HR: 1340h
AN: B13A-0197    [Abstracts]
TI: Fault-hosted hydrothermal breccia at 22\deg40' N on the Mid-Atlantic Ridge
AU: * Bowles, M W
EM: mwb8@duke.edu
AF: Division of Earth and Ocean Science, Duke University, Division of Earth and Ocean Sciences Box 90227 103 Old Chemistry Building, Durham, NC 27705 United States
AU: Hayman, N W
EM: hayman@duke.edu
AF: Division of Earth and Ocean Science, Duke University, Division of Earth and Ocean Sciences Box 90227 103 Old Chemistry Building, Durham, NC 27705 United States
AU: Karson, J A
EM: jkarson@duke.edu
AF: Division of Earth and Ocean Science, Duke University, Division of Earth and Ocean Sciences Box 90227 103 Old Chemistry Building, Durham, NC 27705 United States
AU: Kelley, D S
EM: kelley@ocean.washington.edu
AF: School of Oceanography, University of Washington, 1492 Boat St., Seattle, WA 98105 United States
AB: The SMARK region is approximately 100 km south of the Kane Transform on the Mid-Atlantic ridge (MAR). The region is noteworthy for its west-facing Eastern Median Valley wall (EMVW), a 2300 m-high seismically active fault scarp that exposes the upper 1-2 km of $<$1Ma slow-spread oceanic crust. The oceanic crust at SMARK is pervasively faulted and fractured basaltic pillow lavas and dikes. The setting of the SMARK area and the nature of the samples are ideal for testing hypotheses about the relationship between faulting, hydrothermal systems, and microbial communities. Select samples collected in 1995 by dredging and with Alvin from the EMVW fault zone include fault gouges and breccias with a range of cataclastic and granular textures. Clasts are angular (but in many places with smooth grain boundaries) and consist primarily of fine-grained to glassy basalt. The matrix grains are primarily silt-sized with subordinate clay. The composition of the matrix minerals is equally magnesium and iron rich owing to a combination of orange-brown clay (palagonite) minerals, chlorite, and amphiboles. Preliminary XRD of one sample shows evidence for barite. In contrast, samples with no identified sulfur-bearing phases have concentric alteration rims of iron surrounding clasts. Iron-rich samples also contain disseminated grains of magnetite (and, in some samples, titano-magnetite). The data are interpreted in terms of a model wherein axial faults and fractures initially function as conduits rather than seals to (hydrothermal) fluid flow. Although the fault rocks would be predicted to seal the conduits as they move off-axis and cool, variation in the oxidation of iron (expressed as alteration rims, for example) within the matrix minerals is testament to continuing, and possibly off-axis fluid flow. In a setting of evolving permeability, decreasing temperature, and changing oxidation potential of fluids, we expect biological communities thriving in the sub-surface environment to have several windows of stability over time. Further clarification and tests of this model are planned using both structural and geochemical analyses of the fault rocks.
UR: http://www.duke.edu/~mwb8/GMT.html
DE: 8010 Fractures and faults
DE: 8100 TECTONOPHYSICS
DE: 8135 Hydrothermal systems (8424)
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting