HR: 17:00h
AN: T44A-05 INVITED     [Abstracts]
TI: Spatial and Temporal Variation of Sub-arc Mantle in the Southern Izu-Bonin arc
AU: * Ishizuka, O
EM: o-ishizuka@aist.go.jp
AF: Geological Survey of Japan/AIST, Central 7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Taylor, R N
EM: rex@noc.soton.ac.uk
AF: National Oceanography Centre,Southampton, European Way, Southampton, SO14 3ZH United Kingdom
AU: Yuasa, M
EM: yuasa-m@aist.go.jp
AF: Geological Survey of Japan/AIST, Central 7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Milton, J A
EM: jam2@noc.soton.ac.uk
AF: National Oceanography Centre,Southampton, European Way, Southampton, SO14 3ZH United Kingdom
AU: Nesbitt, R W
EM: rwn2@noc.soton.ac.uk
AF: National Oceanography Centre,Southampton, European Way, Southampton, SO14 3ZH United Kingdom
AU: Sakamoto, I
EM: izumis@jamstec.go.jp
AF: JAMSTEC, 3173-25 Showamachi, Yokohama, 236-0001 Japan
AU: Ohara, Y
T44A-05 AF: Hydrographic and Oceanographic Department of Japan, 5-3-1 Tsukiji, Tokyo, 104-0045 Japan
AB: We present spatial and temporal variation of sub-arc mantle in the southern Izu-Bonin arc and discuss the processes controlling it in conjunction with the tectonic evolution of this area. The southern Izu-Bonin arc is characterised by an Eocene fore-arc massif (Bonin Ridge), an Oligocene rift basin (Ogasawara Trough), active submarine volcanoes and intra-arc rift basins. The Quaternary lavas are mostly low-K tholeiitic basalt and basaltic andesite. Exceptions are highly alkaline shoshonitic rocks from Iojima and the surrounding volcanoes. A southward decrease in ⁸⁷Sr/⁸⁶Sr and increase in ²⁰⁶Pb/²⁰⁴Pb is observed in the northern section of the arc and this continues into the southern arc as far as 27^oN. In Pb-Pb isotope space volcanoes plot systematically closer to the NHRL from north to south. The decoupled behavior of Sr and Pb isotopes led us to propose an along-arc mantle wedge heterogeneity prior to the addition of the slab-derived component. South of 27^oN, the along-arc isotopic trend changes dramatically. Sr isotopic ratio increases southward from 27^oN, and the ²⁰⁶Pb/²⁰⁴Pb becomes highly radiogenic (~19.5). This isotopic signature requires involvement of a slab-derived component with high ²⁰⁶Pb/²⁰⁴Pb with low _elta8/4. Lack of correlation between Nd isotopic composition and Th enrichment implies a major contribution of fluid as a slab component in 27-25^oN, while clear correlation between the two implies a significant melt contribution south of 25^oN (in the vicinity of the Iojima). The estimated slab component south of 27^oN is different from altered MORB-like oceanic crust and pelagic sediment, and this could be derived from the seamounts on the Pacific Plate. New isotopic data for the subducting seamount chains on the Pacific plate demonstrate that one of the chains in the Marcus-Necker seamount chain shows a O`HIMU-likeO_L character, i.e., ²⁰⁶Pb/²⁰⁴Pb>20, _elta8/4<0 and low ⁸⁷Sr/⁸⁶Sr and significant variation occurs among these chains. This isotopic character and heterogeneity of the subducting seamounts could cause large along-arc variation of the slab component in this region. Variation in geometry of the subducting slab (e.g., Miller et al., 2004, 2005) and PT condition in mantle wedge in the southern Izu-Bonin arc (e.g., higher slab surface temperature south of 25^oN may induce melting of sediment) may cause change in the transporting media of slab component. New data from Eocene-Oligocene lavas from the Southern Izu-Bonin arc demonstrate different isotopic characteristics from Quaternary frontal arc volcanoes and remarkable temporal variation in sub-arc mantle composition occurred during this period. Eocene lavas from the Bonin Ridge show higher ²⁰⁶Pb/²⁰⁴Pb (>18.6), while Oligocene lavas from the fore-arc of the southern Izu arc and Kyushu-Palau Ridge show lower ²⁰⁶Pb/²⁰⁴Pb and relatively small along-arc variation (similar to the Quaternary northern Izu-arc in most cases). These results imply that the composition and/or geometry of the subducting plate is clearly different among Eocene, Oligocene and Quaternary arcs.
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8178 Tectonics and magmatism
DE: 8185 Volcanic arcs
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Tectonophysics [T]
MN: Fall Meeting 2005