25. GEOCHEMISTRY OF FLUIDS AND FLOW REGIME IN THE DÉCOLLEMENT ZONE AT THE NORTHERN BARBADOS RIDGE 1

M. Kastner,2 Y. Zheng,3 T. Laier,4 W. Jenkins,5 and T. Ito 6

ABSTRACT

The recent three-dimensional seismic reflection experiment at the décollement zone at the northern Barbados Ridge subduction zone shows variations in the amplitude and polarity of the décollement reflection; this suggests variations in fault-zone velocity and density, hence in porosity and pore-fluid pressure that drives the fluid and influences fluid/rock reactions. The two sites drilled across the décollement zone on Ocean Drilling Program Leg 156, Sites 948 and 949, representative of positive and negative polarity reflections, are located 4 and 2 km, respectively, west of the deformation front.

Geochemical and temperature depth profiles indicate a mainly lateral, focused along faults, flow (eastward) of a warm fluid significantly fresher than seawater at both the positive and negative polarity sites. The fluid with the lower than seawater chloride concentration observed at the top of the décollement zone at Site 948 (~18% dilution) and enriched in I, Ca, Sr, Ba, Mn, Mo, Zn, and Co, is advecting from a source region that is situated deeper than the drilled depths and more arcward. A slight increase in methane concentration relative to background level indicates a source region rich in organic matter. The fluid originates from transformation reactions, including dehydration, of terrigenous clay minerals as indicated by the radiogenic strontium and helium-isotope ratios of the source fluid. In situ volcanic ash alteration reactions considerably overprint the geochemical signatures of the end-member low-Cl fluid. The chemistry and isotopic composition of the carbonate vein minerals indicate that they have precipitated from a geochemically distinct fluid from the in situ pore fluids. Injection of fluid from a deeper and hotter source caused hydrofracturing and mineralization. The multiple generations of vein carbonates imply episodic fluid-flow events, and the strong in situ diagenetic overprinting of the pore fluids suggests that a major fluid-flow episode has not occurred recently. The methane peak, however, signifies active fluid advection.

The pore fluids in the accreted and underthrust sediments are geochemically distinct, indicating little vertical advection.

1 Shipley, T.H., Ogawa, Y., Blum, P., and Bahr, J.M. (Eds.), 1997. Proc. ODP, Sci. Results, 156: College Station, TX (Ocean Drilling Program).
2 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0212, U.S.A. mkastner@ucsd.edu
3 Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, U.S.A.
4 Geological Survey of Denmark, Thoravej 8, DK-2400, Copenhagen, Denmark.
5 Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.
6 Faculty of Education, Ibaraki University, Mito, Ibaraki 310, Japan.