Leg 148 penetrated 111 m farther into Hole 504B, to a total depth of 2111 meters below seafloor (mbsf), and recovered diabase 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 previously 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 plagioclase and high Ti contents of amphiboles. Cu, Zn, and S contents of the more altered rocks are low, and whole-rock delta18O values of 3.3‰–4.5‰ continue the trend of decreasing delta18O 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 consistent 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 87Sr/86Sr 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 secondary phase, the Sr isotopic composition of the rocks is probably dominated by the abundance of secondary plagioclase. The 87Sr/86Sr 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 87Sr/86Sr 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 87Sr/86Sr 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.
Date of initial receipt: 12 August 1994
Date of acceptance: 30 January 1995
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