HR: 1340h
AN: V33A-1157    [Abstracts]
TI: Small-scale Heterogeneity in Pacific Upper Mantle Unrelated to Plumes Evidenced by Sr-Nd- Pb Isotopic Composition of Basalt from Petit-spot Volcanoes
AU: * Machida, S
EM: machida@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, Univ. of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo, 164-8639, Japan
AU: Hirano, N
EM: nhirano@geo.titech.ac.jp
AF: Laboratory for Earthquake Chemistry, Graduate School of Science, Univ. of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033, Japan
AU: Kimura, J
EM: jkimura@riko.shimane-u.ac.jp
AF: Department of Geoscience, Shimane Univ., 1060 Nishikawatsu, Matsue, Shimane, 690- 8504, Japan
AB: Petit-spot is a new type of volcano that is not related to plumes on the northwestern Pacific plate. An important aspect of petit-spot is that this provides a unique window for mantle source material beneath the oldest Pacific plate. To investigate the geochemical features of upper mantle unrelated to plumes in northern hemisphere, we have determined Sr, Nd, and Pb isotopic compositions of basalts collected from petit-spots. The isotope compositions of petit-spot lava fall within the range of compositional variations in MORB. However, the lava is higher in ⁸⁷Sr/⁸⁶Sr, lower in ²⁰⁶Pb/²⁰⁴Pb and ¹⁴³Nd/¹⁴⁴Nd, and higher in ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb at a given ²⁰⁶Pb/²⁰⁴Pb than those of Pacific MORB or Depleted MORB Mantle (DMM). Remarkably, the isotopic compositions are equivalent to Indian and southern Atlantic MORB with strong Dupal signature (MORB from the southwest Indian ridge at 40°E, 43.4°S, and the southern mid-Atlantic ridge at 10°-14°W, 46°-49°S), and are almost identical to the composition of the EM-1 endmember. We propose that recycled material from an ancient subduction system or crustal delamination resides throughout the upper mantle of the Pacific Ocean as minor blobs. Our model of 'small-scale recycled material melting' consistently explains geochemistry and noble gas isotopic composition of magma, and volume of petit-spot volcanism. Furthermore, our results strongly suggest that the Earth's upper mantle is essentially heterogeneous, but its gross composition is controlled by abundances of blobs of recycled material. Upper mantle located beneath (such as the Indian, Atlantic, and possibly Arctic oceans) or on the periphery (such as the Philippine Sea) of the Pangea supercontinent includes abundant recycled material, in contrast to the Pacific Ocean. Because the Dupal signature was discovered in the northern hemisphere, the nature of the Dupal signature must be reconsidered as involving recycling of the entire solid Earth.
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3619 Magma genesis and partial melting (1037)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting