This thesis seeks to improve our understanding of the processes involved in continental collision zones, with a particular focus on subduction-exhumation. The main objective of this work has been to define the tectonometamorphic evolution of the deeply subducted Seve Nappe Complex (SNC) in the Scandinavian Caledonides. I utilize mineralogy, petrology and geochronology to constrain the P-T-t paths of the SNC rocks in Sweden. The research has focused on the high grade rocks of the SNC and resulted in the discovery of metamorphic diamonds within the gneisses in west-central Jämtland and southern Västerbotten. Microdiamonds provided evidence for the ultra-high pressure metamorphism (UHPM) and subduction of continental rocks to mantle depths. The UHPM in these rocks was confirmed by calculations of the P-T conditions. The UHPM is further recorded by eclogites and garnet pyroxenites from northern Jämtland and eclogites from Norrbotten. All these findings provide compelling evidence for regional UHPM of vast parts of the SNC (at least 400 km along the strike of this allochthonous unit). The SNC rocks followed nearly isothermal decompression paths and paragneisses have locally experienced partial melting during exhumation. Formation of the peculiar Ba- and Ti-enriched dark mica in the Tväråklumparna metasediments is related to the latter stage. In-situ monazite dating of the diamond-bearing gneisses from west-central Jämtland supports previous geochronological data inferring that the peak of metamorphism is probably Middle Ordovician and was followed by Early Silurian partial melting. The exact timing of the UHPM here still remains to be resolved. The Lu-Hf garnet and U-Pb zircon dating of eclogite and gneiss from northern Jämtland confirms the Middle Ordovician age of the UHP-HP metamorphism of the SNC rocks. The chemical dating of monazite from the Marsfjället gneiss suggests an earlier UHP history of the Seve rocks in southern Västerbotten as a post-UHP uplift is dated to ca. 470 Ma. Based on the P-T-t data obtained in this thesis, particularly on the evidence for Middle Ordovician UHPM and subsequent Silurian exhumation, a new tectonic model for the Scandinavian Caledonides has been proposed. The outcomes of this thesis therefore improve our understanding of the tectonometamorphic history of the Caledonides.

No description available.

La thèse est basée sur une étude pétrologique et géochimique de xénolites mantelliques provenant du centre du craton sibérien et de l’Asie centrale entre le lac Baïkal et la Mongolie. Le but est d’établir l'état redox du manteau lithosphérique continental dans ces deux domaines géodynamiques distincts (ancien craton, ceinture mobile phanérozoïque) et mettre la fugacité d’oxygène en relation avec le régime thermique et la spéciation des fluides C-O-H. Les fugacités d’oxygène sont calculées sur la base des rapports Fe2+/Fe3+ dans les minéraux (spinelles et grenats) de péridotites, obtenus par spectrométrie Mössbauer. En détail, l’étude porte sur : (i) les microstructures et la composition minéralogique et chimique des xénolites ; (ii) les rapports Fe2+/Fe3+ dans les minéraux par spectrométrie Mössbauer; (iii) les températures et pressions d’équilibration des xénolites; (iv) la fugacité d’oxygène à partir des compositions des minéraux; (v) la spéciation des fluides C-O-H coexistant avec les roches mantelliques. Les résultats supportent les trois conclusions majeures. (1) La fugacité d’oxygène dans le manteau Iithosphérique au centre du craton sibérien décroît de +1 à -4 ΔlogʄO2 (FMQ) entre 70 et 220 km, accompagnée de variations latérales significatives. (2) L’état redox du manteau lithosphérique en Asie centrale est très hétérogène avec une décroissance importante lors de la transition spinelle-grenat de +0 à -3 ΔlogʄO2 (FMQ) à 50-90 km. (3) La spéciation des fluides C-O-H évolue avec la profondeur depuis H2O-CO2 en haut du manteau vers H2O-CH4 à la limite lithosphère-asthénosphère, indépendamment du profil thermique et de l’épaisseur de la lithosphère / The thesis is based on a petrologic and geochemical study of mantle xenoliths from the central Siberian craton and the Baikal-Mongolia region of central Asia. Its goal is to establish the redox regime of the lithospheric mantle in these two domains with distinct tectonic settings and age and relate it to thermal regime and the speciation of C-0-H fluids. Oxygen fugacity is calculated based on Fe2+/Fe3+ ratios in spinel and garnet of mantle peridotites obtained by Mössbauer spectroscopy. The study deals with the following topics: (i) microstructures, chemical and mineralogical composition of the xenoliths; (ii) Fe2+/Fe3+ ratios in minerals by Mössbauer spectroscopy; (iii) equilibration temperatures and pressures using mineral thermo-barometry; (iv) oxygen fugacity from mineral compositions using oxybarometry; (v) proportions of molecular components in C-0-H fluids coexisting with the studied rocks. The three main conclusions of this study are: (1) Oxygen fugacity in the lithospheric mantle in the central Siberian craton decreases from +1 to -4 ΔlogʄO2 (FMQ) at depths from 70 to 220 km and shows significant lateral variations. (2) The lithospheric mantle beneath the Baikal-Mongolia region shows important redox heterogeneities, with a sharp decrease in oxygen fugacity (from +0 to -3 AlogfO2 (FMQ)) during the transition from the spine! to garnet facies peridotites at 50 to 90 km. (3) The speciation of C-O-H fluids changes with depth from essentially H2O-CO2 in the shallow lithospheric mantle to H2O-CH4 at the lithosphere-asthenosphere boundary regardless of the thermal state and the thickness of the lithosphere

Deformační interakce mezi horninami na významném rozhraní k·ra - litosférický pláš' je jedním z hlavních proces·, jejichž pochopení umožní lépe popsat termomechanické procesy probíhající hluboko v rozsáhlých pohořích a především chování kontinentálního pláště během kolize kontinentálních desek. Tato disertační práce obsahuje tři hlavní kapitoly, představující výsledky r·zných geologických metod, využitých při zkoumání p·vodu a deformačního chování peridotitových těles ve spodní k·ře Variského orogenu. První kapitola prezentuje závěry ze strukturní studie Mohelenského peridotitu a okolních granulit· z Náměštského granulitového masívu. Jako jedinou efektivní metodou pro studium vnitřní stavby peridotitu se kv·li rozsáhlé serpentinizaci ukázalo měření přednostní mřížkové orientace (EBSD). V serpentinizovaném peridotitu byly zjištěny dva odlišné olivínové skluzné systémy, jejichž distribuce společně s detailním strukturním mapováním v okolním Náměš'ském granulitovém masívu umožnily vysvětlit reologické chování peridotitového tělesa v prostředí kořene orogenu. V této kapiole jsme se pokusili vytvořit geotektonický model, který popisuje historii peridotitového tělesa v rámci mnoha fází tektonické historie orogenního kořenu. Ve druhé kapitole představujeme výsledky z analýzy serpentinizačních proces· v...

The thesis is based on a petrologic and geochemical study of mantle xenoliths from the central Siberian craton and the Baikal-Mongolia region of central Asia. Its goal is to establish the redox regime of the lithospheric mantle in these two domains with distinct tectonic settings and age and relate it to thermal regime and the speciation of C-0-H fluids. Oxygen fugacity is calculated based on Fe2+/Fe3+ ratios in spinel and garnet of mantle peridotites obtained by Mössbauer spectroscopy. The study deals with the following topics: (i) microstructures, chemical and mineralogical composition of the xenoliths; (ii) Fe2+/Fe3+ ratios in minerals by Mössbauer spectroscopy; (iii) equilibration temperatures and pressures using mineral thermo-barometry; (iv) oxygen fugacity from mineral compositions using oxybarometry; (v) proportions of molecular components in C-0-H fluids coexisting with the studied rocks. The three main conclusions of this study are: (1) Oxygen fugacity in the lithospheric mantle in the central Siberian craton decreases from +1 to -4 ΔlogʄO2 (FMQ) at depths from 70 to 220 km and shows significant lateral variations. (2) The lithospheric mantle beneath the Baikal-Mongolia region shows important redox heterogeneities, with a sharp decrease in oxygen fugacity (from +0 to -3 AlogfO2 (FMQ)) during the transition from the spine! to garnet facies peridotites at 50 to 90 km. (3) The speciation of C-O-H fluids changes with depth from essentially H2O-CO2 in the shallow lithospheric mantle to H2O-CH4 at the lithosphere-asthenosphere boundary regardless of the thermal state and the thickness of the lithosphere

L'accrétion océanique au niveau des dorsales lentes est alimentée par un apport magmatique et par l'exhumation de péridotites mantelliques. Une partie de la chaleur dégagée lors du refroidissement et de la cristallisation des magmas sert de moteur à la circulation hydrothermale qui peut transporter de l'eau de mer jusqu'aux roches mantelliques. Une réaction d'oxydoréduction impliquant l'eau de mer, l'olivine et le pyroxène des péridotites peut alors avoir lieu pour produire de la serpentine, de la magnétite et de l'hydrogène. Cette réaction, dite de serpentinisation, contrôle les propriétés physico-chimiques de la lithosphère océanique et est, de fait, incorporée dans les modèles géophysiques avec une cinétique encore mal contrainte. Ce travail de thèse est centré sur la cinétique d'hydratation de l'olivine de San Carlos, une réaction de serpentinisation simple. Un dispositif expérimental de suivi de la cinétique d'hydratation a été mis au point, basé sur la mesure du signal magnétique associé à la magnétite formée. Cette méthode, couplée à une modélisation thermodynamique, permet également de quantifier indirectement la production d'hydrogène. L'influence de la taille initiale des grains l'olivine et de la température sur la cinétique d'hydratation a aussi été étudiée. Une augmentation de la vitesse de serpentinisation dans les premiers temps de la réaction est mise en évidence et expliquée par la mise en place de microtextures également observées dans le milieu naturel (puits de corrosion et fractures) contribuant à un accroissement de la surface réactive. De manière générale, la cinétique de serpentinisation obtenue ici est de un a deux ordres de grandeur plus lente que celle utilisée jusqu'alors dans les modèles géophysiques. En plus de cette étude expérimentale, une réaction d'oxydoréduction dans un contexte naturel a été étudiée au niveau d'une zone réactionnelle entre des serpentinites et des marbres en Corse alpine. Sur la base d'une approche pétrographique et thermodynamique, la présence de wollastonite, CaSiO3, est expliquée par l'atteinte de conditions très réductrices associées à la présence de fluides riches en hydrogène en contexte de subduction. Ces fluides sont interprétés comme résultant de la conservation du potentiel réducteur des serpentinites au cours de la subduction. / Slow-spreading ridges are fed by a magmatic input and the exhumation of mantle peridotites. Part of the magmatic heat is evacuated through the hydrothermal circulation which can transport seawater up to the mantle rocks. A RedOx reaction between seawater and the olivine and pyroxene from the peridotite can then take place, producing serpentine, magnetite and hydrogen. This reaction, so-called serpentinization, controls the physical and chemical properties of the oceanic lithosphere and is, thus, considered in geophysical models even if its kinetics are poorly constrained. Therefore, this study focuses on the kinetics of San Carlos olivine hydratation, a simple serpentinization reaction. An experimental method has been developed which allows monitoring magnetically the kinetics of the reaction of interest through the amount of magnetite that it produces. This method provides also an indirect estimate of the hydrogen production when coupled to thermochemical modeling. The influence of initial grain size and temperature on the hydration kinetics has also been investigated. An increase in the serpentinization rate at the beginning of the reaction is explained by the formation of microtextures also observed in natural samples (fractures and etch pits) which contribute to the generation of new reactive surface area. The inferred kinetics of serpentinization are by one to two orders of magnitude lower than the one commonly input in geophysical models. In addition to this experimental study, a natural case of RedOx reaction involving serpentinites has been studied which deals with a decimetre wide reaction zone between serpentinites and marbles from Alpine Corsica. The presence of wollastonite, CaSiO3, is explained on a petrological and thermodynamical basis as related to highly reducing conditions characterized by the presence of H2-rich fluids in this subduction zone environment. These fluids are interpreted as resulting from the conservation of the reducing potential of serpentinites throughout the subduction process.

L’objectif de cette thèse est de comprendre (1) si l’exhumation des roches mantelliques aux dorsales océaniques est compatible avec l’enregistrement des inversions de polarité du champ magnétique terrestre, (2) quels sont les processus associés aux anomalies magnétiques observées à l’aplomb des transitions océan-continent et (3) quelles sont les conséquences de ces processus sur les mécanismes de l’océanisation. Afin de travailler avec des données de résolution maximum, une méthode de calibration et d’interprétation des données magnétiques acquises proches du fond est développée dans le premier chapitre. Dans le deuxième chapitre, cette méthode est appliquée aux données magnétiques acquises à l’aplomb des domaines de manteau exhumé de la dorsale Sud-Ouest Indienne (SWIR). Il est ensuite montré que l’exhumation des roches mantelliques est associée à un signal magnétique de faible amplitude et de faible continuité spatiale rendant impossible l’identification des anomalies magnétiques d’accrétion océanique. De fait, il est proposé que contrairement aux basaltes, les roches mantelliques exhumées de la SWIR ne portent pas une aimantation rémanente suffisamment stable pour fossiliser la direction du champ magnétique terrestre. Dans le dernier chapitre, il est démontré que l’anomalie « J », antérieurement interprétée comme la première isochrone associée à la partie distale des marges Ibérie/Terre-Neuve, est en fait identifiée à l’aplomb d’une structure crustale particulière caractérisée par un haut topographique et un épaississement crustal. Par conséquent, il est proposé qu’un évènement magmatique majeur est à l’origine de la création de cette structure crustale et que cet événement est le déclencheur de la mise en place de la première dorsale océanique. / The aim of this work is to constrain (1) whether exhumation of mantle rocks at mid oceanic ridges is compatible with the record of polarity reversals of the Earth magnetic field, (2) what is the origin and the processes responsible for the magnetic anomalies observed at magma-poor rifted margins and (3) what are the consequences of these processes on continental breakup. In a first part, in order to work with high-resolution data, we develop a method for calibration and interpretation of deep-tow three component magnetic data. In a second part, we apply these methods to the data acquired above the large exhumed mantle domains of the ultraslow-spreading Southwest Indian Ridge. We show that, in these areas, neither the magnetic properties of the dredge samples nor the deep-tow magnetic data are consistent with the seafloor-spreading magnetic pattern commonly observed at mid oceanic ridges. We further suggest that in contrast to mid oceanic ridges basalts the exhumed serpentinized mantle rocks do not carry a sufficiently stable remanant magnetization to produce marine magnetic anomalies. In the last part, we show that the “J” anomaly, previously interpreted as the first seafloor-spreading anomaly of the Iberia and Newfoundland passive margins, is associated with locally high topography and thickened crust. We propose that this peculiar crustal structure results from voluminous magma both erupted at the surface and added beneath the exhumed mantle domain. We therefore propose that the J anomaly did not form during seafloor spreading but instead represents a pulse of magmatism that have triggered continental breakup.

Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Determining timescales of the formation and preservation of carbonate alteration products in mantle peridotite is important in order to better understand the role of this potentially important sink in the global carbon cycle and also to evaluate the feasibility of using artificially-enhanced, in situ formation of carbonates in peridotite to mitigate the buildup of anthropogenic CO₂ emissions in the atmosphere. Timescales of natural carbonation of peridotite were investigated in the mantle layer of the Samail Ophiolite, Sultanate of Oman. Rates of ongoing, low-temperature CO₂ uptake were estimated through ¹⁴C and ²³⁰Th dating of carbonate alteration products. Approximately 1-3 x 10⁶ kg CO₂/yr is sequestered in Ca-rich surface travertines and approximately 10⁷ kg CO₂/yr is sequestered in Mg-rich carbonate veins. Rates of CO₂ removal were estimated through calculation of maximum erosion rates from cosmogenic 3He measurements in partially-serpentinized peridotite bedrock associated with carbonate alteration products. Maximum erosion rates for serpentinized peridotite bedrock are ~5 to 180 m/Myr (average: ~40 m/Myr), which removes at most 10⁵-10⁶ kg CO₂/yr through erosion of Mg-rich carbonate veins. / by Evelyn Martinique Mervine. / Ph.D.

Ultramafic rocks are abundant in the Earth’s mantle but rare on the surface. Since no in-situ samples from the mantle can be collected, mantle rocks provide knowledge of mechanisms operating in the mantle and large-scale processes that brought them up to the surface. The mantle is considered chemically homogeneous and is dominated by the ultramafic, olivine-dominated rock - peridotite. Peridotites consist of olivine, clinopyroxene, orthopyroxene, and an Al-bearing phase. Increasing PT conditions alter the Al-bearing phase in peridotites, from plagioclase, through spinel to garnet. The Caledonian Orogeny occurred during the Ordovician to the Devonian period after the collision of the paleocontinents Laurentia and Baltica and shaped what is now the Scandinavian Caledonides. During the orogeny, a large scale of orogenic peridotites was tectonically emplaced on the surface. Scandinavian Caledonides are divided into Allochthons and further subdivided into Nappe Complexes. The Seve Nappe Complex (SNC) in the Middle Allochthon is interpreted as a record of Baltica’s outermost margin and exhibits evidence of ultramafic rocks that have gone under high- and ultra-high-grade metamorphism. From a locality in the SNC, near Lejarfjället, Ankarede in Jämtland, samples of an ultramafic rock were collected during field studies. To analyze the minerals’ chemical composition and thereby distinguish the petrographic properties of the rock, the samples were cut into thin sections. This study aims to characterize the aggregates occurring within the Lejarfjället peridotite and establish their possible origins. Thin sections of 30 μm thickness were analyzed in an optical petrographic microscope under plane-polarized light and cross-polarized light. Thin sections of 120 μm were analyzed with electron microprobe analysis. Through analysis, the rock type was identified to be the ultramafic rock peridotite, and more specific dunite, consisting of the mineral olivine, spinel, amphibole, serpentine, chlorite, and orthopyroxene. Further analyses of chemical composition allowed full identification of end members of olivine, orthopyroxene, and spinel. Obtained data indicate that the rock is an orogenic peridotite, and has previously been garnet-bearing, formed at a high pressure of at least 15 kilobars. The remnant of garnet is present as kelyphites, with pressure shadows surrounding the reaction corona and the garnet has been completely replaced with spinel and amphibole which constitutes the groundmass of the aggregates. Visible brittle deformation of the rock indicates that the rock was possibly exposed to seismic activity. The amphibole and chlorite indicate fluid interaction with the rock, pointing to the fact that some sort of metasomatic event has occurred. Serpentine present in the rock indicates hydrothermal reaction at low pressure and temperature conditions while the rock went through exhumation. The rock is interpreted to originate from exhumation in a lithosphere-scale extensional shear zone, similarly to examples described in the literature. / Ultramafiska bergarter dominerar jordens mantel men är sällsynta på ytan. Manteln är kemiskt homogen och domineras av den ultramafiska bergarten peridotit. Peridotiter består av mineralen olivin, klinopyroxen, ortopyroxen samt en aluminium bärande fas. Ökade tryck- och temperaturförhållanden ändrar den aluminium-bärande fasen i peridotit, från plagioklas till spinel till granat. Eftersom det inte går att ta ett bergprov direkt från manteln, ger stuffer av ultramafiska bergarter kunskap om både mekanismer i manteln och vilka storskaliga tektoniska event som tog dem upp till ytan. Den kaledoniska orogenesen inträffade mellan Ordovicium till Devon efter kollisionen av paleokontinenterna Laurentia och Baltica, och formade det som nu är de skandinaviska kaledoniderna. Under orogenesen pressades peridotiter upp från manteln, och är därmed utbredda längs den svenska fjällkedjan. De skandinaviska kaledoniderna är uppdelade i alloktoner och vidare i skollkomplex. Seveskollkomplexet i den mellersta alloktonen visar på ultramafiska bergarter som har utsatts för hög- och ultrahöggrads metamorfos. Ett ultramafiskt bergprov togs från Seveskollan vid Lejarfjället, Ankarede i Jämtland, under fältstudier. Syftet med denna studie är att identifiera aggregat i en peridotit från Lejarfjället och att försöka identifiera dess ursprung. För att analysera bergartens kemiska komposition och därmed urskilja de petrografiska egenskaperna gjordes tunnslip av provet. Tunnslip på 30 μm analyserades i ett optiskt petrografiskt mikroskop under plan polariserat och kors polariserat ljus. Tunnslip på 120 μm analyserades med hjälp av elektronmikrosond analys. Genom analys kunde bergarten identifieras att vara den ultramafiska bergarten peridotit, och mer specifikt en dunit. Provet består av mineralen olivin, serpentin, amfibol, spinel, orthopyroxen, och klorit. Vidare analys av kemisk komposition möjliggjorde fullständig identifikation av subgrupper av olivin, orthopyroxen, och spinel. Analys av data indikerar att bergarten är en orogen peridotit som tidigare varit granat-bärande, och har därmed formats vid minst 15 kilobar tryck. Granat saknas helt i bergarten men den typiska isometriska formen av kristallerna är intakta med tryckskuggor runtom. Avsaknaden av granat gör att kristallerna är kelyfiter av granat med en grundmassa bestående av spinel och amfibol. Synlig spröd deformation av bergartsprovet indikerar att stenen vid någon tidpunkt utsattes för seismisk aktivitet. Amfibol och klorit indikerar att en fluid har reagerat med stenen och orsakat metasomatism. Serpentin visar på hydrotermal reaktion vid låga tryck- och temperaturförhållanden på vägen upp från manteln. Deformationen av peridotiten tyder på att den formades i en litosfärisk skjuvningszon i jämförelse med liknande exempel beskrivna i litteraturen.

A neutron activation analysis procedure for the simultaneous determination of Au, Pd and Ir, and separate determination of Os is described and applied to the determination of these metals in the Mount Albert ultramafic pluton, Quebec. The Mount Albert is an alpine-type ultramafic body whose petrography, geology and geochemistry has been discussed in some detail by MacGregor (1962, 1964). It consists mainly of serpentinized dunite and peridotite. However, it is distinguished by the presence of an eastern lobe which is essentially unserpentinized. In addition to ferromagnesians, two varieties of spinel including disseminated accessory chrome spinel and massive, discordant chromite veins or schlieren are important mineralogical constituents. Twenty samples were analyzed including 5 fresh dunites and peridotites, 8 serpentinized equivalents, 4 disseminated chrome spinels, 1 clinopyroxenite, and 2 massive chromite. The concentrations (in p. p. b.) of average fresh ultramafics and their serpentinized equivalents are:-------------------------------------- ---------------------Pd----Os----Ir----Au------------------------------------------- Fresh rocks------------7.7----7.2--2.5--0.99------------------------------------------- Serpentinized rocks---10----6.6--2.2---2.4-------------------------------------------- The piuton was intruded as a nearly solid mass of olivine and some orthopyroxene with about 15 weight per cent melt. A small concentration of the precious metals, a factor of 2 to 3, in the melt relative to crystalline phases appears to have occurred. The disseminated chrome spinel and massive chromite appear to concentrate the precious metals. In particular, Os and Ir content of the massive chromite is approximately 10 times that of fresh or serpentinized whole rocks. There is little change in average precious metal content with serpentinization. Pd shows a slight increase, and Ir and Os slight decreases in concentration with degree of serpentinization. Only Au appears significant to be enriched in serpentinized rocks where a twofold increase occurs. The variation in precious metal content of all four metals increase when the rocks are serpentinized: When normalized to chondrite meteorites, the precious metal data give fractionation trends similar to that of average chondrite. These trends are compatible with a model in which alpine ultramafics are regarded as residua from the partial melting of parental material of approximately chondritic composition. / Thesis / Master of Science (MSc)