Gabbroic dikelets in serpentinized peridotites from the Mid-Atlantic Ridge at 23°20′N

Mantle-derived peridotites sampled on the seafloor at 23° N along the MidAtlantic Ridge during the Hydrosnake and R.V. Akademic Mstislav Keldysh cruises show evidence of minor magmatic intrusion that produced millimetre to centimetre thick gabbroic dikelets. The nature of the intruding magma and the time relationships between intrusion events and tectonic evolution of the host peridotites is inferred from the mineralogy and textural features of veins cutting the peridotites. The occurrence of Fe-Ti-rich minerals and of zircon in the veins suggests an evolved, basaltic composition of the fluid. Pyroxene-rich, plagioclase-free veins probably represent pyroxenite dikelets inside the peridotites. In all cases, melt intrusion followed ductile deformation affecting the mantle-derived ultramafics. These observations fit with a model in which gabbroic intrusions are emplaced in mantle rocks at shallow depths beneath the ridge axis.