Dissertations / Theses on the topic 'Ophiolites – Oman'

Le but de l'étude est d'établir les conséquences structurales et métamorphiques du charriage d'une nappe océanique d'Oman, sur une marge continentale. La mise en place des ophiolites du semail (oman) est liée à la fermeture de l'océan téthysien à la fin du crétacé. On etudie les conditions p-t et la datation et on discute des modèles possibles d'évolution tectonique de la marge durant l'obduction.

Un intense metamorphisme de type hydrothermal oceanique affecte la sequence crustale de l'ophiolite de semail en oman. On etudie les flux chimiques et les gradients geothermiques grace aux inclusions fluides pour caracteriser les episodes majeurs des circulations hydrothermales

Les objectifs de cette etude geochimique ont ete: a) la caracterisation des series magmatiques de la nappe ophiolitique de semail en oman, b) l'analyse des processus hydrothermaux ayant affecte ce massif. Les unites volcaniques v1, v2 et v3, identifiees dans ce massif, presentent respectivement les caracteres geochimiques des morb de l'ocean indien, de basaltes d'arcs et de basaltes alcalins. Dans la sequence plutonique de la section du wadi haymiliyah, les donnees geochimiques ont permis de relier respectivement les deux series gabbroique et wehrlitique aux volcanismes v1 et v2. La zone de transition de cette section pesente des caracteres geochimiques originaux (nd(t)21, spectres en terres rares discontinus) qui s'expliquent par un processus d'impregnation d'harzburgites residuelles par des magmas elementaires de compositions variees. Les compositions isotopiques du strontium et de l'hydrogene montrent que la sequence extrusive du wadi salahi a ete alteree par des fluides d'origine marine. Les quatre types d'alteration (basaltes alteres, diabases alteres, epidosites et roches mineralisees), mis en evidence mineralogiquement, presentent des caracteres geochimiques distincts. Ils ont ete alteres dans des conditions differentes, et peuvent correspondre a differentes parties d'un systeme hydrothermal comprenant des zones de recharge et des zones de decharge. Le devenir des elements en trace a ete examine. Les bilans de matiere ont montre l'importance des veines d'epidote et de calcite et des epidosites dans les systemes hydrothermaux. Les caracteres geochimiques de l'alteration de cette ophiolite different de ceux observes dans la croute oceanique actuelle. Ces differences peuvent s'expliquer par la superposition, dans le massif d'oman, de plusieurs stades d'alteration hydrothermale induits par les differents episodes magmatiques

Une vingtaine de lits sulfures de puissance centimetrique a decimetrique, affleurent dans une zone d'environ 300 metres d'epaisseur, au sein des cumulats lites de la chambre magmatique d'haymiliyah (massif de haylayn). Cette zone riche en sulfures marque le passage entre un regime d'alimentation permanente marque par la cristallisation de gabbros lites a gros grain et la fermeture progressive de la chambre magmatique, caracterisee par le fractionnement d'une puissante unite de gabbros norites lamines. L'ensemble des resultats concorde pour demontrer que les sulfures sont clairement d'origine magmatique et que leur fractionnement est la consequence directe de la fermeture de la chambre

Pour vérifier l'existence de la segmentation mantellique suggérée par la chimie des basaltes des rides océaniques actuelles, nous avons échantillonné le manteau de l'ophiolite d'Oman sur une distance de 400km parallèlement au paléoaxe. Ainsi avons nous prélevé 280 harzburgites dans lesquelles nous avons analysé les phases primaires et la roche totale (éléments majeurs et traces). A de rares exceptions, les péridotites étudiées sont toutes très réfractaires et montrent une grande variabilité chimique à l'échelle de l'ophiolite, bien organisée parallèlement à l'axe da la paléodorsale. L'étude de ces variations a permis de montrer l'existence de 3 grands segments (longueur de 100-160km) interprétés comme des dômes asthénosphériques, eux-mêmes divisés en petits segments pouvant correspondre à des diapirs mantelliques plus superficiels. Cela laisse penser que l'ophiolite d'Oman s'est formée dans un contexte instable, dynamique et rapide, ressemblant aux bassins arrière-arc actuels
To test that variations in basalts composition at present oceanic ridges reflect segmentation of the underlying mantle, we sampled the mantle section of the Oman ophiolite over a distance of 400km parallel to the inferred paleo-ridge. Data concern 280 harzburgites in which we have analysed primary phases and the whole rock chemistry including major and trace elements. Except for a few samples that bear some impregnation evidence, the studied peridotites are all strongly residual rocks that have recorded a great variability in their chemical characters at the ophiolitic scale, well-organised when analysed along the paleo-axis. This allows to define 3 large segments 100-160km long, that likely correspond to large asthenospheric upwellings, divided in smaller segments that can represent small superficial mantle diapirs. This likely indicates that the Oman ridge, segmented at a small scale, was operating in an unstable, dynamical and fast evolving context, as present-day back-arc domains

La synthese des donnees gravimetriques collectees en oman depuis 1954, completee par de nombreuses campagnes d'acquisition dans les regions encore non-couvertes conduit a l'etablissement d'une base de donnees homogene couvrant tout le territoire omanais. La carte de l'anomalie de bouguer ainsi obtenue est interpretee en termes de structures regionales le long d'un profil 2d au centre de l'oman. Nous proposons alors un premier modele structural pour les ophiolites de masirah et la geometrie de la croute continentale au centre de l'oman. Localement, les anomalies gravimetriques sont interpretees avec les donnees sismiques et magnetiques afin de localiser des bassins sedimentaires presentant un interet petrolier. Puis, dans la seconde partie de ce travail, nous nous interessons a la chaine nord-omanaise. Sur la base des anomalies gravimetriques au nord de l'oman et dans les regions limitrophes, mais egalement des donnees sismiques d'origine petroliere, nous proposons une structure crustale actuelle pour cette region. Les donnees sismiques et gravimetriques permettent egalement d'estimer les differentes forces agissant sur la lithosphere arabique et liees a des processus de deformation actuels ou passes. Ces differentes forces sont appliquees sur une lithosphere arabique deformee de facon elastique, en deux episodes instantanes, durant l'obduction, et au tertiaire. Ces differents resultats permettent alors de proposer un evolution tectonique de la chaine nord-omanaise depuis pres de 100 ma, en domaine compressif, dans le cadre de la convergence arabie-eurasie.

L'origine de la zone de transition dunitique (DTZ) à l'interface manteau-croûte est mal connue, ainsi que les processus physico-chimiques impliqués dans sa genèse. Pour aborder cette question, ce travail a porté sur l'étude pétrologique, géochimique et structurale de 20 coupes (600 échantillons) levées dans la DTZ du massif de Sumail (ophiolite d'Oman), épaisse de plus de 400 mètres à l'aplomb d'un paléo-diapir mantellique. Au-delà des données in situ sur minéraux (microsonde, LA-ICP-MS) et des compositions en éléments majeurs des roches totales, le développement d'une procédure analytique a permis l'acquisition des compositions en éléments en traces des dunites dont les teneurs sont de l'ordre du ng.g-1. La DTZ est faite de dunites pures (olivine et chromite) et de dunites imprégnées, contenant une quantité variable de minéraux interstitiels ayant cristallisé à partir d'un magma percolant. Ces faciès renferment des minéraux d'une variété insoupçonnée incluant, en plus de ceux clairement issus d'un MORB (clinopyroxène et plagioclase), de l'orthopyroxène, amphibole, grenat, et des diopsides témoignant d'un processus d'hybridation entre le MORB et des fluides hydratés. Les forts rapports Mg# et teneurs en TiO2 des orthopyroxènes et amphiboles ainsi que la composition des clinopyroxènes, intermédiaire entre clinopyroxènes magmatiques et diopsides hydrothermaux, a permis de contraindre la composition du magma hybride qui résulterait du mélange entre un magma d'affinité tholéiitique et un fluide supercritique riche en silice, voire trondhjémitique issu de la fusion incongruente hydratée des orthopyroxènes mantelliques, similaire au produit de fusion hydratée des roches environnantes (péridotites serpentinisées, troctolites, gabbros). Ces minéraux sont observés en position interstitielle et en inclusion dans les chromites, témoignant de leur origine précoce et du fait que les magmas hybrides ont participé à la formation de la DTZ. La combinaison des interprétations des données in situ et des données roches totales a permis la déconvolution du message polyphasé enregistré par les dunites : la signature du protolithe, celles de la dunitisation et du rééquilibrage de la matrice d'olivine avec un MORB percolant (métasomatisme cryptique), la signature de refertilisation par la cristallisation des minéraux interstitiels (métasomatisme modal), ainsi que les effets de la serpentinisation. Il apparaît que les dunites pures, caractérisées par un spectres de terres rares en forme de U ou de V, semblent avoir acquis cette signature très précocement, probablement lors de la phase initiale de leur genèse sous l'effet de rééquilibrages avec des liquides très riches en éléments incompatibles (REE, Th, U, HFSE) et pouvant correspondre au magma hybride. L'étude structurale de la DTZ dans le massif de Sumail a montré l'influence de la tectonique synmagmatique sur la structuration de la DTZ, se traduisant par l'alternance d'horizons imprégnés ou non ainsi que par l'évolution verticale sur plusieurs dizaines de mètres des compositions chimiques à l'approche des zones de failles. On l'observe notamment pour les teneurs en éléments immobiles dans les fluides tels que le Ti, les REE ou le Th. La DTZ semble s'être développée dans un environnement transtensif dont les deux systèmes de failles principaux N130 et N165-180 ont accommodé la percolation des magmas et fluides responsables de la dunitisation ainsi que l'introduction des fluides hydrothermaux pouvant conditionner les échanges globaux avec les enveloppes externes.La comparaison avec les DTZ d'autres massifs en Oman ou à Trinity (Californie), ayant évolué dans un contexte magmatique différent, montre également l'importance des failles synmagmatiques dans la structuration de la DTZ. Les liquides qui ont percolé dans ces DTZ apparaissent systématiquement sous-saturés en Al et saturés en H2O, amenant à interpréter le caractère hydraté comme une condition critique pour la genèse des dunites
The origin of the dunitic transition zone (DTZ) between the mantle and the crust is still largely unknown, as well as the physical and chemical processes involved in its genesis. To address this topic, this thesis focused on the petrological, geochemical and structural study of 20 cross-sections (600 samples) collected along the DTZ from the Sumail massif, Oman ophiolite, 400 meters thick and located above a former paleo-mantle diapir. In addition to mineral compositions acquired using in situ methods (microprobe, LA-ICP-MS) and to whole rock major elements, the development of an analytical procedure permitted to determine trace element contents in dunites that display low concentrations (regularly about one ng.g-1). The DTZ is made of pure dunites (olivine and minor chromites), and of impregnated ones, containing a variable amount of interstitial minerals that crystallized from a percolating melt. These latter rocks contain an unexpected mineralogical variety with, in addition to clinopyroxene and plagioclase showing a MORB affinity, the presence of orthopyroxene, amphibole, garnet and diopsides that highlights a hybridization process between the MORB and hydrated fluids. The high Mg# ratio and TiO2 content in orthopyroxene and amphibole together with the clinopyroxene composition, intermediate between igneous clinopyroxene and pure hydrothermal diopside, allow deciphering the nature of the parent melt as the result of the mixing between tholeiitic melt and a supercritical water enriched in silica, or trondhjemitic fluid issued from the hydrated incongruent melting of mantle orthopyroxene, similar to melts produced by the hydrated melting of country rocks (serpentinized peridotites, troctolites, gabbros). All these minerals are observed both in interstitial position and as inclusions in chromite, showing that they crystallized early and that hybrid melts participated to the genesis of the DTZ. The comparison between mineral and whole rock compositions permitted to highlight the different processes that led to the observed chemical signatures of dunites: the protolithe signature, the dunitization process, chemical reequilibration between the olivine matrix and the percolating MORB, refertilization following the crystallization of interstitial minerals, as well as the effects of later serpentinization. Pure dunites, characterized by U or V-shaped REE patterns, seem to have acquired early the LREE-enriched signature that probably results from the reequilibration with silica- and incompatible trace elements-rich fluids (REE, Th, U, HFSE) generated through the harzburgite orthopyroxenes incongruent melting and probably reflecting the hybrid melt that crystallized interstitial hydrous minerals. The structural study of the DTZ in Sumail highlights the effect of synmagmatic faults on the DTZ development, resulting in the alternation between pure and impregnated horizons as well as in the vertical chemical structuration with compositions evolving on few tens of meters until fault zones. This is particularly true for chemical species expected as immobile during weathering as Ti, REE or Th. The DTZ seems to have been developed in a transtensional environment structured by two main faults systems, oriented N130 and N165-180. These faults spatially constrained both the melt flow, thus the dunitization, and the introduction of hydrothermal fluids probably oceanic in origin. This meeting zone between igneous and hydrothermal fluids can strongly influence the chemical exchanges and distribution between the deep lithosphere and the surface. The comparison between the Sumail DTZ and other ones from Oman or Trinity (California) ophiolites, which evolved in a different magmatic setting, shows the systematic role of synmagmatic faults. Melts that percolated these other DTZ were under-saturated in Al and saturated in water, allowing to interpret the hydrated component as an essential condition for dunites genesis at the mantle-crust transition

Les ophiolites présentent un grand interêt d'une part pour les reconstitutions géodynamiques des chaînes de montagnes, d'autre part pour l'étude détaillée des processus d'accrétion océanique. Ce memoire s'appuie principalement sur l'étude de trois terrains issus de la Téthys et présentant des caractères contrastés: - l'ophiolite de Semail, en Oman, est une des plus belles ophiolites du monde; sa taille permet la mise en évidence des variations longitudinales des processus d'accrétion à une échelle comparable à celle abordée dans les océans; - les ophiolites du Ladakh, préservées sous forme de klippes charriées sur la plaque indienne telle l'ophiolite de Spongtang, ou sous forme de petites écailles dans la suture et substratum de l'arc représentent des traceurs géodynamiques importants et indiquent des conditions opposées depuis l'accrétion jusqu'à l'obduction par rapport à l'ophiolite d'Oman; - les ophiolites tauriques présentent des caractère intermédiaires. Ce mémoire présente les résultats des études pétro-structurales menées dans ces ophiolites et les implications pour leur accrétion, leur histoire tectonique intra-océanique, et leur obduction. Les ophiolites omanaises présentent à l'affleurement une séquence souvent complète, et des structures témoignant d'un taux d'accrétion élévé. Les structures tranverses à l'axe sont rares et aucune n'interrompt la séquence crustale; les relations entre les différents faciès suggèrent une segmentation de la dorsale à l'échelle d'environ 30 km, qui évoluait avec le temps. A plusieurs endroits (Fizh, Haylayn) des 'overlaps' ou des 'propagating rifts' ont pu être mis en évidence. La séquence plutonique est constituée à part presques égales de gabbros et de wehrlites qui sont souvent interlités et pourraient représenter soit des ensembles de sills s'intrudant mutuellement, soit deux liquides coexistants mais immiscibles. Les structures internes de la série plutonique présentent souvent des variations et des images de recoupement à petite échelle; il est probable que ces variations reflètent la superposition de nombreuses petites chambres. Leur formation est expliquée par des cycles d'imprégnation de la zone de transition, celle-ci devenant périodiquement instable et intrudant la section crustale. Ces structures primaires sont en beaucoup d'endroits transposées par un flux visqueux, couplé avec le flux plastique des péridotites sous-jacentes. Ce couplage est particulièrement bien exprimé dans les gabbros de base, en contact direct avec les péridotites. Les autres ophiolites étudiées présentent des caractères bien différents, des séquences rarement complètes et des péridotites souvent moins déprimées, plus abondamment affectées par les zones de cisaillement lithophériques. La Klippe de Spongtang est composée essentiellement de péridotites, dans lesquelles les structures astenosphériques sont reprises par des zones de cisaillement s'organisant en deux familles: l'une à foliations sub-verticales et linéations sub-horizontales indiquant un mouvement décrochant, l'autre à structures sub-horizontales. Des wehrlites et gabbros peu repandus sont recoupés par des mini-complexes filoniens, orientés perpendiculairement aux cisaillements transverses. Ces structures suggèrent que l'accrétion de Spongtang eut lieu près de l'intersection d'une ride avec une zone transformante. Les facies et leur pétrologie présentent des caractères d'une ophiolite accrétée au droit d'une dorsale lente. Les critères structuraux et pétrologiques, ainsi que son âge crétacé inférieur, permettent de la comparer aux ophiolites Tibétaines, suggérant leur accrétion dans un domaine commun. L'obduction de la Klippe de Spongtang est tardive, contemporaine de la collision et résulte du glissement gravitaire d'un petit domaine océanique prédestiné par son relief, dans un bassin de mélanges dévelopé au pied de la marge passive de la platefrome nord-indienne. Ce scenario est complètement différent de celui proposé pour l'ophiolite omanaise. La zone de fracture d'Owen Chaman limite ici des domaines à evolutions différentes. Les structures des ophiolites tauriques suggèrent également une genèse dans un bassin océanique haché de zones transformantes, d'âge comparable à celles du Ladakh. Des cumulats bien développés, au moins localement, indiquent par contre un taux d'accrétion plus élevé. La tectonique intra-océanique est enregistrée par les cisaillements sub-horizontaux dans les amphibolites de semelle d'une part, et dans des brèches supraophiolitiques d'autre part.

Les etudes structurales anterieures dans l'ophiolite d'oman ont mis en evidence une chambre magmatique permanente et continue le long de l'axe des dorsales oceaniques rapides (nicolas et al. , 1996). Les campagnes sismiques realisees sur la dorsale est-pacifique ont permis de deceler une lentille liquide surplombee d'une zone a faible vitesse sismique (lvz) (wilcock et al. , 1995). L'objectif de ce travail est d'effectuer un inventaire detaille des microstructures et petrofabriques des gabbros d'oman, de contraindre la fraction de magma presente dans la lvz et de preciser le mode de circulation du liquide dans la chambre magmatique. La fraction liquide dans la lvz (10-20%) est repartie dans le plan de foliation et sa forme peut etre representee par un ellipsoide dont les parametres sont : x=4, y=4, z=1 (x = lineation, xy = plan de foliation). La modelisation sismique incluant les resultats precedents permet d'expliquer le gradient de vitesse des ondes sismiques obtenue dans la lvz : il est du a l'orientation de la foliation et donc a l'orientation de la phase liquide qui varie entre le haut et le bas de la chambre magmatique (lamoureux et al. , sous presse). Le mode de circulation du magma depuis le manteau jusqu'a la lentille perchee se deduit des caracteristiques structurales des anorthosites (roche monominerale) au sein des gabbros. Ainsi, la circulation se fait probablement par fracturation hydraulique mais la circulation par percolation n'est pas a exclure. En effet, du fait de l'orientation des structures, le magma primitif peut migrer en zigzag du bas vers le haut de la chambre et le magma residuel issu de la compaction peut migrer horizontalement en base de chambre.

L'ophiolite d'Oman permet d'étudier un morceau de lithosphère océanique formé à une dorsale rapide et échoué sur le continent. La structure générale permet de reconnaître la présence de plusieurs diapirs dont certains alimentaient la dorsale d'origine, partiellement obductée avec l'ophiolite. Le diapir de Maqsad est l'exemple type d'un diapir à l'axe. Les linéations verticales couvrent 100 km2 et tournent à l'horizontale au sein d'une zone de transition riche en magma, épaisse de 500 m, juste sous le Moho. Un flux forcé et radial est issu du diapir. Un calcul prédit que ce flux divergent devient passif à 10 km du diapir, comme le suggèrent nos données de terrain. Le diapir de Nakhl a des caractères similaires, tandis que le diapir de Mansah, éloigné de l'axe de la paleodorsale, est bordé par des zones de cisaillement et des injections de diabase qui attestent qu'il poinçonnait une lithosphère refroidie. Dans une seconde partie, les traces de magma dans les péridotites sont étudiées par analyse d'image sur des lames minces. On montre que les poches de liquide ont un allongement préférentiellement parallèle à la linéation et proportionnel à la force de la fabrique cristallographique de l'olivine. Ces données sont utilisées pour calculer les propriétés sismiques des échantillons. Nos résultats suggèrent que la quantité de liquide au Moho de la dorsale Est Pacifique peut être sous estimée à cause de l'anisotropie des roches. Enfin, une modélisation sismique d'un diapir, à partir de nos données structurales et des vitesses sismiques calculées permet de comparer les données d'Oman et de l'EPR.

Cette thèse présente les résultats de l'étude de deux séquences magmatiques continues. La première est une section de laves de l'ophiolite d'Oman. Cette séquence a été étudiée en termes de propriétés physiques, pétrologie et géochimie afin d'accéder à l'évolution temporelle des laves à l'axe de la paléographe omanaise, décrite comme étant rapide. Ces mesures ont montré la présence de cinq cycles magmatiques. La composante temporelle adoptée dans ce travail a permis l'étude des processus volcaniques depuis l'extraction des magmas jusqu'à l'échelle du cycle volcanique. La seconde section continue étudiée est une section forée dans les gabbros de l'Océan Indien sur l'Atlantis Bank. Les données géophysiques en forage ont été complétées par des mesures sur mini carotte en laboratoire. L'analyse des mesures de micro-résistivité enregistrées par le FMS (Formation Micro Scanner) montre une distribution bimodale de l'orientation des structures associées à la déformation
This thesis presents the results of the investigations realized on two continuous magmatic sections. The first section is a basaltic sequence which has been sampled at high-resolution in the Oman Ophiolite. This section has been investigated in terms of physical properties, petrology and geochemistry to study the structure of the volcanics emplaced at the axis of a fast spreading ridge. The Wadi Shaffan section is composed of five magmatic cycles. The temporal variations enhanced by this study allowed the integration of magmatic processes from magma extraction to volcanic cycles. The second section was drilled into gabbroic basement in the Atlantis Bank (South West Indian Ridge). Downhole logging and minicore petrophysical measurements have been investigated. Electrical conduction via electronic processes in oxide-rich gabbros has been taken into account in our analysis of the electrical properties. High-resolution images have been interpreted in terms of deformation orientation

L'ophiolite d'Oman permet d'étudier un morceau de lithosphère océanique formé à une dorsale rapide et échoué sur le continent. La structure générale permet de reconnaître la présence de plusieurs diapirs dont certains alimentaient la dorsale d'origine, partiellement obductée avec l'ophiolite. Le diapir de Maqsad est l'exemple type d'un diapir à l'axe. Les linéations verticales couvrent 100 km2 et tournent à l'horizontale au sein d'une zone de transition riche en magma, épaisse de 500 m, juste sous le Moho. Un flux forcé et radial est issu du diapir. Un calcul prédit que ce flux divergent devient passif à 10 km du diapir, comme le suggèrent nos données de terrain. Le diapir de Nakhl a des caractères similaires, tandis que le diapir de Mansah, éloigné de l'axe de la paleodorsale, est bordé par des zones de cisaillement et des injections de diabase qui attestent qu'il poinçonnait une lithosphère refroidie. Dans une seconde partie, les traces de magma dans les péridotites sont étudiées par analyse d'image sur des lames minces. On montre que les poches de liquide ont un allongement préférentiellement parallèle à la linéation et proportionnel à la force de la fabrique cristallographique de l'olivine. Ces données sont utilisées pour calculer les propriétés sismiques des échantillons. Nos résultats suggèrent que la quantité de liquide au Moho de la dorsale Est Pacifique peut être sous estimée à cause de l'anisotropie des roches. Enfin, une modélisation sismique d'un diapir, à partir de nos données structurales et des vitesses sismiques calculées permet de comparer les données d'Oman et de l'EPR.

Les rides médio-océaniques sont les sites de formation de la lithosphère océanique, qui recouvre les deux tiers de la surface terrestre. Le transport des liquides de fusion jusqu’à la ride reste encore peu compris. Les ophiolites, segments de lithosphère océanique obductés sur les continents, sont des sites idéaux pour étudier les processus mantelliques sous les dorsales. Elles contiennent des corps dunitiques, formés lors de l’interaction de la péridotite encaissante avec un liquide basaltique, entraînant la dissolution des pyroxènes et la précipitation d'olivine. Par l’utilisation de la région sud-est de l'ophiolite d'Oman comme cas d’étude, cette thèse a exploré la relation entre la distribution de la dunite et le transport des liquides vers la ride. La combinaison d'observations structurales et de données géochimiques sur les dunites et leurs harzburgites adjacentes a permis d'identifier trois types de dunite: (1) les dunites de la zone de transition du Moho (MTZ), situées immédiatement sous la croûte, (2) les veines et bandes de dunite, dispersées dans la section mantellique principale, et (3) d'abondantes bandes dunitiques dans l'unité litée à la base de l'ophiolite. Nos travaux géochimiques se sont concentrés sur les isotopes de l'osmium (Os), qui sont particulièrement adaptés au traçage de la migration de liquides à travers des péridotites mantelliques. Nous avons constaté que les harzburgites ont des compositions assez homogènes, similaires aux valeurs des péridotites abyssales, tout le long de la section mantellique. En revanche, les compositions des dunites varient systématiquement avec le contexte structural. Les dunites basales ont un caractère hautement résiduel (par exemple, un Mg# élevé) et des compositions isotopiques en Os équivalentes à celles de leurs harzburgites encaissantes, impliquant leur formation par l’interaction de ces harzburgites avec seulement de petites fractions de liquides. Les dunites de la section mantellique principale ont des compositions en Os plus radiogéniques, distinctes de celles des harzburgites adjacentes, suggérant une interaction avec des liquides plus importante. Les dunites de la MTZ ont interagi avec de grandes quantités de liquides, conduisant à de faibles concentrations en Os couplées à des rapports 187Os/188Os très élevés, dépassant ceux des MORB. La modélisation de la percolation de liquides à travers un chenal de dunite a démontré que ce mode de transport ne peut fournir des liquides avec des compositions en Os équivalentes à celles des dunites de la MTZ et de la croûte inférieure sus-jacente. Les travaux de cette thèse concluent à une migration des liquides, de leur source mantellique vers le Moho, produite sans équilibration avec la dunite ou la harzburgite, et par conséquent rapide, ou du moins isolée chimiquement
Mid-ocean ridges are the formation sites of ocean lithosphere covering two-thirds of the Earth's surface. Yet, melt transport to the ridge remains poorly understood. Ophiolites, segments of oceanic lithosphere thrust onto the continents, are ideal sites for studying mantle processes beneath ridges. They contain dunitic bodies, formed by interaction of the host peridotite with basaltic liquid causing pyroxene dissolution coupled with olivine precipitation. Using the example of the southeastern Oman ophiolite, this thesis explored the relationship between dunite distribution and melt transport to the ridge. The combination of structural observations with geochemical data on dunites and their host harzburgites allowed identification of three dunite types: (1) dunites of the Moho Transition Zone (MTZ), located immediately below the crust, (2) dunite veins and bands, scattered throughout the main mantle section, and (3) abundant dunitic bands in the banded unit at the ophiolite base. Our geochemical work focused on osmium (Os) isotopes, which are particularly suited for tracing melt flow through mantle peridotites. We found that harzburgites have fairly homogeneous compositions, similar to abyssal peridotite values, throughout the whole mantle section. In contrast, dunite compositions vary systematically with structural context. Basal dunites have highly residual character (e.g. high Mg#), and Os isotopic compositions equivalent to those of their host harzburgites, implying formation from these harzburgites by interaction with only small melt fractions. Dunites of the main mantle section have more radiogenic Os compositions, differing from those of adjacent harzburgites, suggesting more extensive melt interaction. MTZ dunites have interacted with large melt quantities, leading to low Os concentrations coupled with very high 187Os/188Os ratios, exceeding those of MORB. Modeling melt percolation through a dunite channel demonstrated that this transport mode cannot deliver melts with Os compositions equivalent to those of MTZ dunites and the overlying lower crust. This thesis works conclude that melt migration, from the mantle source to the Moho, occurred without equilibration with dunite or harzburgite, and was therefore rapid, or at least chemically isolated

I processi di interazione fuso-roccia rappresentano processi chiave nella formazione ed evoluzione chimica della crosta oceanica, come mostrato da diverse evidenze: (1) lo studio delle zonature chimiche e delle inclusioni fluide in fenocristalli in lave (Lissenberg et al., 2013; Laubier et al., 2014; Coumans et al., 2016), (2) “trends” composizionali peculiari, osservati in basalti di dorsale medio-oceaniche (MORBs), non consistenti con processi di pura cristallizzazione frazionata (Collier & Kelemen, 2010; Van den Bleeken et al., 2010, 2011; Paquet et al., 2016), (3) troctoliti arricchite in olivina descritte nei livelli più profondi della crosta oceanica, le quali vengono ritenute rappresentare il prodotto ‘ibrido’ di reazione tra duniti, originariamente localizzate nel limite mantello-ctosta, e magmi MORB percolanti che dissolvono olivina e cristallizzano plagioclasio e clinopirosseno (Suhr et al., 2008; Drouin et al., 2010; Renna & Tribuzio, 2011; Higgie & Tommasi, 2012; Sanfilippo et al., 2014; Rampone et al., 2016). Gli studi sopra citati descrivono la formazione di rocce gabbriche ricche in olivina come risultato di processi multipli di interazioni fuso-roccia a spese di scaglie di peridotiti di mantello incorporate alla base della crosta oceanica (Drouin et al., 2010, Sanfilippo & Tribuzio, 2012). Lo studio condotto in questa tesi ha come obiettivo quello di determinare le variazioni strutturali e geochimiche causate dai processi di interazione fusoperidotite in ambienti di transizione mantello-crosta oceanica, processi che causano la formazione di rocce ibride (duniti a plagioclasio, troctoliti, olivin gabbri). Questi processi sono descritti negli ambienti oceanici attuali (Mid-Atlantic Ridge, Southwest Indian Ridge, East Pacific Rise) dove però sono spesso carenti le relazioni strutturali tra i diversi litotipi, dai protoliti di mantello alle rocce gabbriche “ibride”, non permettendo quindi l’osservazione diretta dell’evoluzione microstrutturale e geochimica che accompagna i processi di interazione fuso-roccia. Questo lavoro di tesi e’ stato sviluppato mediante lo studio di peridotiti ed associate rocce gabbriche in tre sequenze ofiolitiche che preservano le relazioni strutturali primarie tra i diversi litotipi, ed in cui studi precedenti hanno documentato processi di interazione fuso/peridotite a diversi livelli litosferici : i) l’unita’ peridotitica Erro-Tobbio (Alpi Liguri; Rampone & Borghini, 2008; Rampone et al., 2016), ii) le peridotiti di Mt.Maggiore (Corsica Alpina, Francia; Rampone et al., 2008), e iii) la zona di transizione in Oman che presenta un layering alternato di livelli dunitici e livelli di olivin gabbri variabilmente evoluti (Koga et al., 2001; Higgie & Tommasi, 2012). Lo studio è stato sviluppato seguendo un approccio multidisciplinare che combina studi di terreno, strutturali (Electron Backscatter Diffraction, EBSD) e di composizione chimica degli elementi maggiori ed in tracce dei minerali (Electron Probe Micro-Analyzer e Laser Ablation Inductively Coupled Plasma Mass Spectrometer) con l’obiettivo di definire l’origine ibrida o magmatica “sensu strictu” delle rocce gabbriche, e la correlazione tra cambiamenti strutturali e geochimici durante il processo di interazione tra rocce di mantello e fuso, in settings geologici vincolati da chiare evidenze di terreno. Nell’Unità Erro-Tobbio (Alpi Liguri, Italia), le peridotiti di mantello impregnate sono primariamente associate a corpi mafici composti di troctoliti e wehrliti a plagioclasio, entrambi tagliati da dicchi di gabbro tardivi. I corpi troctolitici mostrano un’importante complessità composizionale interna: il corpo principale di troctolite (Troctolite A) è tagliato da una seconda generazione di corpi troctolitici con dimensioni metriche e forma pseudo-tabulare (Troctolite B) (Borghini et al., 2007; Rampone & Borghini, 2008; Rampone et al., 2016). La Troctolite A incassante è caratterizzata da due differenti occorrenze tessiturali di olivina, comprese tra olivine corrose e defornate di taglia millimetrica a centimetrica ed olivine tondeggianti, non deformate e di taglia fine; entrambe le olivine sono incluse in cristalli di plagioclasio e clinopirosseno con forma da interstiziale a poicilitica. La troctolite A mostra inoltre aggregati dunitici di dimensioni da centimetriche a decimetriche, circondati da zone arricchite in plagioclasio. Tali caratteristiche microstrutturali indicano la formazione della troctolite incassante A per impregnazione di una dunite da parte di un fuso sottosaturo in olivina (l’olivina viene corrosa dal plagioclasio interstiziale). L’evoluzione tessiturale durante l’evento di impregnazione è accompagnata da un evidente cambio dell’orientazione preferenziale degli assi cristallografici dell’olivina (CPO), che varia dal caratteristico pattern assiale [100] degli aggregati dunitici a plagioclasio interstiziale (simile al CPO analizzato nelle peridotiti associate) ad un CPO ad orientazione “random” nei campioni arricchiti in plagioclasio dove singoli cristalli di olivina sono inclusi in plagioclasio pecilitico. Quest’ultimo indica la perdita di coesione tra i cristalli di olivina, costituenti la matrice solida, durante l’evento di impregnazione ad elevati rapporti fuso/roccia (Rosenberg & Handy, 2005; Drouin et al., 2010). Le composizioni chimiche dei minerali costituenti la troctolite A mostrano trend evolutivi tipicamente di cristallizzazione “reattiva”, caratterizzati da variabili contenuti in anortite nei plagioclasi a costante contenuto in forsterite nelle olivine della matrice: questo indica che la composizione del fuso viene modificata e tamponata dall’assimilazione di olivina durante il processo d’impregnazione (Borghini et al., 2007; Rampone et al., 2016). I corpi pseudo-tabulari di troctolite B corrispondono ad intrusioni fragile-duttili nella troctolite A preesistente. Il CPO dell’olivina nella troctolite B indica che la sua origine è magmatica e la sua formazione è legata ad un flusso magmatico (orientazione preferenziale della forma dei cristalli; Benn & Allard, 1989; Jousselin et al., 2012). Le troctoliti B sono caratterizzate da estremevariazioni tessiturali, con olivine da dendritiche di dimensioni decimetriche a euedrali di dimensioni inferiori, entrambe incluse in plagioclasio poichilitico. Questa variazione tessiturale è il risultato dell’assimilazione di olivina durante la formazione della troctolite A (percolazione diffusa) ed intrusione della troctolite B, che causa un aumento del grado di raffreddamento del fuso percolante (aumento della temperatura di liquidus del fuso fino a 65°C durante l’assimilazione d’olivina; Faure et al., 2003, 2007; O’Driscoll et al., 2007). Le composizioni dei minerali costituenti la troctolite B sono simili a quelle dei minerali nella troctolite A e indicano che i processi di interazione fusoroccia, che sono responsabili per la dissoluzione di olivina, erano continuamente attivi durante la percolazione focalizzata di fusi. I minerali nelle intrusioni gabbriche che tagliano le peridotiti di mantello ed i corpi troctolitici, mostrano variazioni composizionali consistenti con processi di cristallizzazione frazionataa partire da fusi modificati durante la percolazione reattiva che ha determinato la formazione della troctolite A e troctolite B. I corpi troctolitici dell’Erro-Tobbio registrano quindi una storia evolutiva polifasica legata all’evoluzione termica di questo settore di mantello durante la progressiva esumazione (Borghini et al., 2007; Rampone & Borghini, 2008), dalla percolazione reattiva diffusa e formazione della troctolite incassante A, in aambiente duttile,alla percolazione reattiva focalizzata in ambiente fragile-duttile e formazione della troctolite B, fino alla tardiva intrusione e cristallizzazione frazionata del fuso modificato e formazione dei dicchi gabbrici in ambiente fragile più superficiale. Le peridotiti di Monte Maggiore (Corsica Alpina, Francia) registrano una chiara storia evolutiva di progressiva esumazione litosferica, accompagnata da processi di interazione fusoroccia, da lherzoliti e minori duniti a spinello a litotipi che presentano variabili contenuti in plagioclasio (peridotiti a plagioclasio, troctoliti ricche in olivina e troctoliti) (Rampone et al., 1997, 2008; Muntener & Piccardo, 2003; Piccardo & Guarnieri, 2010).In facies a spinello, la percolazione reattiva di un fuso povero in LREE ha causato la dissoluzione di pirosseni di mantello e la crescita di cristalli d’olivina, fino alla formazione di harzburgiti reattive, e minori corpi di duniti di sostituzione a spinello. Come risultato, aggregati dunitici di dimensioni metriche, caratterizzati da irregolari cristalli di olivina di grandi dimensioni, sono inclusi in harzburgiti a spinello reagite e ricche in olivina (Rampone et al., 1997, 2008; Muntener & Piccardo, 2003; Piccardo & Guarnieri, 2010). L’evoluzione progressiva da lherzoliti a spinello, a harzburgiti fino alla formazione di duniti di sostituzione, è marcata da un cambio del CPO dell’olivina, da un pattern assiale [100] nelle lherzoliti, consistente con deformazione per dislocation creep ad alta temperatura e generalmente descritto in peridotiti di mantello (Tommasi et al., 2000), al pattern assiale [010] del CPO dell’olivina nelle duniti, che tipicamente è interpretato come il risultato di deformazione in presenza di un fuso (Holtzman et al., 2003; Le Roux et al., 2008). La composizione iniziale del fuso di percolazione é consistente con singoli incrementi di fuso prodotti dal 6% di fusione parziale di un mantello depleto (Rampone et al., 1997, 2008). La percolazione reattiva conduce al progressivo arricchimento del fuso in concentrazioni assolute di HREE, mentre preserva la concentrazione povera in LREE (Vernières et al., 1997; Rampone et al., 2008): queste sono consistenti con le composizioni delle olivine analizzate nelle duniti a spinello che sono arricchite in HREE. In facies a plagioclasio, i fusi modificati durante il processo di percolazione (impoveriti in LREE, arricchiti in HREE) impregnano le peridotiti e duniti a spinello, portando alla dissoluzione di olivina e cristallizzazione di plagioclasio e ortopirosseno (+- clinopirosseno) nelle peridotiti (Rampone et al., 1997, 2008; Piccardo & Guarnieri, 2010). Lo stadio di impregnazione è anche osservato nelle duniti a spinello, dove la reazione fuso-roccia porta alla formazione ibrida di troctoliti ricche in olivina e troctoliti successiva alla dissoluzione di olivina e cristallizzazione di plagioclasio interstiziale. La reazione di dissoluzione-precipitazione, che forma le troctoliti ibride, sviluppa variazioni tessiturali progressive correlate con il contenuto modale in olivina (diminuisce durante la reazione). Come dedotto da studi precedenti su troctoliti ibride ricche in olivina formatesi in ambiente oceanico (Suhr et al., 2008; Drouin et al., 2010), le olivine con abito irregolare e di grosse dimensioni costituenti il protolita dunitico sono corrose progressivamente e fragmentate dal fuso reattivo che cristallizza plagioclasio, per finalmente formare le tessiture caratterizzate da numerose olivine tondeggianti incluse in plagioclasio poichilitico. Durante il progressivo processo di impregnazione, si osserva l’aumento di ‘grain number’, il quale è correlato con la diminuzione di ‘grain area’, ‘aspect ratio’ e ‘shape factor’ dei cristalli di olivina. L’evoluzione tessiturale legata all’impregnazione non è accompagnata da chiare variazioni del CPO dell’olivina, quindi indicando che il processo di impregnazione è caratterizzato da bassi rapporti fuso/roccia instantanei. Le composizioni in REE di olivina, plagioclasio e clinopirosseno analizzati nelle troctoliti concordano con un processo di impregnazione associato ad una progressiva chiusura della porosità del sistema (a progressiva diminuzione della massa del fuso), la quale conduce alla cristallizzazione di fuso e, conseguentemente, all’arricchimento in REE nelle fasi cristalline durante gli ultimi incrementi di cristallizzazione (Vernières et al., 1997, Rampone et al., 2008). La cristallizzazione di piccole frazioni di fuso intrappolato sono consistenti con i bassi rapporti fuso/roccia instantanei suggeriti dal fatto che il CPO dell’olivina conserva caratteristiche precedenti la reazione durante il processo di impregnazione. La zona di transizione Moho in Oman è costituita di un layering alternato di duniti (sensu lato; >70 vol% olivina modale) e livelli di olivin gabbri variabilmente evoluti (da 10 vol% a 70 vol% di olivina; Boudier & Nicolas, 1995; Higgie & Tommasi, 2012). In un recente studio petrologico e strutturale, Higgie & Tommasi (2012) hanno dimostrato che i livelli di olivin gabbro sono di origine reattiva di sostituzione, formatesi durante un processo deformativo guidato dalla percolazione reattiva focalizzata in pre-esistenti duniti, durante il quale le olivine della matrice vengono corrose e clinopirosseno e plagioclasio cristallizzano interstiziali. La progressiva evoluzione delle composizioni modali (diminuzione della composizione modale dell’olivina) è accompagnata da un cambio della simmetria del CPO dell’olivina: da un pattern assiale [100] nelle duniti ad un CPO dell’olivina con pattern assiale [010] negli olivin gabbro, gli ultimi osservati in un range di composizione modale dell’olivina tra il 60% e 40 vol%. Higgie & Tommasi (2012) hanno interpretato quest’evoluzione del CPO dell’olivina come evidenza di un’intensa deformazione progressiva concentrata in zone ad elevato rapporto fuso/roccia nei livelli ad olivin gabbro. L’occorrenza di tali variazioni strutturali in scala centimetrica, che sono state osservate nel layering di dunite-olivin gabbro, indica una correlazione positiva tra deformazione e focalizzazione del fuso nei livelli più duttili ad olivin gabbro, come descritto in esperiment precedenti che riproducono deformazione per simple shear in presenza di fuso (Zimmerman et al., 1999, Holtzman et al., 2003b). Forti discrepanze si osservano tra le composizioni chimiche dei livelli dunitici e quelle dei livelli ad olivin gabbro: i “trends” di evoluzione composizionale delle duniti sono indicativi di un processo di impregnazione da fusi in un sistema chiuso, con la composizione del fuso controllata dalla roccia ospite e modificata dall’interazione fuso-roccia e dissoluzione di olivina; i livelli ad olivin gabbro mostrano composizioni dominate da quelle del fuso di percolazione, consistenti con un processo di percolazione reattiva in sistema aperto come descritto precedentemente da Higgie & Tommasi (2012). Abbiamo osservato una buona correlazione tra le microstrutture, che marcano una transizione da pattern assiale [100] nei campioni ricchi in olivina a pattern assiale [010] negli olivin gabbri più evoluti, e le composizioni geochimiche dei minerali che indicano una transizione da sistema chiuso (dominato dalla composizione della roccia) a sistema aperto (dominato dalla composizione del fuso). Le composizioni in REE ed elementi in tracce, che sono consistenti con le composizioni di fusi tipo MORB, sono simili ai clinopirosseni analizzati nei gabbri della sezione di crosta oceanica inferiore in Oman: questo suggerisce che i fusi che migrano nells zona di transizione Moho sono successivamente estratti ed alimentano la crosta oceanica (Kelemen et al., 1997; Korenaga & Kelemen, 1997; Koga et al., 2001). Gli olivin gabbri mostrano una sequenza di cristallizzazione [olivina linopirosseno-plagioclasio] (Browning, 1984), e l’elevato contenuto in anortite nel plagioclasio (An = 87-92 mol%) è associato ad una composizione relativamente evoluta di olivina (Fo = 83-84 mol%) e clinopirosseno (Mg = 86-88 mol%). Studi precedenti (Kelemen et al., 1997; Korenaga & Kelemen, 1997; Koga et al., 2001) hanno invocato la cristallizzazione di fusi idrati per interpretare il peculiare ordine di cristallizzazione e le composizioni chimiche dei minerali, che non sono consistenti con un fuso anidro tipo MORB a 2 kbar di pressione (Koga et al., 2001). In contrasto, considerando l’assenza di fasi idrate nelle rocce investigate, le elevate temperature di equilibrio calcolate tra plagioclasio e clinopirosseno (geotermometro REE elaborato da Sun et al., 2017), e la discordanza tra i contenuti in anortite nel plagioclasio ed in forsterite nell’olivina (entrambi correlati positivamente con il contenuto in acqua nel fuso), riteniamo che la composizione dei minerali che cristallizzano nelle rocce gabbriche non sia primariamente controllata dal contenuto in acqua nel magma parentale, quanto piuttosto dalla sua peculiare composizione chimica, ricca in CaO (elevati CaO/Na2O e CaO/Al2O3), possibilmente acquisita per fusione parziale di una sorgente mista di mantello peridotite-pirossenite (Borghini et al., 2017)
Melt-rock interactions have been increasingly invoked as key processes in the formation and compositional evolution of the oceanic crust, from separate lines of evidence: (1) the study of the chemical zoning and melt inclusions in lava phenocrysts (Lissenberg et al., 2013; Laubier et al., 2014; Coumans et al., 2016), (2) peculiar compositional trends observed in mid-ocean ridge basalts (MORBs), not consistent with a process of pure fractional crystallization (Collier & Kelemen, 2010; Van den Bleeken et al., 2010, 2011; Paquet et al., 2016), (3) olivine-rich troctolites found enclosed in the lowermost oceanic crust, thought to represent the “hybrid” reactive product of the interaction between dunites from the crust-mantle boundary and percolating MORB melts, dissolving olivine and crystallizing interstitial plagioclase and clinopyroxene (Suhr et al., 2008; Drouin et al., 2010; Renna & Tribuzio, 2011; Higgie & Tommasi, 2012; Sanfilippo et al., 2014; Rampone et al., 2016). The latter studies thus describe melt-rock interactions as a rock-forming process, leading to the incorporation of mantle slivers into the lower oceanic crust and formation of olivine-rich lithologies after multiple episodes of melt-rock interaction (Drouin et al., 2010, Sanfilippo & Tribuzio, 2012). The study presented in this thesis aims at constraining the structural and geochemical variations caused by melt-rock interaction processes acting at oceanic peridotite/gabbro transition settings, leading to the replacive formation of hybrid lithologies (plagioclase-bearing dunites, troctolites and olivine gabbros). These processes are often described in oceanic settings (Mid-Atlantic Ridge, Southwest Indian Ridge, East Pacific Rise), where clear field associations from the mantle protolith to the end-product are lacking, thus not allowing the direct observation of the evolution related to melt-rock interaction processes. The work has been developed through investigations on three ophiolitic case studies, fossil analogues of the oceanic lithosphere, preserving the primary association between mantle peridotites and gabbroic rocks: i) the Erro Tobbio peridotitic body and associated troctolites and olivine gabbros, in the Ligurian Alps (Italy), ii) the Monte Maggiore peridotites and associated dunites and troctolites, in the Alpine Corsica (France), and iii) the Maqsad massif Moho Transition Zone, in the Oman ophiolites. In all these ophiolites, previous studies documented a multi-stage melt-rock interaction history. In this study, we have used a multidisciplinary field, structural (EBSD) and geochemical approach (mineral major and trace element analyses) to define the replacive vs magmatic origin of the gabbroic lithologies and the correlation between the structural and geochemical variations induced by melt-rock interaction processes in a field-controlled petrological setting. In the Erro-Tobbio unit (Ligurian Alps, Italy), impregnated mantle peridotites are primarily associated to a hectometre-size mafic body composed of troctolite and plagioclase-bearing wehrlite, both crosscut by late gabbroic dykes. The troctolitic body exhibits high inner complexity, with a host troctolite (Troctolite A) crosscut by a second generation of troctolitic metre-size pseudo-tabular bodies (Troctolite B) (Borghini et al., 2007; Rampone & Borghini, 2008; Rampone et al., 2016). The host Troctolite A is characterized by two different textural types of olivine, between corroded deformed millimetre- to centimetre-size olivine and fine-grained rounded undeformed olivine, both embedded in interstitial to poikilitic plagioclase and clinopyroxene. It includes centimetre- to decimetre-size dunitic pods, surrounded by plagioclase enrichments. The Troctolite A shows meltrock reaction microstructures indicative of formation after impregnation of dunites by an olivineundersaturated melt (olivine being corroded by interstitial plagioclase). The textural evolution during impregnation is accompanied with a clear change of the olivine Crystallographic Preferred. Orientation (CPO), from dunitic aggregates bearing interstitial plagioclase preserving an axial-[100] pattern, similar to the CPO analyzed in the country peridotites, to single olivine grains embedded in poikilitic plagioclase in the most plagioclase-rich samples, showing a randoming of the olivine CPO indicative of the loss of cohesion of the solid matrix during impregnation at high melt/rock ratios (Rosenberg & Handy, 2005; Drouin et al., 2010). The Troctolite A shows reactive mineral compositional trends of evolution, characterized by variable Anorthite contents in plagioclase at constant Forsterite content in the olivine matrix, indicative of the buffering of the melt composition by the assimilation of olivine during impregnation (modelling performed using the pMELTS thermodynamic software; Ghiorso et al., 2002). The Troctolite B pseudo-tabular bodies correspond to brittle-ductile intrusions within the pre-existing Troctolite A, showing olivine CPO indicative of a formation by magmatic flow (shape-related orientation of the crystals; Benn & Allard, 1989; Jousselin et al., 2012). They are characterized by extreme textural variations from decimetre-size dendritic to fine-grained euhedral olivines, embedded in poikilitic plagioclase. We infer that this textural variability is the result of olivine assimilation during formation of the Troctolite A (diffuse percolation) and intrusion of the Troctolite B (focused percolation), and to the related increase in the degree of undercooling of the percolating melt (increase of the melt liquidus temperature up to 65°C during olivine assimilation; Faure et al., 2003, 2007; O’Driscoll et al., 2007). Mineral compositions within the Troctolite B are similar to the ones analyzed in the Troctolite A and indicate that olivinedissolving melt-rock interactions were still involved in the focused melt percolation (pMELTS; Ghiorso et al., 2002). The gabbroic intrusions crosscutting the association between mantle peridotites and the troctolitic body show mineral compositional trends of evolution consistent with the fractional crystallization of the melts modified after the olivine-dissolving reactive melt percolation related to the formation of the Troctolite A and Troctolite B (pMELTS; Ghiorso et al., 2002). Therefore, the Erro-Tobbio troctolitic body shows a poly-phase formation history related to the thermal evolution (Borghini et al., 2007; Rampone & Borghini, 2008), from diffuse reactive percolation leading to the replacive formation of the host Troctolite A in ductile environments, to brittle-ductile focused reactive percolation and formation of the Troctolite B, to the intrusion and fractional crystallization of the modified melt and formation of the gabbroic dykes in brittle environments. The Mt.Maggiore peridotitic body (Alpine Corsica, France) exposes a clear evolution from spinel lherzolite to plagioclase-bearing lithotypes (plagioclase peridotites, olivine-rich troctolites and troctolites) (Rampone et al., 1997, 2008; Muntener & Piccardo, 2003; Piccardo & Guarnieri, 2010) during two continuous episodes of melt-rock interaction. At spinel facies, the reactive melt percolation of LREE-depleted melts, likely representing unaggregated last melt increments produced by fractional melting of underlying asthenosphere mantle, leads to the dissolution of mantle pyroxenes and the growth of olivine crystals, until the formation of replacive spinel dunites in the most reacted peridotites. As a result, metre-size dunitic pods, characterized by irregular coarse grains of olivine, are enclosed in reacted olivine-rich spinel harzburgites (Rampone et al., 1997, 2008; Muntener & Piccardo, 2003; Piccardo & Guarnieri, 2010). The progressive evolution from spinel lherzolite to harzburgite to replacive dunite is accompanied by a change of olivine CPO, from axial-[100] in the lherzolite, consistent with high-temperature low-strain dislocation creep, commonly described in mantle peridotites (Tommasi et al., 2000), to axial-[010] olivine CPO in the dunites, indicative of deformation in presence of melt (Holtzman et al., 2003; Le Roux et al., 2008). The initial percolating melt composition is consistent with single melt increments after 6% partial melting of a depleted mantle source (Rampone et al., 1997, 2008). The reactive melt percolation leads to a progressive enrichment in the melt HREE absolute concentrations, while preserving its LREE depletion (REE modelling using the Plate Model after Vernières et al., 1997), consistent with the enriched analyzed HREE composition of olivine in the spinel dunite. At shallower plagioclase facies, the melts modified after reactive melt percolation (LREEdepleted, HREE-enriched) impregnate the spinel-facies lithotypes, leading to the dissolution of olivine and crystallization of plagioclase and orthopyroxene (± clinopyroxene) in the peridotites (Rampone et al., 1997, 2008; Piccardo & Guarnieri, 2010). The impregnation stage is also observed in the spinel dunites, leading to the hybrid formation of olivine-rich troctolites and troctolites, after dissolution of olivine and crystallization of interstitial plagioclase. The dissolution-precipitation reaction forming the hybrid troctolite develops progressive textural variations correlated with the modal content in olivine (decreasing during reaction). As inferred from previous studies of hybrid olivine-rich troctolites in oceanic settings (Suhr et al., 2008; Drouin et al., 2010), the irregular and coarse olivines from the dunite protolith are progressively corroded and disrupted by the reactive plagioclase-crystallizing melt, to form textures characterized by numerous rounded olivines embayed in poikilitic plagioclase. Therefore, during progressive impregnation, an increase in Grain Number, correlated with a decrease in grain area, aspect ratio and shape sactor of the olivine grains is observed. The textural evolution during impregnation is not accompanied by clear variations in the olivine CPO, indicative of very low instantaneous melt/rock ratios during the impregnation process. Olivine, plagioclase and clinopyroxene REE compositions analyzed in the troctolite fit a process of impregnation with a progressive closure of the porosity (at decreasing melt mass), leading to the crystallization of trapped melt and REE enrichments during the last crystallization increments (Plate Model; Vernières et al., 1997). The crystallization of small trapped melt fractions is consistent with the low instantaneous melt/rock ratio suggested by the preserved olivine CPO during the impregnation process. The Oman Moho Transition Zone (MTZ) is formed of a layering of interfingered dunites (sensu lato; >70 vol% modal olivine) and variably evolved olivine gabbros (from 10 to 70 vol% olivine; Boudier & Nicolas, 1995; Higgie & Tommasi, 2012). A recent petrological and structural study by Higgie & Tommasi (2012) demonstrated the replacive origin of the olivine gabbro layers, formed by a deformation-driven focused reactive percolation within the pre-existing dunite, corroding the olivine matrix and crystallizing interstitial clinopyroxene and plagioclase. The progressive evolution of modal compositions (decreasing modal composition of olivine) is accompanied by a change in symmetry of olivine CPO from axial-[100] patterns in the dunite to axial-[010] CPO in the olivine gabbro, observed over a range of olivine modal compositions from 60 to 40 vol%. Higgie & Tommasi (2012) interpreted this olivine CPO evolution as indicative of the higher cumulated strain and melt/rock ratio integrated over time in the olivine gabbro layers. The centimetre-scale at which the structural variations are observed within the dunite - olivine gabbro layering calls for a positive feedback between deformation and melt focusing in the weaker olivine gabbro layers, as previously described in melt-bearing simple shear experiments (Zimmerman et al., 1999, Holtzman et al., 2003b). Strong chemical discrepancies are observed between the dunite layers showing compositional trends of evolution indicative of a closed system melt impregnation, with the buffering of the melt composition by the host rock and the olivine dissolving melt-rock interaction, and the olivine gabbro layers showing compositions dominated by the percolating melt composition, consistently with the open-system reactive melt percolation previously described by Higgie & Tommasi (2012). We observed a good correlation between the transition from axial-[100] in olivine-rich samples to axial-[010] in more evolved olivine gabbros, and the geochemical transition from closed system (matrix-dominated) to open system (meltdominated) mineral compositions. The clinopyroxene REE and trace elements compositions are consistent with a MORB-type melt composition, similar to the clinopyroxenes analyzed in the Oman lower crustal gabbros, indicating that melts percolating the Moho Transition Zone were then extracted and fed the oceanic crust (Kelemen et al., 1997; Korenaga & Kelemen, 1997; Koga et al., 2001). The olivine gabbros show an [olivine – clinopyroxene – plagioclase] crystallization sequence (Browning, 1984), and high-Anorthite plagioclase (An = 87-92 mol%) is associated with relatively evolved compositions of olivine (Fo = 83-84 mol%) and clinopyroxene (Mg# = 86-88 mol%). Previous studies (Kelemen et al., 1997; Korenaga & Kelemen, 1997; Koga et al., 2001) invoked the crystallization of hydrous melts to explain the peculiar crystallization order and minecompositions, not consistent with the crystallization ofdry MORB-type melt at 2kbar (Koga et al., 2001). However, based on the absence of any hydrous phase, the high calculated plagioclaseclinopyroxene equilibrium temperatures (REE geothermometer after Sun et al., 2017), and the decoupling between Anorthite contents in plagioclase and Forsterite contents in olivine (both positively correlated with the water content in the melt), we infer that the water content of the parental melt is not the controlling factor on the composition of crystallizing minerals, rather a peculiar CaO-rich chemical signature of the primary MORB melt (high CaO/Na2O and CaO/Al2O3 and fractional crystallization modelling performed using the pMELTS thermodynamic software; Ghiorso et al., 2002), possibly formed by partial melting of a mixed pyroxenite-peridotite mantle source (e.g. Borghini et al., 2017). The results of this thesis demonstrate the possible hybrid origin of gabbroic rocks in fieldcontrolled ophiolitic settings, and provide a textural, structural and geochemical methodology to constrain melt-rock interaction processes and the magmatic vs replacive origin of an olivine-rich gabbroic rock. The three case studies highlight a significant control of instantaneous and cumulated melt/rock ratios on both the structural and the geochemical evolution during melt-rock interaction processes. The combination of structural and geochemical analyses allows a detailed understanding of the melt-rock interaction processes from large to small scale, and the related structural and geochemical evolution of the pre-existing mantle matrix and percolating melt.

Les roches mafiques à ultramafiques permettent de tracer les processus de formation et d'évolution des surfaces planétaires. Pour caractériser ces surfaces, la spectroscopie de réflectance visible-proche infrarouge est une technique adaptée de part sa sensibilité aux absorptions du fer présent dans les minéraux. L'objectif de cette thèse est de déterminer la composition modale des roches, ainsi que la composition chimique des minéraux constitutifs.
Nous avons développé une procédure basée sur la mise en oeuvre du Modèle Gaussien Modifié (MGM) qui permet de modéliser les spectres pour des assemblages minéralogiques complexes (olivine - orthopyroxène - clinopyroxène). Après validation de cette approche sur des données simples (poudres), la méthode a été appliquée à des roches naturelles complexes (météorites martiennes et roches d'Oman). Fort de l'expertise acquise dans cette étape intermédiaire, des cartographies minéralogiques ont alors pu être réalisées à partir de données spatiales et aéroportées, respectivement pour le volcan Syrtis Major sur Mars et le massif ophiolitique de Sumail (Oman). Nous avons ainsi montré que les laves de l'édifice volcanique présentent un enrichissement en olivine (Fo50-80) et que les pyroxènes, suivant les conditions de mise en place, peuvent avoir des compositions allant des augites aux enstatites. Pour l'ophiolite d'Oman, nous avons mis en évidence et cartographié pour la première fois des variations spatiales organisées de composition modale au sein même de l'unité harzburgitique. Nous avons également caractérisé des variations de composition dans les clinopyroxènes de la zone crustale ayant des implications pétrogénétiques.
Ces apports sont essentiels dans la caractérisation et la compréhension des processus pétrologiques inhérents à la formation des surfaces planétaires et devraient stimuler l'utilisation de l'imagerie spectroscopique à des fins géologiques.

Dans des conditions naturelles d'exposition, les roches présentent divers types d'altération modifiant leur composition initiale. En climat tempéré, elles se dégradent pour former une altérite. En milieu désertique, une patine d'altération se développe. Ces deux phénomènes masquent la composition de la roche-mère et induisent une détermination erronée des faciès lors de leur cartographie par télédétection. Pour le Muscadet Nantais et l'ophiolite d'Oman, l'étude des altérations passe par la comparaison des spectres V-IR des surfaces fraîches et altérées. Pour retirer les effets de rugosité de surfaces propres à chaque échelle d'observation (laboratoire, terrain, aéroportée), une nouvelle normalisation des spectres est proposée. L'étude minéralogique des sols de la région nantaise, par spectrométrie et DRX, a permis de définir des limites de détectabilité des roches de la Nappe de Champtoceaux et des faciès de gabbro du Pallet. Ces résultats ont été testés sur une simulation de campagne aéroportée. La cartographie du Massif du Sumail (Oman) est réalisée grâce à l'étude des spectres des différents faciès et validée par des études pétrologiques et MEB. Les séquences mantellaires et crustales de l'ophiolite sont dissociées sur l'image HyMap. Le calcul des mélanges synthétiques à partir des spectres de terrains a permis de quantifier la couverture de chaque lithologie. La combinaison de ces cartes aboutit à une nouvelle carte géologique. Dans la séquence crustale, elle révèle la présence d'un nouveau bloc présentant une abondance anormale en orthopyroxene.

Dans des conditions naturelles d’exposition, les roches présentent divers types d’altération modifiant leur composition initiale. En climat tempéré, elles se dégradent pour former une altérite. En milieu désertique, une patine d’altération se développe. Ces deux phénomènes masquent la composition de la roche-mère et induisent une détermination erronée des faciès lors de leur cartographie par télédétection. Pour le Muscadet Nantais et l’ophiolite d’Oman, l’étude des altérations passe par la comparaison des spectres V-NIR des surfaces fraîches et altérées. Pour retirer les effets de rugosité de surfaces propres à chaque échelle d’observation (laboratoire, terrain, aéroportée), une nouvelle normalisation des spectres est proposée. L’étude minéralogique des sols de la région nantaise, par spectrométrie et DRX, a permis de définir des limites de détectabilité des roches de la Nappe de Champtoceaux et des faciès de gabbro du Pallet. Ces résultats ont été testés sur une simulation de campagne aéroportée. La cartographie du Massif du Sumail (Oman) est réalisée grâce à l’étude des spectres des différents faciès et validée par des études pétrologiques et MEB. Les séquences mantellaires et crustales de l’ophiolite sont dissociées sur l’image HyMap. Le calcul des mélanges synthétiques à partir des spectres de terrains a permis de quantifier la couverture de chaque lithologie. La combinaison de ces cartes aboutit à une nouvelle carte géologique. Dans la séquence crustale, elle révèle la présence d’un nouveau bloc présentant une abondance anormale en orthopyroxene
Under natural exposure conditions, rocks display a wide range of weathering modifying initial rock composition. In temperate climates, they form dip alterites, while a rock varnish develops in desert environments. Both phenomena mask the composition of the bedrock leading to a misidentification of rock types by remote sensing. For both Nantes Muscadet and Oman ophiolite, the weathering study necessitates comparisons between fresh and weathered surface V-NIR spectra. A new standardisation of spectra is introduced to remove roughness effect specific to each scale of observation (laboratory, field, airborne). A mineralogical study of sols by spectrometry and XRD, allows to define limit of detection between rocks of the Nappe de Champtoceaux and rock types of the Pallet gabbro. These results are tested with an airborne campaign simulation. The Sumail Massif (Oman) mapping is achieved by studying different facies spectra, validated by petrological and SEM analysis. The mantle and the crustal section of the ophiolite are dissociated from the HyMap image. The calculation of synthetic mixtures from field spectra led to quantify the surface coverage of each lithology. The combination of these maps succeeds to a new geological map. In the crustal section, it reveals the presence of a new bloc with anomalous contain in orthopyroxene

La séquence plutonique du massif de Fizh (partie nord de l'ophiolite d'Oman) offre une complexité crustale, liée à une diversité des corps magmatiques et du système filonien. La cartographie détaillée montre une succession d'événements magmatiques dont les séries principales sont formées de gabbros lités, des wehrlites et pyroxénites, des gabbronorites. Le magmatisme gabbroïque est généré dans une grande chambre magmatique axiale, alors que le magmatisme wehrlitique est dit hors-axe ; ceci est conditionné par la segmentation axiale de la paléodorsale. L'asymétrie ainsi observée dans la séquence crustale est marquée par la distribution et l'abondance des corps ultrabasiques. Les variations longitudinales dans les séquences stratigraphiques montrent des discontinuités au sein de la paléodorsale dans ce massif. Les granitoïdes sous forme de filons ou de stocks, attestent une succession d'événements magmatiques et les modalités de l'accrétion. En effet, les plagiogranites sont liés à la cristallisation fractionnée des gabbros, au cours de l'épisode d'accrétion normale. Les granodiorites et les diorites quartziques sont rattachées spatialement aux ultrabasites et formées par cristallisation fractionnée pendant la phase d'écaillage intra-océanique. Les granites et les leucogranites sont tardifs et à cachet collisionnel. Ils sont issus de la fusion partielle de la semelle métamorphique et liés à l'épisode obductif chaud. Les gisements de chromite sont variés et sont localisés dans le manteau et à la limite manteau-croûte. Les concentrations de chromite dans les ultrabasites sont importantes et présentent un effet sur le comportement des éléments du groupe du platine. Les sulfures magmatiques (pentlandite et chalcopyrite) précipitent dans les wehrlites et les pyroxénites en même temps que le plagioclase. Le comportement des PGE dans les chromitites montre qu'elles sont moins différenciées que celles des autres massifs ophiolitiques. Les PPGE et particulièrement le Pd précipitent dans les ultrabasites en même temps que les sulfures d'origine magmatique

A range of siliceous, ferruginous and ferromanganiferous deposits are intercalated with, and overlie the lavas of the Late Cretaceous northern Oman ophiolite. Most of the deposits lie on the upper surface of the spreading event lavas; spreading event magmatism and later seamount-building events are coeval to relatively small metalliferous sediment deposits. The mineralogical and geochemical characteristics of these sediments are a function of the interaction between local hydrothermal systems, the marine depositional environment, and early diagenetic transformations. Various techniques are employed to objectively determine the actual end-member component compositions from which the metalliferous sediments formed. The sediments are a mixture of primary biosiliceous oozes and hydrothermal metallic components which were deposited at or near a marginal ocean-basin spreading axis during Cenomanian time. Factor analysis, selective acid leaching experiments and linear programming modelling identify six geologically reasonable end-members, which represent biosiliceous sediment, carbonate sediment, detrital sediment, hydrogenous sediment, and hydrothermal sediment. The techniques show that the sediments have a complicated hydrothermal history which is associated with the evolution of the Oman ophiolite. The hydrothermal component is sub-divided into high temperature and low temperature end-members which are characteristic of the proto-seamount and proto- rift event environments respectively. Vent proximal and vent-distal facies are described. The geochemistry of the deposits provides evidence for calcareous pelagic dissolution by hydrothermal fluids, which resulted in the relative concentration of a hyaloclastic component. The deposits which were not early-lithified are epidotized. Metamorphic transformation of the primary sediment occurred prior to eruption of the upper lava unit. The techniques which have been used to describe the range, composition and distribution of the end-member components provide a flexible framework for the characterisation of geological mixing in all marine metalliferous sediments.

Un diapir asthenospherique fige a l'axe d'une dorsale oceanique affleure dans la zone de maqsad au sud de l'ophiolite d'oman. Cette zone offre donc la possibilite de percer l'anatomie de la croute oceanique a l'aplomb d'une zone d'accretion. Au voisinage du paleo-moho, les gabbros lites sont envahis par des corps ultrabasiques (wehrlites-troctolites et dunites). Certains de ces corps s'enracinent au niveau du moho et remontent jusqu'au sommet de la croute, ou vient s'enraciner directement le complexe filonien nw-se. Des intrusions pegmatitiques affleurent a la limite de la zone de divergence du flux asthenospherique. Ces pegmatites forment une suite differenciee: pyroxenites, gabbros-gabbros norites, diorites et plagiogranites. Les plagiogranites profonds ont des plagioclases tres calciques (an% 90-95) temoignant d'une genese par la fusion partielle de l'encaissant. Les filons se sont reveles de bons marqueurs spatiaux et temporels de l'evolution du magma extrait de la fusion partielle du diapir. Ces filons ont permis egalement de suivre le prolongement du diapir sous la croute. Ce resultat est conforte par une structure en dome dans les cumulats lites. L'existence d'une transition entre une croute inferieure plastique et superieure fragile a ete revelee par l'etude des structures d'ecoulement visqueux et de deformation plastique dans les gabbros lites, coherente avec la repartition verticale des filons. Ces nouvelles donnees confortent un scenario de montee d'un diapir asthenospherique, et d'ouverture progressive du centre d'expansion de maqsad. Une conclusion integrant ces arguments aux donnees geophysiques marines permet de confirmer et d'expliquer la grande variete de facies lithologiques, recemment reconnus dans la croute oceanique dans les dorsales actuelles

The Oman ophiolite is the largest and best preserved ophiolite in the world and records a switch from mid-ocean ridge (MOR) to supra-subduction zone (SSZ) setting. This study investigates the geochemical variability of chrome-spinel in the mantle sequence of the poorly known United Arab Emirates (U.A.E.) part of the northern Oman-U.A.E. ophiolite. Extensive field work was carried out and 260 samples collected for petrogenetic studies and geochemical mapping of the U.A.E. mantle. Chrome-spinel geochemistry provides valuable information on bom the residual mantle and on the nature and extent of melt-rock reaction. In particular, it is used to fingerprint the compositions of the magmas that interacted with the mantle lithosphere. This study also develops a new method to analyse gallium in chrome-spinel by Laser Ablation-ICP-MS, and successfully uses it to improve the tectonic discrimination of chrome-spinel. The results show that the U.A.E. mantle lithosphere formed at a MOR-type setting and was modified by melt-rock reaction with MORB-type and SSZ-type melts. This history of melt infiltration strongly resembles the magmatic history of the crustal sequence in each of the Aswad and Khawr Fakkan Blocks. Geochemical mapping illustrates a strong spatial control on the pattern of melt infiltration in the mantle and constrains the proximity of each mantle domain with respect to the subduction zone. The Khawr Fakkan mantle extensively interacted with boninitic melts during subduction initiation. Thus, it was closer to the subduction zone than the Aswad mantle which predominantly interacted with island-arc tholeiite melts. Importantly, this work demonstrates for the first time that the Dibba Zone peridotites originate from pre-existing 'true' MOR mantle lithosphere between the trench and the main body of the ophiolite. A further important conclusion is that the mantle lithosphere of the northern Oman-U.A.E. ophiolite was not the source region for the SSZ magmatism. Previous workers proposed that the plane of detachment and the subduction zone were the same, which implies that the mantle of the ophiolite was the source of the SSZ magmatism. Instead, this study proposes that detachment of the ophiolite took place at a shallower level than the plane of the subduction zone and thus provides strong evidence for a subduction zone at a still deeper level. During detachment, the ophiolite incorporated slices of ultramafic rock near the trench (i.e. the Dibba Zone peridotites) as it bulldozed its way over the underlying plate and onto the continental margin of Arabia.

L'ophiolite d'Oman permet d'observer les roches mantelliques inaccessibles aux dorsales. Avec cinq diapirs à l'axe (dont Maqsad) et un diapir hors-axe (Mansah), elle permet d'étudier les processus magmatiques en jeu dans les diapirs au niveau de la Zone de Transition au Moho, qui est différente entre les deux types de diapirs avec hors-axe des pyroxénites dans la dunite à la place de gabbros lités à l'axe. Le diapir de Mansah est entouré d'intrusions gabbroïques dans le manteau et la croûte. Les roches hors-axe ont des valeurs d'εNd plus faibles que celles à l'axe qui ont des compositions similaires aux MORB, suggérant une source des liquides plus riche en veines de pyroxénite hors-axe. Les εNd montrent aussi que les pyroxénites et les intrusions gabbroïques sont une suite magmatique. L'abondance de clinopyroxenes hors-axe provient de la réaction entre des liquides provenant de la fusion de veines de pyroxénites avec la harzburgite appauvrie de la lithosphère, ce qui donne à ces clinopyroxènes des compositions appauvries en éléments traces. La présence d'eau provenant de la lithosphère hydratée favorise la cristallisation de ces clinopyroxènes à la place du plagioclase qui doit normalement apparaître à cette profondeur dans la MTZ, ce qui est le cas à l'axe. Le diapir hors-axe pourrait fournir une analogie aux seamounts que l'on trouve actuellement à proximité des dorsales rapides et faire la lumière sur les interactions entre la lithosphère appauvrie et le matériel ascendant hors-axe, ainsi que sur les structures internes de ces seamounts. Ce travail offre des évidences probantes pour l'existence de veines de pyroxénites dans le manteau asthénosphérique sous les dorsales
The Oman ophiolite offers the possibility to study mantle rocks which are inaccessible at mid-ocean ridges. The presence of five on-axis diapirs and an off-axis diapir allows comparison of magmatic processes occurring in these different settings. The Moho Transition Zone is dominated by dunite in both cases, but off-axis includes massive pyroxenites instead of layered gabbro. The off-axis diapir is surrounded by gabbroic intrusions in the crust and mantle, which are not found elsewhere in the ophiolite. While the on-axis samples have εNd values similar to those of MORB, all of the off-axis rocks have less radiogenic Nd suggesting a larger contribution from melting of pyroxenite veins in the off-axis source. The abundance of clinopyroxene in the off-axis MTZ results from the reaction between the pyroxenite-derived melts and the depleted harzburgite of the lithosphere, which explains the highly depleted incompatible trace element compositions of the clinopyroxenes. The presence of water from the hydrated lithosphere favors the crystallization of clinopyroxene instead of plagioclase, which should normally appear at this depth in the MTZ, as is the case on-axis. The gabbroic intrusions in the mantle and crust surrounding the off-axis diapir crystallized from the residual magma produced by the interaction between the pyroxenite-derived melts and the harzburgite. The off-axis diapir could be viewed as an analog to seamounts currently found near fast-spreading ridges, and could provide information on their internal structure. More generally, this study provides compelling evidence for the existence of pyroxenite veins in the asthenospheric mantle beneath mid-ocean ridges

The Magmatic Evolution and Crustal Accretion of the northern Oman-U.A.E. ophiolite The Oman-United Arab Emirates (U. A.E.) ophiolite is the largest and best-preserved ophiolite complex in the world and has assumed critical importance in guiding ideas as to the functioning of mid-ocean ridge magma chambers and accretion of lower crust formed at a fast-spreading ridge. The portion of the ophiolite within the United Arab Emirates has received much less study than that within the Sultanate of Oman, particularly with regard to its magmatic evolution. Fieldwork has been conducted in co-operation with the British Geological Survey on the ophiolite sections in the U.A.E. Geochemical and field characteristics of these plutonic and extrusive units provide important constraints on the influence subduction-related fluids have on crustal accretion processes. Initial VI magmatic events exhibit a MORB-like composition and record mid-ocean ridge accretion processes which relate to modern crust, forming from steady-state magma chambers. The Khawr Fakkan Block provides the first 'true' MORB mantle source signature to be documented in the Oman-U.A.E. ophiolite, which represents pre-subduction zone influenced oceanic crust and applies constraints to the mechanisms and timing of subduction initiation and propagation. These primary events were followed by V2 magmatic events in a supra subduction zone (SSZ) setting, with plutonic magmatism localised predominantly along major faults. Each subsequent magmatic event exhibits an increase in the subduction related component to the mantle source. The V2, SSZ, magmatic period of the northern Oman-U.A.E. ophiolite can be regarded as an embryonic volcanic arc, which ceased magmatism and was obducted before reaching maturity. The northern Oman-U.A.E. ophiolite records the transition from spreading- to subduction-related volcanism in a SSZ setting within a marginal (fore-arc) basin, representing the change from steady-state to plutonic magmatism. The SSZ signature increases to the north spatially and temporally.

This study uses the strontium isotope tracer system to constrain hydrothermal flux estimates and investigate the hydrothermal contribution to the oceanic strontium budget. This is achieved through a multi-directional approach. Firstly, strontium isotope data for ocean and ophiolite crust are compiled to investigate the degree of isotopic alteration displayed by modern and ancient crustal profiles; and calculate the cumulative hydrothermal contribution to the oceanic strontium budget. This reveals an apparent imbalance in the oceanic strontium budget, because the hydrothermal contribution calculated is only a third of that required to balance a revised riverine input and the buffering effect of carbonate digenesis. Secondly, a multi-phase hydrothermal system in the northern Oman ophiolite is investigated as an analogue for understanding modern oceanic process and the magnitude of hydrothermal fluxes in arc-related environments. Field mapping and trace elements discrimination methods are combined to identify three hydrothermal regimes which correlate with the complex magmatic-tectonic evolution of the area. The first regime is associated with formation and cooling of the crustal sequence within an oceanic spreading environment; the second and third regimes are associated with later magmatism in an off-axis environment. Strontium isotope geochemistry is used to investigate the character of each regime and predict a time-integrated high-temperature fluid flux of ~4.5±1.1 x 10⁷ kg m^-2 for the initial spreading related hydrothermal event and lower estimates for the later regimes. The flux calculated is significantly higher than a similar prediction made for mid-ocean ridge systems (eg. Teagle et al, 2003), supporting previous suggestion that oceanic spreading systems in supra-subduction settings support greater hydrothermal fluxes than normal oceanic environments.

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.

Ophiolites - thrust sheets of oceanic lithosphere that have been emplaced onto the continental margin - provide the opportunity to explore the structure and genesis of oceanic crust. As many ophiolites formed above subduction zones, they also allow for the investigation of mantle wedge and subduction interface processes. This the- sis examines the Oman-United Arab Emirates (UAE) ophiolite, which is the largest and most intensely studied ophiolite on Earth. Three distinct problems are addressed. (1) Recent research has proposed that the architecture and tectonic evolution of the ophiolite in the UAE differs from in Oman. In Chapter 2, I test this hypothesis by integrating new geological mapping and field observations with previously published maps of the ophiolite in the UAE. My results indicate that the ophiolite is gently folded, but otherwise largely intact. I demonstrate that the architecture of the ophi- olite in the UAE is not significantly different from in Oman. Thus, there is no basis for a different tectonic evolution as recently proposed. (2) Observations from exper- iments and small-scale natural shear zones indicate that volumetrically-minor phases can control strain localization. In Chapter 3, I test the hypothesis that minor phases control strain-localisation at plate boundaries. To do so, I analyzed peridotites from the base of the ophiolite, a palaeosubduction interface. My results demonstrate that minor phases limited olivine grain growth, which led to rheological weakening. (3) The mechanisms by which metamorphic soles detached from the downgoing slab and accreted to the hanging-wall mantle is unclear. In Chapter 4, I examine a transect across the metamorphic sole in the UAE. My results reveal that granulite formation was more extensive than is typically considered. I propose that granulite formation resulted in rheological strengthening, which caused the subduction interface to migrate into the downgoing slab and accrete the metamorphic sole.

Structural field observations, combined with petrological, isotopic and geochemical analysis of metamorphic and igneous rocks associated with the Semail ophiolite of Oman and the United Arab Emirates, have been used in conjunction with geochronology and estimates of metamorphic conditions and PT paths to constrain the ophiolite emplacement history. The ophiolite metamorphic sole was formed at peak conditions of 840 ± 70°C and 11.6 ± 1.6 kbar (THERMOCALC) and 840-870°C and 11.8-13.9 kbar (conventional thermobarometry) and is characterised by an anticlockwise PTt path. Further analysis and structural constraints imply an apparent inverted sole gradient of ~2°C m-1 and ~3.7 MPa m-1. In conjunction with existing geochronology, a peak sole exhumation rate of ~12.5 mm yr-1 is indicated. Geochemical analysis and tectonic constraints suggest that the Semail ophiolite sole formed from neo Tethyan MORB crust similar in composition to the preserved Triassic-Jurassic Haybi tholeiites and Masirah ophiolite crust. The Bani Hamid granulite facies sole metamorphism peaked at ~96.5 Ma and exhibits a similar PTt path and peak conditions, but formed from oceanic island igneous, volcanoclastic and sedimentary protoliths. Anatectic granitoids in the ophiolite mantle sequence have geochemical and isotopic (Rb-Sr and Sm-Nd) characteristics compatible with derivation from the Bani Hamid sole granulites during prograde metamorphism and have ages of 98.8 ± 9.5 Ma, 93.0 ± 10.0 Ma (Sm- Nd) and 105 ± 4 Ma (U-Pb). The Saih Hatat high pressure metamorphic terrane beneath the ophiolite consists of two contrasting structural levels juxtaposed during exhumation following the subduction of the Arabian continental margin beneath the advancing ophiolite. PT analysis shows the HP event culminated at 450-550°C and 20.0 ± 1.5 kbar (THERMOCALC) and was characterised by a clockwise PTt path. In conjunction with ambiguous existing geochronology, a peak exhumation rate of ~4-12 mm yr-1 is indicated, followed by erosion at ~0.5 mm yr-1.

Hyperalkaline (pH >11) spring waters across the Oman-UAE ophiolite have precipitated large-scale surface and subsurface freshwater carbonate deposits within and on mantle-sequence peridotites. Ten carbonate lithofacies have been identified that display a common stratigraphy across the region, with some on-going deposition. Alternating wet and arid climate periods, consistent with previous Quaternary climate data for Oman and the UAE, can be inferred from the morphologies and stratigraphy of carbonate lithofacies. Carbon and oxygen stable isotope ratios for individual lithofacies are consistent across the ophiolite, indicating uniform formation processes throughout the region. Modern carbonates actively precipitate via the drawdown of atmospheric CO2 into hyperalkaline groundwaters, indicated by very negative δ13C values (≈ –25‰) and δ18O (≈ –18‰) caused by kinetic fractionation. In contrast, ancient travertines display a much wider range in δ13C and δ18O, reflecting mixing of several C and O sources including atmospheric CO2, dissolved limestone CO32– and soil DIC. Radiocarbon data for ancient travertines show a series of stratigraphically impossible 14C “ages” which are interpreted to be due to the incorporation of a proportion of 14C-dead carbon. The sources of 14C-dead carbon are a mixture of dissolved limestone CO32–, soil DIC and the re-working of older carbonate deposits. The proportion of carbon from different sources must be taken into account when calculating sequestration rates of atmospheric CO2 into carbonates. The incorporation of 14C-dead carbon into carbonate lithofacies leads to a systematic offset in conventional radiocarbon ages towards older ages. Use of offset, older ages will cause underestimation of the rate of carbon sequestration, whilst the presence of 14C-dead carbon will cause overestimation of the volume of atmospheric carbon being stored. Age offsets in carbonates mean that sequestration rates taking into account all carbon in the system, e.g. from atmospheric or limestone CO32–, will be higher than previously thought.

This thesis presents the results of a combined magnetic fabric and palaeomagnetic analysis of lower crustal rocks exposed in the Oman (Semail) ophiolite. This has long been an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution, but magnetic investigations in the ophiolite have been limited. Analyses presented here involve using: (i) magnetic anisotropies as a proxy for magmatic petrofabrics in lower crustal rocks in order to contribute to outstanding questions regarding the mode of accretion of fast-spread oceanic crust; and (ii) classical palaeomagnetic analyses to determine the nature of magnetization in these rocks and gain further insights into the regional-scale pattern of tectonic rotations that have affected the ophiolite. The extensive layered gabbro sequences exposed in the Semail ophiolite have been sampled at a number of key localities. These are shown to have AMS fabrics that are layer-parallel but also have a regional-scale consistency of the orientation of maximum anisotropy axes. This consistency across sites separated by up to 100 km indicates large-scale controls on fabric development and may be due to consistent magmatic flow associated with the spreading system or the influence of plate-scale motions on deformation of crystal mushes emplaced in the lower crust. Detailed analysis of fabrics in a single layer and across the sampled sections are consistent with either magmatic flow during emplacement of a melt layer into a lower crustal sill complex, or traction/drag of such layers in response to regional-scale stresses (e.g. mantle drag). Together, results support formation of the layered gabbros by injection of melt into sill complexes in the lower crust. New anisotropy data from the overlying foliated gabbros sampled at two key localities also provide insights into the style of melt migration at this crustal level. Fabrics are consistent with either focused or anastomosing magmatic upwards flow through this layer, reflecting melt migration beneath a fossil axial melt lens. Previous palaeomagnetic research in lavas of the northern ophiolitic blocks has demonstrated substantial clockwise intraoceanic tectonic rotations. Palaeomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization. Systematic sampling resolves for the first time a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used previously to infer lack of significant rotation of the southern blocks. In contrast, E/ENE-directed remanences in the uppermost gabbros imply a large, clockwise rotation of the southern blocks, of a sense and magnitude consistent with that inferred from extrusive sections in the northern blocks. Hence, without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times may lead to erroneous tectonic interpretation.

Ce travail repose sur (1) une étude de terrain des cumulats lités de l'ophiolite d'Oman, notamment dans plusieurs massifs où cette unité restait largement inexplorée, (2) une étude pétrographique et géochimique (microsonde électronique et LA-ICP-MS) d'environ 700 échantillons et (3) une simulation en laboratoire de la cristallisation de magmas boninitiques en présence d'eau. La cinquantaine de faciès pétrographiques définie, gabbroïques et ultrabasiques, est indispensable pour décrire rigoureusement la croûte profonde omanaise. Cette variété lithologique est révélatrice de la très grande variabilité des paramètres intensifs (P, T°, PH2O ...) et extensifs (composition) lors de la cristallisation des magmas au sein des chambres magmatiques à l'aplomb d'un centre d'expansion océanique. L'orthopyroxène précoce dans les cumulats primitifs (XMg > 80 %) est beaucoup plus commun que ce qui était proposé jusqu'à présent, ce qui remet en cause certaines idées reçues concernant l'origine de l'ophiolite. Je démontre que cette variété lithologique, l'abondance d'orthopyroxène, et d'autres caractéristiques pétrographiques et géochimiques des cumulats d'Oman s'expliquent par le mélange, dans des proportions variables, de deux principaux magmas : un d'affinité tholéiitique issu de la fusion d'une source asthénosphérique " N-MORB ", et un d'affinité andésitique à boninitique issu de la fusion hydratée d'une source lithosphérique déprimée. Ces deux magmas ont circulé dans des chenaux mantelliques (" filons ") avant d'alimenter les chambres. Le rôle de " mélangeur " des chambres magmatiques est ainsi confirmé. La contribution relative de ces deux magmas évolue au cours de l'ascension de diapirs asthénosphériques dans la lithosphère. Des variations abruptes de la pression partielle d'eau sont également indispensables pour rendre compte de l'ensemble de mes observations. Je montre aussi que les failles syn-accrétions, dont l'importance en Oman restait largement sousévaluée, sont un vecteur principal de la pénétration de fluides hydrothermaux jusque dans la croûte profonde en cours de cristallisation. Mon étude me permet également de mieux contraindre l'origine, encore très discutée, des intrusions " wehrlitiques " et de la zone de transition dunitique.

L'etude petrographique et geochimique de l'ophiolite de semail (massif salahi, oman) qui represente le plus vaste affleurement de lithosphere oceanique fossile permet de distinguer 3 phases de volcanisme, presentant des rapports ta/th anormaux qui sont interpretes comme les temoins d'une anomalie mantellaire regionale. Un modele d'evolution en contexte oceanique franc est propose. L'etude des differents episodes de circulation hydrothermale est egalement abordee

L'ophiolite d'Oman est un fragment de dorsale océanique téthysienne obducté sur la marge continentale arabe. Sa section mantellaire contient de nombreux filons basiques assimilés à un réseau de drainage magmatique mantellique fossile ayant alimenté la croûte. L'étude pétrographique et microsonde d'un millier de filons conduit à définir une quarantaine de types lithologiques différents montrant une grande variété de conditions pétrogénétiques dans le manteau. Ces lithologies appartiennent à deux séries de différenciation dont les caractères pétrographiques et géochimique sont bien distincts. La première s'apparente à une série tholéiitique de type N-MORBs et affleure dans quelques régions limitées dont une bande parallèle à l'axe de la paléo-dorsale centrée sur la zone diapirique de Maqsad. L'alimentation de la croûte (majoritairement de type "N-MORB") s'est probablement focalisé au niveau de ces quelques zones. La chimie de ces cumulats couvre largement le domaine des MORBs, illustrant la faiblesse du mélange dans un réseau de drainage magmatique au sein du manteau et la précocité de la cristallisation dans l'histoire des magmas. Les caractères pétrographiques montrent que ces magmas ont circulé dans un environnement "asthénosphérique" relativement chaud. La seconde série s'apparente à un magma parent plus siliceux et déprimé de type andésitique. Elle affleure sur la plus large part de la section mantellaire et est quasi absente dans les zones "tholéiitiques". Les modes d'affleurement et les caractères pétrographiques de ces filons impliquent un contraste de température liquide-encaissant qui pourrait provoquer une cristallisation rapide du magma. Ce qui entraînerait une difficulté d'extraction à l'origine de leur rareté dans la section crustale omanaise. Certains caractères chimiques de ces roches et l'existence de lithologies métamorphiques magnésiennes sont compatibles avec la refusion d'un manteau déprimé hydraté lors d'un précédent épisode d'hydrothermalisme.

The thesis presents the application of the magnetotelluric (MT) sounding method to image Earth’s crust in Oman and South Australia. The aim of these MT surveys is to provide constraints on the geological interpretation of emplacement scenarios and the tectonic evolution of the geological domain. The thesis concentrates on the methodological aspects of the MT technique, e.g. the data analysis and modelling of electromagnetic fields. The phase tensor approach by Caldwell et al. (2004) is applied to the data and provides insights into the dimensionality of the MT data in even complex and electrically distorted terranes. Modelling and inversion of the MT data is performed with various 2-D and 3-D codes to show how the interpretation of the data can benefit from multiple modelling approaches. Data collected in a 2-D survey across the Oman ophiolite mountains show complex behaviour and 2-D inversion and 3-D forward modelling resolve ambiguities in the emplacement scenario of the Oman ophiolite. It is believed that initial underthrusting of the Jurassic-Cretaceous oceanic lithosphere was followed by exhumation. Further oceanic thrusting subsequently led to rising of lower-plate eclogites and eventually gravitational collapse of the ophiolite onto the margin (Gray et al., 2000). The 3-D inversion code by (Siripunvaraporn et al., 2005a) was expanded to incorporate static shift corrections and inversion model misfits have therefore improved significantly compared to inversion models without static shift correction. 2-D and 3-D surveys across the South Australian Gawler Craton reveal deep crustal conductors which are connected to near surface mineralisation systems of the IOCG Olympic Dam deposit in the north-eastern part of the craton and the Au-dominated central Gawler Craton provinces.
Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2008

The formation of carbonate minerals during alteration of ultramafic rocks represents a geological analogue of mineral carbon sequestration. In the Oman Ophiolite, these carbonation reactions are manifested in (1) active, on-going low-temperature systems involving meteoric water, which result in serpentinization, carbonate vein formation, and travertine precipitation at alkaline springs, and (2) older, higher- temperature systems, which resulted in completely carbonated peridotite, known as listvenite. Employing electron microprobe analysis, X-ray diffraction, stable and clumped isotope thermometry, Sr isotope geochemistry, and geochemical modeling, this study seeks to constrain the conditions under which natural carbonation has occurred in the Oman ophiolite, with the broader goal of understanding what factors and feedbacks control efficient carbonation of peridotite. Near low-temperature alkaline springs emanating from peridotite in Oman, networks of young carbonate veins are prevalent in highly serpentinized peridotite. A notable feature in some carbonate-veined serpentinite samples is the coexistence of Fe-rich serpentine and quartz. At a given pressure, the formation of iron-rich serpentine at the expense of magnetite should be favored at lower temperatures. Calculations of thermodynamic equilibria in the MgO-SiO2-H2O-CO2 system show that serpentine + quartz is stable assemblage at sufficiently low temperatures (e.g., less than ~15-50℃), and is stabilized to higher temperatures by preferential cation substitutions in serpentine over talc. Thus, the observed serpentine + quartz assemblages could result from serpentinization at near-surface temperatures. Clumped isotope thermometry of carbonate veins yields temperatures within error of the observed temperatures in Oman groundwater, while the d18O of water calculated to be in equilibrium with carbonate precipitated at those temperatures is within error of the observed isotopic composition of Oman groundwater. As groundwater geochemistry suggests that carbonate precipitation and serpentinization occur concomitantly, this indicates that both hydration and carbonation of peridotite are able to produce extensive alteration at the relatively low temperatures of the near-surface weathering environment in Oman. Along some locations near the basal thrust of the ophiolite, hydrothermal alteration of peridotite in the Samail Ophiolite of Oman has resulted in the formation of listvenite, characterized by complete carbonation, in which all of the Mg and much of the Fe has been incorporated into carbonate minerals, resulting in a rock composed primarily of magnesite (and/or dolomite where Ca has been added) + quartz. Mineral parageneses and clumped isotope data from magnesite and dolomite suggest that carbonate phases within the listvenite formed at peak temperatures ~100℃. CO2-enriched fluids were likely derived from underlying calcite-bearing sediment during emplacement of the ophiolite. Initial 87Sr/86Sr values in the listvenite vary from 0.7085 to 0.7135, mostly significantly higher than seawater values, and are consistent with values within the underlying allochthonous and autochthonous metasediments. An internal Rb- Sr isochron from one listvenite sample yields an age of 97 ± 29 Ma, consistent with the timing of emplacement of the ophiolite. Release of pore fluid during compaction of subducted sediments may result in similar carbonation of peridotite in the shallow hanging wall of other subduction/obduction environments. These natural systems demonstrate that significant carbonation of peridotite may occur even at low temperatures, but can be much more efficient at higher temperatures. Furthermore, complete carbonation of peridotite may be achieved, in spite of the potential for armoring of reactive surfaces and reduction of permeability, as demonstrated by the formation listvenite. These natural processes of hydrothermal alteration and weathering could potentially be accelerated to provide a permanent storage solution for the disposal of CO2 via the in situ formation of solid carbonate minerals in peridotite.

Tese de mestrado em Geologia Económica, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2017
O Sultanado de Omã situa-se na península arábica e desde sempre despertou grande interesse geológico por nele aflorar a maior e mais bem preservada secção de manto superior, crosta oceânica e sedimentos pelágicos, conhecida como ofiolito de Sumail. A obducção deste ofiolito sobre a plataforma arábica ocorreu durante o Cretácico Superior, relacionando-se com o fecho do oceano Tétis. A sequência obductada pode atingir os 20 km de espessura e ocupa toda a região nordeste do Sultanado (Montanhas de Omã), formando uma faixa de direção NE-SE com ca. de 550 km de comprimento e 150 km de largura. Da base para o topo esta sequência compreende harzburgitos tectonizados (manto superior), uma porção gabroica com texturas variadas cortada por dioritos e trondjemitos, e um complexo de diques que alimenta a sequência extrusiva (lavas em almofada) superior. Esta última pode ser subdividida, dependendo dos autores, até um máximo de 5 unidades principais. Geotimes é a unidade basal desta sequência extrusiva, repousando diretamente sobre o complexo de dique em dique. Sobre esta assenta a unidade Lasail que, de acordo com Kusano et al. (2012), desenvolve interdigitações com a Geotimes sugerindo a possibilidade de corresponder a uma mera subunidade da última. A unidade Alley repousa sobre as duas últimas e partilha uma relação espacial com a unidade Boninítica. Os depósitos de sulfuretos maciços vulcanogénicos localizam-se maioritariamente na região norte de Omã, entre as sequências lávicas, sobretudo na transição Geotimes-Lasail. Barrie e Hannington (1999) descrevem estes depósitos como sendo do tipo máfico devido à natureza das rochas encaixantes típicas nestes sistemas ofiolíticos. Os minérios são ricos em Cu e pobres em Pb quando comparados com outros depósitos do tipo VMS (Volcanogenic Massive Sulphide). As primeiras evidências de mineração destes depósitos remontam à Idade do Bronze, tendo incidido sobretudo nos domínios de enriquecimento secundário (supergénico) de cobre. As minas de Lasail, Bayda e Aarja, no distrito de Sohar, representam as primeiras explorações de minério, rico em calcopirite, conhecidas no país. O depósito de Mandoos 1 localiza-se no norte de Omã, no distrito de Sohar, e foi descoberto em 2009 após campanha de prospeção geofísica (VTEM). Trata-se de um depósito oculto, coberto por gravilhas não consolidadas, possivelmente hospedado na unidade vulcânica Alley. A modelação tridimensional do depósito com o software MICROMINE revela um corpo mineralizado com 550 m de comprimento e largura máxima de 370 m, alongado segundo NW-SE e inclinado para SE; a sua espessura varia entre 1 e 90 m. O cálculo do volume da lentícula de minério aponta para um valor global de 1.8 milhões m3 que, assumindo uma densidade média de 4.44 g/cm3, resulta numa tonelagem de minério de aproximadamente 8 Mt com 1.72 wt% Cu. A modelação espacial da distribuição dos valores de concentração de Cu e Zn no depósito não colocou em evidência qualquer tipo de zonamento composicional relevante. Em Mandoos, duas campanhas distintas de amostragem foram efetuadas: uma amostragem pontual levada a cabo pela Dra. Ana Jesus em 2015, ao longo da frente de exploração Este; e uma amostragem em sondagens cedidas pela Mawarid à Universidade GUtech. Destas amostragens resultou uma coleção de 42 amostras, 27 das quais referentes a minérios sulfuretados, 6 representando jaspes mineralizados, 5 documentando sedimentos metalíferos e 4 basaltos mineralizados. Todas estas amostras foram alvo de caracterização petrográfica e geoquímica detalhada com o intuito de contribuir com informações úteis à compreensão do depósito de Mandoos e ao estabelecimento de um modelo metalogenético coerente. Os minérios associados a depósitos do tipo máfico são geralmente dominados por pirite, apresentando quantidades subordinadas de calcopirite e esfalerite. Tal é observado nos minérios texturalmente e mineralogicamente monótonos de Mandoos 1, os quais incluem ainda brechas cimentadas por sílica e alguns filossilicatos. Os sulfuretos de ferro predominam (principalmente pirite), ocorrendo ainda calcopirite, esfalerite, algumas fases mais ricas em cobre e, raramente, galena. Por sua vez, as fases minerais constituintes da ganga correspondem maioritariamente a quartzo, filossilicatos e laumontite. Os minérios são relativamente pouco retrabalhados termicamente, permitindo a preservação de um grande número de arranjos espaciais primários, sendo bastante comum a presença de texturas framboidais e colomórficas, para além de icnofósseis tipo polychetae piritizados. A origem biogénica de texturas framboidais foi rejeitada, tendo esta sido esta atribuída à propriedade ferromagnética da greigite, precursor da pirite em texturas framboidais. As fases mais ricas em Cu (bornite, covelite e spionkopite) são correlacionáveis com os estádios mais tardios de mineralização, evidenciando um evento mais oxidante. Esta lentícula mineralizada é alimentada por um stockwork caracterizado por basaltos silicificados com disseminações de pirite e alguns veios preenchidos por pirite e calcopirite, não sendo evidente o desenvolvimento de uma rede anastomosada de veios mineralizados. Texturas primárias de marcassite (colomórfica) e crescimentos de wurtzite em espaço aberto (e em contexto proximal de black-smoker) são observadas em algumas das amostras estudadas. Contudo, a coexistência de duas fases de ferro (pirite e marcassite) e de zinco (esfalerite e wurtzite) foi atribuída às variações físico-químicas abruptas que se podem fazer sentir, em poucos centímetros, nestes sistemas. Estas duas fases (marcassite e wurtzite) cristalizam preferencialmente em equilíbrio com soluções hidrotermais ácidas, sendo a mistura destas soluções com a água do mar responsável pelo aumento de pH que, por sua vez, conduz ao desenvolvimento das fases mais estáveis nessas condições (pirite e esfalerite) Relativamente aos sedimentos metalíferos (umbers e ochres), estes evidenciam texturas sedimentares com alternâncias de bandas mais ricas ou empobrecidas em óxidos e hidróxidos de ferro. Adicionalmente, observam-se também pseudomorfoses de sulfuretos, completamente oxidados. Por sua vez os jaspes mineralizados apresentam abundantes disseminações de sulfuretos, sendo ainda cortados por veios selados por pirite e calcopirite. A formação dos jaspes foi interpretada como síncrona do evento evolutivo mais oxidante, responsável pela formação das fases mais ricas em Cu, enquanto a génese dos sedimentos metalíferos foi atribuída a eventos tardios de oxidação da lentícula mineralizada no fundo oceânico e/ou à precipitação de metais libertados para a coluna de água em contexto exalativo e ambiente oxidante. Os dados de natureza geoquímica, mais precisamente a análise dos padrões de concentração normalizada de terras raras, sugere que a deposição dos minérios sulfuretados terá sido governada por misturas de água do mar e fluido hidrotermal, tal como nos jaspes, embora com proporções distintas. Os padrões obtidos para os sedimentos metalíferos (umbers e ochres) sugerem adsorção significativa de terras raras disponíveis na água do mar pelos filossilicatos constituintes dos sedimentos. As terras raras contidas nos minérios sulfuretados e nos jaspes deverão resultar da lixiviação dos basaltos a muro da mineralização, aquando da alteração hidrotermal. Os enriquecimentos em terras raras evidenciados pelas amostras de minério e jaspes podem ser alcançados com uma mistura de água do mar:basalto entre 0.85:0.15 e 0.99:0.01 para basaltos pouco alterados e entre 0.50:0.50 e 0.95:0.05 no caso de basaltos alterados. A modelação das magnitudes características dos fluidos hidrotermais de análogos modernos (TAG e EPR) podem ser simuladas com proporções entre 0.999:0.001 e 0.9999:0.0001, contudo a anomalia positiva em Eu e fracionação positiva em terras raras leves e pesadas típicas nestes fluidos não são completamente reproduzidas. Os enriquecimentos relativos em Cd, W, Sn, Te e In revelados por alguns minérios sulfuretados indicam a possibilidade do fluido hidrotermal mineralizante incluir uma contribuição magmática (episódica? e muito diluída), a qual pode ainda justificar outras diferenças composicionais observadas.
The modelling of the Mandoos 1 massive sulphide deposit revealed an orebody of ca. 8 million tonnes with an average grade of 1.72 wt% Cu. The spatial distribution of Cu and Zn concentrations suggests that the ores are relatively homogenous in composition and were not significantly affected by metal zoning refinement processes. The massive sulphide lens is mainly composed of iron sulphide rich (pyrite ± marcasite) breccias, cemented by silica ± phyllosilicates ± laumontite, having chalcopyrite (commonly altered to Cu-rich phases) and sphalerite as accessory minerals. The ores preserve large number of primary textures (colloform and framboidal) and also tubeworms (polychetae). The observed large amount of trace elements in the majority of mineral phases are consistent with the poor thermal reworking of the ores, also compatible with the monotonous and low diverse mineral assemblage. This Cu-rich VMS deposit is characterised by secondary Cu enrichment, developed during an oxidising event, which might also be synchronous of jaspers formation. When these sulphide ores were not covered, a seafloor weathering produced sequences of umbers and ochres with variable thickness, where massive sulphide breccias are oxidised and impregnated with silica in a more cold and oxidising environment, although these can also be formed by direct precipitation of metals within the water column, derived from exhalative discharges and deposited in such conditions. The REE normalised patterns of sulphide ores are consistent with a mixture of seawater and vent fluids (derived from basalt leaching – SW:B). The relative REE enrichments displayed by the ore samples can be simulated by a ratio of seawater (SW) and basalt (B) scattered between 0.85:0.15 and 0.99:0.01 for less altered basalts, while for altered basalts (B’) the values range from 0.50:0.50 to 0.95:0.05. The same is observed in sulphide-mineralised jaspers, while the REE in umbers and ochres derives entirely from seawater. Present-day vent-fluids REE magnitudes (TAG an EPR) are simulated with SW:B and SW:B’ interactions between 0.999:0.001 and 0.9999:0.0001. Nevertheless the pattern displayed by modern vent fluids, characterised by an evident Eu positive anomaly and positive LREE and HREE fractionation, is not completely reproduced by the performed simulation. Evidence of magmatic contributions to the hydrothermal mineralising fluid is supported by overall enrichments in Cd, W, Sn, Te and In; these contributions may also explain some other observed compositional differences.

Carbon dioxide capture and storage will be necessary to mitigate the effects of global climate change. Mineral carbonation - converting carbon dioxide gas to carbonate minerals - is a permanent and environmentally benign mechanism for storing carbon dioxide. The peridotite section of the Samail Ophiolite is host to exceptionally well-developed, naturally occurring mineral carbonation and serves as a natural analog for an engineered carbon dioxide storage project. This work characterizes the geochemistry and hydrogeology of peridotite aquifers in the Samail Ophiolite. Water samples were collected from hyperalkaline springs, surface waters, and boreholes in peridotite, and recent mineral precipitates were collected near hyperalkaline springs. Samples were analyzed for chemical composition. Geochemical data were used to delineate water-rock-CO₂ reactions in the subsurface and constrain a reaction path model for the system. This model indicates that mineral carbonation in the natural system is limited by the amount of dissolved carbon dioxide in water that infiltrates deep into the aquifer. The amount of carbon dioxide stored in the system could potentially be enhanced by carbon dioxide injection into the aquifer. Reaction path modeling suggests that injection of water at saturation with carbon dioxide at 100 bars pCO₂ and 90⁰C could increase the carbonation rate by a factor of up to 16,000 and bring carbonation efficiency to almost 100%. Dissolved gas samples from boreholes were collected at in situ conditions and analyzed for chemical composition. Boreholes with pH > 10 contain millimolar levels of dissolved hydrogen and/or methane, indicating these boreholes are located near areas of active low temperature serpentinization. Serpentinization rates were calculated using groundwater flow estimates and dissolved gas concentrations, and range from 3x10⁻⁸ to 2x10⁻⁶ volume fraction peridotite serpentinized per year. Additionally, laboratory incubation experiments show dissolved hydrogen can be stored in sealed copper tubes for at least three months with neither diffusive loss nor production of hydrogen from oxidation of the copper. These experiments demonstrate that copper tubes can be practical containers for collecting and storing dissolved hydrogen in freshwater. Groundwater ages in the peridotite section of the Samail Ophiolite are investigated through analysis of tritium, dissolved noble gases, and stable isotopes. Tritium-³Helium dating was used to estimate the age of modern groundwaters (< 60 years old), and helium accumulation was used as relative age indicator for pre-bomb groundwaters (> 60 years old). Waters with pH < 9.3 have ages from 0-40 years, while waters with pH > 9.3 are all more than 60 years in age. Helium accumulation indicates pH < 10 waters contain only atmospheric and tritiogenic helium, while pH > 10 waters have accumulated 30-65% of their helium from radiogenic production or mantle helium. pH > 10 waters are thus significantly older than pH < 10 waters. Noble gas temperatures are generally around 32⁰C, close to the current mean annual ground temperature. One hyperalkaline borehole has noble gas temperatures 7⁰C cooler than the modern ground temperature, indicating the water at that site may have recharged during a glacial period. Stable isotope data (Δ¹⁸O and Δ²H) for waters with pH < 11 plot between the northern and southern local meteoric water lines, in the typical range for modern groundwater. Hyperalkaline boreholes and springs are enriched in Δ¹⁸O, which suggests they recharged when the southern vapor source dominated, perhaps during glacial periods. Lastly, the potential for in situ mineral carbonation in peridotite is investigated through reactive transport modeling of dissolved CO₂ injection into a peridotite aquifer. Injection was simulated at two depths, 1.25 km and 2.5 km, with reservoir conditions loosely based on the peridotite section of the Samail Ophiolite. The dependence of carbonation extent (mass of carbon dioxide sequestered as carbonate minerals per unit volume) on different factors - such as permeability, reactive surface area, and temperature - was explored. Carbonation extent is strongly controlled by reactive surface area (RSA), with geometric RSA models producing 10 to 770 times more carbonation than conservative RSA models with the same initial permeabilities and temperatures. The ratio of carbon dioxide supply to RSA is also a key factor. The ideal relationship between CO₂ supply and RSA appears to be from 5x10⁻⁴ to 0.2 kg CO₂ /day per m²/m³ RSA. Temperature has also has an impact on carbonation rate: for the same initial permeability, carbonation is 7-35% faster at 90⁰C than at 60⁰C. Simulations of a 50-year carbon dioxide injection show that fracture porosity and permeability do not become overly clogged and carbonation continues at a more or less constant rate. We estimate that one dissolved CO₂ injection well in peridotite could store 1.4 Mtons CO₂ in 30 years with a storage cost of $6/ton. This suggests that an engineered carbon dioxide storage project in peridotite could be both feasible and economical. In situ mineral carbonation in peridotite should continue to be investigated as a safe and permanent mechanism for carbon dioxide storage.

Relatório de estágio de mestrado, Geologia Económica (Prospecção Mineral)Universidade de Lisboa, Faculdade de Ciências, 2017
Durante o Cretácico, o complexo contexto geológico e geodinâmico do ofiolito de Sumail, situado no Omã, permitiu o desenvolvimento de sistemas hidrotermais e consequente formação de numerosas mineralizações de sulfuretos maciços, algumas com interesse económico, encontrando-se hospedadas na sequência vulcânica. Esta é dividida em quatro principais unidades vulcânicas (da mais antiga para a mais recente): Geotimes, Lasail, Alley e Boninitic Alley. Apesar da maior parte destes depósitos se encontrarem hospedados na transição entre Geotimes e Alley, qualquer período de quiescência vulcânica, muitas vezes marcados pela presença de sedimentos metalíferos e outros marcadores de actividade hidrotermal exalativa, traduz um horizonte temporal onde a precipitação e acumulação de sulfuretos maciços é possível. Esta província metalogénetica de VMS do tipo máfico/tipo Chipre, é, então, comparável, em dimensão e tonelagem, ao ofiolito de Troodos, Chipre, com 30 a 50Mt de tonelagem total de minérios. O depósito de Mandoos, situado perto de Sohar, na parte Norte do ofiolito de Sumail, foi descoberto em 2009 através de técnicas geofísicas aeroportadas (VTEM), e as operações de extração tiveram início em 2011, em open-pit. Com recursos medidos que perfazem um total de 8 Mt, com 5 Mt de reservas (a 1.8% de Cu), este depósito representa um dos maiores depósitos descobertos neste ofiolito. Apesar disto, a inexistência de estudos detalhados desta mineralização, fazem dele um ótimo caso de estudo, permitindo o desenvolvimento de um estudo metalogenético aprofundado, (possivelmente) contribuindo para a definição de critérios de prospeção deste tipo de mineralizações. Assim, recorrendo a amostras recolhdas no open-pit e em sondagens cedidas pela Mawarid Mining, este trabalho teve como objectivo a caracterização da sequência vulcânica que hospeda esta mineralização, utilizando critérios mineralógicos e geoquímicos, com objectivo de I) atribuir uma posição volcano-estratigráfica ao depósito, e II) caracterizar os padrões de alteração observados, distinguindo entre halos de alteração imputáveis a episódios regionais, de baixa temperatura (“metassomatismo oceânico”) e halos de alteração hidrotermal, associados à precipitação e acumulação de sulfuretos maciços. As rochas vulcânicas que hospedam esta mineralização correspondem a rochas basálticas, de natureza toleiitica, caracterizadas por anomalias positivas, baixas, em Ti. Os teores baixos em ETRL (comparados com os dos ETRP), os padrões normalizados (condrito C1) de ETR e os seus teores em HSFE permitem atribuir estas rochas à unidade Lasail, colocando a possibilidade das rochas vulcânicas a muro da mineralização corresponderem a vulcanismo transicional entre esta unidade e a Geotimes. Considerando as características geoquímicas, particularmente de elementos traço e, também, os dados publicados (e.g. Godard et al., 2003; 2006; Kusano et al., 2012) para esta unidade vulcânica, considera-se que estas derivam da fusão parcial de rochas mantélicas, sujeitas anteriormente a taxas de fusão parcial elevadas, aquando da formação de fundidos silicatados máficos, tipo MORB, cuja extrusão originou a unidade infrajacente, Geotimes, num contexto de rift e formação de crosta oceânica. A sequência vulcânica é composta por basaltos microporfiríticos, sob a forma de escoadas maciças e/ou pillow-lavas, com quantidades e distribuição de vesículas variáveis, traduzindo os canais por onde os gases escaparam, durante a sua extrusão. Estas encontram-se também localmente fragmentadas, sendo esta fragmentação resultante de processos de arrefecimento brusco, relacionados com a interacção com água do mar. Estes processos estão associados a fracturação e fragmentação, criando descontínuidades físicas que permitem o escoamento de fluidos, gerando texturas similares a brechas. Os microfenocristais de clinopiroxena, de composição augítica, e de feldspatos do grupo das plagioclases, de composição bytownítica, encontram-se dispersos, por vezes em aglomerados, numa matriz composta por micrólitos de feldspatos do grupo das plagioclases, de composição semelhante à dos microfenocristais, e óxidos de Fe com quantidades consideráveis de Ti. Um primeiro episódio de alteração, de natureza pervasiva e ubíquo, correspondente a alteração regional decorrente da circulação de água do mar, em condições oxidantes, emm razões água/rocha baixas (metassomatismo oceânico), processo comumente apelidado de espilitização. Este estádio de alteração é marcado pela alteração das fases vulcânicas primárias: no caso dos cristais de clinopiroxena, esta alteração dá-se preferencialmente nos bordos e em fracturas, e esta é materializada por anfíbola e, posteriormente, marcada por clorite+epídoto; por sua vez, os microfenocristais e micrólitos de plagioclase registam um incremento significativo da molécula albítica na sua composição, sendo apenas encontrados núcleos que preservam a composição primária; os óxidos de ferro disseminados na matriz apresentam, frequentemente, bordos corroídos, e, na proximidade destes, são encontrados cristais de titanite. A matriz destas rochas encontra-se substituída por filossilicatos, com características expansivas, possivelmente interestratificados clorite-esmectite. A presença destes interestratificados poderá resultar, assim, da alteração de vidro vulcânico. Este episódio é também caracterizado pelo preenchimento das vesículas por fases minerais neo-formadas, como é o caso dos interestratificados clorite-smectite, calcite e zeólitos cálcicos. Estas ocorrem frequentemente, também, sob a forma de precipitados que preenchem veios. Frequentemente, são também encontrados veios preenchidos por hematite+carbonatos+clorite, que marcam a transição para o segundo episódio de alteração identificado. Na zona SE do depósito, assim como no open pit, são encontrados jaspes e umbers – rochas argilíticas ricas em (hidr)óxidos de Fe e Mn-, resultado de actividade exalativa hidrotermal de baixa tempertura. O desenvolvimento deste sistema hidrotermal está, possivelmente, controlado estruturalmente, e, perto do contacto com estas rochas exalativo-hidrotermais, observa-se, na matriz das rochas vulcânicas, uma forte impregnação de hematite, sendo apenas reconhecível alguns fenocristais de clinopiroxena. Na restante área, longe da influência directa das estruturas por onde os fluidos hidrotermais oxidantes, ricos em Fe e Mn, circulam, este episódio está marcado pela presença de veios de hematite+quartzo. São, também, frequentemente, observados veios de carbonatos calcíticos, que cortam estas rochas exalativas, localmente preenchendo fracturas en echelon, com brechificação sobreimposta. A actividade hidrotermal relacionada com a mineralização de sulfuretos maciços (e a intensidade da alteração associada) é, também, controlada estruturalmente, mas também função da porosidade e permeabilidade das rochas vulcânicas, i.e. abundância e distribuição das vesículas. Com base em geotermómetros baseados na variabilidade composicional da clorite, é possível estimar temperaturas entre 250-400ºC, para o fluido hidrotermal relacionado com a mineralização. De forma geral, distinguem-se diferentes fases de circulação de fluidos hidrotermais, dispostos numa rede de veios e filonetes, preenchidos por: I) Qz+Py+Chl; II) Qz+Py+Ccp; III) Cb+Py/Qz+Zeol; IV e V) Qz microcristalino. O controlo estrutural para a circulação destes fluidos hidrotermais é também reconhecível através das evidências de reabertura e sobreposição destas estádios de precipitados hidrotermais. A acompanhar a alteração hidrotermal associada à mineralização, ocorrem esporádicas disseminações de cristais euédricos, sub-microscópicos, de apatite, sugerindo uma contribuição de fluidos magmático-derivados, associados à cristalização das intrusões sub-vulcânicas doleríticas. Estas rochas apresentam também disseminações de apatite, assim como alguns sulfuretos (sub)microscópicos. Na área S/SE da mineralização, a alteração hidrotermal está marcada por uma silicificação fraca, disseminação de pirite e processos mais intensos de cloritização (comparando com as amostras recuperadas no open pit), com veios de quarto, clorite, pirite e, ocasionalmente, calcopirite. A partir dos dados de quimica mineral de grãos de chlorite, obtidos a partir destas amostras, é possível concluir que o fluido hidrotermal mineralizante, circulou a temperaturas entre os 200-400ºC, com valor mediano de ~270ºC. Por sua vez, nas amostras provenientes do open-pit são observáveis processos alteração hidrotermal marcados por silicificação mais intensos, acompanhados pela deposição de clorite, sob a forma de agregados de granularidade reduzida, disseminação de pirite, e ocasionalmente de calcopirite e esfalerite. Em ambos os casos é possivel encontrar, por vezes, evidências mineralógicas atribuíveis ao primeiro estádio de alteração descrito (metassomatismo oceânico), nomeadamente micrólitos de plagioclase albitizados. Durante a alteração hidrotermal, os balanços de massa que se estabelecem são, geralmente, negativos, com perdas de massa significativas, caracterizados por perda considerável em Si, Ca ou Mg, compatível com a decomposição das fases ferromagnesianas primárias e neo-formação de fases secundárias. Ganhos em Fe, acompanhados por Mg, indicam a presença de fluidos hidrotermais diluídos (misturados com água do mar), enquanto algumas zonas do open-pit são caracterizadas por ganhos em Fe, não acompanhados por ganhos em Mg, sugerindo a circulação de fluidos hidrotermais não diluídos. A estes domínios, registam também ganhos consideráveis em Cu, Zn, As, Ba e ainda em In, Sn, Tl e Cd, na ordem das dezenas de ppm, indicando ainda um input de fluidos tardios de oridem magmática, para o fluido hidrotermal, responsável pela mineralização. É também possível observar veios de hematite, posteriores à alteração hidrotermal, revelando que, durante o colapso térmico do sistema hidrotermal, outro(s) episódio(s) exalativo-hidrotermal de baixa temperatura sejam responsáveis pela deposição de jaspes/umbers. Assim, a mesma fonte de calor que permite sustentar os sistemas produtivos hidrotermais representa também, a fonte de calor que alimenta outros sistemas, de mais baixa temperatura, favorecendo a recorrência de alguns estádios de evolução do sistema, em períodos temporais distintos. Não obstante a amostragem abranger uma área significativa do depósito e permitir a caracterização da evolução do(s) sistema(s) hidrotermal/hidrotermais responsáveis pela formação deste depósito, uma amostragem mais completa, assim como a identificação e mapeamento das principais estruturas tectónicas presentes na área, permitiria a caracterização espacial do padrão de distribuição dos halos de alteração hidrotermal. Por fim, a utilização dos índices CCPI, AI e dos AImaiores e AItraço corroboram os dados mineralógicos e texturais, traduzindo uma boa correlação entre os dados de geoquimica de rocha total com as modificações mineralógicas, texturais e composicionais descritas. Assim, a sua utilização afigura-se interessante na prospecção de mineralizações do tipo VMS para o ofiolito de Omã.
The Sumail ophiolite hosts numerous epi to syn-genetic massive sulphide mineralizations in its Cretaceous volcanic sequence. The Mandoos is one of the largest VMS deposits found in the ophiolite and the lack of detailed studies on its genesis makes it a prime case-study. Thus, this work aims to develop a mineralogical and geochemical characterization of the volcanic sequence that hosts the mineralization, in order to develop a metallogenetic model for the formation of the deposit, characterizing the hydrothermal alteration associated to the massive sulphide formation, the pinpoint of its volcanostratigraphic position and to develop geochemical criteria, helpful for mineral exploration purposes. Considering the REE contents and patterns, and the HSFE contents, these volcanic rocks are considered part of the Lasail volcanic unit, which rocks derive from the partial melting of an already depleted DMM mantle source, which underwent a previous melt extraction, in a ridge setting. These are microporphyritic basaltic rocks, with tholeiitic affinities, with a primary mineralogy is characterized by augitic clinopyroxene and bytownitic to labradoritic feldspar microphenocrysts, set in a plagioclase microlite rich matrix, with disseminated Ti-rich magnetite crystals. Oceanic metasomatic alteration is pervasive and responsible for the alteration and replacement of the primary volcanic minerals by (Ca-)amphiboles+Chl+Ep, Ab and Ttn and by the alteration of volcanic glass to interlayered Chl-Sme, as well as other secondary mineral phases such as calcite and calcic-zeolites, as vesicles and fractures infills. Sub-volcanic intrusions (dolerites) act as heat sources that allow to sustain structurally controlled hydrothermal systems, in different periods of time. Low-temperature hydrothermal systems are responsible for late-developed Hem+Qz±Chl±Cb veins, throughout the sampled area, and, in the SE part of the deposit, for the deposition of jaspers and umbers. Overall, the hydrothermal alteration includes silicification, dissemination of pyrite and chloritization and, in most of the area it is associated to hydrothermal quartz veins, sometimes occurring with sulphides, distinguishing four different hydrothermal vein stages: i) Qz+Py, ii) Qz+Py+Ccp±Sph which are, sometimes, re-opened and Cb+Py or Qz+Zeol precipitate, denoting an evolution of the hydrothermal fluid; iii and iv) Qz. The hydrothermal system, responsible for the mineralization, is characterized by temperatures between 200-430 ºC; it’s circulation is responsible for the development of different alteration facies, sometimes with preservation of oceanic metassomatic alteration effects, such as the albitized microlites. This is the result of the primary volcanic rocks permeability but, more importantly, the proximity to the structural features that act as fluid-flow channels. In extreme cases, the hydrothermal alteration is marked by intense brecciation, with hydrothermal quartz and sulphides-bearing veins, silicification and sulphides dissemination, revealing that the mineralization also took place as sub-seafloor replacement. The lithogeochemical data is coherent with the mineralogical transformations, using the AI and CCPI indexes. Additionally, during the registered alteration stages, elements such as Ti, Zr, Ta, Yb, Y, Nb and Ta are considered to have an immobile behaviour, some of them showing a slight mobility, especially during hydrothermal alteration. Nevertheless, the established mass balances, during hydrothermal alteration, record negative balances. Nevertheless, Fe gains are usually accompanied by Mg additions, highlighting the presence of diluted fluids but, for the sampled area of the open pit, the presence of non-diluted hydrothermal fluids is inferred by gains in Fe (but not in Mg) and metals such as Cu and Zn. Also, gains in In, Cd, Sn and Tl, as well as Ap disseminations in hydrothermal altered samples, reveal an input of late-magmatic fluids into the hydrothermal system. Finally, the use of AImajor and AItrace is found to be useful in distinguishing hydrothermal alteration, related to sulphide deposition, from regional metassomatic alteration, useful for exploration surveys.