1,720,983 research outputs found
Petrology of the micaceous Koidu and Tongo - Tonguma kimberlites, Man Craton, Sierra Leone
Get PDFSierra Leone contains two Jurassic-aged diamondiferous volcanic clusters, namely Koidu and Tongo-Tonguma (hereafter referred to as Tongo), consisting of eruptive pipes at Koidu and NESW trending dikes at both Koidu and Tongo. Petrographically, the diamondiferous rocks from both clusters have features of both micaceous kimberlites and unevolved Kaapvaal lamproite, making classification ambiguous. To successfully classify and constrain the petrogenesis of these rocks, a combination of detailed petrography, and phlogopite, spinel and olivine chemistry is used in this study. The Koidu rocks are predominantly macrocrystic in texture, whereas the Tongo rocks can broadly be separated into macrocrystic and aphanitic varieties. Olivine is present as macrocrysts and microcrysts, while phlogopite is occasionally present as macrocrysts and abundantly present as groundmass microcrysts (33 vol.% average). Other groundmass minerals include spinel, perovskite, apatite, and calcite set in a base of serpentine possibly containing secondary carbonate. These characteristic features can be present in both micaceous kimberlites and unevolved Kaapvaal lamproites, making classification based on petrography ambiguous. Phlogopite and spinel have similar compositional trends for both Koidu and Tongo, comparable to trends observed for archetypal kimberlites rather than lamproites. This in combination with previously studied trace element geochemistry and isotopic compositions from rocks within the Tongo cluster, indicate characteristics of an archetypal kimberlite. The overall phlogopite abundances further indicate that these rocks are micaceous kimberlites. Olivine core compositions are divided into Mg-rich (Fo >89) and Fe-rich (Fo < 89) endmembers, with Koidu and Tongo cores being dominantly Mg-rich with fewer Fe-rich cores. The overall olivine core compositions range between Fo 84.7 to 94.5 for Koidu and between 83.2 to 94.6 for Tongo. The Mg-rich olivine cores often contain intensely resorbed margins and no groundmass inclusions, consistent with a xenocrystic origin, whereas the Fe-rich population have resorbed margins to a lesser extent and are interpreted to be derived from a Cr-poor megacryst suite. Olivine rim compositions have Fo contents which are relatively homogenous, with compositional overlaps observed between Tongo and Koidu. Olivine rims often contain mineral inclusions of groundmass spinels and are interpreted as having crystallized from a magma. Previous models have suggested that olivine chemistry and groundmass mineralogy in worldwide diamondiferous rocks can be used to constrain their origin and evolution. However, the Koidu-Tongo rocks are the first example where multiple kimberlite clusters do not align with these models. The Koidu-Tongo rocks are highly micaceous and contain Mg-rich olivine core-rim compositions and correspond with Kaapvaal lamproites and so an alternative model has been suggested where the rocks are likely derived from the asthenosphere, like a typical kimberlite, but assimilated phlogopite-rich material in the SCLM
A multi-isotopic geochemical investigation of the Lower Zone, Bushveld Complex, South Africa: implications for a crustal component for parental magmas
Get PDFThe current study focuses on the Lower Zone of the Bushveld Complex, South Africa using multiple geochemical and isotopic systems to determine the origin of crustal signatures, i.e. crustal assimilation or recycled crust in the mantle source, present throughout the Rustenburg Layered Suite (RLS) such as elevated 18O values. These geochemical and isotopic systems include major elements, trace elements, highly siderophile elements, oxygen isotopes, 87Sr/86Sr, 143Nd/144Nd, and Os-Os isotopes. Samples come from the Nooitgedacht Borehole 2 (NG2) at Union Section of the western limb of the Bushveld Complex, which sampled the Lower Zone. The 87Sr/86Sri (0.7043 – 0.7086) and Ndi values (-7.40 - -4.97) calculated in this study are in agreement with published data for the Bushveld. The majority of NG2 samples contain 18O greater than mantle peridotite (5.50‰) and MORB (5.70‰), in which NG2 18O ranges from 5.60 up to 8.00‰ for olivine, orthopyroxene, and clinopyroxene separates. These high 18O values suggests the Bushveld magmas sourced a crustal reservoir, either through crust assimilation or recycled crustal materials in the mantle source. This is the first study utilizing the Re-Os isotope system for the Lower Zone. The Osi values for the NG2 suite range from -4.37 to +35.9, which overlap with published data for the Critical Zone and the Platreef, the only previous Re-Os studies on the Bushveld. However, there are no previously reported negative Osi values for the Bushveld. The range in Osi values for the NG2 samples suggest mixing of at least two geochemical reservoirs. In addition to Lower Zone NG2 samples, sample NG2-773.65 is a chilled margin sample at the base of the NG2 borehole that contains high 18O ( 18O = 9.42 – 9.78‰) and radiogenic Osi (Osi = +62.5), in which the crust and/or recycled crust in the mantle source caused these high values. Sample NG2-734.64 contains the second lowest 18O ( 18O = 5.67‰) and most unradiogenic Osi (Osi = -4.37) values for the NG2 suite, evidence for a harzburgitic SCLM (H-SCLM) mantle source component. A lack of correlations for Osi values with 18O and 87Sr/86Sri values are indices for crustal assimilation processes. This lack in correlation may suggest a crustal component in the mantle is more likely than assimilation of the crust during ascent of the magmas toward the surface. The geochemical data presented in this study suggest the Lower Zone parent magma had a H-SCLM mantle source component in addition to the assimilation of the crust and/or the eclogitic SCLM (E-SCLM)
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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