ou à différentes distances du 4°35. Ceci favorise plutôt, pour l'origine du bombement volcanique du Hoggar, les modèles faisant appel à des structures d'assez grande échelle telle qu'un panache mantellique ou une cellule de convection asthénosphérique de type « Edge Driven Convection », plutôt qu'un processus essentiellement lié à la réactivation des failles lithosphériques pan-africaines. / This study aims to characterize the lithospheric mantle of the Hoggar swell (Algeria) and its evolution through time via a multidisciplinary (petrological, geochemical and petrophysical) study of mantle xenoliths sampled by Cenozoic volcanism. The samples were collected in two volcanic districts (Tahalagha and Manzaz) located in the periphery and in the central part of the Hoggar massif, respectively. The Tahalgha sampling also straddles a mega pan-African shear zone (the 4°35 fault) between two major structural domains of the Tuareg Shield basement: the Central Polycyclic Hoggar to the East (LATEA terranes) and the Western Hoggar domain to the West (Iskel block). The studied xenoliths provide information on the evolution of the lithospheric mantle from the Pan-African orogeny – i.e. the period when the Tuareg Shield was structured – to the Cenozoic events responsible for topographic upwelling and volcanism in the Hoggar swell.The Pan-African heritage is found in xenoliths from the peripheral Tahalgha district. These samples are distinguished by low equilibrium temperatures (750-900°C) and LREE-depleted clinopyroxene compositions. They are considered to represent the sub-continental lithosphere after the rejuvenation process that marked the later stages of the Pan-African orogeny. They show well preserved deformation textures (porphyroclastic to equigranular) assigned to these events and characterized by preferential crystallographic orientations (CPOs) of olivine (axial-[010]) consistent with a transpressional regime. The Cenozoic events are marked by partial annealing of these textures, particularly pronounced in the Manzaz samples, as well as in the Tahalgha xenoliths equilibrated at medium to high temperatures (900-1150°C). These samples were affected by different degrees of metasomatism. The Tahalgha xenoliths represent a rather unique case study of mantle metasomatism, where coupled textural, mineralogical and chemical variations occur along local temperature gradients. The Cenozoic events were also responsible for a change in olivine CPOs, resulting from both infiltration of metasomatic fluids and reactivation of Pan-African accidents in a pure-shear regime.Important implications of this study lie in the scale at which the first-order lithosphere modifications ascribed to the Cenozoic event are observed, i.e. either at the scale of the whole Hoggar swell, as shown by the increasing degree of textural annealing and metasomatism from Tahalgha to Manzaz (i.e. from outer to central Hoggar), or at the small scale of magma conduits and their wall rocks, as shown by the local variability registered by the Tahalgha xenoliths. Conversely, our data show little changes at intermediate scales, as might be expected, for instance, among the Tahalgha localities situated on either sides - or at different distances - from the 4°35. As regards the origin of the Hoggar volcanic swell, this result favours the models involving relatively large-scale structures such as a mantle plume or "Edge Driven Convection", rather than a process involving merely the reactivation of pan-African lithospheric faults.

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Subduction zone structure and kinematics are topics of ongoing investigation with broadband seismology. Recent advances in experimental observations of mantle materials at high temperatures and pressure, expanded broadband seismic datasets, new seismic analysis methods, and advances in computational modeling are ever increasing our capacity for synthesized investigation of tectonic environments. With the resulting expanded capability for interpretation, the geophysical community is in a position to build more refined and detailed models of subduction zone processes. This study takes part in these advances by refining structural observations of the subducting slab and making new observations of mantle kinematics in a part of the South American subduction zone in Chile and Argentina (between 30 degrees and 36 degrees S). First, I investigate the utility of multiple-event earthquake relocation algorithms for accurate locations using a regional dataset for seismic events in Nevada and then I apply the observations resulting from this study in the determination of new Wadati-Benioff zone seismicity hypocenter locations for the study area in South America. I interpolate new contours of the top of the subducting slab from this seismicity that, when interpreted with focal mechanism solutions for these events, are consistent with its deformation due to slab pull. I use shear wave splitting of teleseismic earthquake waves and s-waves from local earthquakes to characterize mantle strain within the mantle wedge and within and below the subducting slab. From these observations, I conclude that mantle flow in subduction zones is quite responsive to local changes in slab geometry as well as the thermal state of and shear stresses in the mantle wedge.