Subduction influence of Philippine Sea plate on the mantle beneath northern Kyushu, SW Japan: An examination of boron contents in basaltic rocks

Northern Kyushu, characterized by the subduction of two oceanic slabs (a hot Shikoku basin and a cold Philippine Sea plate) beneath the Eurasian plate, forms a complex portion of Southwestern Japan arc. In order to evaluate the effect of slab-derived fluids from these two contrasting oceanic plates, we determined the boron (B) contents in basaltic rocks from ten volcanoes and three old volcanic fields which erupted since 11 Ma. Since B is distinctly concentrated into slab-derived fluids among the earth's materials, we attempted to estimate the influences of subduction on the sub-arc mantle composition from the interpretation of the B data in basaltic rocks. Old (11–6 Ma) basaltic rocks contain low ratios of B/Sm (0.5–1.3), B/Zr (0.02–0.05) and B/Nb (0.2–0.5), suggesting little influence of subduction. Similarly, backarc basaltic rocks occurring throughout the observed period show little influence of subduction. In contrast, volcanic products from young Aso volcano, located at the volcanic front, show a strong influence of subduction, as indicated by the high B/Sm (1.6–4.3), B/Zr (0.07–0.16) and B/Nb (1.4–3.7) ratios. After 6 Ma, the volcanic arc segment containing Aso volcano is associated with the subduction of the cold Philippine Sea plate; hence a B-rich fluid was probably added to the sub-arc mantle. However, Yufu, Tsurumi and Kuju volcanoes, which are located at the same volcanic front, show small B/Sm (0.9–1.9), B/Zr (0.04–0.07) and B/Nb (0.5–0.9) ratios. This implies that the subduction of the hot Shikoku basin released fluids from the slab at shallow depths; therefore it is depleted of B and other subduction components by the time it reaches the volcanic front.