HR: 0800h
AN: V51B-1482    [Abstracts]
TI: Chlorine Stable Isotopes in Three Subduction Zones, With Inferences For Serpentinization and Fluid Flow
AU: * Wei, W
EM: wewei@ucsd.edu
AF: Scripps Institution of Oceanography, SIO 0212 UCSD, La Jolla, CA 92093 United States
AU: Kastner, M
EM: mkastner@ucsd.edu
AF: Scripps Institution of Oceanography, SIO 0212 UCSD, La Jolla, CA 92093 United States
AU: Spivack, A J
EM: spivack@gso.uri.edu
AF: Graduate School of Oceanography, Univ. of Rhode Island, Narragansett, RI 02882 United States
AB: Pore fluids from Mariana, Nankai Trough, and Costa Rica subduction zones were analyzed for chlorine stable isotope ratios (³⁷Cl/³⁵Cl). At the Mariana subduction zone (ODP Site 1200), δ³⁷Cl values are positive and range from 0 to 1.8‰. In contrast, at Costa Rica (ODP Sites 1039/1253, 1043/1255, 1040/1254) and Nankai (ODP Sites 808, 1174, and 1173) subduction zones, δ³⁷Cl values are negative and exhibit the largest observed range in δ³⁷Cl. At the Costa Rica subduction zone, δ³⁷Cl values range from seawater value to -5.5‰ (Site 1040/1254) along the décollement and a fracture zone in the prism. At the Nankai subduction zone, δ³⁷Cl ranges from seawater value of 0‰ to -7.8‰, the latter is the most negative value of all the ODP pore fluids analyzed so far. Chlorine isotopes fractionate when they are incorporated into diagenetic or metamorphic hydrous minerals where Cl substitutes for OH groups. Due to vibrational energy differences, the formation of high temperature (>250°C) hydrous minerals preferentially consumes ³⁷Cl (Schauble et al., 2003), thus enriching the residual fresher fluid in ³⁵Cl. Accordingly, when the minerals dehydrate, Cl, enriched in ³⁷Cl, is released into the fluid. This behavior is well manifested by the Cl isotope data of the pore fluids and solids, mostly serpentines, at the Mariana subduction zone (ODP Site 1200); both are enriched in ³⁷Cl. The pore fluid δ³⁷Cl increases from seawater value at the seafloor to ~+1.8 % at 71 meter below sea floor, the deepest sample obtained. The pore fluids at this site originate at ~27 km, where dehydration of the subducting slab occurs (Mottl et al., 2003) and the Cl concentration in the upwelling fluid is diluted relative to bottom seawater by ~8-9%. The Mariana serpentines contain hundreds of ppm Cl, having δ³⁷Cl values that range from 1 to 4‰. When they dehydrate, Cl with enriched ³⁷Cl, as well as H2O, are released into the pore fluid. Therefore, the upwelling fluid exhibits the positive δ³⁷Cl but lower than sewater Cl concentration. The uptake of Cl by the formation of hydrous minerals is manifested in the δ³⁷Cl in the pore fluids at Nankai and Costa Rica subduction zones. At Nankai, the negative δ³⁷Cl observed was explained by mixing the in situ fluid with a laterally advecting deep-sourced fluid carrying the negative δ³⁷Cl signal (Spivack et al., 2002). In the Costa Rica subduction zone, the existence of a deep-sourced fluid has been proposed based on other geochemical tracers (Silver et al., 2000; Chan and Kastner, 2000). The negative δ³⁷Cl signature observed in the fluids from these two subduction zones thus reflects the hydrous mineral formations at the fluid source. Cl concentration and isotope systematics can thus distinguish between dehydration concurrent with high temperature hydrous mineral formation at Nankai Trough and Costa Rica, and serpentine dehydration in Mariana.
DE: 1030 Geochemical cycles (0330)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4805 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912)
DE: 4832 Hydrothermal systems (0450, 1034, 3017, 3616, 8135, 8424)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005