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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

MAGMA GENESIS AND COMPOSITION OF THE SLAB-DERIVED FLUIDS BENEATH THE SE MARIANA INTRAOCEANIC ARC, WESTERN PACIFIC

Ribeiro, Julia 17 April 2013 (has links) (PDF)
Subduction zones are places where one hydrated oceanic plate goes underneath another plate, and releases its fluids into the overlying mantle wedge. Slab-derived fluids play a key role in subduction zone processes. They serpentinize the cold forearc mantle at shallow depths; and deeper, they trigger hydrous mantle melting beneath the arc volcanoes and sometimes at backarc basin (BAB) spreading center. Examining the composition of arc and BAB magmas helps understanding genesis of subduction-related magmas, nature and composition of their mantle sources and slab-derived fluids. However, investigating such processes at shallow subduction zones is challenging, because the cold forearc mantle generally does not melt. Here, I investigate an unusual region in the southernmost Mariana convergent margin in the Western Pacific, near the Challenger Deep. The SE Mariana forearc stretched to accommodate opening of the southernmost Mariana Trough ~5Ma ago, opening the SE Mariana forearc rift (SEMFR) and causing seafloor spreading ~2.7-3.7Ma ago. The subducted slab beneath SEMFR deepens from <50km to ~100km, thus studying SEMFR lavas provides a unique opportunity to understand shallow subduction processes. By examining the major and trace element composition, the Pb-Nd-Sr isotopic ratios and the volatile contents (H2O, CO2, Cl, S, F) of SEMFR basalts, associated glassy rinds and olivine-hosted melt inclusions (Ol-MI) collected during three cruises (YK08-08, YK10-12, TN273), I show that: (i) SEMFR lavas were produced by adiabatic decompression melting of depleted asthenospheric BAB-like mantle at ~30±6.6 km depth and 1224±40oC; (ii) Ol-MI represent hydrous melts trapped by forearc mantle olivines. Xenocrysts were entrained with SEMFR basalts during ascent; (iii) SEMFR mantle flowed from the forearc towards the arc volcanoes and was metasomatized by shallow aqueous fluids; (iv) SEMFR shallow fluids are more aqueous than the fluids released beneath the Mariana arc and Mariana BAB; (v) the aqueous slab-derived fluids and the volatile fluxes are greatest at ~50-100km slab depth, suggesting that the minerals from the subducting plate mostly broke down beneath the arc to release their fluids. Such results provide new insights into shallow subduction processes, as previous studies showed that volatile fluxes and aqueous slab-derived fluids should increase toward the trench.
2

Étude expérimentale des réactions de déshydratation de l’antigorite naturelle à haute pression / High pressure experimental study of natural antigorite dehydration reactions

Maurice, Juliette 17 March 2017 (has links)
Les serpentinites sont les roches produites par l’hydratation de la péridotite au niveau du plancher océanique. L’antigorite est la phase de haute température et haute pression appartenant au groupe minéral des serpentines, pouvant contenir dans sa structure jusqu’à 13 wt% H2O, et permet ainsi le transfert de quantités considérables d’eau dans le manteau, à travers les processus de subduction. Sa déstabilisation est fonction du chemin thermique emprunté par la plaque plongeante. Durant cette thèse nous avons étudié deux cas de figure pour la déshydratation de l’antigorite menant soit à la libération des fluides dans le coin mantellique et à la production des magmas d’arc, soit au transfert de l’eau à plus grandes profondeurs).Dans un premier temps, des expériences de déshydratation d’antigorite naturelle ont été conduites sur la presse multi-enclumes à 3 GPa et entre 600 et 900°C. Les conditions oxydantes ou bien réductrices ont été contrôlées par le dispositif expérimental (four en graphite ou en chromite de lanthane). Cette étude a permis de caractériser les produits de déshydratation de l’antigorite dans un système chimique représentatif des systèmes naturels ainsi que de contraindre l’état redox des réactions associées. En effet, les résultats mis en avant par cette étude montrent une fO2 équivalente au tampon Quartz-Magnétite-Fayalite (QFM) +5. Un tel potentiel oxydant des fluides issus de la déshydratation de l’antigorite soutient l’hypothèse de l’oxydation de la source mantellique des magmas d’arcs, présentant des rapports Fe3+/Fetotal plus élevés que les basaltes de ride médio-océanique par exemple.Dans un second temps, nous nous sommes intéressés aux modalités de transfert de l’eau dans le manteau profond. L’antigorite naturelle a cette fois été déstabilisée à de plus fortes pressions allant de 6.5 à 10 GPa pour des températures comprises entre 500 et 850°C. Ces résultats expérimentaux, ainsi qu’une analyse géométrique des relations de phases dans le system FMASH selon la méthode de Shreinemaker, ont mis en avant des modifications dans le diagramme de phase pour un système ultramafique hydraté en comparaison des études précédentes. En effet, la phase A est communément décrite comme le produit de déstabilisation de l’antigorite à haute pression, tandis que la phase E n’apparait qu’à des profondeurs plus importantes. Nos résultats suggèrent, dans le système naturel enrichi en aluminium et en fer, une stabilité continue des phases hydratées, suivant la transition antigorite > phase E > phase A pour des températures inférieures à 750°C. Cette étude a également permis d’affiner les estimations des quantités d’eau pouvant être stockées dans les assemblages de minéraux hydratés stables dans la lithosphère plongeante (slab). Dans le cas des plaques plongeantes relativement froides (<750°C à 8-10 GPa) le transport de l’eau par le biais des « Dense Hydrous Magnesium Silicates » (DHMS) phase A et phase E soutient l’hypothèse de l’hydratation de la zone de transition dans le manteau. / From the Mid-Oceanic-Ridge to the subduction trench, hydration of peridotite minerals in the upper part of the oceanic lithosphere produces hydrous phases such as serpentine. Because of its high-water content (13 wt% H2O) this mineral family is of particular interest for water fluxes. Depending on the thermal path followed by the lithosphere while sinking into the mantle, antigorite destabilization can either lead to fluid release in the mantle wedge or water transfer to deeper levels. During this thesis we conducted experimental investigations of antigorite dehydration in the framework of these two scenarios.First, we investigated antigorite dehydration under conditions relevant to slab water release, known to trigger partial melting and to generate arc magmatism. Multi-anvil experiments were conducted on a natural serpentinite sample, at 3 GPa and between 600 and 900°C under different redox conditions. We were able to constrain phase assemblages produced by antigorite dehydration as well as the fO2 of such reactions to 5 units above the FMQ (Fayalite-Magnetite-Quartz buffer). These results support the oxidizing character of slab released fluids, that could explain the oxidized character of arc magmas compared to Mid-Oceanic-Ridge basalts or Oceanic-Island basalts.The second experimental work conducted during this thesis allowed to refine phase equilibria involving antigorite and the Dense Hydrous Magnesium Silicates (DHMS) phase A and phase E, in a realistic chemical composition for hydrated ultramafic system. Antigorite destabilization was performed between 6.5 and 10 GPa, for temperatures in the range <500-850°C. Our experimental results, together with a Shreinemaker’s analysis in the FMASH system led to establish a modified phase diagram compared to those presented in previous studies. While phase A has been described as the high-pressure product of antigorite breakdown in the literature, followed by phase E stability at higher pressure, we propose the transitionantigorite>phase E>phase A for the aluminous and iron-rich hydrated peridotite system. This study allowed the refinement of water budgets that can be stored in relatively cold slabs (<750°C at 8-10 GPa), supporting the hypothesis of water survival down to the transition zone.

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