Cenozoic basalts distributed along the southwestern Japan arc system have geochemical features different from typical island-arc basalts, suggesting that 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 Takashima, northwestern Kyushu, were measured. Most samples contain MORB-type helium. However, some samples showed 3He/4He ratios significantly higher than the MORB value, from 9.4 up to 16.6 times the atmospheric ratio of 1.4¿10-6. This finding strongly supports the contribution of a mantle plume to the volcanism of this region. ¿ 2000 American Geophysical Union