Alt, J.C., Kinoshita, H., Stokking, L.B., and Michael, P.J. (Eds.), 1996 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 148 5. STABLE AND STRONTIUM ISOTOPIC PROFILES THROUGH HYDROTHERMALLY ALTERED UPPER OCEANIC CRUST, HOLE 504B 1 Jeffrey C. Alt, 2 Damon A.H. Teagle, 2 Wolfgang Bach, 3,4 Alex N. Halliday, 2 and Jörg Erzinger 3 ABSTRACT Leg 148 penetrated 111m farther into Hole 504B, to a total depth of 2111 meters below seafloor (mbsf), and recovered dia- base dikes similar to the immediately overlying rocks. Secondary mineralogy and whole-rock oxygen and strontium isotopic compositions were determined for samples from Leg 148. In addition, strontium isotopic analyses were performed on previ- ously recovered samples of the lower dikes (1550-2000 mbsf). Trends observed in the lower dikes (1550-2000 mbsf) continue in the Leg 148 section (2000-2111 mbsf). These include the local presence of magnesiohornblende and secondary calcic pla- gioclase and high Ti contents of amphiboles. Cu, Zn, and S contents of the more altered rocks are low, and whole-rockzyxwvutsrqponmlkjihgfedcbaZY δ 18 θ values ofzyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 33%<r 4.5%c continue the trend of decreasing δ' 8 O with depth. These trends reflect alteration at higher temperatures (>350°C) and greater extents of recrystallization than in the upper dikes. All of the data for the new Leg 148 section are consis tent with the rocks being a continuation of a subsurface reaction zone, where metals and sulfur are leached from the crust by hydrothermal fluids and transported to form metal sulfide mineralizations on or within the crust or vent into seawater. Sr contents of the lower sheeted dikes (1550 2111 mbsf) are generally 48 62 ppm,  and whole rock  87 Sr/ 86 Sr ratios range from 0.70265 to 0.70304, only slightly elevated relative to fresh MORB values. The rocks were altered by seawater derived hydrothermal fluids, with little change in the Sr concentrations of the rocks. Although amphibole is the most abundant second ary phase, the Sr isotopic composition of the rocks is probably dominated by the abundance of secondary plagioclase. The  87 Sr/ 86 Sr ratios of hydrothermal fluids were higher than measured in variably recrystallized (10% 80%) whole rocks. The seawater component of hydrothermal fluids increased through time,  as documented by  87 Sr/ 86 Sr ratios of 0.7028 0.7034 for amphibole and chlorite,  to 0.7035 0.7038 for vein epidotes. The latter values record the composition of upwelling black smoker type end member hydrothermal fluids at Site 504. The low  87 Sr/ 86 Sr ratios for the lower dikes differ from the uniformly high ratios of the thoroughly recrystallized sheeted dikes from the Troodos ophiolite. The oceanic dikes interacted with much smaller volumes of seawater than the ophiolitic rocks and apparently to a lesser extent. These differences are in some way related to the contrasting tectonic setting, mineralogy, and chemical composition of ophiolites compared to in situ ocean crust. INTRODUCTION Ocean Drilling Program (ODP) Hole 504B in  the eastern Pacific is the deepest scientific drill hole into the ocean crust, with penetra tion to 2111 m below seafloor (mbsf) (1836.5 m sub basement) over the course of seven drilling legs. It is the only hole to penetrate through the volcanic section,  altered at low temperatures,  into the un derlying hydrothermally altered sheeted dike complex. The site has become a reference section  for the petrology, geochemistry,  hydro thermal alteration,  and magnetic and physical properties of the upper oceanic crust (Anderson  et  al.,  1982; Becker et  al.,  1989; Dick,  Er zinger,  Stokking,  et  al.,  1991; Alt,  Kinoshita,  Stokking et  al.,  1993). Hole 504B is located in  5.9 m.y. old crust,  200 km south of the intermediate spreading rate Costa Rica Rift  (Fig. 1). Before Leg 148, Hole 504B had penetrated 274.5 m of sediments (Fig. 2),  a 571.5 m volcanic section,  a 209 m transition zone,  and 945.4 m of sheeted dikes. Core recovery averaged 30% in  the volcanic section, 25% in the transition zone,  and 14% in  the sheeted dikes. During January 1 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; Halliday: anh@umich.edu 3 GeoForschungsZentrum Potsdam,  Projektbereich 4.2, Telegrafenberg,  Haus A50, D 14473 Potsdam, Federal Republic of Germany. Bach: wbach@gfz potsdam.de; Erzinger: erz@gfz potsdam.de 4Present address: Universitát Potsdam, Institut für Geowissenschaften, Postfach 601553, D-14415 Potsdam, Federal Republic of Germany. March 1993, drilling on Leg 148 deepened Hole 504B by 110.6 m (from 2000.4 to 2111.0 mbsf; Fig. 2) (Alt, Kinoshita, Stokking, et al., 1993). Core recovery averaged 14% and recovered diabase dikes, ap- parently similar to the immediately overlying dikes (Alt, Kinoshita, Stokking, et al., 1993). This paper presents secondary mineralogy and oxygen and strontium isotopic analyses of whole rocks and sec- ondary phases for rocks from Hole 504B sampled on Leg 148, plus additional Sr isotope analyses of the lower sheeted dikes recovered from the hole during Legs 137 and 140. The objectives are to con- strain the conditions of alteration, as well as hydrothermal fluid com- positions and previous reaction histories. Of particular interest are the controls on the Sr isotopic compositions of altered rocks and hydro- thermal fluids. Also addressed are whether chemical, isotopic, and mineralogical trends identified in the lower dikes (e.g., increasing Ti and Al in amphibole, decreased whole-rock S, Cu, and Zn contents, and decreased δ 18 θ values; Alt et  al.,  1989b, 1995; Zuleger et  al., 1995; Laverne et  al.,  1995), which are attributed to increasing alter ation temperatures in  the hydrothermal "reaction zone," continue in the Leg 148 section. Whole rock chemical analyses of the same sam ples analyzed in this paper are presented elsewhere (Bach et  al.,  this volume),  and more extensive and detailed mineralogical data are giv en by Vanko et  al. (this volume). SITE 504 The basement rocks sampled in  Hole 504B comprise aphyric to highly phyric tholeiitic basalts, having moderately evolved mid 57