HR: 0800h
AN: V41F-1534    [Abstracts]
TI: Ca, B, Sr and Cl Isotope Study on Pore Fluids in the Japan Trench Subduction Zone
AU: * Deyhle, A
EM: adeyhle@ucsd.edu
AF: Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0212 United States
AU: MacDougall, D
EM: jdmacdougall@ucsd.edu
AF: Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0212 United States
AB: Interstitial pore waters from Ocean Drilling Program (ODP) Site 1150, which recovered ca. 1200 meters of sub-seafloor sediment from the seismically active upper Japan Trench forearc, were analyzed for element concentrations and Cl, Sr, Ca and B isotopes. While chlorinity shows profound downhole freshening to values as low as ~310 mM (0.55 x seawater) in the deeper part of the claystone-dominated succession, both Sr and B concentrations show an overall increase. Sr reaches concentrations up to >250 μM (~3 x seawater), whereas B-enrichment is even stronger (3920 microM, i.e. 9.3 x seawater). The strong variations in concentration correspond to fractionation reactions in the deep, tectonically deformed part of the forearc. The heavily fractured portion of Site 1150 (ca.700 meters to the base of the hole) has two shear zones which very likely act as conduits to expel deep-seated fluids to the seafloor. These fluids do not only show the strongest freshening of Cl, but are also characterized by low d37Cl (down to -1.1‰), low 44Ca/40Ca (-1.2‰) and heaviest d11B (40-46‰), and least radiogenic 86/87Sr. These profound isotope anomalies together with the excursions in element concentrations suggest that diagenetic processes operate at depth. They include clay mineral dewatering, alteration of tephra from the Japan and Izu Arcs, and possibly transformation of biogenic silica from abundant diatoms. Given the distinct isotopic signatures off all proxies applied coupled with enrichments/depletion patterns of other elements, we suggest that enhanced fluid flow through permeable shear zones at Site 1150 may be an efficient mechanism for backflux of those elements from the deep forearc into the hydrosphere.
DE: 0330 Geochemical cycles (1030)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005