[1] Abstract: We propose that across-arc differences in the geochemistry of Izu-Bonin arc magmas are controlled by the addition of fertile-slab fluids to depleted mantle at the volcanic front, and residual-slab fluids to fertile mantle in the back arc without slab melting or contemporaneous back arc spreading. The arc consists of a volcanic front, an extensional zone, and seamount chains (the Western Seamounts) that trend into the Shikoku Basin. Each province produces a distinct suite of arc-like volcanic rocks that have relative Nb depletions and high ratios of fluid-mobile elements to high field strength elements. The volcanic front has the lowest concentrations of incompatible elements and the strongest relative enrichments of fluid-mobile elements (high U/Nb, Ba/Nb, Pb/Zr, Th/Nb, 206 Pb/ 204 Pb, e Nd , and 87 Sr/ 86 Sr). A fluid derived from both sediment and altered oceanic crust explains most of the slabrelated characteristics of the volcanic front. The Western Seamounts and some of the extensional zone rocks have lower e Nd , 87 Sr/ 86 Sr, 206 Pb/ 204 Pb, Ba/Th, and U/Th; moderate Ba/Nb and U/Nb; and similar or higher Th/Nb and Th/Nd. Although the lower e Nd and higher Th/Nd tempt a sediment melt explanation, a lack of correlation between the strongest sediment proxies, such as e Nd , Th/Nb, and Ce/ Ce*, precludes sediment melts. The subduction component for the Western Seamounts is probably a fluid dehydrated from a residual slab that was depleted in fluid-mobile elements beneath (as well as trenchward of) the volcanic front. This depleted fluid is added to elementally and isotopically more enriched mantle beneath the Western Seamounts.

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