HR: 1340h
AN: T13B-0474    [Abstracts]
TI: Long distance migration of fluids in the Cascadia margin: evidence from iodine isotopic composition
AU: * Lu, Z
EM: luzunli@earth.rochester.edu
AF: University of Rochester, 227 Hutchison Hall, University of Rochester , Rochester, NY 14627
AU: Fehn, U
EM: fehn@earth.rochester.edu
AF: University of Rochester, 227 Hutchison Hall, University of Rochester , Rochester, NY 14627
AU: Tomaru, H
EM: hitoshi@earth.rochester.edu
AF: University of Rochester, 227 Hutchison Hall, University of Rochester , Rochester, NY 14627
AB: Hydrate Ridge, well known for its unusually high concentrations of gas hydrates, is located on the Cascadia continental margin. In order to identify potential source formations for the methane in gas hydrates, we applied the 129I system (T1/2 = 15.7 Ma) together with halogen concentrations in pore waters collected from ODP Leg 204. The biophilic behavior of iodine and the close association between iodine and methane during transportation make the iodine isotopic system a powerful tool for identifying source formations and fluid pathways. The 129I/I ratios in most of our pore water samples are between 150 and 250x10-15, corresponding to ages between 45 and 57 Ma after the correction for fissiogenic 129I from uranium. This age range is distinctively older than the ages of the currently subducting sediments (< 10 Ma) or of the formations directly underlying Hydrate Ridge, indicating that fluids contributing to the current pore waters come from formations of Early Eocene age or older in the overriding wedge. Likely source formations are located just to the west of the Siletz Terrane, the crystalline backstop in this area, which are at distances of 40 km or more from Hydrate Ridge. The results indicate that fluids must have traveled considerable distances through the overriding wedge in order to reach their present location, either through some of the ubiquitous fractures present in the overriding wedge or along the decollement. Because the results for Hydrate Ridge are consistent with observations from other hydrate occurrences in active margins, fluid movement over large distances must be an important process for the dewatering of the upper plate and the formation of gas hydrates.
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005