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[1] Possible mechanisms for the production of mantle-derived, high-Mg andesite magmas, including (1) partial melting of mantle wedge peridotite by addition of aqueous fluids from the subducting lithosphere and (2) partial melting of the subducting sediments and altered oceanic crust, and subsequent melt-mantle interaction, were examined by geochemical formulation of dehydration, partial melting and melt-solid reactions. The modeling results demonstrate that both mechanisms can reasonably explain the incompatible trace element characteristics of high-Mg andesites in the Setouchi volcanic belt, SW Japan. However, simple hydrous melting of mantle wedge peridotite cannot account for the Sr-Nd-Pb-Hf isotopic compositions of such andesites. By contrast, the latter mechanism, which is consistent with thermal structures beneath the Setouchi volcanic belt, can well reproduce the isotopic signature of those high-Mg andesites.

[1] Hf isotope compositions, coupled with Pb-Sr-Nd isotopes and trace element compositions, for the Miocene to the Quaternary volcanic rocks from the NE Japan arc have documented the geochemical evolution in the magma source compositions in association with back-arc opening (23-14 Ma). Clear temporal variation in Hf isotope ratios is demonstrated, from the lowest e Hf (+9) in the early Miocene to the highest e Hf (+14) in the late Miocene and the Quaternary, whereas there are little changes in Sr and Nd isotope composition. Hf isotope ratios are clearly correlated with some trace elemental ratios. The observed temporal variation is attributed to a change in metasomatic agents derived from the subducted slab, which could have been caused by cooling of the mantle wedge during the syn and post stages of the back-arc opening. In the early stage of the back-arc opening, when injection of hot asthenospheric mantle induced high-temperature conditions in the mantle wedge, slab melting contributed to low e Hf coupled with low e Nd , relatively low La/Nb, and high Th/Yb in volcanic rocks. On the other hand, the later stage volcanism, characterized by high e Hf , decoupling of e Hf and e Nd , high and variable La/Nb and Ba/La, is best explained by addition of fluids from subducted sediments and oceanic crust to the mantle wedge.

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[1] Combined Pb-Sr-Nd-Hf-Os isotopes, together with major and trace element compositions, were determined from clinopyroxene and olivine phenocrysts, along with whole rocks, for ocean island basalts with high m (m = 238 U/ 204 Pb) (HIMU) and enriched mantle isotopic characteristics from Cook-Austral Islands. Clinopyroxene and olivine separates record reliable isotopic information of the sources because of minimized in situ radiogenic ingrowth and their lower susceptibility to crustal contamination. Coherent isotopic systematics in multi-isotope spaces defined by the HIMU samples are best explained by recent mixing of melts derived from the HIMU reservoir and the local shallow mantle. The isotopic compositions of the HIMU reservoir are constrained to be low " Nd (≤+4), low " Hf (≤+3), and moderately radiogenic 187 Os/ 188 Os (0.14-0.15) in association with radiogenic Pb isotopes ( 206 Pb/ 204 Pb ≥ 21.5). Since ancient oceanic crust would have had exceptionally radiogenic 187 Os/ 188 Os, moderately high 187 Os/ 188 Os precludes recycled oceanic crust as the only contributor to the HIMU reservoir. Instead, mantle metasomatized with partial melts from subducted oceanic crust is a likely candidate for the HIMU reservoir. Moreover, partial melting of oceanic crust in equilibrium with Mg perovskite would fractionate U/Pb, Sm/Nd, and Lu/Hf, which are in accordance with the time-integrated U/Pb, Sm/Nd, and Lu/Hf deduced from Pb, Nd, and Hf isotopic compositions of the HIMU reservoir, respectively, with a formation age of 2-3 Ga. We thus propose that the HIMU reservoir was formed by hybridization of a subducted oceanic crust-derived melt with the ambient mantle and then stored for several billion years in the lower mantle.