PETROLOGY OF ULTRAMAFIC XENOLITHS FROM KISHYUKU LAVA, FUKUE-JIMA, SOUTHWEST JAPAN

Ultramafic xenoliths are found in Kishyuku Lava, Fukue-jima, Southwest Japan. These include spinel Iherzolite, harzburgite and dunite, as we ll as pyroxenite. The compositions of the constituent minerals of the peridotite xenoliths are in the range of upper mantle peridotites. Var iable Cr/(Cr+Al) ratios (0.1-0.5) of spinel, together with a limited r ange in olivine composition (Fo(90)-Fo(92)), indicate that the xenolit hs are derived from slightly to highly depleted residual mantle. The c ombination of previously published clinopyroxene-olivine geothermobaro metry and clinopyroxene-orthopyroxene geothermometry applied to the xe noliths yields a high geotherm of 1070 degrees C at 1.0 GPa up to 1200 degrees C at 2.2 GPa. Existence of such depleted upper mantle is comp atible with the existing model of asthenospheric injection during the rifting of the Northeast China and the Japan Sea. The high geotherm is caused by thermal perturbation due to the injection of the hot asthen osphere and/or post-rifting uprise of mantle diapirs since 11 Ma.