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Volume 69, Issue 11
Displaying 1-3 of 3 articles from this issue
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Kenji Shuto, Ryuichi Yashima1974 Volume 69 Issue 11 Pages 373-380
Published: November 05, 1974
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSRb and Sr concentrations and Sr isotopic compositions (Sr87/Sr86) have been determined for five basalts, four andesitic basalts, one andesite and one obsidian of the Ryozen formation.
These eleven volcanic rocks have initial Sr87/Sr86 ratios ranging from 0.7033 to 0.7046. These ratios may be nearly identical among the different rock types.
The Sr87/Sr86 ratios of the basaltic rocks are slightly higher than those of occeanic ridge tholeiites but very similar to those of tholeiitic basalts from Hawaiian Islands and Type II island arc such as the Izu-Bonin Islands, Mariana Islands, Fiji Islands and New Britain Islands.
On the other hand, the Sr87/Sr86 ratios of acid members are very resemble to those of acid volcanic rocks from Type II island arc and they also lie in the lower part of the range measured in acid volcanic rocks from Type I island arc such as the Japanese Islands and New Zealand, and from Cenozoic orogenic belt in the United States and Central America.
The Rb concentrations and K/Rb ratios of basaltic rocks in the Ryozen formation are
nearly comparable to those of the tholeiitic basalts from Hawaiian Islands and Type II island arc, while the Sr concentrations are close to those of oceanic ridge tholeiites.
Considering from the results of the present work and previous petrological studies, it may be concluded that the siliceous basaltic and acid volcanic rocks of the lower and upper horizons are derived by fractional crystallization of olivine tholeiitic magma which produces olivine basalts in the middle horizon. An alternatively possibility is that olivine tholeiitic, siliceous basaltic and acid magmas may be derived directly from certain upper mantle material presumably of peridotitic composition, which has Sr87/Sr86 ratios around 0.704.View full abstractDownload PDF (412K) -
Shigekazu Udagawa, Kazuyori Urabe, Hideki Hasu1974 Volume 69 Issue 11 Pages 381-389
Published: November 05, 1974
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe crystal structure of muscovite dehydroxylate has been determined by X-ray single crystal analysis. The muscovite dehydroxylate was prepared by heating 2M1 muscovite at 900°C for 30 minutes. The dehydroxylate has the cell dimensions, a=5.244±0.008, b=9.202± 0.016, c=20.233±0.021 (Å), β =95°50′±06′; and the space group C2/c. Integrating Weissenberg photographs were taken using the multiple-film technique and intensities of 415 reflections were measured mainly by microphotometer. An examination was made by three dimensional Fourier methods of three possible models for the structure of the dehydroxylate. The result revealed that one of them was successfully refined to R=14.2% by full-matrix least-squares calculations. The R value of the final structure is considered to be low enough since the dehydroxylate gave diffused and streaked reflections caused by the structural disorders. In the structure, Al atoms are coordinated by five oxygen atoms. The change in coordination from six to five gives rise to a very short distance of dehydroxylate 1.69Å between Al and the oxygen atom which is coordinated to two Al atoms. The form of the five-fold coordination groups mentioned above is approximately a trigonal bipyramid and is quite similar to that found in pyrophyllite dehydroxylate proposed by Wardle and Brindley (1972).
Reaction mechanism of the dehydroxylation of muscovite was discussed in connection with the crystal structure of muscovite and its dehydroxylate.View full abstractDownload PDF (1109K) -
HIROJI HONMA1974 Volume 69 Issue 11 Pages 390-402
Published: November 05, 1974
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESS
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