HR: 0800h
AN: V31A-1425    [Abstracts]
TI: Fluid Flow in the Main and Fore Arc of Central America: $^{129}$I as Tracer of Subduction Processes
AU: * Fehn, U
EM: fehn@earth.rochester.edu
AF: Earth and Environmental Sciences, Univ. of Rochester, Rochester, NY 14627 United States
AU: Snyder, G T
EM: gsnyder@rice.edu
AF: Earth Science, Rice University, Houston, TX 77251 United States
AU: Wallmann, K
EM: kwallmann@geomar.de
AF: IFM-Geomar and SFB 574, Kiel University, Kiel, 24148 Germany
AU: Hensen, C
EM: chensen@geomar.de
AF: IFM-Geomar and SFB 574, Kiel University, Kiel, 24148 Germany
AU: Lu, Z
EM: luzunli@earth.rochester.edu
AF: Earth and Environmental Sciences, Univ. of Rochester, Rochester, NY 14627 United States
AB: Iodine combines the most biophilic behavior with the largest ionic radius among the halogenes, which, together with the presence of the long-lived cosmogenic isotope, $^{129}$I (T$_{1/2}$ = 15.7 Myr), makes it an excellent tracer for fluid movement and the origin of organic material. We have used this system for the detection of recycled marine sediments in subduction zones by determining the isotopic composition in geothermal and volcanic fluids. $^{129}$I /I ratios in fluids collected from main arc settings in Central America, Japan, the Andes and New Zealand result in ages, which are site specific and are compatible with derivation from recycled marine sediments. We report here first results for fluids collected from mud-volcanoes in the marine section of the Central American fore arc and compare them to earlier results from the main arc (Snyder and Fehn, 2002). Shallow pore water samples were collected during several cruises, organized by the SFB 574 at Kiel University (e.g. Hensen et al., 2004). The water depths of the sampling sites ranged from 1000 m at the shelf to 5400 m close to the axis of the Central American Trench. Iodine is enriched by factors of 1000 or more in these fluids compared to sea water and occurs together with methane in these locations. Initial results for $^{129}$I /I ratios in these fluids are between 400 and 200x10$^{-15}$, giving ages between 30 and 45 Ma, which are not compatible with derivation from the currently subducting marine sediments ($<$25 Ma) in this area. These ages are also considerably older than those found in the main arc ($<$20 Ma), indicating different sources for fluids in main arc and fore arc. Our results suggest that the dominant source of iodine in main arc fluids are currently subducting marine sediments, while iodine in fore arc fluids is derived from older sources, probably located in the continental margin wedge, demonstrating fundamental differences in fluid transport between fore arc and main arc settings. Snyder and Fehn, GCA, 66, 3827 (2002) Hensen et al., Geology, 32, 201 (2004)
DE: 4808 Chemical tracers
DE: 4875 Trace elements
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting