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

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 highprecision 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 Δ8/4. 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 Δ8/4 signatures are retained. The remarkable Th enrichment associated with low 143Nd/144Nd, Δl8/4, and Δ7/4 suggests that melting a mixture of HIMU volcaniclastics and pelagic sediment is responsible for the geochemical characteristics of this most southerly section. Copyright 2007 by the American Geophysical Union.