The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 110, Issue 11
Displaying 1-6 of 6 articles from this issue
Articles
  • Yoshihiro Hiraoka
    Article type: scientific monograph
    Subject area: Infomation Science
    2004 Volume 110 Issue 11 Pages 661-670
    Published: 2004
    Released on J-STAGE: February 18, 2005
    JOURNAL FREE ACCESS
    The soil distributed at the boundary between roadway and pavement in urban areas, hereafter designated road sediments, usually contain various kinds of microscopic artificial materials. Road sediment samples were collected from three cities in Kyoto Prefecture. Microscopic observations and chemical analyses were carried out for these samples in order to assess its applicability to forensic science.
    In addition to glass beads, traffic paint segments, rubber fragments and asphalt grains, various kinds of metal fragments such as iron, copper, aluminum and brass are found in the road sediments collected from the urban center of Kyoto City.
    Major and trace element concentrations were determined for the <74μm fraction in the road sediments by X-ray fluorescence spectrometer. The present measurements indicate that heavy metals are mostly enriched in the road sediment samples relative to background samples (surface weathered sediments) distributed around Kyoto City.
    The enrichment factors (EF) of copper and zinc decrease with the increase of grain sizes, whereas the EF values of alkaline elements and silicon increase with the increasing grain size. It is likely that the copper concentrations in the road sediments are proportional to the traffic volume. The source of copper contained in the road sediments may be the breaking pads of vehicles.
    There is no significant annual variation of chemical compositions in the road sediments. Abundance of chromium, nickel and lead in the Maidzuru samples are greater than the Kyoto samples, suggesting that chromium in the Maidzuru samples is derived from chromian spinels contained in ultra-basic rocks.
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  • Fuminori Akagawa, Hidekazu Yoshida, Setuo Yogo, Koshi Yamamoto
    Article type: scientific monograph
    Subject area: Infomation Science
    2004 Volume 110 Issue 11 Pages 671-685
    Published: 2004
    Released on J-STAGE: February 18, 2005
    JOURNAL FREE ACCESS
    In a geological disposal system for high-level radioactive waste (HLW), maintenance of a reducing geological environment is considered to be important for long-term safety. However, formation of a redox front in the near-field of a HLW repository can be accompanied by oxidation of artificial barriers and basement rocks during the operational phase. In order to provide an analogue for secondary elemental migration associated with the formation of such a redox front, we examined granitic rock with a redox front formed along a fracture.
    Geochemical analyses revealed that Fe concentrations in the oxidized zone are higher than those of the parent rock, and that secondary Fe accumulation occurred. Secondary accumulation of Mn was also identified in the oxidized zone, which has about 5 times the Fe concentration of the parent rock. These results show that these elements were firstly carried by reduced groundwater, and then oxidized and precipitated after oxygen diffusion into the matrix of the host rock.
    It is also found that heavy metals such as Pb, Ba and Rb are concentrated in the oxidized zone, and that the concentrations of Pb and Ba are also increased by about 2 times compared with those of the parent rock. The concentration of these heavy metals can be considered to occur due to adsorption on Mn precipitates, and this suggests that Mn precipitates are capable of retaining heavy metals within the oxidized zone. Additionally, it has also been revealed that REE and U have accumulated in parts of the redox front, and are considered to be adsorbed onto Fe precipitates.
    These results of an investigation into a redox front, show that secondary migration and accumulation of heavy metals was accompanied by redox front migration. Coupled phenomenon like these may be analogous to processes that might occur in the near-field of a HLW repository.
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  • Hiroyuki Hoshi, Naohiro Kamiya, Yuu Kawakami, Kazuo Nakashima
    Article type: scientific monograph
    Subject area: Infomation Science
    2004 Volume 110 Issue 11 Pages 686-697
    Published: 2004
    Released on J-STAGE: February 18, 2005
    JOURNAL FREE ACCESS
    A Middle Miocene pyroclastic (tuffite) dike at Nakaoku, central Kii Peninsula, has a northerly remanent magnetization direction of normal polarity with low unblocking temperatures (<400°C). A previous study has interpreted this direction as a primary thermoremanent magnetization (TRM). We propose an alternative interpretation that it is a thermochemical remanent magnetization (TCRM) acquired through hydrothermal processes. Samples were collected from both rhyolitic welded tuff fragments and the matrix. A detailed rock magnetic study, including the analysis of isothermal remanent magnetization (IRM) acquisition curves, thermal demagnetization of a composite IRM, and high- and low-temperature measurements, as well as ore microscopic observation, indicates that the main magnetic carrier is pyrrhotite. The pyrrhotite occurs not only as anhedral minerals enveloping other small grains, but also as those filling open fractures within lithic fragments. These observations indicate that the pyrrhotite is a secondary mineral, precipitated most likely by hydrothermal activity. We also determined directions for nearby dikes of basaltic andesite and quartz porphyry. Similarly to the tuffite, the basaltic andesite contains pyrrhotite as the magnetic carrier and possesses a northerly direction of normal polarity, probably acquired as TCRM at nearly the same time as the tuffite. On the other hand, the quartz porphyry has a reverse magnetization with a southerly declination and a steep inclination. Magnetite is the main carrier. The reverse direction can be interpreted as a primary TRM representing the paleomagnetic direction at the time of deuteric cooling (∼15 Ma).
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  • Yusuke Soda, Hideo Takagi
    Article type: scientific monograph
    Subject area: Infomation Science
    2004 Volume 110 Issue 11 Pages 698-714
    Published: 2004
    Released on J-STAGE: February 18, 2005
    JOURNAL FREE ACCESS
    The Asaji metamorphic rocks have previously been considered as a member of the Ryoke belt. However, the occurrence of ultramafic bodies interlayered in the Chokai Unit and the Hikata Unit has led to an alternative suggestion for the correlation of the Asaji metamorphic rocks with other terranes, such as the Sambagawa terrane or the Kurosegawa terrane. To discuss these possibilities, we examined the petrological characteristics of the ultramafic rocks and chemical compositions of chromian spinel.
    The ultramafic body is composed mainly of pyroxenites and serpentinite. Spinel in the serpentinite has high Cr# (Cr/(Cr+Al) atomic ratio) of 0.7-0.9 and very low TiO2 content (<0.5wt%). Although the ultramafic body suffered weak contact metamorphism, chemical composition of chromian spinel has preserved those in the peridotite stage. The spinel chemistry suggests that the original peridotite is a refractory residue after the extraction of basaltic magma. The pyroxenites are subdivided into clinopyroxenite, olivine-clinopyroxenite and olivinewebsterite. Compositions of clinopyroxene from the pyroxenites are similar to those of arc cumulates. It suggests that the peridotites (now completely serpentinitized) and pyroxenites were formed in a subduction zone.
    Chemical compositions of spinel in the serpentinites from the Saganoseki area (the Sambagawa metamorphic belt), the Sashu Fault on the southern margin of the Sambagawa metamorphic belt, and the Miemachi and Usuki area (the Kurosegawa belt) exposed near the Asaji metamorphic rocks were analyzed for comparison. Spinels from the Saganoseki area are clearly different in Cr# and TiO2 content from those from the Asaji area and also from the Sashu Fault. On the other hand, the chemical compositions of spinels from the Miemachi and Usuki area (the Kurosegawa belt) resemble to those from the Asaji area, suggesting a similar tectonic setting for the formation of those ultramafic rocks.
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  • Makoto Takeuchi, Masaki Kawai, Atsushi Noda, Norihiko Sugimoto, Hideha ...
    Article type: scientific monograph
    Subject area: Infomation Science
    2004 Volume 110 Issue 11 Pages 715-730
    Published: 2004
    Released on J-STAGE: February 18, 2005
    JOURNAL FREE ACCESS
    The Permian Shiroumadake Formation is distributed in the Shiroumadake area of the northeast Hida Gaien belt. The Shiroumadake Formation is redefined as a coherent succession of shallow-marine sediments which are composed mainly of felsic pyroclastic rocks deposited in a volcanic arc with extrusion of basalt and intrusion of gabbro, and divided into three members. A bimodal volcanism which is characterized by felsic tuff, felsic tuff breccia and subaqueous eruption of basalt is recognized in lower member of the Shiroumadake Formation. The volcanism was suspended, and shale and sandstone were deposited in middle member. The bimodal volcanism was reactivated during the deposition of upper member, and felsic tuff breccia with limestone breccias and blocks was accumulated due to slumping or collapse of slope deposits.
    The Shiroumadake Formation was thrust by serpentinite melange containing amphibolite blocks with west vergence. After the thrusting, NW-SE and NE-SW trending and steep-dipping faults were formed. The faulting resulted in an emplacement of the Shiroumadake rocks as tectonic blocks within the serpentinite melange.
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