Extensive ash falls in and around the sea of Japan from large late quaternary eruptions   总被引：4，自引：0，他引：4  

Hiroshi Machida  Fusao Arai   《Journal of Volcanology and Geothermal Research》1983,18(1-4)

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Chromian spinel lamellae in olivine from the Iwanai-dake peridotite mass,Hokkaido, Japan     

Shoji Arai 《Earth and Planetary Science Letters》1978,39(2):267-273

Lamellar inclusions of chromian spinel (Cr/Cr + Al> 0.7), clinopyroxene and chromian spinel-clinopyroxene symplectite occur in olivines from alpine-type peridotites which have equilibrated at relatively low temperature (<700°C). They occur most commonly in dunite with very magnesian olivine (Fo₉₃ to Fo₉₅) and discrete grains of Cr-rich spinel. Olivine which initially equilibrated with magnesian and Cr-rich liquid had contained small amounts of calcium and trivalent chromium in the octahedral site, and lamellar chromian spinel and diopsidic clinopyroxene exsolved during the annealing process. The ordinary depletion of chromium or absence of chromian spinel lamellae in olivines in igneous rocks may be partly due to the effective exclusion of chromium from olivine upon cooling.  相似文献   

Magma mixing model for the genesis of middle crust in the Izu–Bonin–Mariana arc: evidence from plutonic rocks in the Mineoka-Setogawa ophiolitic mélange,central Japan     

Yuji Ichiyama  Hisatoshi Ito  Akihiro Tamura  Shoji Arai 《International Geology Review》2020,62(4):503-521

ABSTRACT

A Paleogene accretionary complex, the Mineoka–Setogawa belt is distributed adjacent to the northern portion of the collision zone between Honshu and Izu–Bonin–Mariana (IBM) arcs in central Japan, comprising a mélange of ophiolitic fragments of various sizes. The Eocene-Oligocene plutonic rocks in this belt (gabbro, diorite, and tonalite) have been interpreted as fragments brought from the deep crust beneath the IBM arc through tectonic collisions. The geochemical characteristics of the gabbro and associated basaltic dike are similar to those of the Eocene IBM tholeiitic basalt; thus, the gabbro was likely formed via the crystallization of the Eocene tholeiitic basaltic magmas, which was produced by the partial meltings of a depleted mantle wedge. A comparison with experimental results and geochemical modeling indicates that the tonalite was generated by 10–30% dehydration melting of the gabbro. Actually, Eocene–Oligocene felsic veins, which are coeval with the plutonic rocks, occur in the Mineoka–Setogawa gabbro. Plagioclase crystals in the diorite comprise Ca-rich and -poor parts in a single crystal. Their compositional characteristics are consistent with those of plagioclase in the gabbro and tonalite, respectively. The textures and chemical composition of plagioclase indicate that the diorite was formed by the mixing between mafic and silicic magmas. The whole-rock composition of the diorite also indicates the evidence for the mixing between basaltic magmas which were fractionated to variable degrees and homogeneous silicic magma. The mixing model proposed from the first direct observations of the IBM middle crust exposed on the Mineoka–Setogawa belt is applied to the genesis of the Eocene to present intermediate rocks in the IBM arc. If the continental crust were created at intra-oceanic arc settings such as the IBM arc, the magma mixing model would be one of the most likely mechanisms for the genesis of the continental crust.  相似文献   

Chronic effects of tributyltin on the caprellid amphipod Caprella danilevskii   总被引：2，自引：0，他引：2  

Ohji M  Arai T  Miyazaki N 《Marine pollution bulletin》2003,46(10):1263-1272

In order to examine the biological effects by TBT exposure, experiments involving the exposure of five levels of TBT concentrations (0, 10, 100, 1000 and 10,000 ngl(-1)) were conducted on the caprellid amphipod Caprella danilevskii, over a generation after hatching. Marked delays in growth and molting during the early developmental stage and maturation were found in both 100 and 1000 ng TBTCl l(-1) concentrations in spite of the sex. All specimens died in 10,000 ng TBTCl l(-1) within 4 days after hatching. Inhibition of maturation and reproduction such as delaying in the achievement of maturity and a decrease in the number of juveniles hatched was apparent in 10 and 100 ng TBTCl l(-1) concentrations. Furthermore, brood loss, and failure in egg formation and hatching were observed as the TBT concentration became higher. No significant changes in sex ratio were seen at any of the TBT exposure levels during hatching and maturation in the present study. This phenomenon was different from a previous study in which the proportion of females was increased with an increase in TBT concentrations in the embryonic exposure experiment. This suggests that sex disturbance is induced during the embryonic stage of the species. A drastic decrease in survival rate was observed at 10 ng TBTCl l(-1) (25%) which corresponds to the mean level in coastal waters. Therefore, the present encountering ambient TBT concentration may influence populations of C. danilevskii in the coastal environment.  相似文献   

A new type of orthopyroxenite xenolith from Takashima, Southwest Japan: silica enrichment of the mantle by evolved alkali basalt     

Shoji Arai  Yohei Shimizu  Tomoaki Morishita  Yoshito Ishida 《Contributions to Mineralogy and Petrology》2006,152(3):387-398

We found fine-grained Fe-rich orthopyroxene-rich xenoliths (mainly orthopyroxenite) containing partially digested dunite fragments of Group I from Takashima, Southwest Japan. Orthopyroxenite veinlets, some of which contain plagioclase at the center, also replace olivine in dunite and wehrlite xenoliths of Group I. This shows high reactivity with respect to olivine of the melt involved in orthopyroxenite formation, indicating its high SiO₂ activity. The secondary orthopyroxene of this type is characterized by low Mg# [= Mg/(Mg + total Fe) atomic ratio] (down to 0.73) and high Al₂O₃ contents (5–6 wt%). It is different in chemistry from other secondary orthopyroxenes found in peridotite xenoliths derived from the mantle wedge. Clinopyroxenes in the Fe-rich orthopyroxenite show a convex-upward REE pattern with a crest around Sm. This pattern is strikingly similar to that of clinopyroxenes of Group II pyroxenite xenoliths and of phenocrystal and xenocrystal clinopyroxenes, indicating involvement of similar alkali basaltic melts. The Fe-rich orthopyroxenite xenoliths from Takashima formed by reaction between evolved alkali basalt melt and mantle olivine; alkali basalt initially slightly undersaturated in silica might have evolved to silica-oversaturated compositions by fractional crystallization at high-pressure conditions. The Fe-rich orthopyroxenites occur as dikes within the uppermost mantle composed of dunite and wehrlite overlying pockets of Group II pyroxenites. The orthopyroxene-rich pyroxenites of this type are possibly common in the uppermost mantle beneath continental rift zones where alkali basalt magmas have been prevalent.  相似文献   

Lack of Excitation by Tsunamis of Normal Modes of Sea-surface Oscillations in Bays     

K. Arai  Y. Tsuji 《Pure and Applied Geophysics》1998,151(1):161-181

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Jadeite, albite and nepheline as inclusions in spinel of chromitite from Hess Deep, equatorial Pacific: their genesis and implications for serpentinite diapir formation   总被引：3，自引：1，他引：2  

Kyoko Matsukage  Shoji Arai 《Contributions to Mineralogy and Petrology》1998,131(2-3):111-122

Polymineralic inclusions which consist of a few grains of diopside, enstatite, jadeite, nepheline, albite, pargasite, phlogopites and olivine were found in chromian spinel in a chromitite pod and in troctolite from Hess Deep, equatorial Pacific. The inclusion mineral suite in chromitite is characterized by Na-Al silicates, such as jadeite, nepheline and albite. Jadeite and nepheline commonly coexist with enstatite, and tend to occur as interstitial grains between subhedral enstatite (or other minerals) and host spinel. Albite, diopside and enstatite occur as equant inclusions. The mafic minerals in the inclusions have similar chemistry to those found in the troctolite and dunite. The modes of occurrence and mineral chemistry of the inclusions are controlled by magmatic precipitation, and subsequent reequilibration due to decrease of temperature in the uppermost mantle. The mafic minerals in spinel inclusions were crystallized from a melt enriched in Cr and some incompatible components formed by melt-mantle interaction process mixed to various extent with subsequently supplied more primary melt. Albite and nepheline could also be formed at the magmatic stage. Jadeite was formed by a subsolidus reaction of albite and nepheline at low temperatures (250–300 °C) at slightly less than 3 kbar. This requires a remarkable temperature decrease, at least locally, of the uppermost mantle and crust. The Hess Deep rocks were formed in the uppermost mantle beneath a spreading-ridge axis at more than 1000 °C, and were transposed outwards from the axis by corner flow. At the off-ridge conditions, the rocks were cooled and serpentinized by circulation of sea water at the mantle depth to form jadeite in chromitite. The serpentinized portion could have risen as a kind of serpentinite diapir through the thin crust up to the ocean floor. Received: 24 January 1997 / Accepted: 6 November 1997  相似文献   

Metamorphism of the Izu-Tanzawa collision zone     

Mitsuhiro Toriumi  Toru Arai 《Tectonophysics》1989,160(1-4):293-303

The difference in pressure condition of progressive metamorphism established by Schreinemakers' analysis of mineral assemblages in metabasalts makes it possible for the low grade metamorphism of the Izu-Tanzawa-Fujigawadani collision zone to be divided into three types. Type I is characterized by prehnite + epidote + hematite, suggesting the lowest pressure type; whereas type II is defined by prehnite + epidote + actinolite—the intermediate pressure type, and finally the distinctive assemblage of type III is pumpellyite + epidote + actinolite—the higher pressure type. The pressure conditions estimated are about 1 kbar for type I, 1–2 kbar for type II and 2–3 kbar for type III. The metamorphic rocks of type III occur in the southwestern part of the Tanzawa Mountains, and the metamorphic rocks of type I occur in the central Izu Peninsula and the northeast Tanzawa Mountains. Therefore, the upward displacement of the accretion mass due to collision deformation is most significant at the southwest Tanzawa Mountains. This suggests that the accretion of the Tanzawa and Izu blocks is accompanied with large-scale tilting of the mass.  相似文献   

Unstable forearc stress in the eastern Nankai subduction zone for the last 2 million years     

A. Yamaji  T. Sakai  K. Arai  Y. Okamura 《Tectonophysics》2003,369(1-2):103-120

Transpressional tectonics characterizes the SW Japan arc. However, we will show in this article that offshore seismic profiles and onshore mesoscale faults indicate that the eastern part of the forearc was subject to transtensional tectonics since ca. 2.0 Ma. Offshore normal faults imaged on the profiles run parallel to the Nankai Trough, and started activity at 1.0 Ma, but transtensional tectonics commenced the onshore area earlier. In order to understand the stress history in the forearc region, we collected fault-slip data from onshore mesoscale faults in Plio-Pleistocene sedimentary rocks in the Kakegawa area at the northeastern extension of the offshore normal faults. Most of the mesoscale faults are oblique-normal, indicating that the area was subject to transtensional tectonics. The faults suggest that the compressional tectonic regime was followed by the transtensional one at 2.0 Ma, in agreement with regional tectonostratigraphic data, which indicate that folding ceased at that time. Present compressional stress followed the transtensional tectonic regime sometime in the late Pleistocene. Transtensional or extensional tectonic zone shifted from the Kakegawa area to the offshore region.These observations indicate that the state of stress just behind the accretionary prism of the eastern Nankai subduction zone has been unstable in the last 2 million years, suggesting that the forearc wedge has been at critical state in that gravitational force and basal shear traction on the wedge have been balanced, but the forearc tectonics has been susceptible to small perturbations. Possible factors compatible with the observed stress history include the change of subduction direction of the plate at 1.0 Ma, and the rapid uplift of Central Japan thereafter.  相似文献   

Closed-system geochemical recycling of crustal materials in alpine-type peridotite     

Tomoaki Morishita  Shoji Arai  David H. Green 《Geochimica et cosmochimica acta》2003,67(2):303-310

We examined aluminous mafic rock (with or without corundum or sapphirine) alternating with peridotite from the Ronda peridotite massif, southern Spain. On the basis of petrographic characteristics, these mafic rocks show a decompression history from high pressure (P > 1.5 GPa), but on the basis of their geochemical characteristics, they are crystal accumulates of plagioclase, clinopyroxene, and olivine formed within the lower crust (P < 1 GPa). A complex evolution history, including higher-pressure recrystallization after initial formation as cumulate gabbros at lower-pressure conditions, is proposed. The aluminous mafic rocks and their peridotite hosts are inferred to be recycled crustal materials now observed as centimeter-scale layered components in alpine-type peridotite. The rocks retained their original cumulate compositions; that is, their compositions were not affected by melting and metasomatic modifications during subduction, intense deformation within the upper mantle, and upwelling to the surface.  相似文献