The Drybones Bay kimberlite has offered an opportunity to compare geophysical observations of the mantle lithosphere beneath the southwest Slave craton with geochemical and petrological interpretations through mantle xenoliths and xenocrysts. Coarse-textured garnet and spinel-garnet peridotite xenoliths, and garnet and garnetclinopyroxene xenocrysts were analyzed for a suite of trace elements. The Ni-in-garnet geothermometer (Canil, 1999) yielded equilibration temperatures between 850-1200 DC, and corresponding depths of 90-160 km, assuming a 41m Wm-2 paleogeotherm. This method estimates the thickness of the southwest Slave craton in the Ordovician at approximately 160 km, within the stability field of diamond. Trace element analysis revealed a distinct geochemical anomaly at a depth of approximately 120 km, which marks the transition from wehrlite to metasomatized wehrlite and lherzolite. This anomalous depth has been observed in geophysical studies, and has been interpreted as a structural discontinuity. / Graduate / 0372

L'altération hydrothermale du manteau lithosphérique dans les dorsales médio-océaniques fournit un mécanisme de transfert de chaleur et de masse entre la terre profonde et l'océan recouvrant. Le manteau lithosphérique est constituée de roches ultramafiques, également appelées péridotites. Ils comprennent plus de 70% d'olivine, de pyroxènes associés et de phases minérales mineures. La percolation de l'eau de mer dans le socle ultramafique produit l'altération de l'olivine et des pyroxènes en serpentine par le processus de serpentinisation et il est associé à des réactions d'oxydation et de carbonatation (lorsque le CO2 est présent dans le fluide). Le processus de serpentinisation présente un intérêt particulier pour la production de H2, le stockage du CO2, le développement de la vie et la production de gisements de minerai économiquement intéressants concentrés dans les fumeroles hydrothermaux. La durabilité et l'efficacité des réactions nécessitent la pénétration et le renouvellement des fluides à l'interface fluide-minéral. Les failles et les fractures des détachements océaniques sont les zones hautement perméables qui permettent à l'eau de mer de pénétrer profondément dans le manteau lithosphérique. Cependant, le processus de serpentinisation conduit à la précipitation de minéraux de faible densité qui peuvent remplir le réseau poreux, colmatant les chemins d'écoulement qui peuvent modifier les propriétés hydrodynamiques et la réactivité des roches réagi.Ces travaux de thèse visent à améliorer la compréhension des effets en retour des réactions sur les propriétés hydrodynamique du milieu dans les zones hautement perméables au cours des premières étapes de l'altération du socle ultramafique. Il se concentre en particulier sur les changements de texture et les réactions chimiques des roches ultramafiques en évaluant les effets du (i) débit et (ii) des fluides salins riches en CO2. Deux séries d'expériences de percolation réactive ont été réalisées à T = 170-190°C et P = 25MPa. La première série d'expériences consistait à injecter de l'eau de mer dans des échantillonnes de poudre d'olivine compressé sur une large gamme de débits constants. La tomographie par rayons X de haute résolution a été acquise avant et après l'expérience avec des débits élevés; afin d'évaluer les changements dans la microstructure de la roche lors de la réaction de serpentinisation. La deuxième série d'expériences consistait à injecter des fluides salins riches en CO2 dans des échantillonnes de péridotite fracturés mécaniquement.Les résultats ont permis de différencier: (1) un contrôle du débit du flux à l'échelle du pore peut contrôler la composition du fluide local et le développement de différents chemins de réaction à l'échelle de l'échantillon. (2) Le développement de différentes chemins réactifs et les changements de texture dans la roche dépend de la concentration de CO2 dissous dans la solution. (3) La formation de minéraux carbonatés (MgCO3) peut stocker du CO2 sous forme stable de minéral à long terme. (4) Un contrôle de la concentration de CO2 dissous dans le fluide et du réseau de fractures peut améliorer / limiter l'efficacité du stockage de CO2 dans les réservoirs de péridotite fracturés.Ces nouvelles données suggèrent un contrôle complexe de la structure des roches ultramafiques dans le processus de serpentinisation et fournissent de nouvelles perspectives pour le stockage potentiel du CO2 dans les réservoirs fracturés à la péridotite. / The hydrothermal alteration of the mantle lithosphere at mid-ocean ridges provides a mechanism for transferring heat and mass between the deep Earth and the overlaying ocean. The mantle lithosphere is constituted by ultramafic rocks, also called Peridotites. They comprise more than 70% of olivine, associated pyroxenes and minor mineral phases. The percolation of seawater into the ultramafic basement produces the alteration of olivine and pyroxenes to serpentine through the so-called serpentinization process and is associated to oxidation and carbonation reactions, the later when CO2 is present. The serpentinization process has special interest on H2 production, CO2 storage, development of life, and the production of economically valuable ore-deposits concentrated at hydrothermal vents. The sustainability and efficiency of the reactions requires penetration and renewal of fluids at the mineral-fluid interface. Oceanic detachment faults and fractures are the highly permeable zones allowing seawater derived fluids to penetrate deeply into the mantle lithosphere. However, the serpentinization process lead to the precipitation of low density minerals that can fill the porous network, clogging flow paths efficiently that may in turn modify the hydrodynamic properties and the reactivity of the reacted rocks.This PhD thesis aims at better understanding the feedback effects of chemical reactions on the hydrodynamic rock properties occurred on highly permeable zones during the earliest stages of alteration of the ultramafic basement. It focuses in particular on the changes in texture and chemical reaction paths of ultramafic rocks by assessing the effects of (i) flow rate and (ii) CO2-rich saline fluids. Two suite of reactive percolation experiments were performed at T=170-190°C and P=25MPa. The first suite of experiments consisted in injecting artificial seawater into porous compressed olivine powder cores over a wide range of constant flow rates. X-Ray µ-tomography of high resolution was acquired before and after the experiment run with high flow rates; in order to evaluate the micro-structural changes of the rock occurred during the serpentinization reaction. The second suite of experiments consisted in injecting CO2-rich saline fluids into peridotite cores mechanically fractured.The results allowed us to differentiate: (1) That, a control of flow infiltration rate at the pore-scale can control the local fluid compositions and the development of different reaction paths at the sample-scale. (2) The development of different reaction paths and textural changes in the rock depends on the concentration of CO2 dissolved in solution. (3) The formation of carbonate minerals (MgCO3) can store CO2 in a form of stable mineral at long-term. (4) A control of the concentration of dissolved CO2(g) and the fracture network can enhance/limit the efficiency of CO2-storage in peridotite fractured reservoirs.These new supporting data suggest a complex control of the structure of the ultramafic rocks in serpentinization process and provides new insights for the potential CO2-storage in peridotite fractured reservoirs.

The upper mantle structure of Swedish part of Baltic Shield with its isotropic and anisotropic seismic velocity characteristics is investigated using telesesismic body waves (i.e. P waves and shear waves) recorded by the Swedish National Seismological Network (SNSN). Nonlinear high-resolution P and SV and SH wave isotropic tomographic inversions reveal velocity perturbations of ± 3 % down to at least 470 km below the network. Separate SV and SV models indicate several consistent major features, many of which are also consistent with P-wave results. A direct cell by cell comparison of SH and SV models reveals velocity differences of up to 4%. Numerical tests show that differences in the two S-wave models can only be partially caused by noise and limited resolution, and some features are attributed to the effect of large scale anisotropy. Shear-wave splitting and P-travel time residual analyses also detect anisotropic mantle structure. Distinct back-azimuth dependence of SKS splitting excludes single-layer anisotropy models with horizontal symmetry axes for the whole region. Joint inversion using both the P and S data reveals 3D self-consistent anisotropic models with well-defined mantle lithospheric domains. These domains of differently oriented anisotropy most probably retain fossil fabric since the domains' origin, supporting the idea of the existence of an early form of plate tectonics during formation of continental cratons already in the Archean. The possible disturbing effects of anisotropy on seismic tomography studies are investigated, and found to be potentially significant. P-wave arrival times were adjusted based on the estimates of mantle anisotropy, and re-inverted. The general pattern of the velocity-perturbation images was similar but changed significantly in some places, including the disappearance of a slab-like structure identified in the inversion with the original data. Thus the analysis demonstrates that anisotropy of quite plausible magnitude can have a significant effect on the tomographic images, and should not be ignored. If, as we believe, our estimates of anisotropy are reasonably correct, then the model based on the adjusted data should give a more robust and correct image of the mantle structure.

Title: Anisotropic tomography of the European upper mantle Author: Helena Žlebčíková Department: Department of Geophysics, Faculty of Mathematics and Physics, Charles University Training institution: Institute of Geophysics of the Czech Academy of Sciences (IG CAS) Supervisor: RNDr. Jaroslava Plomerová, DrSc., IG CAS Consultants: RNDr. Vladislav Babuška, DrSc., IG CAS RNDr. Luděk Vecsey, Ph.D., IG CAS Abstract: Large-scale seismic anisotropy of the continental mantle lithosphere derived from joint inversion/interpretation of directional variations of P-wave travel-time residuals and SKS-wave splitting calls for orientation of the symmetry axes to be treated generally in 3D. Nevertheless, most of the tomography studies neglect the anisotropy of the body waves completely or they are limited to either azimuthal or radial anisotropy. Therefore, we have developed a code called AniTomo for coupled anisotropic-isotropic travel-time tomography of the upper mantle. The novel code allows inversion of relative travel-time residuals of teleseismic P waves simultaneously for 3D distribution of P-wave isotropic- velocity perturbations and anisotropy of the upper mantle. We assume weak anisotropy of hexagonal symmetry with either the 'high-velocity' a axis or the 'low-velocity' b axis. The symmetry axis is allowed to be...