Geochemical Journal, Vol. 44 (No. 5), pp. 359-369, 2010
Masaya Miyoshi,1 Madoka Shimono,1 Toshiaki Hasenaka,1 Takashi Sano,2 Yasushi Mori3 and Takaaki Fukuoka4
1Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan
2National Museum of Nature and Science, Tokyo 169-0073, Japan
3Kitakyushu Museum of Natural History and Human History, Kitakyushu, Fukuoka 805-0071, Japan
4Department of Environmental Systems, Rissho University, Kumagaya, Saitama 360-0194, Japan
(Received February 12, 2009; Accepted January 13, 2010)
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.
Key words: boron, Philippine Sea plate, southern Kyushu, subduction zone, basalt