Hydrothermal alteration of a section of upper oceanic crust in the eastern equatorial Pacific : A synthesis of results from Site 504 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148)

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. 'Alt, J.C. , Kinoshita, H. , Stokking, L.B. , and Michael, P.J. (Eds.), 1996. Proc. ODP, Sci. Results, 148: College Station, TX (Ocean Drilling Program). department of Geological Sciences, 2534 C.C. Little Building, The University of Michigan, Ann Arbor, MI 48109-1063, U.S.A. Alt: jalt@umich.edu; Teagle: teagle@umich.edu Laboratoire de Pétrologie Magmatique, URA CNRS 1277, Faculté des Sciences et Techniques de Saint-Jerome, Avenue Escadrille Normandie-Niemen, F-13397 Marseille Cedex 20, France, pezard@imtmerl.imt-mrs.fr "Department of Geology, Georgia State University, Atlanta, GA 30303-3083, U.S.A. geodav@gsusgil.gsu. edu Dipartimento di Geologia, Paleontologia e Geofisica, Università di Padova, Via Giotto 1, 35137 Padova, Italy, tar@epidote.dmp. unipd.it GeoForschungsZentrum Potsdam, Projektbereich 4.2, Telegrafenberg A50, D-14473 Potsdam, Federal Republic of Germany. Bach: wbach@gfz-potsdam.de; Erzinger:erz@gfz-potsdam.de 'Present address: Universitàt Potsdam, Institut für Geowissenschaften, Postfach 601553, D-14415 Potsdam, Federal Republic of Germany. GEOMAR Forschungszentrum für Marine Geowissenschaften, Wischhofstrasse 1-3, Gebaude 5, D-24148 Kiel, Federal Republic of Germany, ezuleger@geomar.de 9 Institut de Geologie, Universite Louis Pasteur, 1 rue Blessig, 67084 Strasbourg Cedex, France, honnorez@illite.u-strasbg.fr Laboratoire de Mesures en Forage, Institut Méditerranéen de Technologie, Technopole de Chàteau-Gombert, F-13451 Marseille Cedex 20, France. ' Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Sciences, 4600 Rickenbacker Causeway, University of Miami, Miami, FL 33149-1098, U.S.A. kbecker@rsmas.miami. edu Geological Survey of Canada Atlantic, Bedford Institute of Oceanography, Dartmouth, P.O. Box 1006, Nova Scotia, B2Y 4A2, Canada, matts@agc.bio. ns.ca Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, U.S.A. wilkens @ soest.hawaii. edu INTRODUCTION Hole 504B is the deepest scientific drill hole in the oceans, penetrating to 2111 mbsf (1836.5 m sub-basement) over the course of seven Ocean Drilling Program (ODP) and Deep Sea Drilling Project (DSDP) legs since 1979 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148; Cann et al., 1983; Anderson, Honnorez, Becker, et al., 1985; Becker, Sakai, et al., 1988; Becker, Foss, et al., 1992; Dick, Erzinger, Stokking, et al., 1992; Alt, Kinoshita, Stokking, et al., 1993). The hole was also visited during Leg 92 in 1983 for downhole logging and to sample borehole fluids (Leinen, Rea, et al., 1986). It is the only hole to penetrate through the volcanic section into the underlying sheeted dike complex, and the site has become a reference section for the petrology, geochemistry, hydrothermal alteration, and magnetic and physical properties of the upper oceanic crust (Anderson, Honnorez, et al., 1982; Becker et al., 1989). Previous drilling results from the site (Dick, Erzinger, Stokking, et al., 1992) and seismic evidence (Becker, Sakai, et al., 1988; Collins et al., 1989; Mutter, 1992) suggested that the bottom of the hole lay within the lower portion of the sheeted dike complex, close to the seismic Layer 2/Layer 3 boundary. It has been thought that this transition coincides with the change downward from sheeted dikes to underlying gabbros, or that it is a metamorphic boundary within gabbros or sheeted dikes (e.g., Fox et al., 1973; Christensen and Salisbury, 1975). The transition from sheeted dikes to gabbros has been observed by submersible in tectonic exposures of both Atlantic and Pa-