GEOCHEMICAL JOURNAL

Lateral variation in ⁸⁷Sr/⁸⁶Sr ratios of volcanic rocks from central Japan

Kenji Notsu, Yoji Arakawa, Shun Nakano, Masao Yamasaki

1989 Volume 23 Issue 2 Pages 45-55
Published: 1989
Released on J-STAGE: April 08, 2008

DOIhttps://doi.org/10.2343/geochemj.23.45

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A number of volcanoes are located on the continental side of the volcanic front in central Japan, where the Northeast Japan and Izu-Ogasawara arcs join. These volcanoes are distributed not only along the volcanic front but also in an area extending up to 150 km away from the volcanic front. We determined ⁸⁷Sr/⁸⁶Sr ratios, and Rb and Sr contents of 40 volcanic rock samples from 11 volcanoes in this area and summarize, together with data already published, the spatial variation of the ratios. The range of ⁸⁷Sr/⁸⁶Sr ratios of volcanic rocks from volcanoes of the Hakusan group, situated far from the volcanic front, is 0.70648-0.70842 (above 0.7070 for most samples). While, the range of the ratios from volcanoes of the Norikura group, situated between the Hakusan group and the volcanic front, is 0.70515-0.70730 (0.7060-0.7070 for most samples). Since the ⁸⁷Sr/⁸⁶Sr ratios of volcanic rocks sampled along the volcanic front lie around 0.70400, the ratios increase with distance from the volcanic front. This trend is opposite to that observed in the Northeast Japan and Izu-Ogasawara arcs. The spatial variation in chemical composition of volcanic rocks from central Japan is neither systematic nor corresponding to Sr isotopic variation. These features suggest that the crustal processes control the magma compositions near the arc junction in central Japan, rather than the mantle-slab interaction.

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Infrared microspectroscopy analysis of the chemical state and spatial distribution of hydrous species in minerals

Satoru Nakashima, Sadatsugu Ohki, Shukichi Ochiai

1989 Volume 23 Issue 2 Pages 57-64
Published: 1989
Released on J-STAGE: April 08, 2008

DOIhttps://doi.org/10.2343/geochemj.23.57

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Fourier transform infrared microspectroscopy (Micro FT-IR) has been applied to minerals to analyze hydrous microphases. Spectra interpretation and mapping analysis of a thin section of altered granitic rock can give us information on the chemical states of hydrous species and their spatial distribution, respectively, in an area as small as 10×10 μm. Micro FT-IR can be a powerful method of microscopical study of heterogeneous geochemical processes, such as water-rock interactions.

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Hydrogen and carbon isotopic compositions of methane in N₂- and CO₂-rich gases from Central Japan

Yoshihiko Mizutani, Minoru Tokiwa, Hiroshi Satake

1989 Volume 23 Issue 2 Pages 65-73
Published: 1989
Released on J-STAGE: April 08, 2008

DOIhttps://doi.org/10.2343/geochemj.23.65

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N₂- and CO₂-rich bubble gases collected from five springs and four wells in Paleozoic metamorphic rock and Cretaceous granite areas of Central Japan contain up to 22.9% methane. δD and δ¹³C values are between -100 and -180‰, and between -24 and -51‰, respectively, for the methane. A linear correlation between δD and δ¹³C values observed is almost identical with that for the TT(m) type thermogenic methane. The methane in the bubble gases is likely to be originated by thermal decomposition of organic matter in deep-seated metasedimentary rocks. The contribution of volcanic, magmatic and mantle-derived methanes to the bubble gases is probably insignificant, whereas ³He/⁴He values given by previous works indicate the presence of gas components derived from deep-seated sources, such as the magma, lower crust and mantle, in the gases.

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Study of colors and degrees of weathering of granitic rocks by visible diffuse reflectance spectroscopy

Tetsushi Nagano, Satoru Nakashima

1989 Volume 23 Issue 2 Pages 75-83
Published: 1989
Released on J-STAGE: April 08, 2008

DOIhttps://doi.org/10.2343/geochemj.23.75

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Diffuse reflectance spectroscopy and colorimetry have been applied to study the colors of powdered granitic rocks with different degrees of weathering. As a result, FeO(OH) was considered to be the source of the yellow-brownish color of the weathered rock powders. FeO(OH) contents were determined by diffuse reflectance spectroscopy based on the Kubelka-Munk function and found to increase during weathering. The FeO(OH) contents can be hence taken as a quantitative indicator of the degree of weathering.

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K-Ar age of molybdenum mineralization in the east-central Kitakami Mountains, Northern Honshu, Japan: Comparison with the Re-Os age

Shunso Ishihara, Ken Shibata, Shigeru Uchiumi

1989 Volume 23 Issue 2 Pages 85-89
Published: 1989
Released on J-STAGE: April 08, 2008

DOIhttps://doi.org/10.2343/geochemj.23.85

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Three K-Ar ages were determined on rocks and minerals from the mineralized areas in the east-central part of the Kitakami Cretaceous granitic terrain, where a Paleogene mineralization age was reported recently on molybdenite by the Re-Os method. The present result showed an early Cretaceous age of 114±4 Ma for the same molybdenite deposit. A rather large discrepancy was found between the Re-Os ages for molybdenite and the K-Ar and Rb-Sr ages for silicate minerals from the same ore deposits of Mesozoic-Cenozoic ages in the Circum-Pacific region, indicating that the Re-Os method needs further examination.

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