Along- and across-arc variations of Boron systematics of basaltic rocks from Kyushu, SW Japan

Volcanism in Kyushu is associated with subduction of young (26-15 Ma) Philippine Sea Plate (PSP) in the north and old (60-40 Ma) PSP in the south. The Kyusyu-Palau Ridge subducts nearly at right angles to central Kyushu, marking the boundary between these two segments. Prior work has shown that the subducted PSP contributes to the volcanic front however it does not reach the backarc region. We have analyzed boron (B), fluid mobile trace elements and high field strength elements in basaltic rocks from Kyushu to investigate the influence of contrasting PSP segments on the mantle composition. Since B and other fluid-mobile elements are concentrated in slab-derived fluids, we can use these data to estimate the subducted contribution to the subarc mantle composition. Results show that across-arc variations of B ratios of old (late Miocene-Pliocene) basalts are subdued. Although Quaternary basalts in the northern volcanic front show similar B ratios to the old basalts, Quaternary basalts in the central and southern volcanic front show definitely high B ratios. Along-arc variations of B ratios of Quaternary basalts in the volcanic front are not regular. Lavas from Aso and Kirishima volcanoes (central volcanic front) show the highest B/Nb (3.0-3.7), B/Be (5.4-18.6) and B/La (0.5-1.9). B/Nb, B/Be and B/La ratios of the southern Kyushu basalts (Sakurajima and Kaimon volcanoes) are higher than those of the northern Kyushu basalts (Kuju, Yufu, Oninomi). Quaternary basalts from these three regions show distinct trends on a plot of Ba/Nb vs. B/Nb: the Kirishima suite shows the highest slope array, while Aso, and the southern Kyushu volcanoes follow an intermediate trend, Yufu, Kuju and Oninomi in northern Kyushu have the lowest B/Nb and the highest Ba/Nb. The trend of these northern Kyushu basalts overlaps with that of the old basalts. These observations indicate that the addition of slab-derived fluid was limited in the central and southern volcanic front, and are compatible with distinct contributions from the old and young segments of PSP. In northern Kyushu young, hot oceanic slab is probably strongly devolatilized before it reaches the volcanic front. In contrast, slower dehydration of the older and cooler PSP segment likely occurs beneath southern Kyushu Higher B/Nb, B/Zr, B/Be and B/La ratios observed in Aso and Kirishima basalts may reflect the subduction of a seamount chain which is the extension of Kyushu-Palau Ridge.