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791.
U. Altenberger R. Schmid R. Oberhänsli 《International Journal of Earth Sciences》2008,97(6):1301-1314
As part of the Yangtze plate, segments of the Dabie Shan terrane of Central China underwent ultra-high pressure metamorphism
during Triassic subduction. We studied the geochemistry of the abundant eclogites to evaluate the nature of the protoliths
and their geodynamic setting. Although some previous geochemical work exists, the analyses and interpretation herein are based
on a new subdivision of the ultra-high pressure sequence into basement and cover units (Changpu and Ganghe Unit), revealing
new and important results. In addition, eclogites of the so-called HP Unit south of the UHP units were studied. Whereas the
large ion lithophile elements indicate postmagmatic, metasomatic changes of some samples, the high-field strength elements
and the rare earth elements display original magmatic trends. The geochemical characteristics of the eclogites of the ultra-high
pressure areas display a strong dependence on their “structural” and geographic position. The eclogites of the basement and
the Changpu Unit indicate melt intrusion and extrusion in a continental rift system, i.e. during extensional tectonics. In
contrast, the Ganghe Unit is characterized by a pronounced chemical homogeneity. The composition of the eclogites indicates
generation from a mantle source highly influenced by slab-derived fluids. Those of the HP Unit show similar characteristics.
Magmatism of the Ganghe and HP Unit probably occurred in a continental arc setting. A similar age for both units, geographically
and/or tectonically separated, is possible. The geodynamic interpretation based on the geochemistry of the four units points
to a Neoproterozoic scenario in which the protoliths of the HP and the cover units could have been of similar age and deposited
in one evolving geological system. A rift-related larger-scale basin might have formed, e.g. a continental back-arc basin
behind a magmatic arc after or simultaneous to sedimentation and magmatism in the magmatic arc. Alternatively, magmatism occurred
in independent geodynamic settings, distinct in time and space. The units were juxtaposed during exhumation, after subduction
to varying depths. 相似文献
792.
There is a net discharge of water and nutrients through Long Key Channel from Florida Bay to the Florida Keys National Marine Sanctuary (FKNMS). There has been speculation that this water and its constituents may be contributing to the loss of coral cover on the Florida Keys Reef tract over the past few decades, as well as speculation that changes in freshwater flow in the upstream Everglades ecosystem associated with the Comprehensive Everglades Restoration Plan may exacerbate this phenomenon. The results of this study indicate that although there is a net export of approximately 3,850 (±404) ton N year?1 and 63 (±7) ton P year?1, the concentrations of these nutrients flowing out of Florida Bay are the same as those flowing in. This implies that no significant nutrient enrichment is occurring in the waters of the FKNMS in the vicinity of Long Key Channel. Because of the effect of restricted southwestward water flow through Florida Bay by shallow banks and small islands, the volume of relatively high-nutrient water from central and eastern portions of the bay exiting through the channel is small compared to the average tidal exchange. Nutrient loading of relatively enriched bay waters is mediated by tidal exchange and mixing with more ambient concentrations of the western Florida Bay and Hawk Channel. System-wide budgets indicate that the contribution of Florida Bay waters to the inorganic nitrogen pool of the Keys coral reef is small relative to offshore inputs. 相似文献
793.
L. I. Kuncheva J. J. Charles N. Miles A. Collins B. Wells I. S. Lim 《Mathematical Geosciences》2008,40(6):639-652
We develop the classification part of a system that analyses transmitted light microscope images of dispersed kerogen preparation. The system automatically extracts kerogen pieces from the image and labels each piece as either inertinite or vitrinite. The image pre-processing analysis consists of background removal, identification of kerogen material, object segmentation, object extraction (individual images of pieces of kerogen) and feature calculation for each object. An expert palynologist was asked to label the objects into categories inertinite and vitrinite, which provided the ground truth for the classification experiment. Ten state-of-the-art classifiers and classifier ensembles were compared: Naïve Bayes, decision tree, nearest neighbour, the logistic classifier, multilayered perceptron (MLP), support vector machines (SVM), AdaBoost, Bagging, LogitBoost and Random Forest. The logistic classifier was singled out as the most accurate classifier, with an accuracy greater than 90. Using a 10 times 10-fold cross-validation provided within the Weka software, we found that the logistic classifier was significantly better than five classifiers (p<0.05) and indistinguishable from the other four classifiers. The initial set of 32 features was subsequently reduced to 6 features without compromising the classification accuracy. A further evaluation of the system alerted us to the possible sensitivity of the classification to the ground truth that might vary from one human expert to another. The analysis also revealed that the logistic classifier made most of the correct classifications with a high certainty. 相似文献
794.
In order to evaluate the influence of the initial plastic anisotropy in the excavation of a tunnel, a “bubble” bounding surface
model for structured soils, formulated by Kavvadas and Belokas (Proc. 10th IACMAG Conf., 2001), was implemented in the explicit
finite difference code FLAC. Two different initial stress (K
0) conditions were considered. The size and shape of the initial bounding surfaces were specified to be consistent with the
initial stress field. The distorted and rotated shape of the bounding surface, supported by experimental results, defines
the anisotropy of shear strength, which is shown to have a significant influence on the displacements. There is also considerable
sensitivity of the soil model to the initial stress field. 相似文献
795.
We used transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to study magmatic crystals in the Ben
Lomond rhyolite lava dome, Taupo Volcanic Center, New Zealand. Using TEM and SEM to investigate the size distributions of
these crystals, we identified three size populations: microphenocrysts (>1.2 μm wide), microlites (>0.6 μm wide), and smaller
crystals (<0.6 μm wide) which we term "nanolites". The predominant mineral phases of the microlites and nanolites are augites,
pigeonites, and hypersthenes. The compositions and microstructures within these pyroxenes indicate disequilibrium crystallization
at approximately 850–900 °C and undercoolings as high as 300 °C from equilibrium crystallization temperatures. Complex microstructures
resulting from subsolidus reactions in augite and pigeonite are consistent with moderate cooling rates within the upper obsidian
layer of the Ben Lomond rhyolite dome. This study demonstrates the existence of sub-micron magmatic crystals in a rhyolite
and illustrates the potential of TEM to provide unique information about the crystallization and cooling histories of glassy
volcanic rocks.
Received: May 8, 1995 / Accepted: November 27, 1995 相似文献
796.
Ohishi M Ishikawa SI Amano T Oka H Irvine WM Dickens JE Ziurys LM Apponi AJ 《The Astrophysical journal》1996,471(1):L61-L64
A new interstellar molecular ion, H2COH+ (protonated formaldehyde), has been detected toward Sgr B2, Orion KL, W51, and possibly in NGC 7538 and DR21(OH). Six transitions were detected in Sgr B2(M). The 1(1,0)-1(0,1) transition was detected in all sources listed above. Searches were also made toward the cold, dark clouds TMC-1 and L134N, Orion (3N, 1E), and a red giant, IRC + 10216, without success. The excitation temperatures of H2COH+ are calculated to be 60-110 K, and the column densities are on the order of 10(12)-10(14) cm-2 in Sgr B2, Orion KL, and W51. The fractional abundance of H2COH+ is on the order of 10(-11) to 10-(9), and the ratio of H2COH+ to H2CO is in the range 0.001-0.5 in these objects. The values in Orion KL seem to be consistent with the "early time" values of recent model calculations by Lee, Bettens, & Herbst, but they appear to be higher than the model values in Sgr B2 and W51 even if we take the large uncertainties of column densities of H2CO into account. We suggest production routes starting from CH3OH may play an important role in the formation of H2COH+. 相似文献
797.
D. G. Rancourt J. Y. Ping B. Boukili J. L. Robert 《Physics and Chemistry of Minerals》1996,23(1):63-71
We have performed a detailed Mössbauer study of synthetic annites on the (OH, F)-join. Recently developed data treatment and spectral analysis methods were used to extract true intrinsic Fe2+ quadrupole splitting distributions (QSDs) that represent the most information that can be resolved from the spectra. The overall room temperature (RT) QSDs can be consistently interpreted in terms of four QSD contributions (or populations) centered at: QSHH2.55 mm/s for Fe2+O4(OH)2 octahedra (cis and trans not resolved), QSHF 2.35 mm/s for Fe2+O4(OH)F octahedra (cis and trans not resolved), QScFF2.15 mm/s for cis-Fe2+O4F2 octahedra, and QStFF 1.5 mm/s for trans-Fe2+O4F2 octahedra. Each such contribution has a width ( 0.2 mm/s) caused by distortions of the octahedra. Minor contributions due to Fe2+O5(OH) and Fe2+O5F octahedra probably also contribute to the overall Fe2+ QSDs. The ferric iron spectral components were also characterized. Here, two distinct types of octahedral Fe3+ contributions are seen and interpreted as being due mainly to Fe3+O5OH and Fe3+O5F octahedra, respectively. Tetrahedral Fe3+ is seen only in the OH-annite end-member and the total Fe3+ content drops significantly on addition of F.
On leave from: Department of Materials Physics, University of Science and Technology Beijing, 100083 Beijing, China 相似文献
798.
Dislocation configurations in natural single crystals of CaTiO3 perovskite deformed in high-temperature creep were examined and characterized by transmission electron microscopy. Screw dislocations with Burgers vector [100]pc and [011]pc, dissociated on the $(01\bar 1)_{{\text{pc}}} $ plane, form rectangular networks with extended four-fold nodes in the shape of octagons, a configuration never observed in any of the previously investigated perovskites, except CaGeO3. Screw dislocations with Burgers vector [101]pc and $(\bar 101)_{{\text{pc}}} $ , on the (010)pc plane, react to form a twist wall; the dislocations with Burgers vector [002] produced by the reaction decompose into two perfect dislocations [001]pc. This results in a new configuration, never observed before, with three-fold nodes at the corners of rectangles. Both the octagonal extended nodes and the junctions decomposed into perfect dislocations are seen in samples deformed indifferently by slip on {100}pc or {110}pc planes, but they seem to appear only above 1520 K, in the cubic phase. 相似文献
799.
Extreme value analysis provides a semiparametric method for analyzing the extreme long tails of skew distributions which may be observed when handling mining data. The estimation of important tail characteristics, such as the extreme value index, allows for a discrimination between competing distribution models. It measures the thickness of such long tailed distributions, if only a limited sample is available. This paper stresses the practical implementation of extreme value theory, which is used to discriminate a lognormal from a mixed lognormal distribution in a case study of size distributions for alluvial diamonds. 相似文献
800.
J. B. W. Day 《Environmental Geology》1996,27(2):77-78
Predictably, in a country such as Britain, with its preponderance of consolidated, sedimentary, mainly fissure-flow aquifers, there is a very large number of springs, many of which are, or have been, used for public supply. Migratory springs are a feature of the British (Ur. Cretaceous) Chalk, the most important British aquifer. The Chalk's low specific yield and high capillary moisture retention together give rise to very considerable fluctuations (more than 33 m in some areas) of the unconfined water table. Along the gentle dip slopes of the Chalk (North and South Downs of southern and southeastern England) springs may migrate laterally for several miles, giving rise to seasonal streams locally known as bournes or lavants. However, springs such as at Duncton, West Sussex, at the base of the much steeper scarp slopes of the Chalk, form point sources, the flows from which tend to be relatively steady; such springs commonly supply and are the original reason for the existence of many of the small towns and villages which nestle along the bases of the chalk scarps of Sussex and Kent.Where the Chalk forms coastal cliffs, a number of springs break out at the base of the cliff between high and low tide levels; there are major chalk coastal springs, for instance, at St. Margaret's Bay (Kent) and at Arish Mells, east of Lulworth Cove, Dorset. Such springs are not used for direct supply (their salinity is usually too high) but are indicators of the presence of local reserves of groundwater for possible future development. 相似文献