Strontium Isotopic Composition of the Volcanic Rocks from Unzen, Tara and Kumamoto-Kimpo Areas in Kyushu, Southwest Japan

Abstract

The ⁸⁷Sr/⁸⁶Sr ratios and the abundances of rubidium and strontium have been determined for 40 samples of Unzen, Tara and Kumamoto-Kimpo volcanoes, and Neogene basaltic rocks from the surrounding areas in Kyushu, Southwest Japan. The ⁸⁷Sr/⁸⁶Sr ratios in these Quaternary volcanic rocks range from 0.70418 to 0.70509. In each volcano, almost all of the magma seems to be subjected to the assimilation with different ⁸⁷Sr/⁸⁶Sr ratios during crystal fractionation. The ⁸⁷Sr/⁸⁶Sr ratios of the Quaternary volcanic rocks are clearly distinguished between the northern part and the southern part of Kyushu region marked by the Shiobara-Beppu Tectonic Graben in the central Kyushu. The ⁸⁷Sr/⁸⁶Sr ratios in the tholeiitic basalts, alkaline basalts and calc-alkaline basalts from the areas, range from 0.70362 to 0.70397, from 0.70404 to 0.70487 and from 0.70451 to 0.70470 (except for a calc-alkaline basalt from Kumamoto-Kimpo area, 0.70363), respectively. In the southwest Japan, the ⁸⁷Sr/⁸⁶Sr ratios of Cenozoic basalts from the Kyushu region are lower than those of basalts from the San-in region. This means that there exists a regional heterogeneity of Sr isotopic ratios under the Southwest Japan arcs.