1,721,021 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
High-speed imaging of Strombolian explosions: The ejection velocity of pyroclasts
Full text linkExplosive volcanic eruptions are defined as the violent ejection of gas and hot fragments from a vent in the Earth's crust. Knowledge of ejection velocity is crucial for understanding and modeling relevant physical processes of an eruption, and yet direct measurements are still a difficult task with largely variable results. Here we apply pioneering high-speed imaging to measure the ejection velocity of pyroclasts from Strombolian explosive eruptions with an unparalleled temporal resolution. Measured supersonic velocities, up to 405 m/s, are twice higher than previously reported for such eruptions. Individual Strombolian explosions include multiple, sub-second-lasting ejection pulses characterized by an exponential decay of velocity. When fitted with an empirical model from shock-tube experiments literature, this decay allows constraining the length of the pressurized gas pockets responsible for the ejection pulses. These results directly impact eruption modeling and related hazard assessment, as well as the interpretation of geophysical signals from monitoring networks. Citation: Taddeucci, J., P. Scarlato, A. Capponi, E. Del Bello, C. Cimarelli, D. M. Palladino, and U. Kueppers (2012), High-speed imaging of Strombolian explosions: The ejection velocity of pyroclasts, Geophys. Res. Lett., 39, L02301, doi:10.1029/2011GL050404
Water diffusion in natural potassic melt. In Volcanic Degassing: experiments, models, observations and impacts
No full textWater diffusion experiments were performed on a trachytic melt from the Agnano-Monte Spina explosive eruption (Phlegrean Fields, South Italy). Experiments were run in a piston cylinder apparatus at 1 GPa pressure, at temperatures from 1373 to 1673 K and for durations of 0 to 255 s, using the diffusion-couple technique. Water concentration profiles were measured by Fourier transform infrared spectrometry. Water diffusion coefficients at different temperatures and water concentrations were calculated from the total water profiles, using the Boltzmann-Matano technique.
Over the investigated range of temperatures and water concentrations, the diffusivity of water in potassic melts (Dwater), m2/s can be described by Arrhenius equations that can be generalized for water concentrations between 0.25 and 2 wt% as follows: Formula where CH2O is the water concentration in wt%, R is 8.3145 (J K−1 mol.−1) and T is the temperature in Kelvin. Water diffusivities in trachytic melts were compared with water diffusivities in rhyolitic and basaltic melts. The activation energies for water diffusivity in trachyte and basalt are comparable, and higher than the haplogranitic melt. This results in a convergence of water diffusion coefficients in all melts at lower (magmatic) temperatures
High-pressure and high-temperature measurements of electrical conductivity in basaltic rocks from Mount Etna, Sicily, Italy
No full text[1] We have investigated the electrical properties of Etnean rocks by in situ complex impedance spectroscopy using a multianvil apparatus. From these measurements we determined the electrical conductivity of a lava flow sample, whose basaltic composition can be considered close to that of the parent magma and also that of a mafic nodule representative of the high-density cumulates interpreted as responsible for the main high-velocity anomaly observed beneath the volcano. The electrical conductivities of the two samples were measured at pressures of 0.9 and 1.5 GPa and temperatures from 400 to 1100degreesC at frequencies from 0.1 to 10(5) Hz. To investigate the electrical properties of the Etnean products as a function of partial melting, a few experiments were performed in a piston-cylinder apparatus prior to the electrical measurements. The obtained data were approximated using an equivalent circuit fitting technique. For the lava flow sample, electrical conductivity displays Arrhenian behavior over the entire investigated temperature range, with an activation energy of similar to0.8 eV Within the uncertainties of our measurements, we do not observe any effect of pressure on conductivity between 0.9 and 1.5 GPa. On the contrary, experiments performed on the series of partially molten samples indicate that conductivity increases with increasing quantity of glass. While conductivities of samples with minor amounts of glass are comparable to that of the lava flow starting material, with increased melting, conductivity can increase by as much as a factor of 3. The mafic nodule was observed to have a conductivity higher than the lava flow sample (e.g., at 800degreesC and 0.9 GPa, a factor of 4 higher). However, scatter in the data is somewhat higher in comparison to the lava flow sample, most likely because of the coarse texture of the samples and a consequence to their chemical and structural heterogeneity. Using our results, we illustrate the effects that a layer of hot magma surrounded by a cooler wall rock has on apparent resistivities determined by one-dimensional forward calculations. Our modeling demonstrates that both size and depth of magmatic intrusions strongly influence apparent resistivity and that these parameters can be extracted from field data if the electrical properties of the rocks below the surface are well understood
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
High porphyritic calcalkaline basalts from the Cenozoic Capo Marargiu Volcanic District (Sardinia, Italy)
No full textDispelling 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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