Mantle-derived Xenoliths with Hot-spot Type Helium in Cenozoic Alkali Basalt, Northwestern Kyushu, Japan

The 3He/4He ratio of mantle-derived material is not uniform and the distinct 3He/4He signatures are found in association with different tectonic environments. Helium isotopic signature of hot-spot regions such as Hawaii, Iceland, and Rbunion range from 9 to 30 Ra, where Ra is 3He/4He of the atmosphere = 1.4 x 10 -6, which indicates that the hot-spot volcanisms tap a deeper region of the mantle enriched in 3He than MORB reservoir. Cenozoic alkaline basalts from back-arc region of the Japanese island-arc system are less depleted in Ta, Nb and Ti and less enriched in K, Sr, Ba and Rb compared with samples from northeastem Japan. These geochemical features lead to Nakamura et al. (1985) to suggest that the alkali volcanisms of this area were not directly related with subduction processes but were triggered by plumes originated from greater depth. Furthermore, it was suggested that the alkali basalt magmatism may represent the final stage of a mantle diapirism which was most active in the Miocene and caused the opening of the Sea of Japan (Iwamori, 1991). We measured noble gas isotopic compositions of xenoliths in Cenozoic alkali basalt, Takashima, northwestern Kyushu to investigate possible contribution of deep mantle plume.