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Volume 80, Issue 2
Displaying 1-3 of 3 articles from this issue
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KATSUTOSHI TOMITA, TOSHIMASA KANAI, TETSUO KOBAYASHI, NOBORU OBA1985 Volume 80 Issue 2 Pages 49-54
Published: February 05, 1985
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSAccretionary lapilli were formed by the eruption of Sakurajima volcano on the 22nd of May, 1983. The accretionary lapilli have been studied mineralogically. Each accretionary lapillus contains commonly crystals of gypsum and spherical vacant spaces. The gypsum was formed due to the reaction of moisture and sulfide which adhered to the surface of volcanic ash, and the spherical vacant spaces were bubbles formed in the fine particles of ash. Gypsum crystals were not found in the fresh volcanic ash from Sakurajima, but gypsum crystallized after the fresh volcanic ash got wet with water. Presence of gypsum crystals and bubbles in the accretionary lapilli suggests that fine particles of volcanic ash and moist condition are the most basic conditions amongst many conditions for the formation of accretionary lapilli.View full abstractDownload PDF (3437K) -
Kenji Shuto, Ryuichi Yashima, Toshiaki Takimoto1985 Volume 80 Issue 2 Pages 55-72
Published: February 05, 1985
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSHigh magnesian olivine basalts of lower Miocene age are distributed in the Ryozen district of the northeastern part of Fukushima Prefecture, Northeast Japan. Olivine basalts have low FeO*/MgO ratios (0.68-1.17) and high Cr contents (323-1193ppm). The olivine basalt R09 is sparsely phyric (phenocrysts of olivine and chromite 10⟨vol%), and the olivine phenocrysts have narrow compositional range (Fo=85-89) and high NiO contents (0.24-0.32%). The olivine phenocrysts of the olivine basalt R09 contain chromite only as inclusion. Fe-Mg partitioning of the olivine basalt R09 suggests that phenocryst of magnesian olivine (Fo=87) is in equilibrium with coexisting liquid (=groundmass).
These features strongly indicate that the olivine basalt R09 has primitive nature in chemical composition and may represent the primary olivine tholeiitic magma generated in the upper mantle.
Differentiated basalts having relatively high FeO*/MgO ratios may be produced from the primary magma by the fractionation of magnesian olivine, chromite and clinopyroxene.View full abstractDownload PDF (825K) -
Mineral chemistryShintaro Hayashi, Ken-ichiro Aoki1985 Volume 80 Issue 2 Pages 73-82
Published: February 05, 1985
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSChemical compositions of the rock-forming minerals in a high-alumina basalt, calc-alkaline basalts, calc-alkaline andesites from Chokai volcano were analyzed with the electron microanalyzer. The rocks of the Chokai volcano can be divided into olivine-bearing and olivine-free types on the basis of their mineralogical character. The cores of phenocryst (orthopyroxene, clinopyroxene, plagioclase and magnetite) in the olivine-free calc-alkaline andesite of stage I are unimodal in chemical compositions, and the rim of phenocrysts and microphenocrysts have similar composition to the core of the respective phenocryst. These phenocrysts tend to change the chemical composition with increasing differentiation of the host andesite. The olivine-free andesite of stage IIc is also characterized by the unimodal phenocryst compositions. The phenocrysts of olivine-bearing rocks (high-alumina basalt and olivine-bearing calc-alkaline andesite of stage I, and most of the rocks from stage II and stage III) have normally-zoned phenocrysts (olivine, calcic plagioclase and magnesian clinopyroxene) and reversely-zoned phenocrysts (sodic plagioclase, ferriferous clinopyroxene and hypersthene) in the same rock. The cores of the phenocrysts are almost constant in composition regardless of the bulk chemical compositions of the host rocks.View full abstractDownload PDF (639K)
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