1,721,106 research outputs found
Dynamics of volcanic systems. Physical and chemical models applied to equilibrium versus disequilibrium solidification of magmas
No full textThe solidification of magmas can occur by cooling (ΔT/Δt) and/or degassing-induced decompression (ΔP/Δt), as a function of solidus, glass transition, and melting temperatures, respectively Ts, Tg, and Tm . These three parameters strongly depend on the bulk composition (X) of the system and vary with T, P, fO2, and H2 O. In recent decades, physical and chemical models of magmas have been profoundly refined, such as thermal, rheological and density behaviors, the formation conditions of mineral/melt phases, and volatile solubility. However, variations of physical and chemical features of magmas through time are still poorly constrained. This means that mechanisms binding the formation of different phases under equilibrium and disequilibrium conditions are still poorly quantified, although they are of paramount importance for reconstructing magmatic dynamics encrypted in volcanic rocks. Here, the most relevant solidification conditions of magmas leading to possible crystallization paths are tested in physical models. The textural and, especially, chemical attributes of minerals are reviewed in order to summarize the salient features able to discriminate between formation of equilibrium and disequilibrium phases. Finally, the reconstruction of magmatic intensive variables deduced from composition of minerals is discussed
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
A new test for equilibrium based on clinopyroxene–melt pairs: Clues on the solidification temperatures of Etnean alkaline melts at post-eruptive conditions
No full textInfluence of bitumen on the petrophysical properties of the Bolognano formation: a multidisciplinary approach applied to an area of the northern flank of Majella
No full textThe role of cooling rate in the origin of high temperature phases at the chilled margin of magmatic intrusions
No full textBoth large (i.e. from hundreds to thousands of metres thick) and small (i.e. from centimetres to a few metres
thick) magmatic intrusions are characterized by mineral compositional variations proceeding from the outermost
to the innermost part of the intrusive body. However, in the case of large intrusions, mineral compositions
become progressively more primitive (e.g. An-rich plagioclases and En-rich pyroxenes) from the
chilled margin towards the interior; whereas, the opposite occurs for small intrusive bodies.
Since it is unclear to what extent variable cooling rate conditions may alter the phase compositions, we have
performed isothermal and dynamic experiments within a temperature interval of 1250–1100 °C using four
different cooling rates of 150, 50, 10 and 2.5 °C/h. Numerical simulations of thermal regimes in and around
small and large magmatic intrusions have also been performed and compared with phase compositional variations
observed in our laboratory experiments.
Results indicate that, over rapid cooling rate conditions, the crystal compositions faithfully reproduce those of
high-temperature formations, i.e. An-rich plagioclases, En-rich pyroxenes and Usp-poor spinels. However,
such a process is limited to a maximum distance of 2–3 m from the margin of the intrusion. Moreover, in active
volcanic systems, heat fluxes are released from the main regions of magma storage into host rocks; therefore,
only magmas solidifying at the contact of cold wall rocks may develop chilled margins with features
related to rapid cooling rate conditions. In the presence of hot host rocks, thermal gradients are significantly
reduced and the role played by cooling dynamics on textural and compositional variations of minerals is
practically negligible
Unraveling the solidification path of a pahoehoe "cicirara" lava from Mount Etna volcano
No full textThe solidified surface of a lava flow reflects the viscosity of its molten fraction and the crystal content during flow; crystal-poor basaltic lavas produce pahoehoe fields, whereas crystal-rich ones solidify with aa carapaces. At Mount Etna volcano aa morphologies are very common, in turn, pahoehoe lavas are rare. The latter are locally named “cicirara” due to the presence of cm-sized plagioclase phenocrysts much more abundant than in aa lavas. The phenocryst content of “cicirara” lavas contrasts with the low viscosity generally associated with pahoehoe morphology. Therefore, to reconcile the discrepancy between textural and volcanic observations, we have studied the most primitive pahoehoe “cicirara” lava sampled until now. Two samples at 0.5 and 1 m from the bottom of the two-meter thick lava flow were investigated on the basis of their mineral compositional variations and textural features, i.e. size frequency and crystal size distribution (CSD). Results coupled with rheological models indicate that only large phenocrysts of plagioclase (>1 mm) and clinopyroxene have grown before eruption. Thermobarometric models and petrological computations based on the composition of plagioclase and clinopyroxene phenocryst cores highlight that only a small amount (10-15 vol.%) of crystals equilibrated at 12 km of depth. Cumulative size frequency and CSD data also indicate that plagioclase and clinopyroxene phenocryst rims grew heterogeneously and coalesced around their cores at depths <1 km, before eruption. In this view, the “cicirara” lava was erupted with a low crystalline content that favoured the formation of its pahoehoe surface; however, crystals with a size <1 mm (~75 vol.%) solidified at post-eruptive conditions.
Our findings underline that the emplacement of high-viscosity aa or low-viscosity pahoehoe lavas is driven by the degree of undercooling imposed by the volatile exsolution rate in the shallowest portion of the Etnean plumbing system. A slow magma ascent rate promotes significant intratelluric degassing and widespread nucleation; consequently, the viscosity of the suspension significantly increases leading to an aa morphology. In contrast, pahoehoe “cicirara” lavas are associated with a rapid rise to the surface of poorly degassed, undercooled magmas
Experimental solidification of anhydrous latitic and trachytic melts at different cooling rates: The role of nucleation kinetics
No full textTwo sets of cooling experiments were run at atmospheric conditions for two anhydrous starting latitic and trachytic melts: 1) five cooling rates (25, 12.5, 3, 0.5, and 0.125 °C/min) between 1300° and 800 °C, and 2) a 0.5 °C/min cooling rate from 1300 °C with quench temperatures at 1200°, 1100°, 1000° and 900 °C. Trachytic run-products are invariably glassy. Nucleation is also suppressed in the latitic run-products at the three highest
cooling rates. Conversely, in the 0.5 and 0.125 °C/min runs, latites have a crystal content of ∼90 vol.%. The
phases are: plagioclase, clinopyroxene, glass and iron-bearing oxide (in order of abundance). The variable
quench temperatures, investigated by coupling experiments with Pt wire and Pt capsule sample containers in set 2, again did not produce crystallization of trachyte, whereas latitic samples are characterized by ∼10 vol.% of oxides, pyroxenes and plagioclase (in order of appearance), at temperature b1000 °C. Effects of (preferential)
heterogeneous nucleation on sample holders, of superheating degree, and chemical species loss during cooling are absent for both melt compositions. The difference of solidification paths between these two silicate melts can be ascribed only to their small chemical differences. In comparison with calculated equilibrium conditions
all the experimental latitic and trachytic run-products revealed strong kinetic effects, interpretable in the light of the nucleation theory. The glass-forming ability (GFA) of trachyte is higher, whereas their critical cooling rate
(Rc) is lower (b0.125 °C/min), in comparison to latitic melts (RcN0.5 °C/min). The experimental results carried out in this study can be applied to lava flows and domes; trachytic lavas are able to flow for longer period with
respect to latitic ones in a metastable condition. Glass-rich terrestrial lavas, i.e. obsidians, can be the result of sluggish nucleation kinetics due to the relative high polymerisation of evolved silicate melts
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