1,721,053 research outputs found
Sulfur, noble gases, and halogens: solubility relations of the less abundant volatile species in magmas
No full textCrystallization of peralkaline rhyolitic magmas: Rheological implications for the Pantelleria system
No full textCrystallization of peralkaline rhyolitic magmas: Pre- And syn-eruptive conditions of the Pantelleria system
Full text linkPantelleritic magmas are low-viscosity peralkaline rhyolites which exhibit large differences in eruptive style (explosive to effusive). The processes that promote fragmentation and explosive eruptions of pantelleritic magma remain subject to debate, but undoubtedly variations of magma viscosity during magma ascent and degassing contribute to differences in eruptive style. Because crystallization can significantly influence magma rheology, we present a review of equilibrium and disequilibrium crystallization experiments of pantellerites, focusing on the crystallization of the main phases, alkali feldspar, and (lesser) clinopyroxene. Our analysis of data for several explosive pantelleritic eruptions on Pantelleria suggests pre-eruptive pressures of 50-100 MPa, temperatures of 700-800 °C for water-saturated conditions. Given these conditions, we show that the low pre-eruptive crystal fractions (0.08 to 0.15), temperatures between 700 and 800 °C, and the decrease of melt H2O content during magma ascent/decompression can promote a significant change in viscosity (up to 106-107 Pa·s), leading to magma brittle fragmentation and explosive eruptions. Because of their typical range of viscosity, pantelleritic magmas may show greater variations in eruptive style due to differences in ascent (decompression) rate when compared with metaluminous rhyolites
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
The effect of iron and alkali on the nanocrystal-free viscosity of volcanic melts: A combined Raman spectroscopy and DSC study
No full textThe iron coordination, its oxidation state (Fe2+/Fetot.), and alkali ratio [Na/(Na + K)] greatly influence the
structure and thus the viscosity of volcanic melts, which is known to play a key role in the dynamics of volcanic
eruptions. Furthermore, it has been recently reported that volcanic melts can contain iron-bearing nanocrystals
and this makes it difficult to isolate and quantify the chemical contribution to the viscosity of magmas.
Here, we present Raman spectroscopic and differential scanning calorimetry (DSC) data on nanocrystal-free
peralkaline rhyolites with different Fe2+/Fetot. (0.15–0.84) and Na/(Na + K) (0–1) molar ratios. Raman
spectra are used to infer the structural changes occurring with varying iron oxidation state and alkali content,
whereas the combination of Raman spectroscopy and DSC measurements allow the characterization of the
anhydrous nanocrystal-free viscosity as a function of temperature. Results suggest that at similar and high Fe2+/
Fetot. ratio the Raman spectral feature controlled by the iron coordination changes with Na/(Na + K). Conversely,
the change of alkali content at a fixed Fe2+/Fetot. ratio results in a variation of the spectral feature that reflects
the size distribution of rings of tetrahedra in the melt structure.
We further discuss the implications of our findings for magma transport and estimate that the viscosity of
anhydrous peralkaline rhyolites at the eruptive temperature of 750 ◦C can increase up to 3.5 log units when
Fe2+/Fetot. and Na/(Na + K) ratios decrease contemporaneously from 0.84 to 0.15 and from 1 to 0, respectively.
Finally, the comparison of our viscosity data with those from the literature suggests that the DSC-approach
presented and adopted in this study is independent on chemical composition and thus can be used also to
retrieve the effect of nanocrystals on the viscosity of volcanic melts. The results presented here have profound
implications for the modelling of magma viscosity
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
New IR spectroscopic data for determination of water abundances in hydrous pantelleritic glasses
No full textTo aid current work on the genesis of pantelleritic magmas, and the desire to use IR spectroscopy to measure water contents in natural (e.g., melt inclusions) and experimental glasses of pantelleritic composition, we have determined molar absorptivities for near-infrared (NIR) absorption bands related to molecular water (5200 cm-1) and OH groups (4500 cm-1) in synthetic hydrous pantelleritic glasses, with compositions similar to natural pantellerites from the Eburru complex of the Kenya Rift Valley. The experiments were conducted at P = 30 to 150 MPa and T = 850-900 °C using a synthetic pantelleritic starting composition with (wt%) SiO2 = 76.60, Al2O3 = 8.48, FeO∗ = 5.48, K2O = 3.68, Na2O = 4.72, and with molar ratio (Na+K)/Al = 1.38. The experiments were H2O undersaturated (~1.1 to 6.5 wt% H2O), and the run products were analyzed by Karl-Fischer Titration (KFT) for total dissolved H2O abundance. Different combinations of baseline types (GG or TT) and intensity measurements (peak height and peak area) were applied to measure both hydroxyl group (OH) and molecular water (H2O) in the experimental samples. For instance, evaluating the peak heights and using the TT baseline e4500 results to be equal to 0.98(4) (L mol-1cm-1) and e5200 to 1.92(2) (L mol-1 cm-1); these values differ by ~20 to 50% from published values for metaluminous rhyolitic compositions
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