Transformation textures,mechanisms of formation of high‐pressure minerals in shock melt veins of L6 chondrites,and pressure‐temperature conditions of the shock events     

Shin Ozawa  Eiji Ohtani  Masaaki Miyahara  Akio Suzuki  Makoto Kimura  Yoshinori Ito 《Meteoritics & planetary science》2009,44(11):1771-1786

Abstract— The high‐pressure polymorphs of olivine, pyroxene, and plagioclase in or adjacent to shock melt veins (SMVs) in two L6 chondrites (Sahara 98222 and Yamato 74445) were investigated to clarify the related transformation mechanisms and to estimate the pressure‐temperature conditions of the shock events. Wadsleyite and jadeite were identified in Sahara 98222. Wadsleyite, ringwoodite, majorite, akimotoite, jadeite, and lingunite (NaAlSi₃O₈‐hollandite) were identified in Yamato 74445. Wadsleyite nucleated along the grain boundaries and fractures of original olivine. The nucleation and growth of ringwoodite occurred along the grain boundaries of original olivine, and as intracrystalline ringwoodite lamellae within original olivine. The nucleation and growth of majorite took place along the grain boundaries or fractures in original enstatite. Jadeite‐containing assemblages have complicated textures containing “particle‐like,” “stringer‐like,” and “polycrystalline‐like” phases. Coexistence of lingunite and jadeite‐containing assemblages shows a vein‐like texture. We discuss these transformation mechanisms based on our textural observations and chemical composition analyses. The shock pressure and temperature conditions in the SMVs of these meteorites were also estimated based on the mineral assemblages in the SMVs and in comparison with static high‐pressure experimental results as follows: 13–16 GPa, >1900 °C for Sahara 98222 and 17–24 GPa, >2100 °C for Yamato 74445.  相似文献   

Ca–Mg diffusion in diopside: tracer and chemical inter-diffusion coefficients     

Xiaoyu Zhang  Jibamitra Ganguly  Motoo Ito 《Contributions to Mineralogy and Petrology》2010,159(2):175-186

We have experimentally determined the tracer diffusion coefficients (D*) of ⁴⁴Ca and ²⁶Mg in a natural diopside (~Di₉₆) as function of crystallographic direction and temperature in the range of 950–1,150 °C at 1 bar and f(O₂) corresponding to those of the WI buffer. The experimental data parallel to the a*, b, and c crystallographic directions show significant diffusion anisotropy in the a–c and b–c planes, with the fastest diffusion being parallel to the c axis. With the exception of logD*(²⁶Mg) parallel to the a* axis, the experimental data conform to the empirical diffusion “compensation relation”, converging to logD ~ −19.3 m²/s and T ~ 1,155 °C. Our data do not show any change of diffusion mechanism within the temperature range of the experiments. Assuming that D* varies roughly linearly as a function of angle with respect to the c axis in the a–c plane, at least within a limited domain of ~20° from the c-axis, our data do not suggest any significant difference between D*(//c) and D*(⊥(001)), the latter being the diffusion data required to model compositional zoning in the (001) augite exsolution lamellae in natural clinopyroxenes. Since the thermodynamic mixing property of Ca and Mg is highly nonideal, calculation of chemical diffusion coefficient of Ca and Mg must take into account the effect of thermodynamic factor (TF) on diffusion coefficient. We calculate the dependence of the TF and the chemical interdiffusion coefficient, D(Ca–Mg), on composition in the diopside–clinoenstatite mixture, using the available data on mixing property in this binary system. Our D*(Ca) values parallel to the c axis are about 1–1.5 log units larger than those Dimanov et al. (1996). Incorporating the effect of TF, the D(Ca–Mg) values calculated from our data at 1,100–1,200 °C is ~0.6–0.7 log unit greater than the experimental quasibinary D((Ca–Mg + Fe)) data of Fujino et al. (1990) at 1 bar, and ~0.6 log unit smaller than that of Brady and McCallister (1983) at 25 kb, 1,150 °C, if our data are normalized to 25 kb using activation volume (~4 and ~6 cm³/mol for Mg and Ca diffusion, respectively) calculated from theoretical considerations.  相似文献   

Simple monitoring method for precaution of landslides watching tilting and water contents on slopes surface   总被引：2，自引：2，他引：0  

Taro Uchimura  Ikuo Towhata  Trinh Thi Lan Anh  Jou Fukuda  Carlos J. B. Bautista  Lin Wang  Ichiro Seko  Taro Uchida  Akira Matsuoka  Yosuke Ito  Yuichi Onda  Sho Iwagami  Min-Seok Kim  Naoki Sakai 《Landslides》2010,7(3):351-357

A low-cost and simple monitoring method for early warning of landslides is proposed. To detect abnormal deformation of a slope, this method employs a tilt sensor in place of an extensometer on the slope surface. In order to examine the relevance of measuring rotation angle on a slope surface by tilt sensor, model tests were conducted, and rotation on the slope surface was observed together with slide displacement along the surface. The rotation data responded 30 min before failure in a model test, which could be useful as a signal for early warning. However, the behavior of rotation before failure varies from case to case, and thus, criteria to issue warning should be defined more carefully. For a model slope made of uniform loose sand, measurement of slide displacement along the slope surface is sensitive to failure at the toe, while the measurement of rotation on the slope surface is useful to detect the development of progressive failure upward along the slope. Wireless sensor units with microelectromechanical systems (MEMS) tilt sensor and volumetric water content sensor were also examined on a real slope in Kobe City, and a long-term monitoring was attempted. A simple but possible way to define the criteria of judgment to issue warning can be proposed based on combination of data obtained by the tilt sensors and volumetric water content sensors.  相似文献   

Modeling of the branches of the Tsushima Warm Current in the Eastern Japan Sea     

Hideyuki Kawamura  Toshimichi Ito  Naoki Hirose  Tetsutaro Takikawa  Jong-Hwan Yoon 《Journal of Oceanography》2009,65(4):439-454

The branches of the Tsushima Warm Current (TWC) are realistically reproduced using a three-dimensional ocean general circulation model (OGCM). Simulated structures of the First Branch and the Second Branch of the TWC (FBTWC and SBTWC) in the eastern Japan Sea are mainly addressed in this study, being compared with measurement in the period September–October 2000. This is the first numerical experiment so far in which the OGCM is laterally exerted by real volume transports measured by acoustic Doppler current profiler (ADCP) through the Tsushima Straits and the Tsugaru Strait. In addition, sea level variation measured by tide-stations along the Japanese coast as well as satellite altimeters is assimilated into the OGCM through a sequential data assimilation method. It is demonstrated that the assimilation of sea level variation at the coastal tide-stations is useful in reproducing oceanic conditions in the nearshore region. We also examine the seasonal variation of the branches of the TWC in the eastern Japan Sea in 2000. It is suggested as a consequence that the FBTWC is continuous along northwestern Honshu Island in summertime, while it degenerates along the coast between the Sado Strait and the Oga Peninsula in other seasons. On the other hand, a mainstream of the SBTWC exists with meanders and eddies in the offshore region deeper than 1000 m to the north of the Sado Island throughout the year.  相似文献   

Extragalactic MeV γ-ray emission from cocoons of young radio galaxies     

M. Kino  N. Kawakatu  H. Ito 《Monthly notices of the Royal Astronomical Society》2007,376(4):1630-1634

Strong γ-ray emission from cocoons of young radio galaxies is predicted for the first time. Considering the process of adiabatic injection of the shock dissipation energy and mass of the relativistic jet in active nuclei into the cocoon, while assuming thermalizing electron plasma interactions, we find that the thermal electron temperature of the cocoon is typically predicted to be of the order of ∼ MeV, and is determined only by the bulk Lorentz factor of the relativistic jet. Together with the time-dependent dynamics of the cocoon expansion, we find that young cocoons can yield thermal bremsstrahlung emissions at energies ∼MeV.  相似文献   

Large-Scale Mapping Observations of the C i (3P1-3P0) and CO (J = 3-2) Lines toward the Orion A Molecular Cloud     

Ikeda M  Maezawa H  Ito T  Saito G  Sekimoto Y  Yamamoto S  Tatematsu K  Arikawa Y  Aso Y  Noguchi T  Shi SC  Miyazawa K  Saito S  Ozeki H  Fujiwara H  Ohishi M  Inatani J 《The Astrophysical journal》1999,527(1):L59-L62

Large-scale mapping observations of the 3P1-3P0 fine-structure transition of atomic carbon (C i, 492 GHz) and the J=3-2 transition of CO (346 GHz) toward the Orion A molecular cloud have been carried out with the Mount Fuji submillimeter-wave telescope. The observations cover 9 deg2 and include the Orion Nebula M42 and the L1641 dark cloud complex. The C i emission extends over almost the entire region of the Orion A cloud and is surprisingly similar to that of 13CO (J=1-0). The CO (J=3-2) emission shows a more featureless and extended distribution than C i. The C i/CO (J=3-2) integrated intensity ratio shows a spatial gradient running from the north (0.10) to the south (1.2) of the Orion A cloud, which we interpret as a consequence of the temperature gradient. On the other hand, the C i/13CO (J=1-0) intensity ratio shows no systematic gradient. We have found a good correlation between the C i and 13CO (J=1-0) intensities over the Orion A cloud. This result is discussed on the basis of photodissociation region models.  相似文献   

Geohistorical development of the Tochiyama landslide in north-central Japan     

K. Chihara  S. Iwanaga  K. Ito  T. Ogawa  K. Hayatsu  K. Kosuge 《Engineering Geology》1994,38(3-4):205-219

The Tochiyama landslide is one of several complex, deep-seated and large-scale landslides occurring in the Hokuriku Province in central Japan. The landslide is about 2 km long and about 500–1100 m wide; it occupies an area of approximately 150 ha and has a maximum depth of 60 m. The slide developed on a dip-slope structure, and is divisible into three layers in ascending order: older landslide debris and avalanche deposits, younger debris-avalanche deposits, and talus. The landslide complex is still active. A triangulation point on the upper part of the landslide shifted downhill by 3.3 m from 1907 to 1983, indicating an average rate of 4.3 cm/y. In 1991, the average rate of movement on the sliding surface was also 4.3 cm/y as measured by an automatic system with inclinometers installed in borehole No. 1–2. The rate measured for borehole No. 1–3, located 380 m upslope from No. 1–2, was over twice that of No. 1–2 for the same period; it has since accelerated to about 19 cm/y. Thus current movements on the basal sliding surface are inhomogeneous; the head of the slide complex is increasing the horizontal granular pressures on the lower part of the slide block.

On the basis of dating of two tephra layers and¹⁴C dating of carbonized wood intercalated within the landslide body, two stages of slide movement have been distinguished. The earlier occurred between about 46,000 to 25,000 years ago, and the latter occurred since 1361 A.D. The following sequence of events is inferred. During the middle Pleistocene, intense tectonic movements occurred in the Hokuriku Province, and as a consequence dip-slopes were developed in the Tochiyama landslide area. Low-angle fault planes (possibly representing slump features) and fracture zones then developed within flysch deposits underlying the landslide area, causing a reduction in shear strength. The erosion base level was lowered during the Würm glacial age, and due to severe erosion and incision of stream valleys, the surface slope angle rapidly increased, and toe resistance decreased. This combination of causes led to the development of a deep-seated primary landslide. As a result of an accumulation of younger deposits, regional uplift and further local erosion, stability of parts of the region decreased and led to landslide activity of a second stage. Reactivated and locally accelerating creep movements occur today and may forewarn of a stage of reactivated, hazardous rapid sliding, such as occurred with the adjacent and analogous Maseguchi landslide in 1947.  相似文献   

Heat flow and thickness of the oceanic lithosphere     

Keisuke Ito 《Earth and Planetary Science Letters》1976,30(1):65-70

Existing thermal models of the oceanic lithosphere predict too sharp an increase of heat flow towards the ridge axis. A new mathematical model of a thickening lithosphere is presented. The temperature distribution is computed by the use of observed surface heat flow as a boundary condition. If observed heat flow values represent flow of heat from the mantle, the model predicts a rather rapid growth of the lithosphere within the first 30 m.y. and a nearly steady state after 100 m.y. Heat flow from the asthenosphere to the lithosphere shows a minimum near the ridge axis, suggesting a down-going convective flow in the asthenosphere at both sides of a spreading center.  相似文献   

High-pressure transformations in silicates,germanates, and titanates with ABO3 stoichiometry     

Eiji Ito  Yoshito Matsui 《Physics and Chemistry of Minerals》1979,4(3):265-273

High-pressure phase transformations were investigated for two silicates, MgSiO₃ and ZnSiO₃; six germanates, MGeO₃ and six titanates, MTiO₃ (M=Ni, Mg, Co, Zn, Fe, and Mn) at about 1,000°C and pressures up to ca. 30 GPa. CoGeO₃ was found to assume the ilmenite form. The ilmenite phases were confirmed to transform in the following schemes: to perovskite in MgSiO₃ and MnGeO₃, to corundum in MgGeO₃ and ZnGeO₃, to rocksalt plus rutile in ZnSiO₃ and CoGeO₃ and to rocksalt plus TiO₂ (possibly of some denser structure) in NiTiO₃, MgTiO₃, CoTiO₃, ZnTiO₃ and FeTiO₃. In the case of FeTiO₃, the corundum form appeared as an intermediate phase. The possibility that the corundum type MnTiO₃ might transform to some denser modification could not be excluded. The compound NiGeO₃ was nonexistent throughout the pressure range studied. High-pressure phases of ABO₃ (A=Ni, Mg, Co, Zn, Fe, and Mn; B=Si, Ge and Ti) are summarized, and those stabilized at pressures higher than 20 GPa are discussed.  相似文献   

High-pressure decompositions in cobalt and nickel silicates     

Eiji Ito 《Physics of the Earth and Planetary Interiors》1975,10(1):88-93

High-pressure stability relations in cobalt and nickel silicates have been studied over the pressure range 130–330 kbar employing a double-staged split-sphere-type high-pressure apparatus.γ-Co₂SiO₄ and γ-Ni₂SiO₄ decompose directly into their constituent oxide mixtures (rocksalt plus stishovite) 175 kbar and 280 kbar, respectively. The result that γ-Ni₂SiO₄ has a wider stability field in pressure than γ-Co₂SiO₄, is consistent with simple crystal-field theory.The experimental precision is high enough to show that the decomposition boundary of γ-Co₂SiO₄ has a positive slope (dP/dT > 0) and a preliminary determination of the boundary curve is P(kbar) = 0.065 T (°C) + 110.No positive evidence for the existence of high-pressure forms of CoSiO₃ and NiSiO₃ has been obtained in these quenching experiments, and they finally decompose into constituent oxide mixtures as in the cases of orthosilicates.  相似文献