Mineralogy and Stable Isotopic Compositions of the Hydrothermally Altered Lower Sheeted Dike Complex, Hole 504B, Leg 140

Abstract

Drilling during Legs 137 and 140 of the Ocean Drilling Program deepened Hole 504B, the only hole to penetrate through the volcanic section and into the underlying hydrothermally altered sheeted dike complex, by 438.1 m to a total depth of 2000.4 meters below seafloor. This paper presents the secondary mineralogy, bulk-rock sulfur contents, and stable isotopic (O, S) compositions, plus oxygen isotopic compositions of secondary minerals from the lower sheeted dike complex drilled during Legs 137 and 140. Various evidence indicates higher temperatures of hydrothermal alteration in the lower dikes than in the upper dikes, including: the local presence of secondary clinopyroxene in the lower dikes; secondary anorthite and hornblende in the lower dikes vs. mainly actinolite and albite-oligoclase in the upper dikes; generally increasing Al and Ti contents of amphibole downward in the dike section; and greater 18 O depletions of the lower dikes (δ 18 θ = 3.6-5.0‰) compared with the upper dikes. Early high-temperature alteration stages (T = 350°-500°C) resulted in 18 O depletions and losses of metals (Cu, Zn) and sulfur from the rocks. Local incorporation of reduced seawater sulfate led to elevated δ 34 S values of sulfide in the rocks (up to 2.5‰). Quartz + epidote formed in crosscutting veins at temperatures of 310°-320°C from more evolved fluids (δ 18 θ = l‰). Late-stage lower-temperature (~250°C) reactions producing albite, prehnite, and zeolites in the rocks caused slight 18 O enrichments, but these were insufficient to offset the 18 O depletions caused by earlier higher-temperature reactions. Addition of anhydrite to the rocks during seawater recharge led to increased S contents of rocks that had previously lost S during axial hydrothermal alteration, and to further increases in δ 34 S values of total S in the rocks (up to 12‰). Despite the evidence for seawater recharge to near the base of the sheeted dike complex, the paucity of late zeolites in the lower dikes suggests that late-stage, off-axis circulation was mainly restricted to the volcanics and shallowest dikes, or to localized high-permeability zones (faults) at depth.

Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

“…4), and δ 18 θ values progressively decrease throughout the dikes ( Fig. 5; Alt et al, 1986aAlt et al, , 1989bAlt et al, , 1995Zuleger et al, 1995;Laverne et al, 1995). These mineralogical and chemical changes generally reflect higher alteration temperatures and greater extents of reaction with depth, in conditions approaching the amphibolite facies.…”

“…The Fe 3+ /Fe Tot ratio is fairly constant with depth in the lower dikes ( 0.25), but is slightly elevated above the initial igneous value (0.12, vertical line), which indicates a moderate pervasive oxidation. The average secondary mineral proportions were calculated per 25 m depth from shipboard and published point counting and petrographic estimates (Becker et al, 1989;Dick, Erzinger, Stokking, et al, 1992;Alt, Kinoshita, Stokking, et al, 1993;Alt et al, 1995;Zuleger et al, 1995 . Strontium, oxygen, and sulfur isotopic compositions for whole-rock samples and selected secondary minerals with depth for Hole 504B.…”

“…The lower dikes underwent an early high-temperature alteration stage (>400°C), which resulted in the formation of secondary calcic plagioclase and hornblende, mainly in localized centimeter-scale alteration zones. This stage preceded the alteration sequence outlined above for the upper dikes, and alteration at temperatures >350°C was more pervasive and/or persistent in the lower than in the upper dikes (Laverne et al, 1995;Alt et al, 1995;Vanko et al, this volume). Subsequent alteration stages were similar to those in the upper dikes, with greenschist minerals in veins and rocks cut by later quartz plus epidote veins.…”

“…Hblde = hornblende. Data from this work and Alt et al (1985Alt et al ( , 1986aAlt et al ( , 1986bAlt et al ( , 1995.…”

“…Of particular interest are the controls on the Sr isotopic compositions of altered rocks and hydrothermal 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, 1989bAlt et al, , 1995Zuleger et al, 1995;Laverne et al, 1995), which are attributed to increasing alteration temperatures in the hydrothermal "reaction zone," continue in the Leg 148 section. Whole-rock chemical analyses of the same samples analyzed in this paper are presented elsewhere (Bach et al, this volume), and more extensive and detailed mineralogical data are given by Vanko et al (this volume).…”