HR: 17:45h
AN: T44A-08    [Abstracts]
TI: Tracing slab inputs along the Izu-Bonin-Marianas subduction zone: results from volatile emissions
AU: * Fischer, T P
EM: fischer@unm.edu
AF: University of New Mexico, Dept. of Earth & Planet. Sciences, Albuquerque, NM 87131-1116 United States
AU: Hilton, D R
EM: drhilton@ucsd.edu
AF: Scripps Institution of Oceanography, Geosciences Research Division, La Jolla, CA 92093-0244 United States
AU: Shaw, A M
EM: amshaw@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, DC 20015 United States
AU: Hauri, E R
EM: hauri@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, DC 20015 United States
AU: Kazahaya, K
EM: kazahaya-k@aist.go.jp
AF: Geological Survey of Japan, Higashi 1-1-3, Tsukuba, NM 305-8567 Japan
AU: Mitchell, E
EM: mitch@unm.edu
AF: University of New Mexico, Dept. of Earth & Planet. Sciences, Albuquerque, NM 87131-1116 United States
AU: Shimizu, A
EM: aya@eqchem.s.u-tokyo.ac.jp
AF: University of Tokyo, Laboratory for Earthquake Chemistry, Tokyo, NM 113-0033 Japan
AU: de Moor, M
EM: marten@unm.edu
AF: University of New Mexico, Dept. of Earth & Planet. Sciences, Albuquerque, NM 87131-1116 United States
AU: Sharp, Z D
EM: zsharp@unm.edu
AF: University of New Mexico, Dept. of Earth & Planet. Sciences, Albuquerque, NM 87131-1116 United States
AB: The Izu-Bonin-Mariana (IBM) arc system extends 2800 km from the island of Honshu, Japan to Guam and is a type example of an intra-oceanic convergent margin. Subduction began 45 Ma ago and IBM subducts the oldest seafloor on Earth. A number of parameters vary systematically along the strike of the arc: the slab is steeply plunging in the S and gently dipping in the N; the age of the subducted crust varies from Mid-Jurassic in the S to Mid-Cretaceous in the N. Other parameters remain constant: crustal thickness (~20 km); no accretionary prism; no sediment fill in the trench. The sediment outboard of the arc is characterized based on ODP sites 801 (Marianas) and 1149 (Izu islands). 200 m of volcaniclastics are overlain by a 100 m of pelagic clay and chert in the S. In the N, volcaniclastics are lacking and the 400 m sediment sequence is dominated by 200 m of cherts, overlain by 40 m of pelagic clay and 120 m of volcanic ash and diatom/radiolarian clay. There is also a distinct layer (3 m) of hydrothermally altered MORB in the S. Thus, the IBM system is an ideal location to study the inputs and outputs of the subduction factory and to understand the processes occurring within the factory itself. We collected hydrothermal gas samples from 4 volcanic centers in the Marianas (Alamagan, Pagan, Agrigan, Uracas) and 6 centers in the Izu arc (Aogashima, Hachijojima, Niijima, Shikinejima, Oshima, Hakone). With the exception of Uracas (140C) and a well on Hachijojima (170C), all gas discharges were at or below the boiling temperature of water. As is typical for arc-related samples, the major gases are dominated by H₂O, CO₂ and S species. We see the following variations in N₂/Ar and N₂/He ratios of non-air contaminated samples along the arc: Agrigan clearly shows a mantle wedge signature of low N₂/Ar (70) and N₂/He (210) and negative δ¹⁵N (- 2.0 ‰). All other centers have N₂/He ratios characteristic of that resulting from the addition of N from subducted sediments (1000 to 2500). Most Izu samples also show N₂/Ar ratios higher than air (up to 210). Helium isotopes of Mariana samples are MORB-like (7.4 to 7.9 R_A), whereas CO₂/³He varies from 10.1 to 10.7 x 10⁹ with δ¹³C between -0.5 to - 0.7 permil. Based on N-CO₂-He-Ar sytematics, the Izu section of the arc has a signature characteristic of subducted sediment derived fluids. The Mariana section (Agrigan in particular), shows a volatile signature that suggests contribution dominantly from the altered oceanic basement. This is in contrast to studies based on trace elements and radiogenic isotopes that identify Agrigan as the `sediment endmember' of the Mariana arc. Analyses of stable and noble gas isotopes of the samples are currently underway to further constrain the source of volatiles discharging along the arc.
DE: 1025 Composition of the mantle
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Tectonophysics [T]
MN: Fall Meeting 2005