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PhD defenses in December

Thursday, December 6th at 02:00 PM : Carlotta Ferrando "Melt transport and assimilation-precipitation processes through the heterogeneous lower oceanic crust : microstructural and petro-geochemical constraints from drill cores"
University of Montpellier, campus Triolet - amphi 23.01.
Place Eugène Bataillon, Montpellier
Abstract : The continuous cycle of solid Earth begins at mid-ocean ridges where the ascent and crystallization of magmas forms the oceanic crust, which later is destined to return to the mantle at subduction zones. Ocean floor represents 70% of the Earth surface.
About 55% of the mid-ocean ridge system spread at rates lower than 40 mm/y, and is referred to as slow-spreading ridge. The slow-spread oceanic crust presents an extremely heterogeneous structure, which is partly composed of serpentinized mantle rocks intruded by multiple gabbroic bodies. It is locally exposed by long lived detachment faults (Oceanic Core Complex, OCC), which thus provide a unique opportunity to sample in situ oceanic crust.
During deep drilling expeditions at the Atlantis Massif OCC (30°N, Mid-Atlantic Ridge) and the Atlantis Bank OCC (32°S, 57°E, Southwest Indian Ridge) two sections of the heterogeneous oceanic crust were sampled. The study of these two gabbroic sequences allowed to investigate the magmatic and cooling processes that shape the oceanic crust formed at slow-spreading ridges. The combined macro- and microscopic observations, together with detailed microstructural analyses, revealed that the texture and microstructures of gabbroic rocks are the result of extensive melt-rock interaction processes. Mineral geochemical analyses reveal that simple magmatic crystallization is not the solely control on the formation of the oceanic crust. Mineral assimilation associated to melt transport through a pre-existing mineral matrix must be invoked. A reactive percolation numerical model was used to simulate the formation of the most primitive end-member of the plutonic section from Atlantis Massif : results indicate a mantle origin for the precursor olivine-rich matrix.
The reactive process contributes significantly to shaping the lower oceanic crust at slow-spreading ridges. On the other hand, its role on the composition of the erupted basalts is likely controlled by melt productivity in the upwelling mantle.