S. Umino et E. Yoshizawa, PETROLOGY OF ULTRAMAFIC XENOLITHS FROM KISHYUKU LAVA, FUKUE-JIMA, SOUTHWEST JAPAN, Contributions to Mineralogy and Petrology, 124(2), 1996, pp. 154-166
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.