The composition and origin of igneous and hyrothermal veins in the lower ocean crust—ODP Hole 735B, Southwest Indian Ridge

During Legs 118 and 176, Ocean Drilling Program Hole 735B, located on Atlantis Bank on the Southwest Indian Ridge, was drilled to a total depth of 1508 meters below seafloor (mbsf) with nearly 87% recovery.The recovered core provides a unique section of oceanic Layer 3 produced at an ultraslow spreading ridge.Metamorphism and alteration are extensive in the section but decrease markedly downward.Both magmatic and hydrothermal veins are present in the core, and these were active conduits for melt and fluid in the crust.We have identified seven major types of veins in the core: felsic and plagioclase rich, plagioclase + amphibole, amphibole, diopside and diopside + plagioclase, smectite ± prehnite ± carbonate, zeolite ± prehnite ± carbonate, and carbonate.A few epidote and chlorite veins are also present but are volumetrically insignificant.Amphibole veins are most abundant in the upper 50 m of the core and disappear entirely below 520 mbsf.Felsic and plagioclase ± amphibole ± diopside veins dominate between ~50 and 800 mbsf, and low-temperature smectite, zeolite, and prehnite veins are present in the lower 500 m of the core.Carbonate veinlets are randomly present throughout the core but are most abundant in the lower portions.The amphibole veins are closely associated with zones of intense crystal plastic deformation formed at the brittle/ductile boundary at temperatures above 700°C.The felsic and plagioclase-rich veins were formed originally by late magmatic fluids at temperatures above 800°C, but nearly all of these have been overprinted by intense 1 Robinson, P.