Serpentinisation des péridotites exhumées aux dorsales lentes : approches microstructurale, minéralogique et géochimique

At slow spreading ridges, fluid-rock interactions cause abundant serpentinization of the peridotites exhumed in the footwall of axial detachment faults. The conditions under which these interactions occur may be variable and influenced by the presence of variable amounts of melt or magmatic lithologies. Because the easternmost Southwest Indian Ridge (SWIR, 62°-65°E) is nearly amagmatic, it represents an end-member to study serpentinization in the absence of magmatic influences. In this thesis, we used an integrated petrological approach combining microstructural, mineralogical and chemical observations on a set of variably serpentinized peridotites primarily dredged along the SWIR (Smoothseafloor cruise, 62°-65°E). We also considered samples drilled (ODP site 920 at 23°N and ODP site 1274 at 15°N) and dredged (Serpentine cruise at 13° and 15°N) along the Mid-Atlantic Ridge (MAR). It allows us to compare our results on the eastern SWIR samples, to samples from a more magmatically robust ridge. Based on these observations and results, we addressed: • the conditions of temperature and fluid-rock ratio associated with serpentinization; • the physicochemical nature of the serpentinizing fluids; • the geometry of the preferential fluid pathways in the detachment footwalls during serpentinization. Most of our eastern SWIR samples exhibit the typical magnetite-bearing serpentine mesh texture and bastites. However, in ~25% of these samples, significant serpentine recrystallization and veins overprint these textures. These are associated with increasing fluid-rock ratios, from nearly stoichiometric during mesh texture formation to higher than 10 during recrystallization, at relatively high temperatures (280-350°C). The serpentinizing fluids become progressively supersaturated relative to serpentine leading to successive emplacement of lizardite, chrysotile and polygonal or polyhedral serpentines. We also observe late antigorite replacement due to moderate Si-metasomatism of the preexisting serpentine minerals, probably following pyroxenes serpentinization. This contrasts with the higher degrees of Si-metasomatism required to produce the talc-bearing serpentinites that are common in MAR sample sets. The serpentinizing fluids appear to be seawater-derived and to have interacted with serpentinizing ultramafics but not with magmatic lithologies. They enrich serpentine minerals in some fluid-mobile major (Cl) and trace elements (B, U). They also enhance major elements (Al, Fe, Si), trace elements (Cu, As, Sb), and REE mobility and redistribute them at local scales, and remove calcium and sodium from the peridotite. These fluids were also probably mildly alkaline (rare europium anomalies), reducing (following magnetite crystallization) and H2-enriched. These fluids may be similar, though warmer, to those venting at the ultramafic-hosted Lost City hydrothermal fluid (30°N, MAR). Our observations allow us to propose that serpentinization is heterogeneous in space and time in the exhuming footwall of detachment faults, from the base of the hydrothermal domain to late rock emplacement on the seafloor. The scales of preferential fluid pathways change with serpentinisation. They evolve from pervasive and close-spaced microfractures in the nearly fresh (~60-80 µm spacing) and mesh-textured (100-500 µm spacing) serpentinized peridotites, to thicker planar domains of enhanced fluid fluxes, spaced by ~10 cm, in the most serpentinized peridotites. The early microfracture networks are found in both the eastern SWIR and the MAR samples and probably formed by a combination of tectonic and thermal contraction stresses in the fresh peridotites, followed by reaction-induced microfracturing at the first stages of serpentinization. The distribution and geometry of the more widely spaced (~10 cm) planar domains of enhanced fluid flow and serpentine recrystallization in the eastern SWIR samples recall similar domains of enhanced alteration that have been described in serpentinized peridotites from the MAR next to altered amphibole + chlorite ± talc bearing veins.