IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3W_3C-P9Date/Time: July 23 Poster

The origin and magmatic evolution of Quaternary lavas of Sakurajima volcano, southern
Kyushu Island, Japan: inferred from Geochemical and Sr-Nd-Pb isotopic constraints
Tomoyuki Shibata1 , Jun Suzuki1 , Masako Yoshikawa1 , Tetsuo Kobayashi2 , Daisuke Miki3 , Keiji
Takemura1
1IGS, Kyoto University, Japan, 2 Graduate School Sci. Eng., Kagoshima University, Japan, 3 SVRC,DPRC, Kyoto University, Japan
E-mail: tomo@bep.vgs.kyoto-u.ac.jp

Sakurajima volcano represents a post-caldera volcano linked with the Aira caldera, situating at the volcanic frontof the Ryukyu arc, southern end of Kyushu Island, Japan (Fukuyama, 1978), where the Philippine Sea Plate(PSP) is subducting, and is considered to be one of the most suitable volcanoes for studying the links between
caldera formation and catastrophic silicic magmatic activity. We collected 25 lava samples, from almost all of thevolcanostratigraphic units (Fukuyama and Ono, 1981), for a major and trace element geochemical and Sr-Nd-Pb
isotopic petrogenetic investigation of Quaternary lavas of Sakurajima volcano.
The Sakurajima lavas are porphyritic andesites or dacites contains Opx-Cpx-Pl with or without Ol as phenocrysts.
The Nb depletion along with enrichments in Rb, K, and Pb show the typical island arc magma characters causing
by the addition of aqueous fluids to the mantle wedge. The Sr, Nd, and Pb isotopic compositions plot close to amixing curve between MORB-type mantle and sediments of the Philippine Sea Plate, (PSP) but displaced a bit
towards more radiogenic compositions. Plots of Zr v.s. Nb concentration yield a linear trend that falls on a mixing
line between the values for MORB and average continental crust. These observations indicate that the primarysource magmas were initially generated by partial melting of MORB-type mantle hydrated by fluids derived fromthe subducting PSP. The contribution of crustal material during magma evolution is also evident from the Zr/Nbratios and Sr-Nd-Pb isotopic compositions. The mixing relations of Sr-Nd-Pb isotopic compositions suggest thatthe sedimentary rocks of Shimanto Group can be a source of the crustal materials. Although most of the major
element compositions show a single linear trend on each of the Harker diagrams, two different trends are discernible
on each of the P2 O5 , and TiO2 v.s. silica diagrams, and are subdivided into low-P and high-P geochemical groups.
The magma mixing trends of Sakurajima lavas, which seem to be extended from mono andesitic end-member totwo different deictic end-members, are observed from the relationships of major element contents and 87Sr /86Sr Srratios. In addition, the low-P versus high-P groups of lavas show distinctive distribution patterns, whereby the high-Plavas are surrounded by low-P lavas in the central to southern parts of the Sakurajima volcano. These observations
indicate that mixing of andesitic and dacitic magmas played an important role in the genesis of lavas of Sakurajima
volcano, and that multiple dacitic magma chambers with different geochemical characteristics once existed beneaththe Sakurajima area at relatively shallow levels in the crust. From the relations between SiO2 and Sr isotope ratios,an AFC process is required to originate the andesite and dacite end-members.

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