The Subsurface Structure of a Submarine Hydrothermal System, ODP/IODP Hole 1256D

Hole 1256D penetrates a full upper oceanic crustal section through lavas, sheeted dikes, and ~100 m into upper plutonic rocks that formed at a superfast spreading rate at the East Pacific Rise 15 m.y. ago. Electron microprobe analyses of secondary minerals plus oxygen isotope analyses of vein minerals document the thermal structure of the hydrothermal system. The ~800 m volcanic section consists of mainly sheet and massive flows, partly altered to saponite and celadonite, typical of altered submarine basalts but with little oxidation effects. Saponite and Celadonite (n=9) have δ18O values of 13.2-17.8‰, and quartz/chalcedony (n=12) has values of 20.1-30.9‰. These indicate temperatures of 50-110°C in the volcanic section, generally increasing downward. There is a stepwise increase in alteration grade downward across the 60 m lithologic transition from lavas to dikes, which are variably altered to chlorite and other greenschist minerals at temperatures ~250-350°C. Alteration intensity and grade increase downward in the 350 m sheeted dikes, with more amphibole than chlorite below 1300 mbsf, and common retrograde effects. The lower 60 m of sheeted dikes are variably recrystallized to granoblastic textures (Ca-plagioclase, clinopyroxene, orthopyroxene, ilmenite, magnetite). Plagioclase-amphibole and two-pyroxene thermometry indicate temperatures of up to >800°C for this contact metamorphism. Oxygen isotope data indicate formation of quartz veins in the dikes at temperatures of 300°C up to 400-500°C, depending on fluid compositions. Two fractionated gabbro units intrude into granoblastic sheeted dikes beginning at 1407 mbsf. The gabbroic rocks are moderately to highly altered to amphibole, secondary plagioclase, epidote, chlorite, prehnite and laumontite. Plagioclase-amphibole thermometry indicates maximum temperatures of 400- >800°C in the plutonic section, whereas oxygen isotope analyses of quartz veins in these rocks indicate temperatures of 260°C up to 400-500°C, depending on fluid compositions. The stepped thermal gradient in the upper section is similar to that in Hole 504B, but the thermal gradient in the thin sheeted dike complex at Site 1256 was steep. The transition to the plutonic section reveals a complex history of repeated cycles of magmatism and hydrothermal alteration, with contact metamorphism influenced by prior hydrothermal effects.