HR: 09:45h
AN: V11C-08    [Abstracts]
TI: Mantle Diapirs and Genesis of Arc Magmas: Evidence From the Sumisu Caldera Volcano, Izu-Bonin arc, Japan.
AU: * Tamura, Y
EM: tamuray@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Tani, K
EM: kentani@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Ishizuka, O
EM: o-ishizuka@aist.go.jp
AF: GSJ/AIST, Namiki-cho, Tsukuba, 237-0061 Japan
AU: Chang, Q
EM: qchang@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Shukuno, H
EM: shukuno@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Fiske, R S
EM: rfiske@volcano.si.edu
AF: National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560 United States
AB: Are arc basalts dry, wet or both? What are the relationships between flux melting and pressure release melting beneath arc volcanoes? The example we discuss is the Sumisu caldera volcano, Izu-Bonin arc. We present evidence that (1) there exist two kinds of basalts in the volcano, which have petrographical, mineralogical and geochemical differences. (2) These systematic and interrelated differences could have been resulted from different water contents in these magmas and differing degrees of melting in the source mantle, and thus dry and wet basalts can coexist in a single volcanic system, and (3) wet basalts have been derived from a more depleted source than dry basalts, which resulted from different degrees of melting. These degrees of melting are $\sim$20 % and $\sim$10%, respectively. We further present a mantle diapir model beneath the volcano, which could produce dry and wet basalts simultaneously in the same volcano. Our findings may have relevance to magma genesis models in other subduction zones. Basalt-basaltic andesite ($<$55 wt % SiO$_{2}$) and dacite-rhyolite (66-74 wt % SiO$_{2}$) are predominant eruptive products in Sumisu caldera volcano, Izu-Bonin arc, Japan. The most-magnesian basalt (8.5 % MgO), as well as some of the other basalts, contain low Zr (20-30 ppm), which cannot yield basalts containing higher Zr (30-40 ppm) through fractionation and/or assimilation. On the other hand, we recognised that high- and low-Zr basalts have differing phenocryst assemblages, distinct phenocryst chemistries of olivine, plagioclase and pyroxene, different depletion of REE (rare earth element) patterns, and differing fluid mobile-element/immobile-element ratios. Estimated primary olivine compositions are more magnesian ($>$Fo$_{91}$) and thus more depleted in low-Zr basalts compared to those in high-Zr basalts (Fo$_{90}$). Low-Zr basalts contain up to 5 vol % augite, but many high-Zr basalts are free of augite, which appears only in their evolved stage. Hydrous basalts crystallize olivine followed by augite and plagioclase, producing the former assemblage. Moreover, the low-Zr basalts have higher Ba/La and Ba/Zr ratios than the high-Zr basalts. We suggest that both dry and wet primary basalts existed in the Sumisu magmatic system, each having different trace element concentrations and mineral chemistry and assemblages. The lower content of Zr and light REE and magnesian primary olivines in the wet basalt could therefore have resulted from a higher degree of partial melting ($\sim$20 %) of a hydrous source mantle compared to $\sim$10 % melting of a dry source mantle. Interestingly, Sr, Nd and Pb isotopes between these wet and dry basalts are similar and are limited in range. These lines of evidence indicate that mantle diapir model might be applicable to satisfy the configuration of such a mantle source region beneath a single volcanic system such as Sumisu.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 1749 Volcanology, geochemistry, and petrology
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting