Trace element compositions of jadeite (+omphacite) in jadeitites from the Itoigawa-Ohmi district, Japan: Implications for fluid processes in subduction zones   总被引：3，自引：0，他引：3  

Tomoaki  Morishita  Shoji  Arai  Yoshito  Ishida 《Island Arc》2007,16(1):40-56

Abstract   Trace-element compositions of jadeite (±omphacite) in jadeitites from the Itoigawa-Ohmi district of Japan, analyzed by a laser-ablation inductively coupled plasma mass spectrometry technique showed chemical zoning within individual grains and variations within each sample and between different samples. Primitive mantle-normalized patterns of jadeite in the samples generally showed high large-ion lithophile element contents, high light rare earth element/heavy rare earth element ratios and positive anomalies of high field strength elements. The studied jadeitites have no signatures of the protolith texture or mineralogy. Shapes and distributions of minerals coupled with chemical zoning within grains suggest that the jadeitites were formed by direct precipitation of minerals from aqueous fluids or complete metasomatic modification of the precursor rocks by fluids. In either case, the geochemical characteristics of jadeite are highly affected by fluids enriched in both large-ion lithophile elements and high field strength elements. The specific fluids responsible for the formation of jadeitites are related to serpentinization by slab-derived fluids in subduction zones. This process is followed by dissolving high field strength elements in the subducting crust as the fluids continue to circulate into the subducting crusts and serpentinized peridotites. The fluids have variations in chemical compositions corresponding to various degrees of water–rock interactions.  相似文献   

A New Coastal Marine Ecosystem Model Study Coupled with Hydrodynamics and Tidal Flat Ecosystem Effect   总被引：3，自引：0，他引：3  

Akio Sohma  Yasuyuki Sekiguchi  Hiroshi Yamada  Tatsuaki Sato  Kisaburo Nakata 《Marine pollution bulletin》2001,43(7-12):187-208

A new coastal marine ecosystem model was developed, which was composed of pelagic and benthic ecosystems, and was applied to Mikawa Bay, Japan. This model deals with variations of biochemical and physical interactions among dissolved oxygen and C–N–P species (composition formed out of carbon, nitrogen and phosphorus elements) so that it resolves the flux dynamics of carbon, nitrogen, phosphorus and oxygen elements. The physical and biochemical mechanism figured in this model is constructed for the purpose of simulating the estuarine lower trophic ecosystem, in areas where the sea was too deep for light to reach the sea-bottom. As a result of coupling the benthic with pelagic system, the effect of process of sedimentation and nutrient diffusion back to the pelagic system could be indicated. In addition, by implementing the tidal flat ecosystem model's calculation result, the integrated model can include the effect of water purification in tidal flats where the light can reach the sea-bottom, and where sea-weed, sea grass and benthic algae exist. In this study, the model indicates that oxygen-depleted water exists at the sea-bottom especially in summer mainly caused by an increase of oxygen consumption in the benthic system and a decrease of the vertical mixing water process. Furthermore, by comparing the case – with the tidal flat ecosystem model and the case without it, the effect of water purification of tidal flat estuaries was indicated. From the viewpoint of a short time scale, the tidal flat has the potential to restrict red tide (rapid increase of phytoplankton), and from the viewpoint of a long time scale, it restricts the sedimentation of detritus. Restricting the sedimentation prevents oxygen-depleted water occurring in the coastal marine system of Mikawa Bay.  相似文献   

Ozone variation in the lower stratosphere and its mechanism     

Sekiguchi  Yoshiro 《Pure and Applied Geophysics》1973,106(1):1600-1610

Seasonal variations of ozone are studied by taking into consideration both photochemical and dynamical processes. Assuming that the seasonal variations of total ozone amounts are linear combinations of photochemical equilibrium variations and those due to atmospheric motions, the observed variations of the total ozone amount seem reasonably to be explained.The concept mentioned above implies that the time scale of ozone might be rather short in the lower stratosphere. The order of the half restoration time was estimated to be 10 to 10² days in the lower stratosphere. Thus the estimated time scale of ozone must be somewhat longer than that of the temperature in the lower stratosphere, of which variation usually shows its maximum in the midwinter in middle latitudes, while that of ozone shows its maximum in the late winter or early spring. As the maximum values of both quantities would appear in the early summer without air motions, the similarity in the phase differences of temperature and ozone suggests the validity of the concept of this paper.  相似文献   

Photometry of the lunar surface during lunar eclipses     

Naosuke Sekiguchi 《Earth, Moon, and Planets》1980,23(1):99-107

The photometric observations of the lunar surface during lunar eclipses were carried out on four nights between 1972 to 1978, using the 91 cm reflector of the Dodaira Station of the Tokyo Astronomical Observatory. The photometry was performed in B-, V-, and R-colours, and arranged in accordance with the angular distance from the centre of the Earth's shadow. The results do not show any large systematic differences between the four nights, showing no support for Danjon's proposition.  相似文献   

Convective instability and overstability in the sunspot umbra     

Mamoru Saitō  Shoji Kato 《Solar physics》1968,3(4):531-542

Convective instability and overstability of a plane-parallel medium in the presence of a uniform vertical magnetic field is studied. We take into account both the effect of penetration of disturbances into sub-adiabatically stratified semi-infinite upper and lower layers, and the effect of thermal damping of disturbances introduced by the compressibility of the medium. If the degree of super-adiabaticity in temperature stratification in the middle layer is relatively large compared with that of sub-adiabaticity in the upper and lower layers, the range of physical conditions in which overstable convection occurs is very narrow. However, if it is not the case the range is rather wide. In the normal sunspot umbra, only disturbances with selected sizes in vertical dimension will be able to become overstable. The overstable convection might be a cause of some dynamical features (such as dots) in the sunspot umbra, but will not be the main contributor for transporting mechanical energy in the sunspot region.  相似文献   

An anomalous brightening of the lunar surface observed on March 26, 1970     

Naosuke Sekiguchi 《Earth, Moon, and Planets》1971,2(4):423-434

An anomalous enhancement of brightness of the lunar surface was observed on March 26, 1970, during photometric and polarimetric observations of the Moon made by the 91-cm reflector of the Dodaira Station of the Tokyo Astronomical Observatory. The region near the Aristarchus was 0.3 mag. brighter compared with that corresponding to the ordinary brightness variation curve. The polarimetric results also showed evidence quite consistent with the phenomenon of the luminescence. The colour index diminished by about 0.1 mag. The results of the photographic photometry also confirmed the enhancement of brightness over a wide part of the lunar surface that night. This brightening may be related to the large solar flare observed on March 25, 12h UT, 1970, twenty-nine hours before our observations.  相似文献   

Wide-Field Camera for Gravitational Microlensing Survey: MOA-cam2     

Toshifumi Yanagisawa  Yasushi Muraki  Yutaka Matsubara  Fumio Abe  Kimiaki Masuda  Sachiyo Noda  Takahiro Sumi  Yuji Kato  Mitsuaki Fujimoto  Shuji Sato  Ian Bond  Nicholas Rattenbury  Philip Yock  Pam Kilmartin  John Hearnshaw  Micheal Reid  Denis Sullivan  Brian Carter  Richard Dodd  Garry Nankivell  Noiman Rumsey  Morihiro Honda  Maki Sekiguchi  Masanori Yoshizawa  Takashi Nakamura  Humitaka Sato  Seiji Kabe  Makoto Kobayashi  Yoshiyuki Watase  Jun Jugaku  Toshiharu Saito  Barbel Koribalsky 《Experimental Astronomy》2000,10(4):519-535

We have constructed a large, mosaic CCD camera called MOA-cam2 which has 4096 × 6144-pixelsto search for gravitational microlensing events. MOA-cam2 has three4096 × 2048-pixel SITe CCD chips, which have a very high quantum efficiency (nearly 80% in the wave region 500 to 800 nm),and three buttable sides. We have placed the threechips side by side with 100 m dead space. MOA-cam2 has been installed on the 61 cm Boller and Chivens telescope of the MOA collaboration at the Mt. John University Observatory (MJUO) in NewZealand since July 1998. The field coverage is 0.92° × 1.38° per exposure. The technical details of MOA-cam2 and the first images obtained with the Boller and Chivens telescope are presented. MOA-cam2 introduces a second phase of research on gravitational microlensing by the MOA collaboration.  相似文献   

Molecular and isotopic characteristics of gas hydrate-bound hydrocarbons in southern and central Lake Baikal   总被引：4，自引：4，他引：0  

Akihiro Hachikubo  Oleg Khlystov  Alexey Krylov  Hirotoshi Sakagami  Hirotsugu Minami  Yutaka Nunokawa  Satoshi Yamashita  Nobuo Takahashi  Hitoshi Shoji  Shin’ya Nishio  Masato Kida  Takao Ebinuma  Gennadiy Kalmychkov  Jeffrey Poort 《Geo-Marine Letters》2010,30(3-4):321-329

We investigated the molecular composition (methane, ethane, and propane) and stable isotope composition (methane and ethane) of hydrate-bound gas in sediments of Lake Baikal. Hydrate-bearing sediment cores were retrieved from eight gas seep sites, located in the southern and central Baikal basins. Empirical classification of the methane stable isotopes (δ¹³C and δD) for all the seep sites indicated the dominant microbial origin of methane via methyl-type fermentation; however, a mixture of thermogenic and microbial gases resulted in relatively high methane δ¹³C signatures at two sites where ethane δ¹³C indicated a typical thermogenic origin. At one of the sites in the southern Baikal basin, we found gas hydrates of enclathrated microbial ethane in which ¹³C and deuterium were both highly depleted (mean δ¹³C and δD of –61.6‰ V-PDB and –285.4‰ V-SMOW, respectively). To the best of our knowledge, this is the first report of C₂ δ¹³C–δD classification for hydrate-bound gas in either freshwater or marine environments.  相似文献   

Application of an ecosystem model for the environmental assessment of the reclamation and mitigation plans for seagrass beds in Atsumi Bay   总被引：1，自引：0，他引：1  

Akio Sohma  Yasuyuki Sekiguchi  Kisaburo Nakata 《Estuarine, Coastal and Shelf Science》2009

An ecosystem model was used to evaluate the effects of reclaiming seagrass beds and creating artificial shallows with seagrass beds to mitigate the effects of the reclamation. The applied model can simulate the pelagic and benthic ecosystems including seagrass beds and tidal flats. The objectives of this study were (a) to investigate the likelihood of cultivating and maintaining seagrass beds in artificial shallows (Part 1), and (b) to understand the effects of the reclamation of seagrass beds and the creation of artificial shallows on the water quality in the estuary (Part 2). In Part 1, first, the nutrient turnover rates due to both biochemical and physical processes in the natural seagrass beds where reclamation is proposed were analyzed. Biological processes rather than physical processes were the most significant driving forces of nutrient cycles in seagrass beds. Second, the effects of filter feeding benthic fauna (suspension feeders) in the seagrass beds were analyzed. The scenario with suspension feeders resulted in higher transparency of the water column (8.7% decrease in the light attenuation coefficient) and an increase in nutrient supply (24.9% increase in NH₄-N in the water column) contributing to the high specific growth rate of seagrass. Third, the specific growth rate of seagrass on the proposed artificial shallows was measured. The value on the artificial shallows set at a depth of datum line minus 0.8 m (D.L. − 0.8 m) was approximately the same as that of the natural seagrass beds. In Part 2, first, water quality in the estuary was compared among the scenarios with/without natural seagrass beds and artificial shallows. Then, the defined values of the water purification capability of (a) artificial shallows with/without seagrass beds, and (b) natural seagrass beds per unit area were evaluated. The reclamation of the natural seagrass beds resulted in a decrease of the removal of phytoplankton and detritus from the pelagic system (i.e. resulted in a loss in the purification rate). In contrast, the creation of artificial shallows resulted in an increase of the removal of phytoplankton and detritus from the pelagic system (i.e. resulted in a gain in the purification rate). Based on an annual average, approximately twice as much phytoplankton was removed from the artificial shallows at the depth D.L. − 0.8 m, than at the depth, D.L. − 1.5 m, and the artificial shallows with seagrass beds removed pelagic DIN and DIP at a rate 120% higher than that without seagrass beds.  相似文献   

Hydrous peridotites with Ti-rich chromian spinel as a low-temperature forearc mantle facies: evidence from the Happo-O’ne metaperidotites (Japan)   总被引：1，自引：0，他引：1  

Mohamed Zaki Khedr  Shoji Arai 《Contributions to Mineralogy and Petrology》2010,159(2):137-157

The Happo-O’ne peridotite complex is situated in the northeastern part of the Hida Marginal Tectonic Zone, central Japan, characterized by the high-P/T Renge metamorphism, and is considered as a serpentinite mélange of Paleozoic age. Peridotitic rocks, being massive or foliated, have been subjected to hydration and metamorphism. Their protoliths are mostly lherzolites to harzburgites with subordinate dunites. We found a characteristic mineral assemblage, olivine + orthopyroxene + tremolite + chlorite + chromian spinel, being stable at low-T, from 650 to 750°C, and high-P, from 16 to 20 kbar, tremolite–chlorite peridotites of the tremolite zone. Olivines are Fo₈₈–Fo_91, and orthopyroxenes (Mg# = 0.91) show low and homogenous distributions of Al₂O₃ (up to 0.25 wt%), Cr₂O₃ (up to 0.25 wt%), CaO (up to 0.36 wt%) and TiO₂ (up to 0.06 wt%) due to the low equilibration temperature. Chromian spinels, which are euhedral and enclosed mainly in the orthopyroxenes, have high TiO_2, 3.1 wt% (up to 5.7 wt%) on average, and high Cr# [=Cr/(Cr + Al) atomic ratio], 0.95 on average but low Fe³⁺ [=Fe³⁺/(Cr + Al + Fe³⁺) atomic ratio, <0.3]. The bulk-rock chemistry shows that the Happo-O’ne metaperidotites with this peculiar spinel are low in TiO₂ (0.01–0.02 wt%), indicating no addition of TiO₂ from the outside source during the metamorphism; the high TiO₂ of the peculiar spinel has been accomplished by Ti release from Ti-bearing high-T pyroxenes during the formation of low-T, low-Ti silicates (<0.1 wt% TiO₂) during cooling. Some dunites are intact from hydration: their olivine is Fo₉₂ and spinel shows high Cr#, 0.72. The Happo-O’ne metaperidotites (tremolite–chlorite peridotites), being in the corner of the mantle wedge, are representative of a hydrous low-T, high-P mantle peridotite facies transitional from a higher T anhydrous peridotite facies (spinel peridotites) formed by in situ retrograde metamorphism influenced by fluids from the subducting slab. They have suffered from low-T (<600°C) retrogressive metamorphism to form antigorite and diopside during exhumation of the Renge metamorphic belt.  相似文献