1,720,988 research outputs found
Potassic glass and calcite carbonatite in lapilli from extrusive carbonatites at Rangwa Caldera Complex, Kenya
No full textThe ~16 Ma Rangwa Caldera Complex, part of the large Kisingiri nephelinite-carbonatite volcano, Homa Bay District, western Kenya (0º34'S; 34º09'E) contains carbonatitic lapilli and ash tuffs, agglomerate and tuffisite, and a number of intrusive calcite carbonatites. A detailed petrographic and electron microprobe study has been performed on 20 fresh samples from the collection at The Natural History Museum, London. Most of the juvenile lapilli and ash particles are either predominantly composed of devitrified silicate glass (now biotite/phlogopite but probably also originally potassic silicate) or calcite carbonatite, which suggests that two molten liquids were erupted simultaneously. Some 10 mm-diameter lapilli contain quench-textured calcite crystals set in devitrified glass. They are interpreted as having crystallized from a molten silicate-carbonate melt at, or very near, the surface. The extrusive carbonate is mostly composed of calcite, consistent with intrusive calcite compositions at Rangwa. Other key minerals are magnetite, two types of mica (magnesian-biotite phenocrysts and phlogopite xenocrysts) and fluorapatite. The pyroclastic rocks contain many calcite carbonatite clasts, and fragments of calcite, aegirine and diopside, fluorapatite, magnetite, plus some phlogopite, titanite, K-feldspar, fenite and glimmerite; ijolite lithics are rare. Thus, there is no evidence for a cognate nephelinitic (ijolitic) or melilitic magma nor evidence for a direct relationship with the nephelinites of the Kisingiri volcano. Two hypotheses are discussed. A rising silicate and K-rich carbonatite liquid may have evolved towards a carbonate-rich K-silicate liquid after crystallization of calcite, phlogopite, apatite and magnetite. Preservation of the potassic component may be rare, with a more usual scenario being that potassic component separates as fenitizing fluids. The alternative is that the silicate component is remobilized fenite, formed from country rock that was mobilized by supercritical K-rich, fenitizing fluids associated with the carbonatite. Both scenarios require generation of a K-rich carbonatite magma, probably from a carbonated phlogopite-rich metasomatized mantle
Is there a mantle plume below Italy?
No full textSome of the most diverse igneous rocks found on Earth occur along the length of Italy and in many of the islands in the southeastern Tyrrhenian Sea, all the result of Cenozoic magmatism. Magmas extremely rich in alkalis, particularly potassium, and many undersaturated with respect to silica, were erupted, as well as others of calc-alkalic affinity . Their origin has been the subject of heated debate, and there is still no general consensus about how they formed. Most attribute them to subduction-related processes (Beccaluva et al. 2004, for a review); others consider them to be the result of within-plate magmatism [e.g., Vollmer, 1976; Lavecchia and Stoppa, 1996]. Still others consider magmatism the result of a deep, mantle upwelling within a slab window coupled with mixing between isotopically different reservoirs [Gasperini et al., 2002]
Innovative methodological approach integrating SEM-EDS and TXRF microanalysis for characterization in materials science: A perspective from cultural heritage studies
No full textMaterial science is an interdisciplinary field that draws on elements of chemistry, physics, engineering and deals with designing, producing, and using a wide range of materials. The methodological approach of materials investigation is very significant in cultural heritage. The study of Cultural Heritage materials is useful for dating, unraveling production technologies, sources and trading, and their restoration and preservation. Archaeometry is the tool for material characterization, but it cannot always be applied in a non-destructive way: as a result, analytical techniques requiring minimum sampling are of great interest. For this, Total reflection X-ray fluorescence (TXRF) spectrometry is an effective technique, thanks to its microanalytical capabilities and the possibility of preserving the sample after its analysis, either for additional investigations or archiving purposes. It is very sensitive to period four transition elements but less effective with lighter ones. Thus, to gather the most comprehensive analysis of historic enamelled ceramics, Scanning Electron MicroscopyEnergy Dispersive Spectroscopy (SEM-EDS) and TXRF spectrometry were combined in a novel analytical approach. Soil standards and natural soil pigment samples were tested to validate the novel analytical approach. Na, Mg, Al, Si, K and Ca were determined with SEM-EDS analysis and used for TXRF quantification of heavier elements without adding any internal standard. The methodology to determine the total composition of artifacts, by integrating the concentrations of light elements in EDS with the data of the heavier elements obtained with TXRF, is here developed expressing elements as oxides recalculated to 100%. Recovery values for standards were found mostly within 20% of the certified values for MgO, Al2O3, SiO2, K2O, TiO2, Cr2O3, MnO, Fe2O3, NiO, CuO, PbO and SrO. The detection capabilities for major, minor and trace elements in soil pigments prove that this novel, practically non-destructive, analytical approach has a high potential for obtaining the elemental composition of Cultural Heritage materials with broad applications
New aqueous nanolime formulations for fully compatible consolidation treatments of historical mortars for hypogeum environment
No full textThe religious buildings characterizing by hypogea environments need of proper restoration interventions by using compatible materials. At now, organic and inorganic products proved to be inappropriate for this scope, in relation to the lack of chemical compatibility with the original substrate, to poor penetration depth and to the irreversibility of the treatments. Aim of this paper, is to present an innovative approach to safeguard the hypogea environment, by using new aqueous nanolime formulations, properly chemically tailored, for eco-friendly and green consolidation treatments. Phase composition, stability, and reactivity of the nanolime formulations were investigated, and chemical, petrographic and mechanical analyses of the mortar samples from the hypogeum were carried out. The treatments effectiveness was evaluated in terms of aesthetical features, superficial cohesion and mechanical improvement. The aqueous nanolime suspensions, pure and tailored, applied by airbrush, in-crease the superficial cohesion up to 95 % and improve the mechanical resistance of the surface and up to at least 1 cm in depth, without causing appreciable surface chromatic alterations
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
- …
