Fluid Flow in Subduction Zones: Comparison of I-129 Results From the Main and Fore Arc of Central America
Abstract
Aqueous fluids play a major role in the transport of elements during processes associated with subduction processes, but source and pathways of these fluids often are not known. The iodine isotopic system is well suited for hydrologic studies in subduction zones due to the biophilic characteristics of iodine and the presence of the long-lived cosmogenic isotope ^{129}I (T1/2 = 15.7 Myrs). We report here 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 of this subduction zone (Snyder and Fehn, 2002). During several cruises, carried out by IFM-GEOMAR over the last years, shallow pore water samples were collected from mud volcanoes and similar features of the marine fore arc section of Costa Rica and Nicaragua,. 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. In all of the samples, iodine is enriched by factors of 500 or more compared to sea water and occurs together with methane in these locations. ^{129}I/I ratios in these fluids are between 400 and 200x10^{- 15}, corresponding to minimum ages between 30 and 45 Ma, which are not compatible with derivation from the currently subducting marine sediments (<25 Ma) in this area. Iodine concentrations in the fore arc fluids are considerably higher and ^{129}I/I ratios are lower than in the main arc fluids, where iodine ages were generally younger than 20 Ma, indicating that subducting marine sediments are a major source of iodine in main arc fluids. Our results demonstrate 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, located in the upper plate. Sources and pathways of fluids are thus fundamentally different in fore arc and main of the Central American subduction zone, an observation that is in good agreement with findings in other subduction zones such as the Nankai Trough off Japan and the Hikurangi Trough off New Zealand. Snyder and Fehn, GCA, 66, 3827 (2002)
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V41B1731F
- Keywords:
-
- 1030 Geochemical cycles (0330);
- 1031 Subduction zone processes (3060;
- 3613;
- 8170;
- 8413);
- 1040 Radiogenic isotope geochemistry;
- 3004 Gas and hydrate systems