Processes controlling along-arc isotopic variation of the southern Izu-Bonin arc

Ishizuka, O., Taylor, R.N., Yuasa, M., Milton, J.A., Nesbitt, R.W., Uto, K. and Sakamoto, I. (2007) Processes controlling along-arc isotopic variation of the southern Izu-Bonin arc. Geochemistry, Geophysics, Geosystems, 8, Q06008. (doi:10.1029/2006GC001475).

Abstract

The southern Izu-Bonin arc is the volcanic representation of Pacific Ocean lithosphere subduction beneath the oceanic crust of the Philippine Sea plate. We present new geochemical data including high-precision Pb isotopic measurements from the 700 km length of this intraoceanic arc. An aim of this study is to link the along-arc characteristics with variations in the subducting Pacific crust. Chemical variations have been previously recognized along the northern section of the arc as a southward decrease in 87Sr/86Sr and increase in 206Pb/204Pb. This trend continues into the southern arc as far as 27.5°N. An overall correlation between 87Sr/86Sr and fluid-mobile element enrichment between 35 and 27.5°N implies a contribution of a slab-derived fluid, mainly from altered oceanic crust and pelagic sediment. However, the observation that volcanoes plot systematically closer to the Northern Hemisphere Reference Line (NHRL) with increasing 206Pb/204Pb indicates that a component other than pelagic sediment and ocean crust plays a key role toward the south. South of 27.5°N, the along-arc isotopic trend changes dramatically, with 87Sr/86Sr increasing southward from 27.5°N and the 206Pb/204Pb becoming highly radiogenic (?19.6). This isotopic signature requires involvement of a component with high 206Pb/204Pb and low ?8/4. Volcaniclastic sediments originating from HIMU oceanic islands on the subducting Pacific Plate are possible candidates to introduce such a component into the mantle wedge. This is consistent with the sedimentary section of ODP Site 801, located outboard of the Mariana arc which has >40% mass of HIMU volcaniclastics. An aqueous fluid, not melt, has played a major role in the source magma compositions in the 27.5–25°N segment. South of 25°N the isotopic characteristics again change significantly. 143Nd/144Nd decreases down to 0.51280, but the high 206Pb/204Pb and low ?8/4 signatures are retained. The remarkable Th enrichment associated with low 143Nd/144Nd, ?8/4, and ?7/4 suggests that melting a mixture of HIMU volcaniclastics and pelagic sediment is responsible for the geochemical characteristics of this most southerly section.

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Contributors

Author: R.N. Taylor

Author: J.A. Milton

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