Primary and Secondary Variations in Major and Trace Element Geochemistry of the Lower Sheeted Dike Complex: Hole 504B, Leg 140

Abstract

Rocks of the lower sheeted dike complex of Hole 504B sampled during Leg 140 were analyzed for major and trace element compositions to investigate the effects of igneous processes and hydrothermal alteration on the compositions of the rocks. The rocks are relatively uniform in composition and similar to the shallower dikes. They are moderately evolved mid-ocean-ridge basalts (MORB) with relatively high MgO (7.9-10 wt%) and Mg# (0.60-0.70), and have unusually low incompatible element contents (TiO 2 = 0.42-1.1 wt%, Zr = 23-62 ppm). Discrete compositional intervals in the hole reflect varying degrees of differentiation, and olivine and plagioclase accumulation in the rocks, and may be related to injection of packets of dikes having similar compositions. Systematic depletions of total REE, Zr, Y, TiO 2 , and P 2 O 5 in centimeter-size patches are most likely attributed to exclusion of highly differentiated, late-stage interstitial liquids from small portions of the rocks.The rocks exhibit increased H 2 O + reflecting hydrothermal alteration. Replacement of primary plagioclase by albite and oligoclase led to local gains of Na 2 O, losses of CaO, and slightly positive Eu anomalies. Some mobility of P 2 O 5 led to minor increases and decreases in P 2 O 5 contents, and some local mobility of Ti may have occurred during alteration of titanomagnetite to titanite. Higher temperatures of alteration in the lower sheeted dikes led to breakdown of pyroxene and sulfide minerals and losses of Zn, Cu, and S to hydrothermal fluids. Later addition of anhydrite to the rocks in microfractures and replacing plagioclase caused local increases in sulfur contents. The lower sheeted dikes are a major source of metals to hydrothermal fluids for the formation of metal sulfide deposits on and within the seafloor.

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.…”

“…In addition to a selection of least-altered samples, several pairs of highly altered zones plus associated host diabase were investigated petrographically and analyzed for their oxygen and strontium isotopic compositions. Because of the lack of strontium isotopic data from the Leg 137 and 140 sections of Hole 504B (1550-2000 mbsf), we also determined Sr isotopic compositions of rocks from this interval.These samples have been previously characterized for their mineralogy, oxygen, and sulfur isotopic and major and trace element compositions Zuleger et al, 1995).…”

“…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.…”

“…5. The lower dike section or reaction zone (1600-2111 mbsf) is characterized by higher alteration temperatures approaching amphibolite facies conditions, greater intensity of alteration, and depletions of base metals and sulfur Laverne et al, 1995;Zuleger 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).…”