1,720,959 research outputs found
A recipe for the use of rutile in sedimentary provenance analysis
No full textRutile has received considerable attention in the last decade as a valuable petrogenetic indicator mineral. Based on both new and previously published data, we carve out advantages and pitfalls regarding TiO2-minerals in sedimentary provenance analysis. This results in a recipe for the use of rutile in provenance studies. The main points are: Rutile geochemistry from different grain size fractions does not differ systematically, and hence rutiles should be extracted from the fraction containing the most rutile grains (usually 63-200 mu m). Similarly, different magnetic susceptibility of rutile does not systematically imply different trace element composition. Before interpretation of TiO2-mineral data, it is important to determine the polymorph type. Rutile, anatase and brookite appear to differ systematically in trace element composition. As an alternative to Raman spectroscopy, chemical classification according to Nb, Cr, Sn, Fe, V, and Zr concentrations can be applied. For rutile, a new host lithology discrimination scheme based on Cr-Nb systematics is introduced (x = 5( ) (Nb [ppm] - 500) - Cr [ppm]), which leads to better classification results than previously published discrimination methods. According to this equation, metamafic rutiles have negative values of x, while metapelitic rutiles have positive values. Evaluation of the growth temperature calculations of metamorphic rutile after different authors shows that the equations given by Tomkins et al. (2007) should be applied to both metamafic and metapelitic rutiles. Although there is a pressure effect on the Zr incorporation in rutile, the pressure range for most rutiles of 5-15 kbar introduces an uncertainty in calculated temperature of no more than +/- 35 degrees C. The distribution of calculated temperatures from detrital rutiles is crucial; only well-defined temperature populations should be used for thermometry interpretation. (c) 2012 Elsevier B.V. All rights reserved.Deutsche Forschungsgemeinschaft [EY 23/10-2, ZA 285/4-2
Anorthite-calibrated backscattered electron profiles, trace elements, and growth textures in feldspars from the Teide-Pico Viejo volcanic complex (Tenerife)
No full textMagma chamber dynamics at the phonolitic to basanitic Teide-Pico Viejo volcanic complex on Tenerife, Canary Islands, have been investigated using feldspar zonation and chemistry. Backscattered electron-image grey value profiles, obtained by electron microprobe and calibrated on anorthite content, allow the investigation of much better spatially resolved zonation profiles than quantitative point analysis alone. On the basis of these zonation patterns and trace element analyses by electron microprobe, magma chamber processes such as magma recharge and mingling, as well as whole-chamber overturn events, can be identified and characterized in more detail. Samples from all investigated eruption centres contain feldspars, which have experienced growth in a deep reservoir, followed by ascent, resorption, and growth of mantles in more shallow levels in the crust just prior to eruption. Two Pico Viejo samples exhibit feldspars with a characteristic drop in anorthite content, which is not accompanied by resorption. Trace element behaviour in these samples shows that cores and mantles have grown from the same magma reservoir: The conclusion from these data is that there occurred complete overturn events in layered magma chambers, presumably induced by cold plumes from the cupola layers. A mass balance calculation shows that the proportions of the layers are similar to 75 wt.% lower part and similar to 25 wt.% cupola layer. Correlations between zonation patterns of single crystals in different rocks show that the Teide magma chambers are larger and chemically buffered compared with the smaller Pico Viejo system. Feldspars from Pico Viejo exhibit rather complex anorthite-profiles and more frequently display disturbances likely to be caused by arrival of new magma batches. (c) 2006 Elsevier B.V. All rights reserved
Deducing source rock lithology from detrital rutile geochemistry: An example from the Erzgebirge, Germany
No full textThis study evaluates the applicability of rutile trace element geochemistry to provenance studies. The study area is the Erzgebirge in eastern Germany, where metamorphic rocks ranging from lower greenschist facies conditions up to granulite facies conditions are exposed. We collected sand and rock samples from small catchment areas for a comparative analysis of rutile geochemistry using wavelength-dispersive electron microprobe. Our results show that rutile geochemistry is a powerful tool in provenance studies, allowing for the identification of source lithologies and an evaluation of the host orogen's metamorphic history. The log (Cr/Nb) ratio has proven to be decisive in discriminating between mafic and metapelitic lithologies. It is also useful for identifying different source rocks when plotted versus a third element or proxy. Furthermore, our results suggest that rutile thermometry can be applied to a much wider range of lithologies than previously assumed. A quantification of temperature populations within single sand samples shows that at high-grade metamorphic conditions, such as those found in the Erzgebirge, more than 65% of rutiles do not re-equilibrate during retrograde metamorphism and thus retain their peak temperature chernistry. Such samples, which have equilibrated at recent metamorphic conditions, can be identified by their 2-sigma standard deviations of less than 120 degrees C. Below 550-600 degrees C, no complete equilibration is reached. Rutiles from greenschist facies and lower metamorphic conditions in the Erzgebirge still inherit relict temperatures from a former metamorphic cycle. They partly record very high temperatures >950 degrees C and supposedly derive from erosion of the west African craton in Ordovician time. (C) 2007 Elsevier B.V. All rights reserved
Rutile occurrence and trace element behavior in medium-grade metasedimentary rocks: example from the Erzgebirge, Germany
No full textMetamorphic textures in medium-grade (~500–
550°C) metasedimentary rocks from the Erzgebirge give
evidence of prograde rutile crystallization from ilmenite.
Newly-crystallized grains occur as rutile-rich polycrystalline
aggregates that pseudomorph the shape of the
ilmenites. In-situ trace element data (EMP and SIMS) show
that rutiles from the higher-grade samples record large
scatter in Nb content and have Nb/Ti ratios higher than
coexisting ilmenite. This behavior can be predicted using
prograde rutile crystallization from ilmenite and indicates
that rutiles are reequilibrating their chemistry with remaining
ilmenites. On the contrary, rutiles from the lowest grade
samples (~480°C) have Nb/Ti ratios that are similar to the
ones in ilmenite. Hence, rutiles from these samples did not
equilibrate their chemistry with remaining ilmenites. Our
data suggest that temperature may be one of the main
factors determining whether or not the elements are able to
diffuse between the phases and, therefore, reequilibrate.
Newly-crystallized rutiles yield temperatures (from ~500 to
630°C, Zr-in-rutile thermometry) that are in agreement with
the metamorphic conditions previously determined for the
studied rocks. In quartzites from the medium-grade domain
(~530°C), inherited detrital rutile grains are detected. They
are identified by their distinct chemical composition (high
Zr and Nb contents) and textures (single grains surrounded
by fine grained ilmenites). Preliminary calculation, based
on grain size distribution of rutile in medium-grade
metapelites and quartzites that occur in the studied area,
show that rutiles derived from quartzites can be anticipated
to dominate the detrital rutile population, even if quartzites
are a minor component of the exposure
Discrimination of TiO2 polymorphs in sedimentary and metamorphic rocks
Full text linkInvestigation by Raman spectroscopy of samples from different geological settings shows that the occurrence of TiO2 polymorphs other than rutile can hardly be predicted, and furthermore, the occurrence of anatase is more widespread than previously thought. Metamorphic pressure and temperature, together with whole rock chemistry, control the occurrence of anatase, whereas variation of mineral assemblage characteristics and/or fluid occurrence or composition takes influence on anatase trace element characteristics and re-equilibration of relict rutiles. Evaluation of trace element contents obtained by electron microprobe in anatase, brookite, and rutile shows that these vary significantly between the three TiO2 phases. Therefore, on the one hand, an appropriation to source rock type according to Nb and Cr contents, but as well application of thermometry on the basis of Zr contents, would lead to erroneous results if no phase specification is done beforehand. For the elements Cr, V, Fe, and Nb, variation between the polymorphs is systematic and can be used for discrimination on the basis of a linear discriminant analysis. Using phase group means and coefficients of linear discriminants obtained from a compilation of analyses from samples with well-defined phase information together with prior probabilities of groupings from a natural sample compilation, one is able to calculate phase grouping probabilities of any TiO2 analysis containing at least the critical elements Cr, V, Fe, and Nb. An application of this calculation shows that for the appropriation to the phase rutile, a correct-classification rate of 99.5% is obtained. Hence, phase specification by trace elements proves to be a valuable tool besides Raman spectroscopy.Postprint (published version
Die Geochemie von detritischem Rutil - Folgerungen für sedimentäre Provenienzstudien und die Rekonstruktion metamorpher Bedingungen
No full textDie in dieser Studie untersuchten Proben stammen aus einer großen Bandbreite an metamorphen Entstehungsbedingungen (von diagenetisch bis 950°C und 4.5GPa) und geologischen Hintergründen. Haupt-Probenahmegebiete waren das Variszische Erzgebirge (Deutschland) und die Alpen (Schweiz und Italien). Andere Proben entstammen verschiedenen Lokationen in Europa, Nord- und Südamerika und Afrika. Sande aus Liefergebieten zwischen ca. 100m2 und ca. 2300km2 werden mit assoziierten Gesteinsproben verglichen.Rutil-, Anatas- und Brookit-Kristalle mit Korngrößen von 63µm bis in den cm-Bereich werden mit der Elektronenstrahl-Mikrosonde (EMS), Sekundärionen-Massenspektrometrie (SIMS), Raman-Spektroskopie und Laser-Ablations-Induktiv gekoppeltem Plasma-Massenspektrometrie (LA-ICPMS) auf Spurenelement-, Isotopen- und Phasenzusammensetzung analysiert.Die Ergebnisse zeigen, daß bei Rutilen mit Korngrößen zwischen 80 und 250µm kein Zusammenhang zwischen der Korngröße und der chemischen Zusammensetzung der Rutile besteht. Ebenso besteht kein Zusammenhang zwischen magnetischer Suszeptibilität und Rutilchemismus.Im Vergleich mit LA-ICPMS liefert SIMS-Analytik die richtigeren Ergebnisse für Spurenelemente im ppm-Bereich in kleinen, eingebetteten Körnern, wie sie in dieser Studie untersucht werden. Weil die relativen Sensitivitätsfaktoren in der SIMS-Analytik auf EMS-Ergebnisse kalibriert wurden, sind Analysen aus SIMS und EMS zudem gut miteinander vergleichbar.Diese Studie belegt, dass detritischer Rutil ein präziser Indikator für Liefergesteinscharakteristiken in sedimentären Systemen ist. Anhand der Gleichung x = 5 * (Nb[ppm] - 500) - Cr[ppm] (ppm = Masse-ppm) kann eine Unterscheidung zwischen Rutilen aus mafischen und metapelitischen Liefergesteinen vorgenommen werden. Rutile aus mafischen Gesteinen haben demnach negative x-Werte, Rutile aus Metapeliten haben positive x-Werte.Obwohl die Voraussetzung für die Anwendung von Zr-in-Rutil-Thermometrie eine Koexistenz von Rutil mit Quarz (oder SiO2) und Zirkon während des Rutil-Wachstums ist, zeigt ein Vergleich von maximalen Temperaturen errechnet aus Rutilen mafischer und metapelitischer Herkunft, daß mafische Rutile zur Thermometrie genutzt werden können, solange die Si-Aktivität durch eine Silikatphase auf einen Wert nahe 1 gepuffert wird.Rutiltemperatur-Häufigkeitspopulationen aus Zr-in-Rutil-Thermometrieergebnissen, verglichen mit Nb- und Cr-Gehalten, z.B. mit Hilfe des log(Cr/Nb)-Verhältnisses, sind eine Hilfe bei der Klassifizierung und Charakterisierung von Rutilen aus verschiedenen Provenienz-Milieus. Temperatur-Häufigkeitsverteilungen equilibrierter Rutilproben sind durch 2-sigma-Standardabweichungen von kleiner als 120°C charakterisiert und teilen sich in zwei Populationen auf. Von diesen zwei Populationen ist die bei Höchst-Temperaturen unter mittel- bis hochgradig-metamorphen Bedingungen equilibrierte aus 60% der Rutile zusammengesetzt und besitzt somit den ausgeprägteren Peak.Rutile aus Erzgebirgsproben zeigen, dass unter regionalen metamorphen Bedingungen und Gradienten reliktische Rutile bis ca. 550°C überlebt haben. Unterhalb dieser Temperatur sind Zr-Gehalte im Rutil nicht vollständig im Gleichgewicht mit den herrschenden metamorphen Bedingungen. Im Gegensatz dazu zeigen U-Pb-Datierungen dieser Relikte, dass die Alter zum größten Teil schon unter niedrig-gradiger Metamorphose während der Variszischen Orogenese umgestellt wurden. Daraus läßt sich eine niedrige Schließtemperatur für Pb-in-Rutil-Diffusion von ca. 400°C ableiten. Ein Vergleich mit U-Pb-Altern von Zirkonen unterstützt eine gemeinsame West-Afrikanische Provenienz von reliktischen Rutilen und Zirkonen.Quarzite haben sich im Vergleich mit Metapeliten als relativ inert in Bezug auf Zr-in-Rutil-Temperatur-Equilibrierung erwiesen. Dazu ist der Einfluß von Rutilen aus Quarziten auf das Sediment-Budget vergleichsweise groß, denn Quarzite liefern mehr Rutile im untersuchten Korngrößenbereich (80-200µm) als Metapelite bei gleichem anstehenden Volumen.V-, Cr-, Nb- und Fe-Gehalte variieren systematisch zwischen den TiO2-Polymorphen Rutil, Anatas und Brookit. Eine Lithologieklassifizierung anhand der Cr- und Nb- Gehalte ist auf Anatas genausowenig anwendbar wie Zr-in-Rutil-Thermometrie. Aus diesem Grund ist eine Identifizierung der jeweils untersuchten Kristalle unumgänglich. Neben der Identifikation durch Raman-Spektroskopie bietet eine Unterscheidung aufgrund von Spurenelementgehalten eine verläßliche und einfache Methode.Das Vorkommen von Anatas ist weiter verbreitet als bisher angenommen und kann nicht vorhergesagt werden. Es wird sowohl von metamorphen Bedingungen wie Temperatur und Druck beeinflußt, als auch von der Gesamtgesteinszusammensetzung. In der initialen Phase der Rekristallisation von Rutil zu Anatas, die im Erzgebirge in Form von Anatas-Rutil-Verwachsungen auftritt, sind Anatas-Bereiche in diesen Verwachsungen von der Spurenelementzusammensetzung her ähnlich wie Rutil und nicht wie Anatas. Vermutlich ist eine Equilibrierung von Spurenelementgehalten in Anatas, z.B. auf niedrige Cr-, V- und Zr-Gehalte, erst bei räumlicher Separierung von Rutil und Anatas möglich.Samples investigated in this study derive from a large range of metamorphic conditions (from diagenetic up to 950°C and 4.5GPa) and geological settings. Main sampling localities were the Variscan Erzgebirge (Germany) and the Alps (Switzerland and Italy). Other samples derive from different locations in Europe, North- and South America, and Africa. Sand samples from ca. 100m2 to ca. 2300km2 catchment areas are compared to associated rock samples.Rutile, anatase and brookite crystals with grain sizes of 63µm to cm-scale are analysed by Electron Microprobe (EMP), Secondary Ion Mass Spectrometry (SIMS), Raman Spectroscopy, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) for trace elements, isotopes and phase composition.The results show that for rutile grains of 80-250µm, no interdependance exists between grain size and chemical composition in Erzgebirge samples. Similarly, no interdependance is observed between magnetic susceptibility and rutile composition.In comparison with LA-ICPMS, SIMS analysis has turned out to deliver more accurate results for trace elements in the ppm range in small embedded crystals as used in this study. Furthermore, since SIMS relative sensitivity factors have been calibrated on EMP analyses, SIMS and EMP results are well comparable.This study confirms that detrital rutile is an accurate tracer of source rock characteristics in sedimentary systems. Discrimination of rutiles from mafic and metapelitic source rocks can be done according to the equation x = 5 * (Nb[ppm] - 500) - Cr[ppm] (ppm = ppm by mass), where rutiles from mafic host rocks will have negative values of x, whereas rutiles from metapelitic host rocks will have positive values of x.Although the precondition for the application of Zr-in-rutile thermometry is the coexistence of rutile with quartz (or SiO2) and zircon during growth, comparison between maxiumum temperatures derived from rutiles of mafic and metapelitic host rock composition supports that mafic rutiles can be used for thermometry as well, at least as long as silica activity is buffered to a value close to unity by any silica phase.Rutile-temperature frequency populations derived from Zr-in-rutile thermometry and compared to Nb and Cr compositions, e.g., with the aid of log(Cr/Nb) values, help classifying and characterising rutiles from different provenance settings. Temperature frequency distributions of equilibrated rutile samples are characterised by 2-sigma standard deviations of less than 120°C and divide into two temperature populations. The metamorphic peak-temperature population under medium- and high-grade metamorphic conditions is made up of >\,60\,\% of rutiles and exhibits the more pronounced peak of the two.Rutiles from Erzgebirge samples reveal that under regional Variscan metamorphic conditions and gradients, relict rutiles survived up to ca. 550°C. Below this temperature, Zr contents in rutiles are not completely equilibrated at metamorphic conditions. In contrast, U-Pb dating of these relicts shows that ages were for the most part already reset during low-grade metamorphism of the Variscan orogeny, implying a low closure temperature for Pb diffusion in rutile of ca. 400\textdegree C. Comparison with U-Pb ages of zircons supports a common provenance of relict rutiles and zircons from the West African craton in Ordovician time.Quartzites have turned out to be relatively unreactive with respect to Zr-in-rutile temperature equilibration in comparison with metapelites. Furthermore, the impact of rutiles from quartzites on the sediment budget is large compared to rutiles from metapelites, because at similar exposed volume, quartzites deliver larger quantities of rutiles in the investigated grain sizes (80-200µm).V, Cr, Nb and Fe contents vary systematically between the TiO2 polymorphs rutile, anatase and brookite. Both, lithology discrimination based on Cr and Nb contents and Zr-in-rutile thermometry, cannot be applied to anatase. For this reason, indentification of the analysed polymorphs is inevitable in rutile studies. Besides Raman spectroscopy, phase specification by trace element contents has proved to be a reliable and easy method.The occurrence of anatase is more widespread than previously thought and cannot be predicted. It is influenced by metamorphic pressure and temperature, and as well by whole rock chemistry. In the initial stages of rutile recrystallising as anatase (anatase-rutile intergrowths in the Erzgebirge), anatase regions in these intergrowths are chemically similar to rutile and not to anatase. Supposedly, an equilibration of trace elements in anatase towards, e.g., low Cr-, V-, and Zr concentrations, is obtained only when anatase is spacially separated from rutile
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
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