1,721,021 research outputs found
Keuper (Late Triassic) sediments in Germany - indicators of rapid uplift of Caledonian rocks in southern Norway
No full textK/Ar ages of detrital Keuper micas in the Central European Basin provide new information about the provenance of siliciclastic sediments. All samples of the Erfurt Formation (Lower Keuper) and the Stuttgart Formation (Middle Keuper) indicate Caledonian (445 - 388 Ma) ages. These sediments originate from the Caledonides of southern Norway. The older Fennoscandian Shield and the Russian Platform did not supply any material. In combination with zircon fission track data, we show that uplift of the Caledonides of southern Norway accelerated dramatically during the Carnian, most likely as a result of rifting of the Viking Graben. This push of Scandinavian-derived sediments ceased in the Upper Keuper (Rhaetian). Instead of Scandinavian sediments, micas of Panafrican age were transported from southeast Poland and Slovakia into the Central European Basin. Only micas in the vicinity of the south German Vindelician High and the Bohemian Block have Variscan ages
In situ 238U-230Th disequilibrium dating of pyrochlore at sub-millennial precision
No full textWe report the application of high-resolution in situ U-238-Th-230 disequilibrium dating to pyrochlore [(Ca,Na,REE,Th,U)(2)(Nb,Ti,Ta,Zr)(2)O-6(OH,F)]. Compositionally zoned pyrochlore is an accessory phase in intrusive carbonatitic syenites, which were ejected as cognate clasts during the eruption of the phonolitic Laacher See volcano, East Eifel, Germany, 12.9 ka ago. Separated individual pyrochlore crystals were analyzed for U-Th isotopes and elemental abundances at high spatial resolution (similar to 25 mu m) using secondary ion mass spectrometry (SIMS) with an internal precision for the U-238-Th-230 isochron of similar to 700 a (2 sigma). U and Th were also analyzed by electron microprobe in SIMS spots. Instrumental mass fractionation for Th/U was corrected using an EMP vs. SIMS calibration curve, and the resulting pyrochlore isochron age of 26.1 +/- 1.4 ka (including external calibration uncertainty) is concordant with the 23 +7/-6 ka U-Th zircon ages for the same sample. By contrast, AM206 pyrochlore from a Cretaceous alkali granite shows significant disequilibrium with (Th-230)/(U-238) = 0.90 +/- 0.04. The tightly defined isochron implies that U-Th in Laacher See pyrochlore remained a closed decay system during protracted residence in the magma chamber carapace, and was unaffected by hydrothermal alteration or post-eruptive weathering over time scales of tens of thousands of years. By contrast, disequilibrium in AM206 pyrochlore indicates U-Th mobilization during protracted near-surface residence
Magmatic Longevity of Laacher See Volcano (Eifel, Germany) Indicated by U-Th Dating of Intrusive Carbonatites
No full textUranium-series dating of carbonatitic ejecta clasts constrains the crystallization and differentiation timescales of the Laacher See volcano, which erupted 6 center dot 3 km(3) of magma (dense rock equivalent) during one of the largest Late Quaternary eruptions in Central Europe. Carbonatites form a distinct population among plutonic ejecta that are present in the middle and late erupted Laacher See tephra. Characteristic trace element patterns of the carbonatites, including negative Eu anomalies, and mantle-like oxygen isotopic compositions preserved in zircon indicate that the Laacher See carbonatites are cogenetic with their phonolite host. Carbonatite U-Th zircon isochron ages range from 32 center dot 6 +/- 4 center dot 1 ka (2 Sigma; MSWD = 1 center dot 7; n = 24) to near-eruption age (12 center dot 9 ka). Uranium-series carbonatite ages qualitatively agree with alkali feldspar compositions that lack prominent magmatic zonation, but show evidence for perthitic unmixing during subsolidus residence at elevated temperatures ( 720 degrees C). Model differentiation ages and crystallization ages for the carbonatites overlap within a few thousand years as resolved by U-Th dating and indicate rapid crystallization following carbonatite segregation from its parental phonolite. Model differentiation and zircon isochron ages peak at similar to 17 ka, suggesting a major phase of differentiation of the Laacher See magma system at this time, although the onset of phonolite differentiation dates back to at least similar to 10-20 kyr prior to eruption. Phenocrysts in the middle and late erupted phonolite magma crystallized shortly before eruption, and the lack of older crystals implies crystal removal through settling or resorption. Crystal ages from both crystal-rich and liquid-dominated parts of a magma system are thus complementary, and reveal different aspects of magma differentiation and residence timescales
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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