All issues
Volume 30 (1996)
- Issue 6 Pages 323-
- Issue 5 Pages 261-
- Issue 4 Pages 201-
- Issue 3 Pages 139-
- Issue 2 Pages 71-
- Issue 1 Pages 1-
Volume 30, Issue 5
Displaying 1-3 of 3 articles from this issue
- |<
- <
- 1
- >
- >|
-
Tomoharu Miyamoto, Takeru Yanagi1996 Volume 30 Issue 5 Pages 261-271
Published: October 20, 1996
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSU-Pb and Rb-Sr radiometric dating has been done on zircon samples and mineral concentrates, respectively, separated from psammitic schists of the Sangun metamorphic rocks in central Kyushu, Japan. Based on their shapes and colors, zircon grains were grouped into six. Rounded and short prismatic zircons define, independently of their colors, a common discordia with an upper intercept at 1.91 ± 0.04 Ga. Elongated zircons define another discordia with an upper intercept at 0.93 ± 0.06 Ga. A Rb-Sr internal isochron defined by the mineral concentrates gives an age of 197.7 ± 6.5 Ma, which represents the time of the Sangun metamorphism in central Kyushu. The U-Pb age of 1.91 Ga coincides with that of zircons from the Oki metamorphic rocks (1.96 ± 0.04 Ga: Yamashita and Yanagi, 1994). This and the great abundance of 1.91 Ga grains in the zircon populations seem to indicate a prominent plutonic activity at 1.9–2.0 Ga in the provenance common for the Sangun and Oki metamorphic rocks. The U-Pb ages of zircon from the Sangun metamorphic rocks indicate the episodic growth at 1.91 and 0.93 Ga of the East Asian continent close to the Japanese Islands.View full abstractDownload PDF (2821K) -
Takao Oi, Kyutaro Ikeda, Makiko Nakano, Tomoko Ossaka, Joyo Ossaka1996 Volume 30 Issue 5 Pages 273-287
Published: October 20, 1996
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSCompositions of major dissolved components and of boron isotopes were investigated for hot spring waters in Ibusuki and adjacent areas (the Ibusuki region), Kyushu, Japan. The chemical compositions of the hot spring waters examined were consistent with the previous contention that major dissolved components of hot spring waters in the region are the resultant of the interaction of seawater with heated rocks underground. The δ11B values (relative to the isotopic ratio of NBS SRM 951) of the hot spring waters ranged from +2.1‰ to +39.4‰. A close inspection of the boron isotopic data identified two boron sources; one is seawater with a δ11B value of +39‰ and the other is volcanic gases with a δ11B value of ca. +6‰, which supply hot spring waters with boron with δ11B = ca. +2‰.View full abstractDownload PDF (3613K) -
John C. Bailey1996 Volume 30 Issue 5 Pages 289-321
Published: October 20, 1996
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESS87Sr/86Sr ratios of ocean-floor sediments from the NW Pacific result from mixing between island-arc detritus (0.7037) and open-ocean detritus derived from the upper continental crust (0.7175). Sediments lying within and immediately in front of the Kurile-Kamchatka trench change from a c. 500 m sequence rich in open-ocean detritus in front of the south-central Kuriles to a c. 1000 m sequence, the Meiji sediment tongue, rich in island-arc detritus in front of the northern section of the arc. Overall, the sediments of this northern section have lower Th/U but higher K/Cs and Rb/Cs ratios, and these features are mirrored by all basalts from the northern section of the arc, presumably through sediment contamination of the subarc mantle sources. In detail, however, a simple mechanical addition of any type or mixture of known sediments to the sub-arc source mantle fails to reproduce the overall geochemistry of the arc basalts and selective transfer of mobile elements (Cs, Rb, K, Ba, Pb, Sr) and a low-87Sr/86Sr component (altered MORB, sub-arc mantle) seem required. Although 87Sr/86Sr ratios of the bulk northern pre-arc sediments (0.7073) are lower than in the south-central section (0.7104), the flux of Sr into the subduction zone is thought to be greater in the north and could explain the higher 87Sr/86Sr ratios of basalts from the northern section of the arc.View full abstractDownload PDF (8996K)
- |<
- <
- 1
- >
- >|