HR: 1340h
AN: T53A-1400    [Abstracts]
TI: Pb isotope systematics of the Izu-Bonin-Mariana arc: revelations from double spike Pb measurements
AU: * Taylor, R N
EM: rex@noc.soton.ac.uk
AF: National Oceanography Centre, Southampton, Empress Dock, Southampton, SO14 3ZH United Kingdom
AU: Ishizuka, O
EM: o-ishizuka@aist.go.jp
AF: GSJ/AIST, Tsukuba Central 7, Tsukuba, 305-8567 Japan
AB: The Izu-Bonin-Mariana arc volcanoes include a range of isotope compositions that encompass the global diversity recognised in intra-oceanic arcs. Lead isotope ratios of the IBM arc almost span the range of MORB compositions but have previously been recognised as being deflected towards subducted material, and hence promoted as a tracer for sediment input to the volcanic arc. However, interpretation of the composition and proportion of subducted material has been hampered by imprecise Pb isotope analysis. In this study we present new high-precision double spike Pb isotope data from the length of the Izu-Bonin-Mariana arc. The northern section of the Izu-Bonin arc forms tight Pb isotope trends toward the local Pacific Plate sediment which has high Δ 7/4 and 8/4 and low ²⁰⁶Pb/²⁰⁴Pb (~18.6). Further south, between 30^o-25^oN, the Pb compositions change dramatically and progressively vector to a low Δ 7/4 and 8/4 and high ²⁰⁶Pb/²⁰⁴Pb (~19.8) composition similar to HiMu. The 25^o-21^oN section has a similar HiMu trend, but with higher Δ 7/4 and 8/4 for a given ²⁰⁶Pb/²⁰⁴Pb. The Mariana section, 21^o-16^oN, has little evidence for the high Δ 7/4 sediment component recognised in previous studies but indicates the addition of a HiMu-pelagic sediment mix composition. Generally, the Pb isotopes of individual volcanoes are found to have trends coincident with the section of the arc in which they are located. This suggests that the process that generates the heterogeneity within the arc also operates on a local scale. The progressive change to higher ²⁰⁶Pb/²⁰⁴Pb towards the south of the IBM arc can not be linked to a variation in the mantle wedge, which remains at low ²⁰⁶Pb/²⁰⁴Pb (17.8-18.2) Indian Ocean-like compositions. To generate the southward trend requires the subducted material providing Pb for the arc to change sympathetically. Measurements of the isotope composition of the main components within the slab (pelagic and volcaniclastic sediments, seamounts and ocean crust) indicate a viable mixing models can be constructed to represent the southward variation. On the scale of each section of the arc a close correspondence is recognised between the bulk composition of the subducting plate and the overriding arc.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Tectonophysics [T]
MN: Fall Meeting 2005