Hydrothermal alteration of a 1-km section through the upper oceanic crust, Deep Sea Drilling Project Hole 504B; mineralogy, chemistry, and evolution of seawater-basalt interactions

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doi: 10.1029/JB091iB10p10309
Author(s): Alt, Jeffrey C.; Honnorez, Jose; Laverne, Christine; Emmermann, Rolf
Author Affiliation(s): Primary:
Wash. Univ., Dep. Earth and Planet. Sci., St. Louis, MO, United States
Other:
Univ. Miami, United States
Ec. Nat. Ing., Tunisia
Justus-Liebig Inst., Federal Republic of Germany
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 91(B10), p.10,309-10,335. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 130 refs.; illus. incl. 1 table, charts, sketch map
Summary: Deep Sea Drilling Project hole 504B penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes. Alteration of the crust at site 504 occurred in four stages, related to the movement of the crust away from the spreading axis: (1) Dikes reacted with seawater (200->300°C) in the upwelling zone of an axial convection cell at the spreading axis, resulting in the formation of greenschist facies parageneses in veins and host rocks. Mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologie transition zone, causing a steep temperature gradient at the base of the pillow section. Secondary minerals formed in the lower pillow section from the resultant reducing "mixed" fluids at temperatures of around 100°C. At the same time, the initial effects of "seafloor weathering" began in the upper 320 m of the pillow section at low temperatures (<50°C) and under conditions of open circulation of oxidizing seawater. (2) Following refracturing of the dikes, a second stage of axial upwelling occurred; hydrothermal fluids (200°-380°C) were probably similar to those presently sampled from spreading ridges on the seafloor. Mixing of these fluids with seawater in the lithologie transition zone caused deposition of quartz, epidote, and sulfides in veins and formation of a sulfide-rich stockworklike zone within this transition zone. Alteration of the pillow section proceeded under the prior conditions, with the effects of seafloor weathering extending progressively downward into the crust. (3) Seawater recharge penetrated to depths of at least 1075.5 m subbasement and deposited anhydrite locally in veins. (4) Off-axis alteration of the dikes was characterized by formation of zeolites, calcite, and prehnite in veins and host rocks from more highly evolved and lower temperature (100°-250°C) fluids. Alteration in the upper pillow section evolved from "seafloor weathering" conditions to more reducing and rock-dominated, as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment. Zeolites and calcite were the last phases to form throughout the pillow section. Sealing of the crust to convective cooling also allowed conductive reheating of the crust from below. Calculations of seawater-crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt-seawater interactions are a sink for seawater Mg and K and a source for Si and Ca.
Year of Publication: 1986
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; 18 Geophysics, Solid-Earth; Alteration; Basalts; Crust; DSDP Site 504; Deep Sea Drilling Project; East Pacific; Fractures; Geochemistry; Geothermal gradient; Halmyrolysis; Heat flow; Hydrothermal alteration; IPOD; Igneous rocks; Lava; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Metasomatism; Mineral composition; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Permeability; Processes; Sea water; Sea-floor spreading; South American Pacific; South Pacific; Southeast Pacific; Tectonophysics; Upper crust; Volcanic rocks; Wall-rock alteration
Coordinates: N011338 N011338 W0834349 W0834349
Record ID: 1987008560
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