Influence of seawater circulation on the evolution of ultramafic spreading ridges

A mid-oceanic ridge is an underwater mountain range formed by the diverging motion of tectonic plates that triggers the production of new lithosphere and the spreading of seafloor. The mid-ocean ridges of the world are connected and form a single global ridge system. It represents the longest mountain chain in the world, totaling 60,000 km. Mid-ocean ridges are geologically active, with new magma constantly intruded, and erupted onto the ocean floor, along the ridge axes. If for some reason the magma supply stops for a period of time, the crust must stretch to match the plate motion producing multiple cracks (i.e. faults) in the lithosphere and large exposure of deeper mantle rocks at the seafloor. These rocks, known as abyssal peridotites, due to the continuous interaction with seawater, are variably hydrated and transformed in serpentinite. Low-angle detachment faults, exposing the serpentinized mantle rocks at the seafloor, act as pathways for seawater allowing complex interactions with ultramafic/mafic rocks at depth with formation of serpentinitic and talc-rich rocks. Here, we illustrate our recent petrographic, geochemical and isotopic studies on a peridotite-dominated underwater massif, the Atlantis Massif, located at 30°N on the Mid-Atlantic Ridge. The consequences of its hydratation and alteration in relation to the geochemical and geodynamical evolution of this area are discussed in details