Mineralogical, chemical, and isotopic results from seven drilling legs that visited DSDP/ODP Hole 504B over 14 years are compiled here to present an integrated view of hydrothermal alteration of oceanic crust at Site 504. Hole 504B reaches to 2111 mbsf, through 274.5 m sediment, 571.5 m of volcanic rocks, a 209 m transition zone, and 1050 m into a sheeted dike complex. The volcanic section was altered through a series of processes involving interaction with seawater at low temperatures, with the effects of cold, oxidizing seawater decreasing downward. These processes and their effects on the volcanic section are generally similar to those in other oceanic upper crustal sections.The transition zone and upper dikes were altered in a subsurface mixing zone, where hydrothermal fluids upwelling through the dikes mixed with cooler seawater circulating in the overlying more permeable volcanic rocks. Alteration of the transition zone and upper dikes (down to 1500 mbsf) occurred in a series of stages, reflecting the thermal and chemical evolution of the hydrothermal system from (1) early chlorite, actinolite, albite-oligoclase, and titanite, to (2) quartz, epidote and sulfides, to (3) anhydrite, and finally to (4) zeolites and local calcite. The maximum temperature estimated for the first two stages is 350°-380°C, and the inferred mineral assemblages for these early stages are typical of the greenschist facies.The lower dikes (1500-2111 mbsf) underwent an early, high-temperature (>400°C) alteration stage, resulting in the formation of hornblende and calcic secondary plagioclase, consistent with reactions inferred to occur in deep subsurface reaction zones, where hydrothermal vent fluids acquire their final compositions. Much of the subsequent reactions produced greenschist assemblages at ~300°-400°C. The lower dikes have lost metals and sulfur and are a source of these elements to hydrothermal vent fluids and seafloor sulfide deposits. The lower dikes underwent subsequent alteration stages similar to the upper dikes, with rare epidote + quartz veins recording the presence of upwelling hydrothermal fluids, and limited late off-axis effects (zeolites and prehnite). Anhydrites in the lower dikes indicate more reacted fluid compositions than in the upper dikes.Alteration of the sheeted dikes from Hole 504B is heterogeneous, with recrystallization controlled by fracturing and access of fluids. Defining the position of the seismic Layer 2/3 transition depends upon the scale of observation, but the change at Site 504 occurs within the sheeted dikes and is correlated with progressive changes in porosity and hydrothermal alteration. However, we still do not know the nature of the transition from sheeted dikes to gabbros in in situ ocean crust, or the nature of the inferred fault at the base of Hole 504B and its role in fluid flow and alteration.

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D