Felsic veins in gabbros drilled by IODP at Atlantis Bank (Southwest Indian Ridge; Expedition 360): Formation, metamorphism and their role for fluid and mass transfer: first results

Hole U1473 (32° 42.3622’ S; 057° 16.6880’ E), located on the summit of Atlantis Bank at the ultra-slow spreading Southwest Indian Ridge was drilled to 789.7 m below seafloor (mbsf) during IODP Expedition 360. It consists of massive gabbros cut by nearly 400 felsic veins, which are evolved, SiO2 -enriched lithologies comprising ~1.5 vol% of the drill core. They vary in composition from diorite to trondhjemite (Dick et al., 2015). For their formation 3 endmember models are discussed: (1) fractional crystallization as consequence of oxide saturation at a late stage of tholeiitic magma evolution; (2) anatexis of mafic rocks in the deep oceanic crust, triggered by hydrothermal fluids; (3) liquid immiscibility in an evolved MORB system. This project aims to clarify their formation, their metamorphic evolution and their role to act as pathways for transporting fluids into the deep crust.