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11.
H. Anawar K. Komaki J. Akai J. Takada T. Ishizuka T. Takahashi T. Yoshioka K. Kato 《Environmental Geology》2002,41(7):816-825
A stratigraphic profile of solid phase As was measured to investigate the diagenetic cycling of arsenic and related elements in fluvial sediments of the Meghna River delta plain. The distributions of Fe, Mn, and Al are typically characterized by surficial solid phase enrichment, and As is distributed down to 36.6 m showing similar alternate layers of maxima and minima with Fe, Mn and TOC, which reflects the diagenetic remobilization and periodical differences in source materials of As. Lithological characteristics and geochemical data suggest that elevated levels of As are found in organic-matter-rich clay and silty sand rather than sand samples, with occasionally enriched As content in iron-oxyhydroxide-coated sand grains. Arsenic demonstrates a positive and significant co-variation with total organic carbon in sediments, which suggests the important role of particulate and colloidal organic matter and biological activity in controlling the distribution of arsenic in the Bengal delta. However, the concentrations of Fe and Mn weakly correlate with As contents, whereas Al contents show no relationship with As. The results of this study suggest that reactive oxides or hydroxides of Fe and Mn, rather than Fe and Mn with other minerals, might control arsenic distribution. 相似文献
12.
Fuji volcano is the largest active volcano in Japan, and consists of Ko-Fuji and Shin-Fuji volcanoes. Although basaltic in composition, small-volume pyroclastic flows have been repeatedly generated during the Younger stage of Shin-Fuji volcano. Deposits of those pyroclastic flows have been identified along multiple drainage valleys on the western flanks between 1,300 and 2,000 m a.s.l., and have been stratigraphically divided into the Shin-Fuji Younger pyroclastic flows (SYP) 1 to 4. Downstream debris flow deposits are found which contain abundant material derived from the pyroclastic flow deposits. The new14C ages for SYP1 to SYP4 are 3.2, 3.0, 2.9, and 2.5 ka, respectively, and correspond to a period where explosive summit eruptions generated many scoria fall deposits mostly toward the east. The SYP1 to SYP4 deposits consist of two facies: the massive facies is about 2 m thick and contains basaltic bombs of less than 50 cm in size, scoria lapilli, and fresh lithic basalt fragments supported in an ash matrix; the surge facies is represented by beds 1 to 15 cm thick, consisting mainly of ash with minor amount of fine lapilli. The bombs and scoria are 15 to 30% in volume within the massive facies. The ashes within the SYP deposits consist largely of comminuted basalt lithics and crystals that are derived from the Middle-stage lava flows exposed at the western flanks. SYP1 to SYP4 were only dispersed down the western flanks. The reason for this one-sided distribution is the asymmetric topography of the edifice; the western slopes of the volcano are the steepest (over 34 degrees). Most pyroclastic materials cannot rest stably on the slopes steeper than 33 degrees. Therefore, ejecta from the explosive summit eruptions that fell on the steep slopes tumbled down the slopes and were remobilized as high-temperature granular flows. These flows consisted of large pyroclastics and moved as granular avalanches along the valley bottom. Furthermore, the avalanching flows increased in volume by abrasion from the edifice and generated abundant ashes by the collision of clasts. The large amount of the fine material was presumably available within the transport system as the basal avalanches propagated below the angle of repose. Taking the typical kinetic friction coefficient of small pyroclastic flows, such flows could descend the western flanks where scattered houses are below 1,000 m a.s.l. A similar type of pyroclastic flow could result if explosive summit eruptions occur in the future.Editorial responsibility: R Cioni 相似文献
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14.
Osamu Ishizuka 《Resource Geology》2008,58(3):206-219
In the Izu–Bonin Arc, hydrothermal activities have been reported from volcanoes along present‐day volcanic front, a rear arc volcano and a back‐arc rift basin as well as a remnant arc structure now isolated from the Quaternary arc. It is widely known that characteristics of hydrothermal activity (mineralogy, chemistry of fluid etc.) vary depending upon its tectonic setting. The Izu–Bonin Arc has experienced repeated back‐arc or intra‐arc rifting and spreading and resumption of arc volcanism. These characteristics make this arc system a suitable place to study the tectonic control on hydrothermal activity. The purpose of the present paper is, therefore, to summarize volcanotectonic setting and history of the Izu–Bonin Arc in relation to the hydrothermal activity. The volcanotectonic history of the Izu–Bonin Arc can be divided into five stages: (i) first arc volcanism (boninite, high‐Mg andesite), 48–46 Ma; (ii) second arc volcanism (tholeiitic, calc‐alkaline), 44–29 Ma; (iii) first spreading of back‐arc basin (Shikoku Basin), 25–15 Ma; (iv) third arc volcanism (tholeiitic, calc‐alkaline), 13–3 Ma; and (v) rifting in the back‐arc and tholeiitic volcanism along the volcanic front, 3–0 Ma. Magmas erupted in each stage of arc evolution show different chemical characteristics from each other, mainly due to the change in composition of slab‐derived component and possibly mantle depletion caused by melt extraction during back‐arc spreading and prolonged arc volcanism. In the volcanotectonic context summarized here, hydrothermal activity recognized in the Izu–Bonin Arc can be classified into four groups: (i) present‐day hydrothermal activity at the volcanic front; (ii) active hydrothermal activity in the back arc; (iii) fossil hydrothermal activity in the back‐arc volcanoes; and (iv) fossil hydrothermal activity in the remnant arc. Currently hydrothermal activities occur in three different settings: submarine caldera and stratocones along the volcanic front; a back‐arc rift basin; and a rear arc caldera. In contrast, hydrothermal activities found in the back‐arc seamount chains were associated with rear arc volcanism in Neogene after cessation of back‐arc spreading of the Shikoku Basin. Finally, sulfide mineralization associated with boninitic volcanism in the Eocene presumably took place during forearc spreading in the initial stage of the arc. This type of activity appears to be limited during this stage of arc evolution. 相似文献
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Abstract The mineral assemblages of the pumpellyite–actinolite facies such as pumpellyite + actinolite + epidote + chlorite or actinolite + epidote + hematite + chlorite occur in the Sanbagawa low-grade metamorphic region, central Shikoku, southwest Japan. Chemical compositions of these minerals from the eight newly studied areas were analyzed in order to evaluate the areal extent and thermal structure of the region. In the buffered assemblage of pumpellyite + actinolite + epidote + chlorite, the Fe3+ /(Fe3+ + Al) values of epidote decrease slightly with decreasing Fe2+ /(Fe2+ + Mg) values for chlorite. The changes in these values show a general correlation with temperature. The presence of this relationship implies that the Fe3+ /(Fe3+ + Al) values of epidote can be used to divide the Sanbagawa low-grade metamorphic region into low-, medium- and high-grade subzones. The areal distribution of these subzones indicates that: (i) the temperature seems to decrease in the same sense as envisaged by the zonal mapping of the higher-grade pelitic schists; and (ii) there is no significant gap of metamorphic conditions through the boundary between the two structural units (Besshi and Oboke units). It follows that the Sanbagawa low-grade metamorphic region decreases in temperature going up the structural section, and tectonic discontinuities have not affected the thermal structure. 相似文献
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Reina Nakaoka Soko Kado Shuichi Hasegawa Keiko Suzuki-Kamata Osamu Ishizuka Shun Sekimoto Hiroshi Kawabata Yoshiyuki Tatsumi 《Island Arc》2021,30(1):e12378
Ar-Ar ages, and petrographical and geochemical characteristics of pyroclastics and an overlying lava from Teshima Island, southwest Japan are presented. Although previous geological and age data suggested Teshima pyroclastics were products of magmatism > 3 my prior to lava flows of Setouchi volcanic rocks generated in association with southward migration of the southwest Japan arc sliver during opening of the Sea of Japan backarc basin at ~ 15 Ma, the present results led to the conclusion that a sequence of Setouchi volcanism, induced by slab melting and subsequent melt-mantle reactions, produced both pyroclastics and lava at 14.6–14.8 Ma. This age is oldest among those reported so far and may represent the timing of onset of characteristic Setouchi magmatism immediately posterior to and hence as a result of the mega-tectonic event including rotation of the southwest Japan arc sliver. 相似文献
17.
Palau Islands, 7°30′N, are the only emergent feature on the more than 2500‐km‐long Kyushu–Palau Ridge. Small islands are mainly uplifted reef carbonate. Larger islands are volcanic with basalt to dacite and rare boninite. Polymict breccia is abundant: sills, flows, and dykes are common but pillows are rare. Palau Trench samples include all types found on the islands as well as high‐Mg basalt. Volcanism began in the late Eocene and ended by early Miocene. All igneous rocks comprise a low‐K primitive island arc‐tholeiite series. None are mid‐ocean ridge basalts. Rare earth elements and high field‐strength elements indicate a depleted mantle source. Elevated large ion lithophile elements and light rare earth elements indicate influx of ‘dehydration fluid’. Ce/Ce* and Eu/Eu* ratios show no evidence for recycling of arc‐derived clastics. Plate reconstructions and paleomagnetic data suggest that the arc probably formed on the trace of a transform fault that migrated northward and rotated clockwise up to 90°. Episodes of transtension caused upwelling of hot mantle into depleted mantle and sheared altered rocks of the transform. Episodes of transpression may have initiated subduction of old seafloor with a thin cover of pelagic sediments deposited far from terrigenous sediment sources. 相似文献
18.
Kiyoshi Fujimoto Kumiko Kawase Shigehiro Ishizuka Koji Shichi Akio Ohira Hiroshi Adachi 《Quaternary Science Reviews》2009,28(15-16):1472-1480
We estimated stored sediment and carbon during the Holocene for each layer of the Yahagi River Delta, central Japan and discussed the provenance of stored carbon. To estimate the bulk density and the carbon content of each layer, we collected two 30 m deep undisturbed cores. The volume of each layer was calculated using ArcView 3D analyst. Although the volume ratio of each layer to the total volume was calculated to be 9.5% for the top mud layer, 34.9% for the upper sand layer, 32.8% for the middle mud layer and 22.9% for the lower sand layer, the mass ratio of each layer to the total mass was calculated to be 8.5, 40.9, 25.2 and 25.4%, respectively, and the stored carbon ratio in each layer to the total stored carbon was 20.4, 4.7, 55.9 and 18.9%, respectively. These results suggest that the top mud and middle mud layers have a significant role as a place for carbon sequestration during postglacial time. Total stored carbon in the study area of only 92.1 km2 was estimated at 26 Tg C, which is equivalent to 0.003% of atmospheric carbon. This suggests that deltas on the globe have accumulated a massive amount of carbon during the evolution. The inorganic carbon ratio to total carbon reached more than 45% around the boundary between the middle mud and lower sand layers. The increasing trend in the Corg/Ntotal ratio accompanied with a decrease in δ13C from the bottom to the top horizon in the middle mud layer indicates a gradual increase in terrestrial organic matter contribution. The relative proportion of terrestrially derived materials decreases with increasing distance seaward. 相似文献
19.
Motion of a charged particle around a black hole immersed in magnetic field is calculated. It is shown that this motion has a chaotic property depending on initial parameters. 相似文献
20.
Prof. M. Oda T. Yamabe Y. Ishizuka H. Kumasaka H. Tada K. Kimura 《Rock Mechanics and Rock Engineering》1993,26(2):89-112
Summary An elastic stress-strain relation is formulated in terms of crack tensors which makes it possible to take into account explicitly the effect of joints on elastic behavior of rock masses. The present study is to discuss some related topics which may be encountered in its practical application. Two problems are solved by incorporating the elastic stress-strain relation into a program for three-dimensional finite element analyses; i. e., stress concentration by surface loading and displacement by excavation of an intersecting tunnel. Validity of the results is checked by comparing them with a laboratory model test and a field test, with the following conclusions: The overall distribution of stress definitely depends on a joint stiffness ratio (i. e., normal stiffness to shear stiffness). If the ratio is chosen as unity, the stress concentration occurs mainly in the direction parallel to major joints. If the ratio is high, say 10, then the stress concentrates along the perpendicular as well as the parallel directions to major joints. It can be said, on the basis of the fairly good agreement of the calculations using the high stiffness ratio with the field and laboratory measurements, that the elastic solution by crack tensors provides a practical tool for estimating the stress and strain in strongly jointed rock masses. 相似文献