HR: 0800h
AN: V51B-1488    [Abstracts]
TI: The Influence of High Seawater Fluxes on Sulfur Compositions of the Serpentinized Peridotites at the Lost City Hydrothermal Field
AU: * Delacour, A
EM: adelie.delacour@erdw.ethz.ch
AF: Dept. of Earth Sciences, ETH-Zurich, IMP Sonneggstr. 5, Zurich, 8092 Switzerland
AU: Frueh-Green, G L
EM: frueh-green@erdw.ethz.ch
AF: Dept. of Earth Sciences, ETH-Zurich, IMP Sonneggstr. 5, Zurich, 8092 Switzerland
AU: Bernasconi, S M
EM: stefano.bernasconi@erdw.ethz.ch
AF: Dept. of Earth Sciences, ETH-Zurich, Geologisches Institut Universit„tstr. 16, Zurich, 8092 Switzerland
AU: Kelley, D S
EM: kelley@ocean.washington.edu
AF: School of Oceanography, University of Washington Box 357940, Seattle, WA 98195 United States
AB: The discovery of the actively venting carbonate chimneys at the Lost City hydrothermal vent field (LCHF) on the Atlantis Massif (MAR 30°N) has stimulated great interest in the role of serpentinization in driving hydrothermal circulation in peridotite-hosted systems and in the biological communities that may be supported in these systems. The southern wall of the massif exposes serpentinized peridotites with interspersed gabbroic rocks that have undergone several phases of serpentinization, talc-metasomatism and carbonate veining related to the uplift history and to the formation of the LCHF. We present petrological and isotope data from the serpentinized peridotites and gabbros that provide constraints on the history of seawater-rock interaction, changes in oxygen and sulfur fugacities during serpentinization, and the role of serpentinization as a sink for seawater sulfur. Sr- and Nd-isotope analyses of the basement rocks of the Atlantis Massif show large, systematic changes towards seawater compositions and indicate high seawater fluxes during successive phases of serpentinization. The consequence of these high fluid-rock ratios is a change in the sulfur mineralogy and chemistry of the rocks. Most of the analyzed basement rocks show lower sulfide-sulfur and higher sulfate-sulfur contents compared to fertile mantle. Sulfate in the serpentinites is present as barite and various hydroxysulfates. The distinct absence of anhydrite provides important constraints on upper temperature limits of late-stage serpentinization and hydrothermal activity at the LCHF. The sulfates are dominated by seawater sulfur isotope signatures, which indicate that serpentinization is an important sink of seawater sulfur at the Atlantis Massif. A few samples with lower sulfur isotope compositions suggest an additional local contribution of sulfate produced by sulfide oxidation. Sulfide assemblages are dominated by pyrite, pentlandite, pyrrhotite in the serpentinites and by pyrite, pyrrhotite, chalcopyrite, pentlandite in the gabbros, and indicate high oxygen and sulfur fugacity during progressive alteration of the basement rocks at the LCHF. The sulfides in the serpentinites are enriched in ³⁴S, whereas the gabbros show typical mantle δ³⁴S values. Some samples close to the actively venting chimneys are enriched in sulfate as well as sulfide, both of which are depleted in ³⁴S, and suggest that additional processes, possibly microbially mediated, locally take place in the basement of the LCHF.
DE: 1021 Composition of the oceanic crust
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3035 Midocean ridge processes
DE: 7220 Oceanic crust
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005