HR: 11:05h
AN: T31H-04 INVITED     [PDF]
TI: Processes controlling along-arc isotopic variation of the 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@soc.soton.ac.uk
AF: Southampton Oceanography Centre, 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@soc.soton.ac.uk
AF: Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH United Kingdom
AU: Nesbitt, R W
EM: rwn2@soc.soton.ac.uk
AF: Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH United Kingdom
AU: Uto, k
EM: K.uto@aist.go.jp
AF: Geological Survey of Japan/AIST, Central 7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Sakamoto, I
EM: izumis@jamstec.org
AF: JAMSTEC, 1133 21 St. Street, NW, Suite 400, Washington, DC 20036 United States
AB: We present along-arc variation of isotopic and trace element composition for $>$2000km section of the Izu-Bonin arc including new high-precision Pb isotope measurements. New data mainly come from the southern Izu-Bonin arc, where the Sofugan Tectonic Line (Yuasa, 1985) intersects the volcanic front and is characterised by intra-arc rifting and thinner arc crust compared to the north. The lavas studied are mostly low-K tholeiitic basalt and basaltic andesite. Exceptions are highly alkaline shoshonitic rocks from Iojima and the surrounding volcanoes. Most of the volcanoes have been active in late Quaternary. A southward decrease in $^{87}$Sr/$^{86}$Sr and increase in $^{206}$Pb/$^{204}$Pb is observed in the northern section of the arc and this continues into the southern arc as far as 27$^{o}$N. In Pb-Pb isotope space volcanoes plot systematically closer to the NHRL from north to south. The decoupled behavior Sr and Pb isotopes led us to propose an along-arc mantle wedge heterogeneity prior to the addition of the slab-derived component. In terms of the slab-derived component, the overall correlation between $^{87}$Sr/$^{86}$Sr and fluid-mobile element enrichment implies a contribution of fluid from altered oceanic crust and pelagic sediment in the section between 35 and 27$^{o}$N. South of 27$^{o}$N, the along-arc isotopic trend changes dramatically. Sr isotopic ratio increases southward from 27$^{o}$N, and the $^{206}$Pb/$^{204}$Pb becomes highly radiogenic (~19.5). This isotopic signature requires involvement of component with high $^{206}$Pb/$^{204}$Pb with low $\Delta$8/4.The volcaniclastics of the oceanic islands on the subducting Pacific Plate are a possible candidate to introduce such a component into the mantle wedge. Lack of correlation between Nd isotopic composition and Th enrichment relative to Ce implies that fluid, not melt, has played a major role in the source magma compositions in this segment of the arc. South of 25$^{o}$N, in the vicinity of the Iojima, the isotopic characteristics again changes significantly. $^{143}$Nd/$^{144}$Nd decreases down to 0.51280 but the high $^{206}$Pb/$^{204}$Pb and low $\Delta8/4$ signatures are retained. The remarkable Th enrichment associated with low $^{143}$Nd/$^{144}$Nd, $\Delta$8/4 and $\Delta$7/4 implies that melt of a high $\mu$ subductued volcaniclastics is required. In this respect the samples from ODP Hole 801, located outboard of the Mariana arc, have a high $\mu$ volcanic sequence, and may provide a potential source for high $\mu$ signature in the arc south of 27$^{o}$N.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1040 Isotopic composition/chemistry
DE: 1749 Volcanology, geochemistry, and petrology
DE: 5480 Volcanism (8450)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting