Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 95 , Issue 8
Showing 1-4 articles out of 4 articles from the selected issue
  • Hiroyuki HOSHI, Daisuke TANAKA, Masaki TAKAHASHI, Toshiyuki YOSHIKAWA
    2000 Volume 95 Issue 8 Pages 203-215
    Published: 2000
    Released: November 29, 2005
    A paleomagnetic study was conducted for well-dated volcanic rocks from the Middle Miocene Nijo Group in the Mt. Nijo area, southwest (SW) Japan. Samples were collected from 33 sites. Detailed stepwise demagnetization with both alternating-field and thermal techniques revealed well-determined site-mean directions from 29 sites. Tilt correction was applicable to 13 sites, yielding an overall mean direction of D=352.2°, I=51.7°, and α95=7.4° and a paleomagnetic pole at 84.3°N, 35.5°E, and A95=8.5°. The bedding-tilt test passed with 95% probability. Although the reversal test was negative, the presence of dual polarities and a study of angular standard deviation of virtual geomagnetic poles imply that the overall mean direction averages out, to a certain extent, the past geomagnetic secular variation. Magnetic polarity stratigraphy determines the age of the Nijo Group; the younger age limit is 14.6 Ma (top of polarity subchron C5ADr) and the older age limit is 15.2 Ma (base of polarity subchron C5Bn.2n). Comparison of the paleomagnetic pole with those from the Asian continent shows no rotation in the Mt. Nijo area since 14.6 Ma, suggesting that SW Japan has not experienced rotation since that time. We conclude that the clockwise rotation of SW Japan, thought to have occurred in Miocene time in relation to the opening of the Japan Sea, had already ended by 14.6 Ma. Two clockwise-deflected directions of the Middle Miocene rocks (Muro Pyroclastic Flow Deposit and Kumano Acidic Rocks) in the Kii Peninsula are probably instantaneous recordings of a geomagnetic field excursion or transition.
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  • Yukiyasu TSUTSUMI, Kazumi YOKOYAMA, Kentaro TERADA, Yuji SANO
    2000 Volume 95 Issue 8 Pages 216-227
    Published: 2000
    Released: November 29, 2005
    Radiometric ages of zircons in psammitic rocks from the Nishiki and Tsuno groups were determined from 238U/206Pb ratio and isotopic compositions of Pb which are analyzed by Sensitive High Resolution Ion Microprobe (SHRIMP II). Detrital zircons in the Nishiki sandstones show a continuous distribution from 320 Ma to 240 Ma, which is similar to those from the Tsuno psammitic schists; about 380-220 Ma. The youngest zircons from the Nishiki and Tsuno groups are 238.7±5.9 Ma and 218.6±11.4, respectively. It was estimated as upper limits of the depositional ages.
        Detrital zircon age in the psammitic rocks shows continuous variation from 380 Ma to 220 Ma, whereas K-Ar age of white mica corresponding to metamorphic age is around 220 Ma in both the groups. Hence, it is realistic to conclude that the youngest age is close to the depositional age and clastic rocks in the Tsuno Group is Triassic in origin. Probability distributions for zircon ages in psammitic rocks from both groups, calculated by a multi-peak Gauss fitting, indicate that provenances for the clastic materials in both the groups overlap widely and are similar to those in the Mino and Hida belts.
        Acidic tuff in the Nishiki group occurs in a hemi-pelagic unit different from the sandstone of trench-fill sediment. Zircons in the tuffs show apparently two distinctive U-Pb ages; about 2700-2400 Ma and 320-260 Ma. The former is considered to be accidental materials derived from Archean basement which will be a part of the Sino-Korean craton with 2700-2500 Ma. Least squares fitting for the latter younger cluster yields a concordia intercept age of 270.3±8.5 Ma (2σ) which probably represents a depositional age for the tuff.
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  • Aierken SIDIKE, Kyue-Hyung LEE, Isao KUSACHI, Nobuhiko YAMASHITA
    2000 Volume 95 Issue 8 Pages 228-235
    Published: 2000
    Released: November 29, 2005
    Photoluminescence properties of a natural fluorite from China are investigated at temperatures of 10, 80 and 300 K. The color of the fluorite is green under the daylight and it emits bluish violet fluorescence under UV light. The band peak (423 nm) at room temperature and the vibronic structure at low temperatures show that the bluish violet fluorescence originates from Eu2+ ion in the fluorite. In the excitation spectrum obtained by monitoring the bluish violet fluorescence, not only the Eu2+ band but the Ce3+ band are observed as broad bands. They are attributed to the f-d electronic transitions in Eu2+ and Ce3+. It was interpreted that the excitation processes of the bluish violet fluorescence are due to the direct excitation of the Eu2+ ion and the energy transfer from Ce3+ to Eu2+ in fluorite. The abundance of rare-earth impurities in the fluorite was analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) after the treatment with the cation-exchange column. Relationship between the photoluminescence and rare-earth impurities is discussed.
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  • Yoshiki HAMAHATA, Eiji OHTANI, Katsuyuki KAWAMURA
    2000 Volume 95 Issue 8 Pages 236-244
    Published: 2000
    Released: November 29, 2005
    We performed the molecular dynamics simulation of the elastic properties of MgSiO3 perovskite under the lower mantle conditions to estimate the temperature anomalies in the Earth's interior. The simulated P and S wave velocity anomalies corresponding to the temperature anomaly of 500 K are ±1.11% and ±1.13% respectively at the top of the layer, and are ±0.75% and ±0.78% respectively at the core-mantle boundary. Seismologically observed ratios of the P and S wave velocity anomalies R (=∂lnVs/∂lnVp), due to temperature difference are consistent with the simulated R values, after an anelastic correction, of MgSiO3 perovskite or pyrolite (a mixture of MgO and MgSiO3 perovskite). Both P and S wave velocity anomalies at the top of the lower mantle correspond to the cold temperature anomalies of 120 K for the cold slabs in which MgSiO3 perovskite is dominant. The magnitude of the P wave velocity anomaly observed, after the anelastic correction, at the core-mantle boundary corresponds to about +270 K. The large S wave velocity anomalies at the core-mantle boundary may be accounted for partial melting and/or the chemical heterogeneity due to relatively high iron contents and the temperature anomaly in this region.
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