Résumé
This PhD work is based on field, petrographic, and geochemical observations of rocks originated at the base of the sheeted dike complex, in the Oman ophiolite and at IODP Site 1256, coupled with an experimental study. It provides new constrains on processes that occur at the magma / hydrothermal system transition in oceanic crust formed at fast spreading ridges. The base of the sheeted dike complex is truncated by intrusive isotropic gabbros, and therefore reheated and recrystallized to the "granoblastic dikes" under temperatures up to 1030°C. Xenoliths of granoblastic microgabbros and microgabbronorites derived from the granoblastic dikes are commonly observed in the about 100 meters thick horizon of isotropic gabbro that underlies the sheeted dike complex. These features can be explained by upward migrations of the melt lens that is present at fast spreading centers. The occurrence of several assimilation features (xenoliths and granoblastic patches) in the isotropic gabbro horizon supports the hypothesis that this horizon represents the fossilization of the upper melt lens. The experimental study was designed to simulate experimentally the effect of partial melting of hydrothermally altered sheeted dikes. The results show that melting starts at 850°C, confirm the residual origin of granoblastic dikes and xenoliths, and attest to the anatectic origin of the oceanic plagiogranites that are commonly present close to the base of the sheeted dike complex. The major and trace element composition of the experimental anatectic melt that represents the main contaminant for primitive MORBs at fast spreading ridges has been determined. The upper axial melt lens at fast spreading mid-ocean ridges is herein described as a dynamic system that can migrate vertically, and which fossilizes when moving off-axis.