Exsolved garnet-bearing pyroxene megacrysts from some South African kimberlites     

Ken-Ichiro Aoki  Hirokazu Fujimaki  Masao Kitamura 《Lithos》1980,13(3):269-279

Clinopyroxene and orthopyroxene megacrysts containing garnet lamellae up to 1.2 mm thick as an exsolved phase are found rarely in kimberlites from Frank Smith and Bellsbank. Chemically the clinopyroxenes are characteristically subcalcic, being within the range of 100 Ca/Ca + Mg + Fe = 27 to 36, and the orthopyroxenes are characterized by high Al₂O₃ and Cr₂O₃. Immediately after crystallization during very slow cooling, clinopyroxene and orthopyroxene exsolve wide-spaced orthopyroxene and clinopyroxene phases parallel to (100) of the host phases, respectively, then both host and exsolved phases exsolve garnet lamellae. Topotactic relations between pyroxenes and garnet are determined by X-ray for the first time. Partitioning of major and minor elements among the coexisting clinopyroxene, orthopyroxene and garnet in pyroxene megacrysts is the same as that of the granular-type garnet peridotite xenoliths in Lesotho and South African kimberlies. Mineralogy and chemistry indicate that subcalcic clinopyroxene and orthopyroxene megacrysts contain respectively about 10 and 3 mole % of the garnet molecule in solid solution.  相似文献   

Late Pleistocene and Holocene sea-level change in the Australian region and mantle rheology   总被引：8，自引：0，他引：8  

Masao Nakada  Kurt Lambeck 《Geophysical Journal International》1989,96(3):497-517

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Variations in seawater osmium isotope composition since the last glacial maximum: A case study from the Japan Sea     

Tarun K. Dalai  Katsuhiko Suzuki  Masao Minagawa  Yoshiyuki Nozaki 《Chemical Geology》2005,220(3-4):303-314

Concentrations of Re and Os, and the isotopic composition of Os have been measured in the Japan Sea sediments to assess the response of the Japan Sea to glacial–interglacial climate change and associated weathering fluxes. The osmium concentrations in the sediment samples analyzed vary from 59 to 371 pg/g, and ¹⁸⁷Os/¹⁸⁸Os from 0.935 to 1.042. Only ¹⁸⁷Os/¹⁸⁸Os of sediment samples from dark laminations deposited under suboxic to anoxic conditions and having elevated concentrations of Re and Os, and with ≥ 80% hydrogenous Os are explained in terms of seawater composition. Lower ¹⁸⁷Os/¹⁸⁸Os were observed for sediments deposited during the last glacial maximum (LGM) when planktonic foraminifera from the Japan Sea recorded lighter oxygen isotopic composition. Decrease in dissolved Os fluxes from continents and/or change in the composition of the dissolved load to the Japan Sea are suggested as the driving mechanisms for the observed lower LGM ¹⁸⁷Os/¹⁸⁸Os. The results of this study, coupled with lower ¹⁸⁷Os/¹⁸⁸Os during the last glacial observed at other sites from ocean basins with different lithology and contrasting sediment accumulation rates, suggest that this trend is characteristic of the global oceans.

Data from this study show that the Japan Sea recorded higher ¹⁸⁷Os/¹⁸⁸Os during the current interglacial coinciding with excursions of oxygen isotopic compositions of planktonic foraminifera to heavier values. This is explained in terms of preferential release of ¹⁸⁷Os during deglacial weathering and/or higher continental Os flux driven by warm and wet climate. This study demonstrates that Os isotopic composition of reducing margin sediments has immense potential to track variations in the seawater composition. In addition, ¹⁸⁷Os/¹⁸⁸Os of reducing sediments may be used to draw inferences about local paleoceanographic processes in semi-enclosed basins such as the Japan Sea.  相似文献   

Convective coupling between ductile lower crust and upper mantle, and its tectonic implications   总被引：1，自引：0，他引：1  

Masao Nakada 《Geophysical Journal International》1994,118(3):579-603

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Electron microscopy of clinoenstatite from a boninite and a chondrite     

Masayo Yasuda  Masao Kitamura  Nobuo Morimoto 《Physics and Chemistry of Minerals》1983,9(5):192-196

Clinoenstatite crystals from a boninite and the Yamato-74191 chondrite have been studied with an analytical electron microscope. (100) twins and cracks perpendicular and parallel to the c axis are characteristic of their submicroscopic textures. The frequency in appearance along the c axis and widths of the cracks have been explained by the dimensional change of the c axis in the direct transformation of protoenstatite to clinoenstatite and by the cooling rate around the transformation temperature. The cracks in the crystals from the boninite are filled with fibrous crystals of talc, while those from the chondrite are open or filled with glass in which fine crystals of plagioclase are common.  相似文献   

Crustal structure, fault segmentation, and activity of the Median Tectonic Line in Shikoku, Japan     

Michiharu Ikeda  Dapeng Zhao  Yuki Ohno 《Tectonophysics》2006,412(1-2):49-60

We conducted a seismic tomographic analysis to estimate the crustal structure beneath the Shikoku and Chugoku regions in Japan. The Philippine Sea slab (PHS slab) subducts continuously in a SE–NW direction beneath this region, and the crustal structure is complex. Furthermore, the Median Tectonic Line (MTL), one of the longest and most active arc-parallel fault systems in Japan (hereafter, the MTL active fault system), is located in this area, and the right-lateral strike–slip movement of this fault system is related to the oblique subduction of the PHS slab. The MTL active fault system has ruptured repeatedly during the last 10 000 years, and has high seismic potential. Our tomographic analysis clarified the heterogeneous crustal structure along the MTL active fault system. This fault system in Shikoku can be divided into two segments, an east segment and a west segment, on the basis of the velocity structure. This segmentation model is consistent with other such models that have been determined from geological and geomorphological data such as fault geometry, slip rate, and faulting history. This consistency suggests that the surface characteristics of the MTL active fault system are related to structural properties of the crust. In particular, a prominent low-velocity (low-V) zone is present in the lower crust beneath the east segment. Our tomographic images show that the lower crust structure beneath the east segment is obviously different from that of the other segment. Furthermore, this low-V zone may indicate the presence of fluid, possibly related to dehydration of the PHS slab. As the presence of fluid in the lower crust affects the activity of the fault, stress accumulation and the fault failure mechanism may differ between the two segments of the MTL active fault system.  相似文献   

Axial and equatorial rotations of the Earth's cores associated with the Quaternary ice age     

Masao Nakada 《Physics of the Earth and Planetary Interiors》2006,154(2):113-147

The differential axial and equatorial rotations of both cores associated with the Quaternary glacial cycles were evaluated based on a realistic earth model in density and elastic structures. The rheological model is composed of compressible Maxwell viscoelastic mantle, inviscid outer core and incompressible Maxwell viscoelastic inner core. The present study is, however, preliminary because I assume a rigid rotation for the fluid outer core. In models with no frictional torques at the boundaries of the outer core, the maximum magnitude of the predicted axial rotations of the outer and inner cores amounts to ∼2° year⁻¹ and ∼1° year⁻¹, respectively, but that for the secular equatorial rotations of both cores is ∼0.0001° at most. However, oscillating parts with a period of ∼225 years are predicted in the equatorial rotations for both cores. Then, I evaluated the differential rotations by adopting a time-dependent electromagnetic (EM) torque as a possible coupling mechanism at the core-mantle boundary (CMB) and inner core boundary (ICB). In a realistic radial magnetic field at the CMB estimated from surface magnetic field, the axial and equatorial rotations couple through frictional torques at the CMB, although these rotations decouple for dipole magnetic field model. The differential rotations were evaluated for conductivity models with a conductance of 10⁸ S of the lowermost mantle inferred from studies of nutation and precession of the Earth and decadal variations of length of day (LOD). The secular parts of equatorial rotations are less sensitive to these parameters, but the magnitude for the axial rotations is much smaller than for frictionless model. These models, however, produce oscillating parts in the equatorial rotations of both cores and also in the axial rotations of the whole Earth and outer and inner cores. These oscillations are sensitive to both the magnitude of radial magnetic field at the CMB and the conductivity structure. No sharp isolated spectral peaks are predicted for models with a thin conductive layer (∼200 m) at the bottom of the mantle. In models with a conductive layer of ∼100 km thickness, however, sharp spectral peaks are predicted at periods of ∼225 and ∼25 years for equatorial and axial rotations, respectively, although these depend on the strength of radial magnetic field at the CMB. While the present study is preliminary in modelling the fluid outer core and coupling mechanism at the CMB, the predicted axial rotations of the whole Earth may be important in explaining the observed LOD through interaction between the equatorial and axial rotations.  相似文献   

Advanced Micro-XRF Method to Separate Sedimentary Rhythms and Event Layers in Sediments: Its Application to Lacustrine Sediment from Lake Suigetsu,Japan     

Nagayoshi Katsuta  Masao Takano  Shin-ichi Kawakami  Shoji Togami  Hitoshi Fukusawa  Mineo Kumazawa  Yoshinori Yasuda 《Journal of Paleolimnology》2007,37(2):259-271

Event-related sedimentary layers, which are deposited occasionally due to volcanic eruptions, earthquakes or heavy rains, are often contained in the rhythmical sequences of lacustrine and marine sediments. We have developed an analytical method for separating the sedimentary rhythms and the event layers identified using the scanning X-ray analytical microscope (SXAM) and obtained sequential profiles of seven elements Al, Si, K, Ca, Ti, Mn, and Fe in the lacustrine sediment from Lake Suigetsu, Japan. Two types of event layers could be detected from the elemental composition of 33 layers of sediment: three known volcanic ash layers and 30 clay layers containing 12 turbidites. The recurrence interval of the latter, which may potentially be initiated and archived by locally important earthquakes, is estimated to be an average of 640 ± 160 years by using Sompi event analysis (SEA) based on an autoregressive (AR) model. After removing those portions that represented event layers from the elemental profiles, we obtained event-removed (ER) temporal profiles based on the tephrochronology of the three volcanic ash layers. The ER temporal profiles of manganese and iron, probably representing the siderite content, showed a millennial-scale variation in the Holocene that corresponded well with ice-rafting events in the North Atlantic.  相似文献   

Characteristics of deep currents off Cape Shiono-misaki before and after formation of the large meander of the Kuroshio in 1981     

Masao Fukasawa  Toshihiko Teramoto 《Journal of Oceanography》1986,42(1):53-68

Deep currents measured by moored current meters over the shelf-slope off Cape Shiono-misaki, Kii Peninsula during the period from 28 April, 1981 to 4 May, 1982 are analyzed to determine characteristics of the deep current before and after the large meander of the Kuroshio formed. The observed deep currents show some different characteristics between the periods before and after the formation of the large meander of the Kuroshio,i.e.:

1. The mean current direction over the shelf slope changed to westward after the meander was formed, though it was eastward at two offshore stations before the meander was formed.
2. The eddy kinetic energy, \(ke((\overline {u'^2 } + \overline {\upsilon '^2 } )/2)\) became large at all stations after the meander formed.
3. It appears that there were current variations in the period band shorter than 10 days which propagated offshore before the meander formed but inshore after the meander formed.
4. After the meander formed, the current variations with a period of O(25 days) were amplified at two of the three stations. The current variations in this period band showed high coherence among the three stations.

Data from tidal stations showed that sea level variations with a period of O(30 days) were also amplified along the south coast of Japan after the meander was formed. But sea level variations were not coherent with current variations in this period band.  相似文献   

Seasonal and Interannual Variation of DIC in Surface Mixed Layer in the Oyashio Region: A Climatological View     

Tsuneo?OnoEmail author  Hiromi?Kasai  Takashi?Midorikawa  Yusuke?Takatani  Kazuhiro?Saito  Masao?Ishii  Yutaka?W.?Watanabe  Katsuyuki?Sasaki 《Journal of Oceanography》2005,61(6):1075-1087

Eight-year observation results of DIC from 1996 to 2003 in the Oyashio region have been analyzed to obtain a climatological view of its seasonal variation and interannual variation. Data of DIC obtained by several institutes are synthesized to give a dataset with an uncertainty lower than 5 μmol/kg. The obtained climatology of NDIC seasonal variation in the Oyashio mixed layer shows a seasonal amplitude of 176 μmol/kg, with a maximum in January and a minimum in September. These features closely resemble those observed in the southern half of the western subarctic North Pacific (WSNP) including Station KNOT, although the timing of the NDIC maximum is slightly advanced in the case of the Oyashio. Analysis using a quasi-conservative tracer Cp0 (NDIC - 106NP) shows that among 176 μmol/kg of NDIC seasonal variation, only 16 μmol/kg is attributed to hydrographic processes while the remaining 160 μmol/kg is attributed to biological processes. The Cp0 value in the Oyashio mixed layer also resembles that of the WSNP mixed layer during the months May to November, suggesting further resemblance of the Oyashio water mass to that of WSNP in terms of carbon dynamics. The present results also suggest that a single data obtained in Oyashio mixed layer contains 30 μmol/kg of potential uncertainty for the representativity of this region, which leads to a note about a need to treat with caution results obtained by a single observation in this region.  相似文献