1,720,961 research outputs found
Modeling clinopyroxene and plagioclase growth kinetics at Mt. Etna and Stromboli: a time-integrated, polybaric and polythermal perspective
Full text linkBasaltic volcanoes (e.g., Mt. Etna, Stromboli, Hawaii, etc.) are characterized by a range of effusive to explosive activities with variable intensity, which can pose different type of threats to local populations. Challenges in modern volcanology and petrology involve the attempt to constrain pre-eruptive magmatic processes, which provide the basis for volcanic hazard assessment. Although the recent literature has reported constant advancements in this respect, several key questions remain unanswered. Understanding how magma is stored, migrates and feeds eruption is not a trivial task, requiring for renewed improvements over the years. In this context, both textural maturation and compositional variability of minerals crystallizing in basaltic systems represent valuable sources of information to quantify the physio-chemical conditions experienced by magmas upon the effect of changing and complex plumbing system dynamics.
This study aims to provide new insights on the solidification behavior of mafic alkaline magmas erupted at Mt. Etna and Stromboli (Italy). Such open conduit volcanoes are characterized by the ubiquitous stability of clinopyroxene from mantle depths to shallow crustal levels. More evolved magmas are also saturated with plagioclase, especially at lower temperatures, melt-water contents, and pressures. Thus, clinopyroxene and plagioclase crystals represent powerful recorders of the intricate ascent dynamics explored by mafic alkaline magmas during their ascent paths towards the surface. By focusing on textural and chemical features of natural/synthetic clinopyroxene, plagioclase and coexisting glasses, I have provided new tools for interpreting polythermal-polybaric changes of magmas, together with several guidelines and a secure methodology to model pre- and syn-eruptive conditions. The temporal evolution of Etnean and Strombolian magmas has been also tracked via timescale modeling to better constrain the cooling-decompression paths of magmas rising and accelerating through the vertically extended, highly dynamic plumbing systems.
In the first part of this PhD thesis, I have experimentally explored the role of supersaturation and relaxation phenomena on clinopyroxene nucleation and growth processes, which affect the final crystal cargo of variably undercooled magmas. A certain degree of undercooling is pivotal to promote the growth and textural maturation of crystals. With increasing crystallization time, however, the crystal growth rate decreases as the system approaches to near-equilibrium conditions that minimize the effect of melt supersaturation. By quantifying the textural features of synthetic and natural crystals it has been possible to parameterize clinopyroxene growth kinetics under a broad range of isothermal-isobaric, decompression, and cooling conditions representative of crystallization scenarios typically encountered in open-conduit volcanoes. This model parameterization has been combined with the textural analysis of natural clinopyroxene crystals erupted during lava fountain events at Mt. Etna allowing to unlock timescale of growth for microphenocryst and microlite populations. The retrieved temporal information has been used to develop a new conceptual model for the timescale of magma dynamics recorded by the (dis)equilibrium textural evolution of clinopyroxene and for the rapid acceleration of magma ascending within the volcanic conduit, immediately before eruption at the vent.
A more comprehensive work, focusing on plagioclase textural and compositional features, characterized the second part of my PhD thesis with the aim to identify disparate aspects of plagioclase growth scenarios. Following the same approach discussed above, timescale of plagioclase crystallization from mafic alkaline magmas has been parameterized as a function of growth rate by integrating experimental (i.e., isothermal-isobaric, decompression, and cooling experiments) and natural textural data from literature. Timescales of eruptive processes at Mt. Etna and Stromboli volcanoes have been quantified by considering phenocryst/microphenocryst and microlite crystals growing during lava flow and explosive eruptions. Statistical methodologies have been employed to assess the correlation between plagioclase growth rate and other system parameters governing the crystallization process. Special attention has been paid to disambiguate the role of temperature and melt-H2O content on plagioclase chemical zoning patterns at Stromboli and Mt. Etna. By using plagioclase components and major cation substitution mechanisms, I have refined and readapted equilibrium, thermometric, and hygrometric models for future investigations
Parameterization of clinopyroxene growth kinetics via crystal size distribution (CSD) analysis. Insights into the temporal scales of magma dynamics at Mt. Etna volcano
No full textThere is increasing recognition that both textural and compositional changes of clinopyroxene crystallizing from mafic alkaline magmas are the direct expression of complex dynamic processes extending over a broad range of spatial and temporal scales. Among others, supersaturation and relaxation phenomena play a key role in controlling the final crystal cargo of variably undercooled magmas erupted from active alkaline volcanoes. Following this line of reasoning, we have carried out isothermal-isobaric, decompression, and cooling rate experiments on a basalt interpreted as the parental magma of mafic alkaline eruptions at Mt. Etna volcano (Sicily, Italy). The main purpose is to reconstruct and quantify the textural changes (i.e., length of major and minor axes, surface area per unit volume, area fraction, and maximum growth rate) of clinopyroxene at variable pressures (30-300 MPa), temperatures (1,050-1,100 °C), volatile contents (0-5 wt.% H2O and 0-0.2 wt.% CO2), and equilibration times (0.25-72 h). Melt supersaturation, corresponding to a degree of undercooling variable from 14 to 125 °C, drives the formation of clinopyroxene crystals with different textures and sizes as a function of growth rate and relaxation time. By integrating experimental data and thermodynamic modeling, the transition between interface-controlled (euhedral morphologies) and diffusion-controlled (anhedral morphologies) growth regimes has been determined to occur at degrees of undercooling higher than 30 °C. The decrease of clinopyroxene growth rate with increasing the equilibration time is combined with the crystal size distribution (CSD) analysis of naturally undercooled clinopyroxene crystals erupted during 2011-2012 lava fountain episodes at Mt. Etna volcano. Results indicate that the crystallization of microlites and microphenocrysts is on the order of ~100-101 min (large undercooling, short equilibration time) and ~101-102 h (small undercooling, long equilibration time), respectively. This temporal information allows to better constrain the cooling-decompression paths of Etnean magmas rising and accelerating along a vertically extended, highly dynamic plumbing system. While clinopyroxene microlites develop during the fast ascent of magmas (~100-101 m s-1) within the uppermost part of the conduit or immediately before ejection from the vent, the onset of microphenocryst crystallization occurs at depth and continues within the plumbing system during the slow ascent of magmas (~10-2 m s-1) that migrate through interconnected storage regions
Carrier and dilution effects of CO2 on thoron emissions from a zeolitized tuff exposed to subvolcanic temperatures
Full text linkRadon (222Rn) and thoron (220Rn) are two isotopes belonging to the noble gas radon (sensu lato) that is frequently employed for the geochemical surveillance of active volcanoes. Temperature gradients operating at subvolcanic conditions may induce chemical and structural modifications in rock-forming minerals and their related 222Rn-220Rn emissions. Additionally, CO2 fluxes may also contribute enormously to the transport of radionuclides through the microcracks and pores of subvolcanic rocks. In view of these articulated phenomena, we have experimentally quantified the changes of 220Rn signal caused by dehydration of a zeolitized tuff exposed to variable CO2 fluxes. Results indicate that, at low CO2 fluxes, water molecules and hydroxyl groups adsorbed on the glassy surface of macro- and micropores are physically removed by an intermolecular proton transfer mechanism, leading to an increase of the 220Rn signal. By contrast, at high CO2 fluxes, 220Rn emissions dramatically decrease because of the strong dilution capacity of CO2 that overprints the advective effect of carrier fluids. We conclude that the sign and magnitude of radon (sensu lato) changes observed in volcanic settings depend on the flux rate of carrier fluids and the rival effects between advective transport and radionuclide dilution
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
Micro-Raman water calibration in ultrapotassic silicate glasses:Application to phono-tephrites and K-foidites of Colli Albani Volcanic District (Central Italy)
No full textThis study investigates the potential use of micro-Raman spectroscopy for the quantification of water in ultrapotassic silicate glasses. A calibration was developed using experimental phono-tephritic glasses with water content ranging from ~ 1 to ~ 3 wt%. The calibration curve showed a typical direct proportionality between water content and the ratio of high- (3100–3750 cm−1) and low-wavenumber (100–1500 cm−1) spectral regions, with a linear fit coefficient m = 1.74. The comparison with the m coefficients available in literature for other silicate compositions showed a deviation of our composition as a function of some major oxides such as FeO, TiO2 and K2O, highlighting the possible influence of the polymerization degree (NBO/T: non-bridging oxygens per tetrahedron) on m coefficient. In this respect, we observed a linear relationship between m coefficient and NBO/T and a positive correlation between the area underneath the silicate region (100–1500 cm−1) and NBO/T for the phono-tephrite of this study and for other compositions spanning from basalts to phonolite and rhyolites available in literature. For ultrapotassic natural and experimental glasses characterized by the presence of CO2, documented by the carbonate peak at 1062–1092 cm−1, it has been possible to extrapolate the CO2 content by using the model of Morizet et al. (2013) obtaining values of ~ 1.1 ± 0.3 and ~ 1.7 ± 0.2 wt%, respectively. The obtained m coefficient was applied to estimate water content of natural phono-tephritic glasses belonging to the Colli Albani Volcanic District. Moreover, we estimated water content also for some natural K-foiditic glasses from the same volcanic district. Since the m coefficient results to be strongly dependent on the chemical composition of the sample of interest, the coefficient estimated for the phono-tephrites of this study could result in significant overestimation or underestimation of the water content of the Colli Albani Volcanic District K-foiditic natural samples. Thus, we extrapolated the m coefficient for the K-foiditic samples by means of an equation obtained in this study as function of the polymerization degree (NBO/T).</p
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
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