High He-3/He-4 ratio in xenoliths from Takashima: evidence for plume type volcanism in southwestern Japan

Cenozoic basalts distributed along the southwestern Japan are system have g eochemical features different from typical island-are basalts, suggesting t hat the volcanism might have been triggered by a mantle plume. To identify the source materials in these volcanics, noble gas isotopic compositions of olivines separated from dunite xenoliths in alkaline basalts from Takashim a, northwestern Kyushu, were measured. Most samples contain MORE-type heliu m. However, some samples showed He-3/He-4 ratios significantly higher than the MORB value, from 9.4 up to 16.6 times the atmospheric ratio of 1.4x10(- 6). This finding strongly supports the contribution of a mantle plume to th e volcanism of this region.