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

4W_1B-P10

Date/Time: July 24 Poster

The redox states of basaltic and boninitic magmas throughout the early stage of theBonin arc formation; Fe-K edge XANES study
Hidemi Ishibashi1 , Shoko Odake2 , Kyoko Kanayama3 , Morihisa Hamada4 , Hiroyuki Kagi5
1

Department of Geoscience, Faculty of Science, Shizuoka University, Japan, 2 GIA Tokyo, Japan,Department of Earth Science, Graduate School of Natural Science and Technology, Kanazawa
University, Japan, 4 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Japan,5Geochemical Research Center, Graduate School of Science, the University of Tokyo, Japan
3E-mail: shishib@ipc.shizuoka.ac.jp

One of the significant features of arc magmas is their relatively oxidized nature compared to those from othertectonic settings. Previous studies discussed that the nature is attributed to relatively high oxygen fugacity (fO2 )conditions in their source mantle region, and accession of subducting slab-derived fluid may have raised fO2 ofthe MORB source-like mantle. However, it is unknown how the fO2 condition of melting region in mantle wedge
develops throughout the early stage of arc formation. In this study, we investigated the fO2 conditions of basaltic
and boninitic magmas erupted during the initial ca. 12 million years of the Bonin arc formation based on the Fe
valence states of quenched silicate glasses, which is a sensitive indicator of magmatic fO2 , to examine how fO2 oftheir source mantle region developed with time.
Fe-K edge micro-XANES (X-ray Absorption Near Edge Structure) measurements enable us to determine valence
state of Fe in silicate glass with spatial resolution of several microns. In this study, Fe-K edge XANES spectra weremeasured for quenched silicate glasses included in pillow lavas and hyaloclastites of basalt and boninite from Boninarc, using a X-ray micro-beam system at Beam Line 4A in Photon Factory, KEK, Japan. Mole ratios of ferric to total
iron, Fe3+ /Fetotal , were determined from the spectra, and then the ratios were used to estimate fO2 of silicate melts.
Precision of our analyses was evaluated to be within standard deviation of 0.4 log unit in fO2 using basaltic andandesitic standard glasses synthesized at controlled fO2 conditions.
The measured samples include forearc basalt (FAB) collected at Bonin Ridge, which is the earliest volcanic productfrom Bonin arc and has MORB-like geochemical feature, erupted right after the initiation of subduction (ca. 51Ma),
boninites from Chichijima, Otojima, and Mukojima, erupted at ca. 44-48 Ma, and arc basalts from Anejima andHahajima, erupted at ca. 37-44Ma. The measured Fe3+ / Fetotal ratios of quenched glasses are ca. 0.20 for FAB,
0.17-0.24 for boninites from Chichijima, Otojima, and Mukojima, and 0.20-0.22 for arc basalts from Anejima andHahajima, respectively. All measured samples show Fe3+ / Fetotal ratios higher than that of MORB. fO2 of themeasured glasses are estimated to be near Ni-NiO buffer which is consistent with the range of arc magmas. The
results suggest that fO2 of magmas erupted at Bonin arc has been higher than that of MORB since the initiation ofarc volcanism. In addition, the similarity of fO2 among MORB-like FAB, boninites, and arc basalts implies that theoxidation process may be independent of accretion of subducting-slab derived aqueous fluid.

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