Japan Geoscience Union Meeting 2016

Presentation information


Symbol S (Solid Earth Sciences) » S-CG Complex & General

[S-CG59] Ocean Floor Geoscience

Thu. May 26, 2016 1:45 PM - 3:00 PM 301B (3F)

Convener:*Kyoko Okino(Ocean Research Institute, University of Tokyo), Keiichi Tadokoro(Research Center for Seismology, Volcanology and Earthquake and Volcano Research Center, Nagoya University), Osamu Ishizuka(Geological Survey of Japan, AIST), Tomohiro Toki(Faculty of Science, University of the Ryukyus), Narumi Takahashi(Research and Development Center for Earthquake and Tsunami, Japan Agency for Marine-Earth Science and Technology), Chair:Norikatsu Akizawa(College of Science and Engineering, Kanazawa University), Tetsuo Matsuno(Earthquake Research Institute, The University of Tokyo), Kazuya Kitada(Research and Development Center for Submarine Resources Japan Agency for Marine-Earth Science and Technology)

2:30 PM - 2:45 PM

[SCG59-27] Two types of basalts erupted in Active rift, Izu-Bonin arc, Japan

*Yasuhiro Hirai1, Satoshi Okamura1, Izumi Sakamoto2, Ryuichi Shinjo3, Keiji Wada4, Takanori Yoshida5 (1.Hokkaido Education University, Sapporo, 2.Tokai University, 3.University of the Ryukyus, 4.Hokkaido Education University, Asahikawa, 5.Niki Junior High School, Hokkaido)

Keywords:Izu-Bonin arc, Active rift, Back-arc basin basalts, Mantle heterogeneity

We observed that two types of basalts have erupted in Sumisu rift, Izu-Bonin arc, Japan. High-Zr type basalts have higher concentration of K2O, Na2O, Y, Zr, Ni than Low-Zr basalts at similar FeO*/MgO ratio. High-Zr type basalts have higher Nb/Yb, Ce/Yb, Zr,Yb ratio and lower Ba/Th ratio than Low-Zr type basalts. The Sr isotope composition tends to be higher in direction from High-Zr type to Low-Zr type, although the Nd isotope composition of both types are similar. The Hf isotope composition (176Hf/177Hf) tends to be lower in direction from High-Zr type to Low-Zr type. The Hf isotope composition of Low-Zr type basalts similar to basalts from Sumisu Caldera in the volcanic front.
Olivines in High-Zr type basalts have higher wt. % NiO than those in Low-Zr type basalts at given Fo contents. Estimated primary olivine compositions are more magnesian (Fo = 92.3) in Low-Zr type basalts compared with those in High-Zr type basalts (Fo = 89.6).
Major element compositions of the calculated primary magmas indicate that the primary High-Zr type magmas segregated from source mantle at deeper than those of Low-Zr basalts (High-Zr: 1-2 GPa; Low-Zr: 2-3.5 GPa).
From the petrological and geochemical factors mentioned above, it is difficult to explain the differences of two types of basalts from different degrees of partial melting of the same source mantle and addition from slab-derived components (for example, sediment melt). Thus, we concluded that the two types of basalts are derived from different source mantle. This may indicate that the mantle beneath Sumisu rift have heterogeneities in the vertical direction (High-Zr type sources are in the shallow part of the mantle and Low-Zr type sources are in the deep part of the mantle). Moreover, Low- and High-Zr type basalts similar to West Philippine Basin MORBs and Shikoku basin basalts, respectively. The mantle heterogeneities beneath the Sumisu rift might have been related to the development of Izu-Bonin arc.
In this presentation, we also discuss the basalts from Myojin and Aogashima rift near the Sumisu rift.