HR: 0800h
AN: S51B-0151    [Abstracts]
TI: Pore-Water Freshening in the Nankai Trough, Japan: Implications for Lateral Fluid Flow
AU: * McKiernan, A W
EM: awm@uwyo.edu
AF: University of Wyoming, Department of Geology and Geophysics, Dept. 3006 1000 E. University, Laramie, WY 82071 United States
AU: Saffer, D M
EM: dsaffer@uwyo.ed
AF: University of Wyoming, Department of Geology and Geophysics, Dept. 3006 1000 E. University, Laramie, WY 82071 United States
AB: Pore-water freshening has been observed in ocean sediments cored along the Muroto Transect in the Nankai Trough, offshore Japan during ODP Legs 190/196. Chloride concentrations up to $\sim$11% below normal seawater values ([Cl-] $\sim$555 mM) were documented at drill site 1173, 13 km seaward of the trench. Updip, lateral fluid flow from depth, and in situ clay dehydration have been proposed as freshening sources. Placing realistic bounds on the relative contributions of these potential sources has importance for quantifying fluid and chemical fluxes in subduction zones. Here, we evaluate the hypothesis that freshening is due solely to in situ clay dehydration. We simulate the general shape and magnitude of the down hole [Cl-] anomaly observed at site 1173 using a one-dimensional model which combines sedimentation, thermal history, clay dehydration, and Cl- diffusion. Sediment accumulation during arcward transport and high heat flow for the area ($\sim$180 mW/m2), result in temperatures sufficient to transform smectite to illite + H20. The volume of released water scales with initial smectite abundance (here we assume 50% by weight based on mineralogy at site 1177, where heat flow is low and clays are less reacted). In the model, released water is added at each time step creating unstable [Cl-] profiles. Chloride diffusion is modeled implicitly in a reference frame fixed to the interstitial water column, through which consolidating sediment nodes move. Basal boundary conditions used in the model are: constant [Cl-], no-flow, and the inclusion of oceanic crust in diffusion. Simulated reaction progress matches observed clay mineralogy for site 1173. Minimum simulated [Cl-] scales with initial smectite content, ranging from 4% freshening for 20% initial smectite, to 14% freshening for 70% initial smectite. Simulated [Cl-] profiles for all boundary conditions mimic the general [Cl-] profile for site 1173, but no single boundary matches observed data exactly. A no-flow basal boundary reaches a minimum [Cl-] of $\sim$500 mM (10% freshening) but does so at the sediment-oceanic crust interface, $\sim$100 m below the observed minimum. Constant-concentration basal conditions show a return to normal [Cl-] near the base of the section, but lack [Cl-] minima and maxima comparable to those observed. Oceanic crust is included in some models to simulate communication between basement and sediment. These models show similar behavior to no-flow and constant-concentration boundaries, differing mainly in magnitude.
DE: 7230 Seismicity and seismotectonics
DE: 4825 Geochemistry
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 1050 Marine geochemistry (4835, 4850)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting