Prediction of deuterium abundance in comets     

Vladimír Vanýsek  Petr Vanýsek 《Icarus》1985,61(1):57-59

Using a chemical scheme based on ion-molecule reactions in cool interstellar clouds, the possibility of a deuterium enrichment of volatile material in comets is discussed. It is assumed that the hydrogen-containing molecules are deuterated before accretion and condensation on the coremantle dust particles from which the cometesimals are formed. The D/H ratio in comets may be enhanced in respect to the average value by a factor of 10². Therefore comets are not promising objects for testing the primordial deuterium abundance.  相似文献   

Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite   总被引：1，自引：0，他引：1  

R. L. Romer  S. -A. Smeds  P. Černý 《Mineralogy and Petrology》1996,57(3-4):243-260

Summary Columbite-tantalite can accommodate considerable amounts of uranium and excludes lead almost completely. It is thus potentially suited for U-Pb dating. Discordias generally yield closely constrained upper intercept ages. Yet, in a few cases, the U-Pb system of columbite-tantalite gives poorly constrained ages due to excess scatter or inverse discordance. These anomalous features do not show any relation with gross mineral chemistry or pegmatite fractionation. Recrystallization or the presence of exsolutions and inclusions generally do not result in anomalous and scattered U-Pb systematics, unless the inclusions are uraninite or secondary Nb, Ta-bearing phases. Open-system behavior of uraninite, whose occurrence seems to be related to the local host-rock-influenced redox conditions during pegmatite crystallization, often results in inverse discordance. Secondary Nb, Ta-bearing phases that accept Pb and U derived from uraninite inclusions into their structure may inherit this inverse discordance.

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Kristallchemische und bildungshedingte Einflüsse auf die U-Pb Systematik von Columbit-Tantalit

Zusammenfassung Columbit-Tantalit kann bedeutende Mengen Uran aufnehmen and schließt Blei beinahe vollständig aus. Deshalb ist das Mineral potentiell zur U-Pb Datierung geeignet. Im allgemeinen ergeben Diskordias gut definierte obere Schnittalter. Doch in einigen Fällen ergibt das Kolumbit-Tantalit U-Pb System aufgrund von gestreuten order invers diskordanten Daten, welche keinen systematischen Zusammenhang mit der Mineralchemie oder dem Fraktionierungsgrad des Pegmatites zeigen, schlecht bestimmte Schnittalter. Rekristallisation oder Einschlüsse bedingen im allgemeinen keine anomale oder gestreute U-Pb Systematik, ausser in Fällen wo die Einschlüsse Uraninit oder sekundäre Na, Ta-Phasen sind. Uraninit, dessen Auftreten mit dem lokalen Nebengestein und den Redoxbedingungen während der Pegmatitkristallisation zusammenzuhängen scheint, kann als offenes System eine inverse Diskordanz verursachen, welche von sekundären Nb, Ta-Phasen, die Pb und U von Uraninit in ihre Kristallstruktur einbauen, geerbt wird.

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With 5 Figures  相似文献   

Reconnection of magnetic field of active regions by the interaction of their extensions in interplanetary space     

S. Gopasyuk  L. Křivský 《Solar physics》1967,1(1):145-147

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On the origin of type II and IV radio sources during flares observed by a radioheliograph on 80 MHz     

L. Křivský 《Solar physics》1969,9(1):194-197

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Nevada underground nuclear explosions recorded at Kašperské hory seismological station     

Jiří Zahradník  Reviewer J. Vaněk 《Studia Geophysica et Geodaetica》1972,16(4):367-372

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Field of electric dipole above two-layer anisotropic medium     

Oldřich Praus 《Studia Geophysica et Geodaetica》1965,9(4):359-380

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Numerical simulation of fracture flow with a mixed-hybrid FEM stochastic discrete fracture network model     

Jiří Maryška  Otto Severýn  Martin Vohralík 《Computational Geosciences》2005,8(3):217-234

We introduce a discrete fracture network model of stationary Darcy flow in fractured rocks. We approximate the fractures by a network of planar circle disks, which is generated on the basis of statistical data obtained from field measurements. We then discretize this network into a mesh consisting of triangular elements placed in three-dimensional space. We use geometrical approximations in fracture planes, which allow for a significant simplification of the final triangular meshes. We consider two-dimensional Darcy flow in each fracture. In order to accurately simulate the channeling effect, we assign to each triangle an aperture defining its hydraulic permeability. For the discretization we use the lowest order Raviart-Thomas mixed finite element method. This method gives quite an accurate velocity field, which is computed directly and which satisfies the mass balance on each triangular element. We demonstrate the use of this method on a model problem with a known analytical solution and describe the generation and triangulation of the fracture network and the computation of fracture flow for a particular real situation.  相似文献   

U‐Pb Detrital Zircon Analysis – Results of an Inter‐laboratory Comparison     

Jan Košler  Jiří Sláma  Elena Belousova  Fernando Corfu  George E. Gehrels  Axel Gerdes  Matthew S. A. Horstwood  Keith N. Sircombe  Paul J. Sylvester  Massimo Tiepolo  Martin J. Whitehouse  Jon D. Woodhead 《Geostandards and Geoanalytical Research》2013,37(3):243-259

Inter‐laboratory comparison of laser ablation ICP‐MS and SIMS U‐Pb dating of synthetic detrital zircon samples provides an insight into the state‐of‐the art of sedimentary provenance studies. Here, we report results obtained from ten laboratories that routinely perform this type of work. The achieved level of bias was mostly within ± 2% relative to the ID‐TIMS U‐Pb ages of zircons in the detrital sample, and the variation is likely to be attributed to variable Pb/U elemental fractionation due to zircon matrix differences between the samples and the reference materials used for standardisation. It has been determined that ~ 5% age difference between adjacent age peaks is currently at the limit of what can be routinely resolved by the in situ dating of detrital zircon samples. Precision of individual zircon age determination mostly reflects the data reduction and procedures of measurement uncertainty propagation, and it is largely independent of the instrumentation, analytical technique and reference samples used for standardisation. All laboratories showed a bias towards selection of larger zircon grains for analysis. The experiment confirms the previously published estimates of the minimum number of grains that have to be analysed in order to detect minor zircon age populations in detrital samples.  相似文献   

Unmanned aerial vehicles (UAV) for assessment of qualitative classification of Norway spruce in temperate forest stands   总被引：1，自引：0，他引：1  

Olga Brovkina  Emil Cienciala  Peter Surový  Přemysl Janata 《地球空间信息科学学报》2018,21(1):12-20

Abstract

The study investigates the potential of UAV-based remote sensing technique for monitoring of Norway spruce health condition in the affected forest areas. The objectives are: (1) to test the applicability of UAV visible an near-infrared (VNIR) and geometrical data based on Z values of point dense cloud (PDC) raster to separate forest species and dead trees in the study area; (2) to explore the relationship between UAV VNIR data and individual spruce health indicators from field sampling; and (3) to explore the possibility of the qualitative classification of spruce health indicators. Analysis based on NDVI and PDC raster was successfully applied for separation of spruce and silver fir, and for identification of dead tree category. Separation between common beech and fir was distinguished by the object-oriented image analysis. NDVI was able to identify the presence of key indicators of spruce health, such as mechanical damage on stems and stem resin exudation linked to honey fungus infestation, while stem damage by peeling was identified at the significance margin. The results contributed to improving separation of coniferous (spruce and fir) tree species based on VNIR and PDC raster UAV data, and newly demonstrated the potential of NDVI for qualitative classification of spruce trees. The proposed methodology can be applicable for monitoring of spruce health condition in the local forest sites.  相似文献   

Early Paleozoic accretionary orogens along the Western Gondwana margin     

Sebastián Oriolo  Bernhard Schulz  Silvana Geuna  Pablo DGonzález  Juan E.Otamendi  JiíSláma  Elena Druguet  Siegfried Siegesmund 《地学前缘(英文版)》2021,12(1):109-130

Early Paleozoic accretionary orogens dominated the Western Gondwana margin and were characterized by nearly continuous subduction associated with crustal extension and back-arc basin development.The southwestern margin is represented by Famatinian and Pampean basement realms exposed in South America,both related to the protracted Paleozoic evolution of the Terra Australis Orogen,whereas the northwestern margin is mainly recorded in Cadomian domains of Europe and adjacent regions.However,no clear relationships between these regions were so far established.Based on a compilation and reevaluation of geological,paleomagnetic,petrological,geochronological and isotopic evidence,this contribution focuses on crustal-scale tectonic and geodynamic processes occurring in Western Gondwana accretionary orogens,aiming at disentangling their common Early Paleozoic evolution.Data show that accretionary orogens were dominated by high-temperature/lowpressure metamorphism and relatively high geothermal gradients,resulting from the development of extended/hyperextended margins and bulk transtensional deformation.In this sense,retreating-mode accretionary orogens characterized the Early Paleozoic Gondwana margin,though short-lived pulses of compression/transpression also occurred.The existence of retreating subduction zones favoured mantle-derived magmatism and mixing with relatively young(meta)sedimentary sources in a thin continental crust.Crustal reworking of previous forearc sequences due to trenchward arc migration thus took place through assimilation and anatexis in the arc/back-arc regions.Therefore,retreating-mode accretionary orogens were the locus of Early Paleozoic crustal growth in Western Gondwana,intimately associated with major flare-up events,such as those related to the Cadomian and Famatian arcs.Slab roll back,probably resulting from decreasing convergence rates and plate velocities after Gondwana assembly,was a key factor for orogen-scale geodynamic processes.Coupled with synchronous oblique subduction and crustal-scale dextral deformation,slab roll back might trigger toroidal mantle flow,thus accounting for bulk dextral transtension,back-arc extension/transtension and a large-scale anticlockwise rotation of Gondwana mainland.  相似文献