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791.
792.
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.
With 5 Figures 相似文献
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.
With 5 Figures 相似文献
793.
794.
795.
796.
Foraminiferal and conodont faunas at the Devonian–Carboniferous (D–C) boundary in the southern part of the Moravian Karst (Czech Republic) were studied in different facies of the basin slope. The joint presence of foraminifers and conodonts in calciturbidites along with a positive δ13C excursion of the Hangenberg anoxic event enabled the high‐resolution calibration of the late Famennian–early Tournaisian interval (Upper expansa–crenulata conodont zones). The conodont stratigraphic and biofacies succession reveals a strong correlation with other European areas. The Siphonodella sulcata morphotype (close to Group 1 sensu Kaiser and Corradini and “nov. gen. nov. sp. 1” sensu Tragelehn) enters prior to the Hangenberg Event, which resembles Upper and Uppermost Famennian conodont successions from Franconia, Bavaria and Morocco. The diversification of the early siphonodellids takes place after the Hangenberg Event and after the protognathodid radiation. In terms of foraminiferal biostratigraphy, the D–C boundary interval is characterized by the first appearance datum (FAD) of Tournayellina pseudobeata close below the D–C boundary followed by a sequence of Tournaisian bioevents, where apart from the last appearance datums (LADs) of quasiendothyrs, the FADs of the Neoseptaglomospiranella species and chernyshinellids play an important role in a similar manner as in Eastern Europe. The correlation of these bioevents elsewhere is often hindered by glacioeustatically‐driven unconformities and widespread occurrences of unfavourable facies for plurilocular foraminifers (Malevka beds and Bisphaera beds). Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
797.
Oliver Wetter Christian Pfister Johannes P. Werner Eduardo Zorita Sebastian Wagner Sonia I. Seneviratne Jürgen Herget Uwe Grünewald Jürg Luterbacher Maria-Joao Alcoforado Mariano Barriendos Ursula Bieber Rudolf Brázdil Karl H. Burmeister Chantal Camenisch Antonio Contino Petr Dobrovolný Rüdiger Glaser Iso Himmelsbach Andrea Kiss Oldřich Kotyza Thomas Labbé Danuta Limanówka Laurent Litzenburger Øyvind Nordli Kathleen Pribyl Dag Retsö Dirk Riemann Christian Rohr Werner Siegfried Johan Söderberg Jean-Laurent Spring 《Climatic change》2014,125(3-4):365-367
798.
B. Vršnak T. Žic D. Vrbanec M. Temmer T. Rollett C. Möstl A. Veronig J. Čalogović M. Dumbović S. Lulić Y.-J. Moon A. Shanmugaraju 《Solar physics》2013,285(1-2):295-315
We present the “Drag-Based Model” (DBM) of heliospheric propagation of interplanetary coronal mass ejections (ICMEs). The DBM is based on the hypothesis that the driving Lorentz force, which launches a CME, ceases in the upper corona and that beyond a certain distance the dynamics becomes governed solely by the interaction of the ICME and the ambient solar wind. In particular, we consider the option where the drag acceleration has a quadratic dependence on the ICME relative speed, which is expected in a collisionless environment, where the drag is caused primarily by emission of magnetohydrodynamic (MHD) waves. In this paper we present the simplest version of DBM, where the equation of motion can be solved analytically, providing explicit solutions for the Sun–Earth ICME transit time and impact speed. This offers easy handling and straightforward application to real-time space-weather forecasting. Beside presenting the model itself, we perform an analysis of DBM performances, applying a statistical and case-study approach, which provides insight into the advantages and drawbacks of DBM. Finally, we present a public, DBM-based, online forecast tool. 相似文献
799.
The Tuolumne batholith, Sierra Nevada, California, consists of several nested granitoid units and is an example of upper-crustal
normally zoned intrusions. The two outermost units of the batholith are separated by a wide gradational contact in what is
interpreted to represent a large magma chamber. In the Potter Point area near the eastern margin of the batholith, the gradational
contact is cross-cut by a network of interconnected mafic–felsic sheets, which grade into zones of magmatic erosion by stoping
where the host granodiorite between the sheets was entirely removed and replaced by younger enclave-rich quartz diorite. We
interpret these features to record disruption of a steep solidification front, which migrated inwards from the eastern batholith
margin and separated the mushy to solidified margin from the remaining active magma chamber. When intersecting the gradational
contact, the solidification front started to break up via a network of tectonically driven fractures accompanied by simultaneous
injection of localized magma pulses. The solidification front break-up is interpreted here as an initial stage of a “recycling”
process, whereby older magma mush is disrupted and incorporated into younger magma batches, a process we propose to have been
widespread along internal contacts in the Tuolumne magma chamber. 相似文献
800.
Summary We have investigated the influence of heating in the D-layer on the convection dynamics for the Rayleight number Ra=106. Strong heating, which may represent a local small-scale heat transfer in the D-layer, results in an increase of lateral heterogeneities near the upper and lower boundaries, the blob-like structure of upwellings and the stabilization of the convection pattern. The influence of the electromagnetic heating was found to be too weak to produce any substantial effect which is in contrast with the idea of Braginskii and Meitlis [1].Dedicated to the Memory of K. P 相似文献