Sulphur, carbon and oxygen isotope studies of early Variscan mineralisation and Pb-Sb vein deposits in the Cornubian orefield: implications for the scale of fluid movements during Variscan deformation     

R. E. Clayton  B. Spiro 《Mineralium Deposita》2000,35(4):315-331

The strongly deformed Middle Devonian-Lower Carboniferous metasedimentary-volcanic successions of the Trevone Basin (SW England) contain stratiform and Pb-Sb vein deposits that reveal a wide variation in δ³⁴S and δ¹³C, reflecting mineral deposition during diagenesis, regional metamorphism and basin inversion. Pre-Variscan metasedimentary sulphide (δ³⁴S=−33.7 to −26.7‰) and metabasite sulphide (δ³⁴S=+4.0 to +10.8‰) suggest two accessible source reservoirs for sulphur which were available for Sb-As-(Au) and Pb-Zn-(Ag) mineralisation (δ³⁴S=−3.3 to −15.0‰) during late Variscan semiductile-brittle shear. On the basis of pressure-corrected fluid inclusion temperatures, the calculated composition of fluid sulphur reveals an enrichment in δ³⁴S_H2S in the individual vein parageneses and depletion of the fluid sulphur reservoir during evolution of the vein systems. Carbonates in the same veins are partly contemporaneous with Pb-Sb mineralisation and late tensional deformation; their isotopic composition (δ¹³C=−3.2 and −13.4‰) appears strongly influenced by the host formation. Fluid inclusions in post-tensional quartz show a marked reduction in CO₂, suggesting that episodes of CO₂ degassing in response to punctuated reductions in pressure during uplift and brittle deformation was an important mechanism for vein carbonation. An origin for the Pb-Sb mineralisation involving local remobilisation of sulphur from the mixed metasedimentary-volcanic succession is probably inseparable from processes connected with Variscan metamorphism and deformation. Although the N Cornish Variscan deformation is part of a spatially large-scale event, the isotopic evidence suggests compartmentalisation of sulphur and carbon isotope features and short distances between sources and sinks. Received: 15 August 1998 / Accepted: 8 October 1999  相似文献   

An Evaluation of Proposed Mechanisms of Slab Flattening in Central Mexico     

Steven M. Skinner  Robert W. Clayton 《Pure and Applied Geophysics》2011,168(8-9):1461-1474

Central Mexico is the site of an enigmatic zone of flat subduction. The general geometry of the subducting slab has been known for some time and is characterized by a horizontal zone bounded on either side by two moderately dipping sections. We systematically evaluate proposed hypotheses for shallow subduction in Mexico based on the spatial and temporal evidence, and we find no simple or obvious explanation for the shallow subduction in Mexico. We are unable to locate an oceanic lithosphere impactor, or the conjugate of an impactor, that is most often called upon to explain shallow subduction zones as in South America, Japan, and Laramide deformation in the US. The only bathymetric feature that is of the right age and in the correct position on the conjugate plate is a set of unnamed seamounts that are too small to have a significant effect on the buoyancy of the slab. The only candidate that we cannot dismiss is a change in the dynamics of subduction through a change in wedge viscosity, possibly caused by water brought in by the slab.  相似文献   

Trichoptera remains from early Holocene river deposits in the Great Belt, Denmark     

PETER WIBERG-LARSEN  OLE BENNIKE  JØRN BO JENSEN  WOLFRAM LEMKE 《Boreas: An International Journal of Quaternary Research》2001,30(4):299-306

Analyses of a sediment core from the Great Belt documented the local presence of fluvial deposits. The sediments consisted of silt, clay, sand and organic detritus and they contained macroscopical remains of a mixture of terrestrial, telmatic, lacustrine and fluvial plants and animals. Most noteworthy was the abundance of remains of caddisfly larvae, totalling at least 24 species, which were dominated by fluvial species. Thus, filter-feeding hydropsychids made up approximately 95% out of 1496 identified specimens, the dominating species being Hydropsyche contubernalis and H. pellucidula . The present-day requirements of the recorded species point to a fairly fast-flowing river, at least locally, with areas of stones (some with moss growth) and gravel, but also to more slowly-flowing parts with sand, fine and coarse detritus. This river may have been considerably larger than any present-day Danish river. It appears that the fluvial deposits formed locally in connection with flooding of the area south of the core position. AMS radiocarbon dating shows that the sequence was deposited very rapidly during the early Holocene, about 10650 to 10250 cal. years BP. The occurrence of the lentic Leptocerus tineiformis points to a climate at least as warm as today.  相似文献   

ACCEPTANCE OF THE LEONARD MEDAL OF THE METEORITICAL SOCIETY, 1982     

Robert N. Clayton 《Meteoritics & planetary science》1982,17(4):175-176

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The fall,recovery, and classification of the Park Forest meteorite     

S. B. Simon  L. Grossman  R. N. Clayton  T. K. Mayeda  J. R. Schwade  P. P. Sipiera  J. F. Wacker  M. Wadhwa 《Meteoritics & planetary science》2004,39(4):625-634

Abstract— On the night of March 26, 2003, a large meteorite broke up and fell upon the south suburbs of Chicago. The name Park Forest, for the village that is at the center of the strewnfield, has been approved by the nomenclature committee of the Meteoritical Society. Satellite data indicate that the bolide traveled from the southwest toward the northeast. The strewnfield has a southeast‐northwest trend; however, this is probably due to the effects of strong westerly winds at high altitudes. Its very low ⁵⁶Co and very high ⁶⁰Co activities indicate that Park Forest had a preatmospheric mass that was at least ～900 kg and could have been as large as ～7 times 10³ kg, of which only ～30 kg have been recovered. The average compositions of olivine and low‐Ca pyroxene, Fa_{24.7 ± 1.1} and Fs_{20.8 ± 0.7}, respectively, and its bulk oxygen isotopic composition, δ¹⁸O = +4.68%o, δ¹⁷O = +3.44%o, show that Park Forest is an L chondrite. The ferromagnesian minerals are well equilibrated, chondrules are easily recognized, and maskelynite is mostly ≤50 μm across. Based on these observations, we classify Park Forest as type 5. The meteorite has been strongly shocked, and based on the presence of maskelynite, mosaicism and planar deformation features in olivine, undulatory extinction in pyroxene, and glassy veins, the shock stage is S5. The meteorite is a monomict breccia, consisting of light‐colored, angular to rounded clasts in a very dark host. The light and dark lithologies have essentially identical mineral and oxygen isotopic compositions. Their striking difference in appearance is due to the presence of a fine, pervasive network of sulfide veins in the dark lithology, resulting in very short optical path lengths. The dark lithology probably formed from the light lithology in an impact that formed a sulfide‐rich melt and injected it into cracks.  相似文献   

Trichinella sp. in polar bears from Svalbard, in relation to hide length and age     

THOR LARSEN  BJØRN KJOS-HANSSEN 《Polar research》1983,1(1):89-96

DiaPhragma and masseter muscles from 376 polar bears ( Ursus maritimus ), 252 ringed seals ( Phoca hispida ), 84 bearded seals ( Erignathus barbatus ), and 77 arctic foxes ( Alopex lagopus ) from Svalbard were examined for Trichinella . Infection rates in polar bears varied between 23 and 58%, and between 3 and 67% in arctic fox- None of tne seals were infected. Trichinella in polar bears is probably transferred through cannibalism and scavenging upon polar bear carcasses. Infection rate in arctic fox was high when they preyed upon polar bear carcasses before polar bear hunting was prohibited in 1973. A low infection rate seems more natural when such prey is not available. No difference could be found in infection rate between male and female polar bears. There is only a slight increase in infection rate with age, as calculated from hide lengths, and many adult animals remain uninfected. Geographical isolation of polar bear populations may explain differences in Trichinella infection rates between bears from arctic America and arctic Europe. Possible explanations are that discrete polar bear populations have different food habits, or that they are exposed to different Trichinella strains.  相似文献   

Flow slides run-out prediction using a sliding-consolidation model   总被引：1，自引：1，他引：0  

S. F. Qiao  C. R. I. Clayton 《Landslides》2013,10(6):831-842

The estimation of maximum travel distance of flow slides is an important topic to assess the consequence of natural disasters caused by landslides. During debris transportation, dissipation rules of pore-water pressure determine movement properties of flow slides. Based on 1-D Terzaghi consolidation theory, expressions of excess pore-water pressure with three cases of initial conditions are deduced and are programmed using Mathematica^® language. Furthermore, the factors affecting the distribution of pore-water pressure are studied using nondimensional method interactively, such as z/h, u _b /u _a , and T _v , which are fairly significant to investigate soil consolidation during the movement of flow slides. On the basis of the sliding-consolidation model first provided as reported by Hutchinson (Can. Geotech. J. 23(2):115-126, 1986), equations of pore-water pressure, velocity, and travel distance of flow slides are obtained and the physical quantities are coded as mathematical functions using Mathematica^® language characterized by its user-friendly interfaces to study run-out properties of flow slides very easily. The program can be used to compute velocity of flow slide, time, and pore-water pressure at a certain position, and thus judge automatically when and where flow slide will stop on slopes with different slope angles, solving the computing difficulties encountered during the Hutchinson's model application, especially in the last decades when computing technique with computers did not develop so rapidly as at present. At last, back analysis for properties of the 1966 flow slide at Aberfan, South Wales is done to test the model and the program, whose results are compared with those as reported by Hutchinson (Can. Geotech. J. 23(2):115-126, 1986). The results show that the program developed by the authors makes the application of Hutchinson's model more correct and easier.  相似文献   

Non-stationary Geostatistical Modeling Based on Distance Weighted Statistics and Distributions   总被引：3，自引：0，他引：3  

David F. Machuca-Mory  Clayton V. Deutsch 《Mathematical Geosciences》2013,45(1):31-48

A common assumption in geostatistics is that the underlying joint distribution of possible values of a geological attribute at different locations is stationary within a homogeneous domain. This joint distribution is commonly modeled as multi-Gaussian, with correlations defined by a stationary covariance function. This results in attribute maps that fail to reproduce local changes in the mean, in the variance and, particularly, in the spatial continuity. The proposed alternative is to build local distributions, variograms, and correlograms. These are inferred by weighting the samples depending on their distance to selected locations. The local distributions are locally transformed into Gaussian distributions embedding information on the local histogram. The distance weighted experimental variograms and correlograms are able to adapt to local changes in the direction and range of spatial continuity. The automatically fitted local variogram models and the local Gaussian transformation parameters are used in spatial estimation algorithms assuming local stationarity. The resulting maps are rich in nonstationary spatial features. The proposed process implies a higher computational effort than traditional stationary techniques, but if data availability allows for a reliable inference of the local distributions and statistics, a higher accuracy of estimates can be achieved.  相似文献   

Geometallurgical Modeling at Olympic Dam Mine, South Australia     

Jeff B. Boisvert  Mario E. Rossi  Kathy Ehrig  Clayton V. Deutsch 《Mathematical Geosciences》2013,45(8):901-925

Modeling of geometallurgical variables is becoming increasingly important for improved management of mineral resources. Mineral processing circuits are complex and depend on the interaction of a large number of properties of the ore feed. At the Olympic Dam mine in South Australia, plant performance variables of interest include the recovery of Cu and U₃O₈, acid consumption, net recovery, drop weight index, and bond mill work index. There are an insufficient number of pilot plant trials (841) to consider direct three-dimensional spatial modeling for the entire deposit. The more extensively sampled head grades, mineral associations, grain sizes, and mineralogy variables are modeled and used to predict plant performance. A two-stage linear regression model of the available data is developed and provides a predictive model with correlations to the plant performance variables ranging from 0.65–0.90. There are a total of 204 variables that have sufficient sampling to be considered in this regression model. After developing the relationships between the 204 input variables and the six performance variables, the input variables are simulated with sequential Gaussian simulation and used to generate models of recovery of Cu and U₃O₈, acid consumption, net recovery, drop weight index, and bond mill work index. These final models are suitable for mine and plant optimization.  相似文献   

Spread and status of seven submerged pest plants in New Zealand lakes     

Mary D. deWinton  Paul D. Champion  John S. Clayton  Rohan D.S. Wells 《新西兰海洋与淡水研究杂志》2013,47(2):547-561

The distribution of seven submerged aquatic pest plants is reported. Lake vegetation surveys recorded pest plants in 27.9% of 344 lakes, with two species co‐occurring in 5.8%, and three species in 2.6% of lakes. Egeria densa was most frequent (15.4% of lakes), followed by Ceratophyllum demersum (9.0%), Lagarosiphon major (7.3%), and Utricularia gibba (5.5%). Spread since 2000 has continued for five pest plants, with 34 lakes invaded by U. gibba over 2004–08 alone. Early regional sites in proximity to human population centres were likely plant liberations and numerous potential founder colonies remain in garden ponds. Human activities were important for inter‐lake dispersal, with the exception of bird‐dispersed U. gibba. Significant lake associations between pest plants, and with presence of six exotic fish species, suggest common dispersal pathways and similar introduction risks. Therefore, predictions of future spread should be possible based on sources, dispersal pathways, and identifying key risk factors for lakes.  相似文献