Drilling the oceanic mantle lithosphere: A window on melt extraction and mantle metasomatism at ridges

The oceanic crust represents more than 70% of the Earth’s surface and its continuous formation along spreading centres is one of the most notable demonstrations of plate tectonics. It is widely accepted that partial melting of the upwelling mantle beneath spreading centres feeds the dominantly basaltic ridge magmatism. Abyssal peridotites exposed at avolcanic segments and tectonic windows along slow and fast spreading ridges have long been considered as simple mantle residues of mid-oceanic ridge basalt formation. Over the last 25 years, petrological and geochemical studies of abyssal peridotites revealed more complex petrogenetic processes (mantle metasomatism, melt impregnation, …) and suggested that the transport of melts from the mantle toward the surface contributed also to the composition and architecture of the shallow mantle and of the lower crust. Scientific drilling provides the only means to document, from the micro- to tens of meter scale, the structural and textural heterogeneities resulting from these magmatic processes. So, although drilling expeditions targeting successfully mantle peridotites along ridges were rare (ODP Legs 153 and 209 along the Mid-Atlantic Ridge, ODP Leg 147 at Hess Deep), scientific drilling was, and remains, determinant for our understanding of the processes driving the construction of the oceanic lithosphere.