ABSTRACT

To evaluate the effect of slab-derived fluids from subducted Philippine Sea plate on mantle composition beneath southern Kyushu, we determined the boron (B) contents in basaltic rocks from Hisatsu volcanic province and Kirishima volcano, which erupted since 7.6 Ma. Since B is distinctively concentrated into slab-derived fluids, we have attempted to estimate the influence of subduction-related inputs on the composition of the sub-arc mantle based on the B systematics of the basaltic rocks. Old (7.6-0.4 Ma) basalts from Hisatsu volcanic province possess relatively low B/Sm (0.8-4.2), B/Zr (0.03-0.12) and B/Nb (0.6-2.5) ratios. In contrast, volcanic products from the comparatively young (<0.3 Ma) Kirishima volcano, located at the volcanic front, show significantly higher B/Sm (4-8), B/Zr (0.2-0.3) and B/Nb (3-5) ratios. In addition, basalts from these two regions show distinct trends on plots of Ba/Zr vs. B/Zr and Ba/Nb vs. B/Nb: the Hisatsu basalts have lower B/Zr, B/Nb and higher Ba/Zr, Ba/Nb ratios than those of the Kirishima basalts. These observations indicate that B-rich fluids from Philippine Sea plate were added to the mantle beneath Kirishima volcano between 0.4 and 0.3 Ma. Therefore the modern fluid input from the subducted slab to the subarc mantle may be limited beneath the volcanic front in this area.