Deep Crystallization Differentiation of Arc Tholeiite Basalt Magmas from Northern Honshu Arc, Japan
1Faculty of Engineering and Resource Science, Akita University, 1-1 Tegatagakuen-Machi, Akita, Japan
2Mitsubishi Material Techno Corporation, 3-7-13, Toyo, Koto Ward, Tokyo, Japan
3Nippon Telegraph and Telephone East Corporation, 3-19-2, Nishishinjuku, Shinjuku Ku, Tokyo, Japan
4Sumiko Consultants Co., Ltd, 2-9-7, Ikenohata, Taito Ku, Tokyo, Japan
5Graduate School of Science, Tohoku University, 6-3, Aramaki Aza Aoba, Aoba Ku, Sendai, Japan
RECEIVED MAY 25, 2007; ACCEPTED MAY 1, 2009
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Abstract |
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The depth of crystallization differentiation was investigated for arc tholeiite basalts from the Takada Ohdake and Nishimori cones, within the volcanic front of the northern Honshu volcanic arc, Japan, based on the petrography of crystal-bearing melt inclusions, whole-rock trace element chemistry, and the simulation of fractional crystallization with MELTS. Olivine-hosted melt inclusions contain aluminous clinopyroxene, spinel, plagioclase (An60–65), and rare garnet, suggesting melt-entrapment under garnet-granulite facies conditions. Removal of the observed phenocrysts (olivine, orthopyroxene, and anorthitic plagioclase) cannot account for either the major or trace element variations within the basalts. However, the major element variations are typical of a tholeiitic differentiation trend, showing an increase in Fe/Mg ratio with increasing SiO2. The results of trace element modelling, assuming Rayleigh fractionation, indicate fractionation of clinopyroxene, plagioclase, and spinel. The estimated fractionated mineral assemblage is consistent with that in the melt inclusions except for the presence of garnet. The occurrence of garnet in some of the more differentiated andesitic melt inclusions implies its crystallization within the inclusions during the later stages of cooling. Fractional crystallization modelling using MELTS closely approximates the major element variations at 10 kbar under anhydrous conditions. The MELTS modelling is consistent both with the melt-inclusion mineralogy and with the trace element modelling. An estimated depth of crystallization of 34 km (
10 kbar) corresponds to the seismic Moho in the region (35–36 km). Our data also suggest that the relatively anhydrous magmas that were emplaced at Moho depths became hydrous concurrent with differentiation, implying incorporation of water from the surrounding crustal rocks.
KEY WORDS: aluminous clinopyroxene; arc basalt; crystal-bearing melt inclusions; crystallization-differentiation; garnet; northern Honshu
*Corresponding author. E-mail: t-ohba{at}gipc.akita-u.ac.jp