Pb isotopic composition,colour, and microstructure of monazites from a polymetamorphic rock in Antarctica   总被引：1，自引：0，他引：1  

Lance P. Black  John D. Fitzgerald  Simon L. Harley 《Contributions to Mineralogy and Petrology》1984,85(2):141-148

A moderate- to high-grade regionally metamorphosed paragneiss from Antarctica contains monazites of several different colours — brown, yellow and grey. Each colour type has a distinctive U-Pb isotopic composition which appears to result from different proportions of radiogenic Pb loss. Isotopic differences are neither related to La, Nd, Ce, P, Ca, Ti (and/or Ba), nor to U or Th content. All colour types have similar structures at the submicron scale, as determined by both conventional and high-resolution transmission electron microscopy (TEM). These show that the grains are essenttially non-metamict but are composed of 100 Å crystalline domains misoriented from each other by no more than 2× 10^–3 radians, and separated by narrow confused boundary regions where misorientation is probably accommodated by imperfect atomic arrangements. These regions of mismatch form potential zones of high permeability/diffusivity which are believed to be fundamental to the isotopic and colour differences between grains. Colour type is apparently related to the capacity of different minerals to shield included monazite grains from fluids circulating in the rock system.The well aligned monazite U — Pb analyses produce concordia intercepts of 2429 _–16 ⁺¹⁷ Ma and 1087±29 Ma. Both ages are comparable to those of major geological events in this part of Antarctica. They are interpreted in terms of isotopic resetting through Pb loss, and original monazite crystallisation is thought to have occurred somewhat earlier, possibly at the time this terrain first underwent granulite-facies metamorphism, about 3070 Ma ago.  相似文献   

Ultraviolet absorption spectrum of trifluoro-bromo-methane,difluoro-dibromo-methane and difluoro-bromo-chloro-methane in the vapor phase     

D. Gillotay  P. C. Simon 《Journal of Atmospheric Chemistry》1989,8(1):41-62

Ultraviolet absorption cross-sections of trifluoro-bromo-methane (CF₃Br-Halon 1301), difluoro-dibromo-methane (CF₂Br₂-Halon 1202) and of difluoro-bromo-chloro-methane (CF₂BrCl-Halon 1211) are measured in the wavelength interval 172–304 nm for temperatures ranging from 210 to 295 K with uncertainties of between 2 and 4%. They are compared with previous measurements available at room temperature. Temperature effects are discussed and parametrical formulae are proposed to compute the absorption cross-sections for wavelengths and temperatures useful in atmospheric modelling calculations. Photodissociation coefficients are presented and their temperature dependence is discussed.  相似文献   

Space-time variability of low frequency oscillations and Asian summer monsoon precipitation     

Anu Simon  C. K. Rajan  T. N. Krishnamurti 《Meteorology and Atmospheric Physics》2006,93(3-4):177-199

Summary In this study, we provide an observational synthesis of several aspects of MJO (Madden Julian Oscillation) and ISO (Intra Seasonal Oscillation) waves and the manner in which they seem to affect the Asian summer monsoon variability. We examine a number of features such as the passage of MJO waves around the globe from west to east, excitation of ISO waves over the equatorial Indian Ocean, meridional propagation of ISO waves and the role of the local Hadley cell over the monsoon domain in its propagation. The geographical distribution of the amplitude and phase speed of ISO waves over the Asian summer monsoon domain shows that the ISO has prominent centers of action, with small scales of the order of a few 1000 km and that these features vary interannually in amplitude and in their geographical locations. It was also seen that some of the interesting features of the WET and DRY season rainfall distribution can be obtained by compositing the extremes of the amplitude oscillations of these ISO centers. We also noted that the amplitudes of ISO waves generally decay as they propagate north but have an occasional built up in their amplitudes when they seem to encounter and interact with heavy rain producing monsoon disturbances. The issue of these low frequency modes and the modulation of monsoonal dry and wet spells are examined in the context of this broad scenario.  相似文献   

Particulate kinematic simulations of debris avalanches: interpretation of deposits and landslide seismic signals of Mount Saint Helens, 1980 May 18     

Steven N. Ward  Simon Day 《Geophysical Journal International》2006,167(2):991-1004

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Fault-induced perturbed stress fields and associated tensile and compressive deformation at fault tips in the ice shell of Europa: implications for fault mechanics     

Simon A. Kattenhorn  Scott T. Marshall   《Journal of Structural Geology》2006,28(12):2204

Secondary fractures at the tips of strike-slip faults are common in the ice shell of Europa. Large magnitude perturbed stress fields must therefore be considered to be a viable driving mechanism for the development of part of the fracture sequence. Fault motions produce extensional and compressional quadrants around the fault tips. Theoretically, these quadrants can be associated with tensile and compressive deformational features (i.e. cracks and anti-cracks), respectively. Accordingly, we describe examples of both types of deformation at fault tips on Europa in the form of extensional tailcracks and compressional anti-cracks. The characteristics of these features with respect to the plane of the fault create a fingerprint for the mechanics of fault slip accumulation when compared with linear elastic fracture mechanics (LEFM) models of perturbed stress fields around fault tips. Tailcrack kink angles and curving geometry can be used to determine whether opening accompanies sliding motion. Kink angles in the 50–70° range are common along strike-slip faults that resemble ridges, and indicate that little to no opening accompanied sliding. In contrast, tailcrack kink angles are closer to 30° for strike-slip faults that resemble bands, with tailcrack curvatures opposite to ridge-like fault examples, indicating that these faults undergo significant dilation and infill during fault slip episodes. Anti-cracks, which may result from compression and volume reduction of porous near-surface ice, have geometries that further constrain fault motion history, corroborating the results of tailcrack analysis. The angular separation between anti-cracks and tailcracks are similar to LEFM predictions, indicating the absence of cohesive end-zones near the tips of Europan faults, hence suggesting homogeneous frictional properties along the fault length. Tailcrack analysis can be applied to the interpretation of cycloidal ridges: chains of arcuate cracks on Europa that are separated by sharp kinks called cusps. Cusp angles are reminiscent of tailcrack kink angles along ridge-like strike-slip faults. Cycloid growth in a temporally variable tidal stress field ultimately resolves shear stresses onto the near-tip region of a growing cycloid segment. Thus, resultant slip and associated tailcrack development may be the driving force behind the initiation of the succeeding arcuate segment, hence facilitating the ongoing propagation of the cycloid chain.  相似文献   

Composite nature of the North China Granulite-Facies Belt: Tectonothermal and geochronological constraints   总被引：13，自引：3，他引：13  

Guochun Zhao  Min Sun  Simon A. Wilde  Li Sanzhong  Shuwen Liu  Jian Zhang 《Gondwana Research》2006,9(3):337-348

Granulite-facies rocks are intermittently exposed in a roughly E–W trending belt that extends for approximately 2000 km across the North China Craton, from the Helanshan, Qianlishan, Wulashan–Daqingshan, Guyang and Jining Complexes in the Western Block, through the Huai'an, Hengshan, Xuanhua and Chengde Complexes in the Trans-North China Orogen, to the Jianping (Western Liaoning), Eastern Hebei, Northern Liaoning and Southern Jilin Complexes in the Eastern Block. The belt is generally referred to as the North China Granulite-Facies Belt, previously interpreted as the lowest part of an obliquely exposed crust of the North China Craton. Recent data indicate that the North China Granulite-Facies Belt is not a single terrane. Instead, it represents components of three separate terranes: the Eastern and Western Blocks and Trans-North China Orogen. Each of these units records different metamorphic histories and reflect the complex tectonic evolution of the NCC during the late Archean and Paleoproterozoic. Mafic granulites in the Eastern Block and the Yinshan Terrane (Western Block) underwent medium-pressure granulite-facies metamorphism at about 2.5 Ga, with anticlockwise P–T paths involving near isobaric cooling following peak metamorphism, reflecting an origin related to intrusion and underplating of mantle-derived magmas. Pelitic granulites in the Khondalite Belt (Western Block) underwent medium-pressure granulite-facies metamorphism at about 2.0–1.9 Ga, with clockwise P–T paths, which record the Paleoproterozoic amalgamation of the Yinshan and Ordos Terranes to form the Western Block. Mafic and pelitic granulites in the Trans-North China Orogen experienced high- to medium-pressure granulite-facies metamorphism at 1.85 Ga, with clockwise P–T paths involving nearly isothermal decompression following peak metamorphism, which are in accord with the final collision between the Eastern and Western Blocks to form the North China Craton at 1.8 Ga. The NCGB cannot therefore represent a separate unique terrane; instead it reflects the amalgamation of three separate granulite terranes that evolved independently and at different times.  相似文献   

Utility of Landsat 7 satellite data for continued monitoring of forest cover change in protected areas in Southeast Asia   总被引：1，自引：0，他引：1  

Simon N. Trigg  Lisa M. Curran  Alice K. McDonald 《Singapore journal of tropical geography》2006,27(1):49-66

Satellite instruments, particularly the Landsat TM (Thematic Mapper) and ETM+ (Enhanced Thematic Mapper Plus) series of sensors, are important tools in the interdisciplinary study of tropical forests that are increasingly integrated into studies that monitor changes in vegetation cover within tropical forests and tropical protected areas, and also applied with other types of data to investigate the drivers of land cover change. However, further advances in the use of Landsat to study and monitor tropical forests and protected areas are threatened by the scan line corrector failure on the ETM+ sensor, as well as uncertainty about the continuity of the Landsat mission. Given these problems, this paper illustrates how ETM+ data were used in an interdisciplinary study that effectively monitored forest cover change in Gunung Palung National Park in West Kalimantan, Indonesian Borneo. Following 31 May 2003, when the ETM+ sensor's scan line corrector failed, we analysed how this failure impedes our ability to perform a similar study from this date onwards. This analysis uses six simulated post-scan line corrector failure (SLC-off) images and reveals that data gaps caused by SLC-off introduce maximum errors of 1.47 per cent and 4.04 per cent in estimates of forest cover and rates of forest loss, respectively. The analysis also demonstrates how SLC-off has transformed ETM+ data from a complete inventory dataset to a statistical sample with variable sample fraction, and notes how this data loss will confound the use of Landsat data to model land cover change in a spatially explicit manner. We discuss potential limited uses of SLC-off data and suggest alternative sensors that may provide essential remotely sensed data for monitoring tropical forests in Southeast Asia.  相似文献   

Determining hydraulic resistance in gravel‐bed rivers from the dynamics of their water surfaces     

James R. Cooper  Simon J. Tait  Kirill V. Horoshenkov 《地球表面变化过程与地形》2006,31(14):1839-1848

Traditionally, approaches to account for the effect of the boundary roughness of a gravel‐bed river have used a grain‐size index of the bed surface as a surrogate for hydraulic resistance. The use of a single grain‐size does not take into account the spatial heterogeneity in the bed surface and how this heterogeneity imparts resistance on the flow, nor the way in which this relationship changes with variables such as flow stage. A new technique to remotely quantify hydraulic resistance is proposed. It is based on measuring the dynamics of a river's water surface and relating this to the actual hydraulic resistance created by a rough sediment boundary. The water surface dynamics are measured using a new acoustic technique, grazing angle sound propagation (GRASP). This proposed method to measure hydraulic resistance is based on a greater degree of physical reasoning, and this is discussed in the letter. By measuring acoustically the temporal dynamics of turbulent water surfaces over a water‐worked gravel bed in a laboratory flume, a dependency is demonstrated between the temporal variation in the reflected acoustic pressure and measured hydraulic resistance. It is shown that the standard deviation in acoustic pressure decreases with increasing hydraulic resistance. This is shown to apply for a range of relative submergences and bed slopes that are typical of gravel‐bed rivers. This remote sensing technique is both rapid and inexpensive, and has the potential to be applied to natural river channels and to other environmental turbulent flows, such as overland flows. A whole new class of low‐cost, remote and non‐intrusive instruments could be developed as a result and used in a wide range of hydraulic and hydrological applications. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Automated metric calibration of colour digital cameras     

Simon  Cronk  Clive  Fraser  Harry  Hanley 《The Photogrammetric Record》2006,21(116):355-372

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Anharmonicity of inner-OH stretching modes in hydrous phyllosilicates: assessment from first-principles frozen-phonon calculations     

Etienne Balan  Michele Lazzeri  Simon Delattre  Merlin Méheut  Keith Refson  Bjoern Winkler 《Physics and Chemistry of Minerals》2007,34(9):621-625

The theoretical anharmonicity of the inner-OH stretching modes of kaolinite and lizardite is determined using a frozen-phonon approach. The anharmonicity parameter, defined as the difference between half the first overtone frequency and the frequency of the fundamental transition, is −95 and −92 cm⁻¹ for kaolinite and lizardite, respectively. These values are consistent with the experimental measurements. The very good agreement usually observed between harmonic calculations of vibrational frequencies at the density functional theory (DFT) level, using the generalized gradient approximation (GGA), and experimental positions of vibrational bands results from the fortuitous, but almost exact, cancelation of two types of errors. One is related to the GGA approximation and the other to the neglect of anharmonicity.  相似文献