Across-arc geochemical trends in the Izu-Bonin arc: Contributions from thesubducting slab

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 m antle at the volcanic front, and residual-slab fluids to fertile mantle in the back arc without slab melting or contemporaneous back arc spreading. Th e arc consists of a volcanic front, an extensional zone, and seamount chain s ( 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, Pb-206/Pb-204, epsilon (Nd), and Sr-87/Sr- 86). A fluid derived from both sediment and altered oceanic crust explains most of the slab-related characteristics of the volcanic front. The Western Seamounts and some of the extensional zone rocks have lower epsilon (Nd), Sr-87/Sr-86, Pb-206/Pb-204, Ba/Th, and U/Th; moderate Ba/Nb and U/Nb; and s imilar or higher Th/Nb and Th/Nd. Although the lower epsilon (Nd) and highe r Th/Nd tempt a sediment melt explanation, a lack of correlation between th e strongest sediment proxies, such as epsilon (Nd), Th/Nb, and Ce/Ce*, prec ludes sediment melts. The subduction component for the Western Seamounts is probably a fluid dehydrated from a residual slab that was depleted in flui d-mobile elements beneath ( as well as trenchward of) the volcanic front. T his depleted fluid is added to elementally and isotopically more enriched m antle beneath the Western Seamounts.