HR: 1330h
AN: T52C-0293    [PDF]
TI: Halogen Concentrations and Stable Isotopes (O, Sr, and Cl) in the Nankai Muroto Transect and Their Implication for Fluid-Sediment Interactions and Fluid Flow
AU: * Wei, W
EM: wewei@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Kastner, M
EM: mkastner@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Spivack, A
EM: spivack@gso.uri.edu
AF: Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882 United States
AB: The halogen concentrations and stable isotopic abundances of Sr, O, and Cl in the pore fluids at ODP sites 808, 1174 and 1173, provide new insights on the geochemical reactions and fluid flow at Nankai Trough. The sharp reversal in the Cl concentration gradient and oxygen isotope data (-3.4 \permil SMOW at Site 808, and -2.0 \permil SMOW at Site 1174) at the boundary between the trench-wedge and upper Shikoku Basin sediments is likely maintained by flow of a slightly more saline fluid than seawater or by in situ hydration reactions that outpace diffusion. The curvature of the $^{18}$O profile and the depth of the glacial maximum Cl concentration suggest vertical advection in the upper part of turbidite section. F concentration of the pore fluids remains about seawater value throughout the turbidite facies at Sites 808 and 1174, then increases dramatically at the volcanic ash-rich versus ash-poor lithological boundary (90 ppm at Site 808 and 60 ppm at Site 1174). The maximum F concentrations observed are significantly higher than previously reported values from all other ODP sites. A lithological control on F concentration profiles is suggested by the preliminary data of F concentration in the solids at Site 808, indicating F concentration may be related with volcanic ash abundance. Br is associated with organic matter diagenesis in the turbidite section, thus Br concentration reaches a maximum value at a rather shallow burial depth where organic matter is more abundant. At all three sites, Br concentrations decrease with depth but Br/Cl ratios do not return to, and remain higher than, the seawater value of 1.5, implying fractionation between Br and Cl. The Sr isotopic ratio exhibit similar depth profiles at the three sites, controlled by the same lithology. $^{87}$Sr/$^{86}$Sr reaches the most non-radiogenic value of about 0.7071 at Site 808 at the same lithological boundary of minimum F concentration. The more radiogenic maxima, below the d"Ýcollement at Site 808 and 1174, which coincide with the Cl minima depths, most likely reflect transformation reactions of predominantly detrital minerals having radiogenic-continental-Sr composition. These Sr and oxygen isotope data together with F and Cl concentration profiles could be explained by either 1) in situ clay dehydration and transformation controlled by lithology, temperature, and pressure; 2) transport of a deep-sourced fluid enriched in F, radiogenic Sr and $^{18}$O, but depleted in Cl which is uptaken by hydrous silicate reactions; for example, serpentine, chlorite, or talc incorporate considerable amounts of Cl in their structure, forming at temperatures of 250 to $450\deg$C; 3) a combination of both. The Cl isotope data help to differentiate between the two possible options. The highly negative Cl isotopic values observed at all three sites could not be explained by smectite dehydration alone, which would produce $^{37}$Cl enriched pore fluids. In contrast, hydrous mineral transformation would result in $^{37}$Cl depleted pore fluids, as observed. The chemical and isotopic signatures observed in the pore fluids most likely reflect a mixed signal of in situ reactions and fluid flow from depth, probably from the seismogenic zone. The extent of mixing between the two fluids is being modeled.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting