The Petrological Full Picture of Back-arc Volcanism along the en Echelon Seamounts: Mantle Heterogeneity and Across Arc Variations of Slab-derived Components beneath the Izu-Bonin Arc

Detailed petrological and geochemical analyses of sampled volcanic rocks from the en echelon seamounts has enabled the clarification of petrological full picture for back-arc volcanism associated with spreading system in an oceanic arc. The en echelon seamounts in the northern Izu-Bonin arc, a typical example of back-arc seamount chains, consist of three distinct volcanic rock suites (Machida & Ishii, 2003, G-Cubed; Machida et al., in prep). More Enriched Suites (MES) have enriched HFSE, higher Nb/Zr, and low Cr# in spinel. Less Enriched Suites (LES) have depleted HFSE, lower Nb/Zr, and high Cr#. More Depleted Suites (MDS) have the lowest Nb/Zr values, Nb and Zr contents, and slightly enriched LILE. Mineralogical and geochemical analyses show that petrological variations (basalt to rhyolite) within volcanic rock suites are explained by fractionation with open system magma mixing. However, magma mixing do not critically affect for distinction of three volcanic rock suites. HFSE enrichment and depletion cannot be explained by differences in degree of melting and/or subduction inputs by slab melting and dehydration. Instead, these results require distinct mantle sources having different fertility for the three volcanic rock suites: fertile mantle for MES lavas, less enriched mantle for LES lavas, and more depleted mantle for MDS lavas. Geochemical features indicate that slab derived components are Ba-poor slab-derived fluids and sediment melts. The chemical signatures of slab components are strongest in the MDS rocks, followed by LES, with the MES rocks having the weakest signal. Furthermore, evidence for the influence of a fluid component tends to increase toward the quaternary volcanic front. In contrast, sediment melt signature reach a maximum at a distance of about 120-130 km from the quaternary volcanic front, where K-rich magmas (Alkaline andesites) were erupted. We suggest that these across arc geochemical variations should illustrate dehydration and melting processes of deep-subducted slab beneath island arc.