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Origine De La Diversité Géochimique Des Magmas Équatoriens : De L’Arc Au Minéral

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Origine De La Diversité Géochimique Des Magmas Équatoriens : De L’Arc Au Minéral N°d’Ordre : D.U. 926 UNIVERSITÉ CLERMONT AUVERGNE U.F.R. Sciences et Technologies Laboratoire Magmas et Volcans ÉCOLE DOCTORALE DES SCIENCES FONDAMENTALES N°178 THÈSE présentée pour obtenir le grade de DOCTEUR D’UNIVERSITÉ spécialité : Géochimie Par Marie-Anne ANCELLIN Titulaire du Master 2 Recherche Terre et Planètes Origine de la diversité géochimique des magmas équatoriens : de l’arc au minéral Thèse dirigée par Ivan VLASTÉLIC Soutenue publiquement le 17/11/2017 piouiMembres de la commission d’examen : Janne BLICHERT-TOFT Directrice de recherche à l’Ecole Normale Supérieure de Lyon Rapporteur Gaëlle PROUTEAU Maitre de Conférence HDR à l’Université d’Orléans Rapporteur Kevin BURTON Professeur à l’Université de Durham Examinateur Hervé MARTIN Professeur à l’Université Clermont Auvergne, Clermont-Ferrand Examinateur Silvana HIDALGO Professeur à l’Escuela Politécnica Nacional, Quito Invité François NAURET Maitre de Conférence à l’Université Clermont Auvergne, Clermont-Ferrand Invité Ivan VLASTÉLIC Chargé de recherce HDR à l’Université Clermont Auvergne, Clermont-Ferrand Directeur de thèse Pablo SAMANIEGO Chargé de recherche à l’IRD, Université Clermont Auvergne, Clermont-Ferrand Co-encadrant de thèse Laboratoire Magmas et Volcans Université Clermont Auvergne Campus Universitaire des Cézeaux 6 Avenue Blaise Pascal TSA 60026 – CS 60026 63178 AUBIERE Cedex ii RÉSUMÉ Origine de la diversité géochimique des magmas équatoriens : de l’arc au minéral Les laves d’arc ont une géochimie complexe du fait de l’hétérogénéité des magmas primitifs et de leur transformation dans la croûte. L’identification des magmas primitifs dans les arcs continentaux est difficile du fait de l’épaisseur de la croûte continentale, qui constitue un filtre mécanique et chimique à l’ascension des magmas. En Équateur, cette problématique est particulièrement critique du fait de la grande épaisseur de la croûte (≈ 50-60 km) et de la rareté des magmas primitifs arrivant en surface. Cette thèse a pour but de déterminer la composition des liquides primitifs dans l’arc équatorien, à l’échelle de l’arc entier, et à celle de deux édifices volcaniques : le Pichincha et le Tungurahua. Elle vise également à mieux comprendre comment ces liquides primitifs évoluent à travers la croûte continentale. En Équateur, le pendage, le relief et l’âge de la plaque plongeante varient du nord au sud de l’arc. Ainsi, la première partie de la thèse aborde la question de l’influence de ces paramètres sur la géochimie des magmas, via une étude sur roches totales couvrant la totalité de l’arc. Elle confirme les variations géochimiques décrites à travers l’arc par les études précédentes : augmentation de la teneur en éléments incompatibles et diminution de l’enrichissement en éléments "mobiles" d’est en ouest. L’étude identifie des variations géochimiques le long du front volcanique (e.g., rapport Ba/Th), liées au changement de nature des fluides métasomatiques, qui sont aqueux au centre de l’arc (environ 0,5°S) et silicatés au Nord et possiblement au Sud. Ce changement est attribué à la jeunesse du plancher océanique dans le nord de l’arc, qui pourrait promouvoir la fusion de la plaque plongeante. Enfin, il semble que la contamination par la croûte inférieure augmente vers le sud du front volcanique. Dans un deuxième temps, les produits émis par le Tungurahua lors de ses derniers 3000 ans d’activité sont étudiés. À cette échelle de temps, les paramètres tectoniques de la première étude sont constants. Ce travail détaille le rôle de la croûte dans la production des magmas différenciés, qui sont systématiquement associés à des éruptions plus explosives. Elle conclut que les andésites ont des compositions isotopiques hétérogènes (206Pb/204Pb = 18,834 - 19,038), acquises en profondeur (manteau ou croûte inférieure), qui se restreignent lors de la différenciation des andésites en dacites (206Pb/204Pb = 18,965 - 19,030), par cristallisation fractionnée et assimilation de la croûte supérieure locale (7-9 %). Enfin, la troisième partie de la thèse se focalise sur l’hétérogénéité des magmas primitifs. Des études sur minéraux individuels ont été effectuées au Pichincha et au Tungurahua, et montrent que la majorité des minéraux sont en déséquilibre avec la roche hôte (jusqu’à 8600 ppm en 206Pb/204Pb). Au Pichincha, la diversité des minéraux échantillonnés permet d’identifier la diversité des liquides mantelliques (206Pb/204Pb = 18,816 - 19,007), qui s’alignent dans les espaces Pb-Pb. Comme dans le cas des roches totales du Tungurahua, l’assimilation crustale écrase cette diversité isotopique lors de la différenciation des liquides primitifs, dont la signature n’est pas préservée dans les roches. Au Tungurahua, les minéraux individuels montrent que l’hétérogénéité des signatures est acquise en profondeur. L’analyse de deux lots d’olivines met en évidence une signature radiogénique dans les liquides primitifs du Tungurahua, interprétée comme la présence de croûte délaminée dans la source mantellique du Tungurahua. Enfin, l’étude de lots d’olivines provenant de sept volcans équatoriens montre qu’il n’existe pas de signature primitive unique dans l’arc. La totalité de l’hétérogénéité isotopique des magmas est héritée du manteau (206Pb/204Pb = 18,583 - 19,000). Les compositions des liquides primitifs sont ensuite déviées par la contamination crustale, dans la majorité des cas, vers des signatures plus radiogéniques. Mots-clés : magmas primitifs, subduction, arcs continentaux, Équateur, source, isotopes, Pichincha, Tungurahua, analyse de minéraux individuels, amphibole, plagioclase, pyroxène, olivine iii iv ABSTRACT Origin of the geochemical diversity of Ecuadorian magmas: from the arc to the mineral Arc lavas display a complex geochemistry resulting from the heterogeneity of primitive magmas and their transformation within the crust. Identifying primitive magma compositions in continental arcs is challenging because continental crust is thick and acts as a mechanical and chemical filter for ascending magmas. This issue is particularly criticial in Ecuador owing to the great thickness of the continental crust (≈ 50-60 km) and the scarcity of erupted primitive magmas. This thesis aims to determine the composition of primitive silicate melts in the Ecuadorian arc, on the scale of the whole arc, as well as on the scale of two volcanic edifices: the Pichincha and the Tungurahua. This study also intends to better understand how those primitive melts evolve during their journey through the continental crust. In Ecuador, slab dip, relief and age change from north to south. Hence, the first part of the PhD focuses on the influence of those parameters on magma geochemistry, through a whole rock study covering the entire arc. It confirms the across-arc geochemical variations described by previous studies: an increase of incompatible element contents and a decrease of fluid-mobile over fluid-immobile element ratios from west to east. We identify along-arc geochemical variations in the volcanic front (e.g. Ba/Th), related to the changing nature of metasomatic fluids, which are aqueous fluids at the centre of the arc (around 0.5°S) and silicate melts to the north and probably to the south. This change may be due to the subduction of a younger and warmer oceanic crust to the north, which might promote slab melting. Lastly, it seems that deep crustal contamination increases towards the south of the volcanic front. Secondly, volcanic products emitted for the last 3,000 years at Tungurahua are studied. On this timescale, the tectonic parameters of the first study are constant. This work details the role of continental crust in the production of differentiated magmas, which are systematically associated with more explosive eruptions. We conclude that andesites have heterogeneous isotopic compositions (206Pb/204Pb = 18.834 - 19.038), acquired at depth (mantle or deep crust), that homogeneize through andesite differentiation to dacite (206Pb/204Pb = 18.965 - 19.030) by fractional crystallization and assimilation of the local upper crust (7-9 %). Lastly, the third part of the PhD focuses on the heterogeneity of primitive magmas. We study individual minerals from Pichincha and Tungurahua volcanoes and show that most minerals are in disequilibrium with their host rock (up to 8,600 ppm for 206Pb/204Pb). The diversity of Pichincha minerals allows the identification of mantle melt diversity (206Pb/204Pb = 18.816 - 19.007), with compositions forming a tight trend in Pb-Pb isotope spaces. As for Tungurahua whole rocks, crustal assimilation erases the diversity of primitive melt isotope signatures through differentiation, so that primitive melt signatures are not preserved in whole rock samples. At Tungurahua, individual minerals show that the heterogeneity of isotope compositions is acquired at depth. The analysis of two olivine fractions reveals the existence of a radiogenic signature in the mantle source of Tungurahua volcano, interpreted as the presence of delaminated crust within the mantle beneath its edifice. Finally, olivine fractions from seven Ecuadorian volcanoes highlight the fact that no unique primitive signature exists in the arc. Isotopic heterogeneity is entirely inherited from the mantle (206Pb/204Pb = 18.583 - 19.000). Primitive melt compositions are then shifted by continental crust contamination which, in most cases, results in more radiogenic signatures. Keywords: primitive magmas, subduction, continental arcs, Ecuador, source, isotopes, Pichincha, Tungurahua, single mineral analysis, amphibole, plagioclase, pyroxene, olivine v vi REMERCIEMENTS Voilà un peu plus de trois ans que je suis arrivée dans cette ville de Clermont-Ferrand, prête pour de nouvelles rencontres et l’aventure de la thèse, et je dois l’avouer... je n’ai pas été déçue ! Ces années ont sûrement été les meilleures de ma vie. J’ai tant de monde à remercier, ça ne va pas être facile! Les premières personne que je tiens à remercier de tout cœur sont mes trois encadrants : Ivan, Pablo et François. Merci de m’avoir fait confiance et choisi pour mener à bien le projet que vous aviez imaginé, et surtout pour m’avoir autant appris pendant ces années de doctorat! Vous avez toujours su être présents à juste dose.
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