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K. C. WELTEN C. ALDERLIESTEN K. VAN DER BORG L. LINDNER T. LOEKEN L. SCHULTZ 《Meteoritics & planetary science》1997,32(6):775-780
Abstract— We measured the concentrations of the cosmogenic radionuclides 10Be (half-life = 1.51 × 106 a), 26Al (7.05 × 105 a) and 36Cl (3.01 × 105 a) in Lewis Cliff (LEW) 86360, an L-chondrite from the Lewis Cliff stranding area, East Antarctica. In addition, the concentrations and isotopic compositions of He, Ne and Ar were measured. The combined results yield a terrestrial age of 2.35 ± 0.15 Ma. Only one other stony meteorite with a similar terrestrial age (～2 Ma) is known from the Allan Hills stranding area (ALH 88019), whereas all previously dated stony meteorites from Antarctica are younger than 1 Ma. We argue that LEW 86360 spent most of its terrestrial residence time deep inside the ice, near the base of the glacier, where ice flow rates are much lower than at the surface. The terrestrial ages of LEW 86360 and ALH 88019 are consistent with existing hypotheses concerning the stability and persistence of the East Antarctic ice sheet. 相似文献
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LESZEK LINDNER LESZEK MARKS KAZIMIERZ PEKALA 《Boreas: An International Journal of Quaternary Research》1984,13(1):35-47
On the basis of field and laboratory studies supported by thermoluminescence dates, an occurrence of marine and glacial sediments is described from South Spitsbergen. These deposits are thought to date from the Holsteinian interglacial and the Saalian glacial stages respectively. In addition, the evidence and extent of Vistulian and Holoccne glacial advances in South Spitsbergen are presented. These advances occurred at 50,000–43,000, 30,000–10,000, 3,000–2,500 and 600–100 years B.P. The latter have been tentatively correlated with those recorded in other parts of Svalbard, as well as in East Greenland, the Barents Sea, Scandinavia, the Baltic Sea, North Poland and the Russian Plain. 相似文献
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A particular methodology adapted to crystalline formations with a thin weathered zone was developed for a village hydrological project, in Benin. A combination of electrical profiles, Schlumberger and square arrays, was able to locate the most fractured zones in the basement. We present some results obtained from theoretical models as well as from field data. The suggested methodology uses both measurements of resistivity and anisotropy. Strong anisotropy and low resistivity indicate the most productive hydrogeological areas. 相似文献
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K. C. WELTEN L. LINDNER C. ALDERLIESTEN K. van der BORG 《Meteoritics & planetary science》1999,34(4):559-569
Abstract— We determined terrestrial ages of ordinary chondrites from the Lewis Cliff stranding area, East Antarctica, on the basis of the concentrations of cosmogenic 10Be (t½; = 1.51 Ma), 26Al (t½; = 0.705 Ma), and 36Cl (t½; = 0.301 Ma). After an initial 26Al γ-ray survey of 91 meteorites suggested that many have terrestrial ages >0.1 Ma, we selected 62 meteorites for 10Be and 26Al measurements by accelerator mass spectrometry (AMS) and measured 36Cl in twelve of those. Low terrestrial ages (<0.1 Ma) were found for ～60% of the meteorites, whereas all others have ages between 0.1 and 0.5 Ma, except for one exceptional age of >2 Ma (Welten et al., 1997). Our major conclusions are: (1) The Lewis Cliff H-chondrites show similar ages to those from the Allan Hills icefields, but the L-chondrites are about a factor of 2 younger than those from Allan Hills, which indicates that Lewis Cliff is a younger stranding area. (2) The terrestrial age distributions at different parts of the Lewis Cliff stranding area generally agree with simple meteorite concentration models, although differences in weathering rate may also play a role. (3) We confirm that meteorites with natural thermoluminescence (TL) levels >80 krad are associated with low terrestrial ages (Benoit et al., 1992) but conclude that natural TL levels <80 krad can not be used to calculate the terrestrial age of a meteorite. Natural TL levels do seem useful to estimate relative terrestrial ages of large groups of meteorites and to determine differences in the surface exposure age of paired meteorite fragments. (4) Of the 62 meteorites measured with AMS, 31 were assigned to 11 different pairing groups, mainly on the basis of their cosmogenic nuclide record. The meteorites are estimated to represent between 42 and 52 distinct falls. 相似文献
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Rheological Transitions During Partial Melting and Crystallization with Application to Felsic Magma Segregation and Transfer 总被引：26，自引：7，他引：19
We consider the rheological behaviour of felsic magma in thezone of partial melting and during subsequent crystallization.We also introduce and combine concepts (mushy zone, percolationtheory, granular flow, shear localization) derived from thenon-geological literature and apply them to field observationson migmatites and granites. Segregation and transportation offelsic magmas is commonly observed in association with non-coaxialdeformation, suggesting that gravity forces have limited influenceduring magma segregation. Solid to liquid and liquid to solidtransitions are shown to be rheologically different, which infirmsthe concept of a unique rheological critical melt percentagefor both transitions. Four stages are examined, which dependon the melt fraction present. (1) A minimum of 8% melt by volume must first be produced toovercome the liquid percolation threshold (LPT) above whichmelt pockets can connect, thus allowing local magma displacement.Transport of the liquid phase is amplified by deformation towarddilatant sinks and is restricted to a very local scale. Thiscorresponds to partially molten domains illustrated by incipientmigmatites. (2) When more melt (20–25%) is present, a melt escapethreshold (MET) allows segregation and transport of the meltand part of the residual solid phase, over large distances.This corresponds to segregation and transfer of magma towardsthe upper crust. (3) Segregation of magma also occurs during granite emplacementand crystallization. In a flowing magma containing few particles(20%), particles rotate independently within the flow, defininga fabric. As soon as sufficient crystals are formed, they interactto construct a rigid skeleton. Such a random loose packed frameworkinvolves 55% solids and corresponds to the rigid percolationthreshold (RPT). Above the RPT, clusters of particles can sustainstress, and the liquid fraction can still flow. The only remainingpossibilities for rearranging particles are local shear zones,often within the intrusion rim, which, as a consequence, developsdilatancy. This stage of segregation during crystallizationis totally different from that of magma segregation during incipientmelting. (4) Finally, the system becomes totally locked when random closepacking is reached, at 72–75% solidification; this isthe particle locking threshold (PLT). The introduction of four thresholds must be viewed in the contextof a two-fold division of the cycle that generates igneous rocks,first involving a transition from solid to liquid (i.e. partialmelting) and then a transition from liquid to solid (i.e. crystallization).Neither transition is simply the reverse of the other. In thecase of melting, pockets of melt have to be connected to afforda path to escaping magma. This is a bond-percolation, in thesense of physical percoloation theory. In the case of crystallization,randomly distributed solid particles mechanically interact,and contacts between them can propagate forces. Building a crystalframework is a site-percolation, for which the threshold ishigher than that of bond-percolation. For each transition twothresholds are applicable. The present approach, which basicallydiffers from that based on a unique critical melt fraction,expands and clarifies the idea of a first and a second percolationthreshold. One threshold in each transition (LPT and RPT, respectively)corresponds to a percolation threshold in the sense of physicalpercolation theory. Its value is independent of external forces,but relies on the type and abundance of minerals forming thematrix within which melt connectivity is developing. The exactvalue of the second threshold (MET or PLT) will vary accordingto external forces, such as deformation and the particle shape. KEY WORDS: migmatites; partial melting; granites; magma segregation; magma solidification
*Corresponding author. Telephone: 33 03 83 44 19 00. Fax: 33 03 83 44 00 29. e-mail: jlv{at}cregu.cnrs-nancy.fr 相似文献
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KEES C. WELTEN LOUIS LINDNER KLAAS VAN DER BORG THOMAS LOEKEN PETER SCHERER LUDOLF SCHULTZ 《Meteoritics & planetary science》1997,32(6):891-902
Abstract— We determined the cosmic-ray exposure age of 20 diogenites from measured cosmogenic noble gas isotopes and calculated production rates of 3He, 21Ne and 38Ar. The production rates were calculated on the basis of the measured chemical composition and the cosmogenic 22Ne/21Ne ratio of each sample. The shielding conditions of each sample were also checked on the basis of the measured 10Be and 26AI concentrations. The exposure ages range from 6 to 50 Ma but do not form a continuous distribution: ten ages cluster at 21–25 Ma and four at 35–42 Ma. The two diogenite clusters coincide with the 22 Ma and 38 Ma peaks in the exposure age distribution of eucrites and howardites. After the selection from literature data of 32 eucrites and 11 howardites with reliable ages, we find a total of 23 howardite, eucrite and diogenite (HED) group meteorites at 20–25 Ma and 10 at 35–42 Ma. The shape of the two peaks is consistent with single impact events, and random number statistics show that they are statistically significant at the 99% level. Altogether, this provides strong evidence for two major impact events 22 Ma and 39 Ma ago. Although these two events can explain more than half of all HED exposure ages, it takes at least five impact events to explain all ages <50 Ma. An impact frequency of one per 10 Ma corresponds to projectiles of at least 2–4 km in diameter for Vesta and of 60–300 m for the 100× smaller Vesta-derived “vestoids.” Based on the HED exposure-age distribution, the size distribution of the main-belt asteroids and the difference in size between Vesta and the kilometer size vestoids, we favor Vesta as the major source of HED meteorites, although some of the meteorites may have been ejected from the vestoids rather than directly from Vesta. 相似文献
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KEITH W. LINDNER 《Geographical review》2012,102(3):372-381
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Résumé Les données recueillies à 13 stations hydromètriques des réseaux du SENAMHI et de ENDE, de 1975 à 1983, permettent d'estimer les flux de matières particulaires et dissoutes dans les bassins andins des rios Pilcomayo et Bermejo, formateurs du Rio Paraguay. Les résultats obtenus en Bolivie sur le bassin versant du Rio Pilcomayo, montrent que l'essentiel de l'exportation est le fait des matières en suspension (90%). Le flux de sédiments mesuré à la sortie des Andes sur le Rio Pilcomayo (80 × 106 t an?1) est du même ordre de grandeur que Celui observé 2000 kilomètres à l'aval sur le Moyen Parana. 相似文献