雲仙・多良・熊本金峯地域火山岩類の Sr 同位体組成

概要

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The ^<87>Sr/^<86>Sr 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 ^<87>Sr/^<86>Sr 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 ^<87>Sr/^<86>Sr ratios during crystal fractionation. The ^<87>Sr/^<86>Sr 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 ^<87>Sr/^<86>Sr 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 ^<87>Sr/^<86>Sr 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.
特定非営利活動法人日本火山学会の論文
1985-12-28