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A New Model to Estimate Deep-level Magma Ascent Rates, with Applications to Mt. Etna (Sicily, Italy)
No full textMagma ascent velocities, v (dH/dt; where H is depth and t is time), can be determined from decompression rates (dP/dt), and rates of cooling (dT/dt): Graphic, where ρ is magma density, P is pressure, T is temperature and g is the acceleration due to gravity. This equation for v provides a key to investigating the relationships between the initial ascent velocity of magmas and the depths of magma dehydration. Ascent velocities can be calculated using pressure and temperature (P–T) estimates from mineral–liquid thermobarometry and cooling rates inferred from crystal size distribution (CSD) theory. For recent Mt. Etna lava flows, both dP/dT and dT/dt have been characterized for the portion of the feeding system between the Moho (∼27 km) and 6 km based, respectively, on clinopyroxene thermobarometry and clinopyroxene CSDs. Deep-level (>6 km) magma ascent velocities range from practically zero (where clinopyroxene P–T estimates form a cluster, and so dP/dT ≈ 0), to about 10 m h–1 for flows that yield very steep P–T trajectories. Many lava flows at Mt. Etna yield P–T paths that follow a hydrous (∼3% water) clinopyroxene saturation surface, which closely approximates the water content inferred from melt inclusions. Independent assessments of deep-level water contents have been obtained by means of a new geohygrometer and yield ascent rates of ∼1 m h–1, in agreement with the slowest rates derived for magma effusion or vapor-driven ascent (∼0·001 to >0·2 m s–1, or 3·6–720 m h–1). Changes in P–T slope, as determined by pyroxene thermobarometry, indicate an upward acceleration of magma, which may be due to the onset of deep-level magma dehydration linked to the non-ideal behavior of water and CO2 mixtures that induce a deep-level maximum of water loss at P ≈ 0·4 MPa and T ≈ 1200°C for a CO2 content >1000 ppm
A new model for estimating deep-level magma ascent rates from thermobarometry: an example From Mt. Etna and implications for deep-seated magma dehydration
No full textWe propose a method to estimate deep-level magma ascent rates, and potentially, the depths of initial magma dehydration, using pressure
and temperature (P-T) estimates from mineral-liquid thermobarometers, and cooling rates inferred from Crystal Size Distribution
theory. Ascent rates can be ascertained by recognizing that the slope of a given P-T path (dP/dT), rates of cooling (dT/dt), and
magma ascent rates (dP/dt) are interrelated, and ascent velocity, v, is given as: v = 1 ⎛dP⎞ ⎛dT⎞, where Ú is magma density and
Ú g ⎝dT⎠ ⎝dt ⎠
g is the acceleration due to gravity. Preliminary applications of this method are provided for Mt. Etna lava flows, where both dP/dT
and dT/dt have been well characterized based on, respectively, clinopyroxene thermobarometry, and clinopyroxene CSDs (the latter
yields dT/dt = 2×10-6). Deep-level (>20 km) magma ascent rates range from effectively 0 (where clinopyroxene P-T estimates form a
cluster, and so dP/dT ≈ 0), to about 10 m/hr for flows that yield very steep P-T trajectories. Many lava flows at Mt. Etna yield P-T paths
that follow a hydrous (3% water) clinopyroxene saturation surface, as calculated by pMELTS (Ghiorso et alii 2002), which closely approximates
water contents obtained from melt inclusions; these slopes yield ascent rates of ~1 m/hr, and are comparable to the very
slowest rates derived for magma effusion or vapor-driven ascent (~0.001 to > 0.2 m/s, or 3.6 to 720 m/hr). The initiation of such upward
movements, however slow, may be key to understanding eruption triggering mechanisms. At Mt. Etna, certain flows exhibit two
kinds of clinopyroxene crystallization behavior: from a single flow, those clinopyroxenes that form at the greatest depths either follow
a clinopyroxene saturation surface (as calculated from pMELTS) or cluster along such a curve, but clinopyroxenes from these
same flows that yield more shallow depth estimates fall on near-vertical P-T paths. Such changes in slope appear to indicate an acceleration
of upward magma transport, which may be due to the initiation of deep-level magma dehydration
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 textPlagioclase???melt (dis)equilibrium due to cooling dynamics: Implications forthermometry, barometry and hygrometry
No full textThe compositional variation of plagioclase and the partitioning of major elements between plagioclase and melt have been experimentally measured as a function of the cooling rate. Crystals were grown from a basaltic melt at a pressure of 500 MPa under (i) variable cooling rates of 0.5, 2.1, 3, 9.4, and 15 °C/min from 1250 °C down to 1000 °C, (ii) quenching temperatures of 1025, 1050, 1075, 1090, and 1100 °C at the fixed cooling rate of 0.5 °C/min, and (iii) isothermal temperatures of 1000, 1025, 1050, 1075, 1090, and 1100 °C. Our results show that euhedral, faceted plagioclases form during isothermal and slower cooling experiments exhibiting idiomorphic tabular shapes. In contrast, dendritic shapes are observed from faster cooled charges. As the cooling rate is increased, concentrations of Al + Ca + Fe + Mg increase and Si + Na + K decrease in plagioclase favoring higher An and lower Ab + Or contents. Significant variations of pl???liqKd are also observed by the comparison between isothermal and cooled charges; notably, pl???liqKdAb???An, pl???liqKdCa???Na and pl???liqKdFe???Mg progressively change with increasing cooling rate. Therefore, crystal???melt exchange reactions have the potential to reveal the departure from equilibrium for plagioclase-bearing cooling magmas. Finally, thermometers, barometers, and hygrometers derived through the plagioclase???liquid equilibria have been tested at these non-equilibrium experimental conditions. Since such models are based on assumption of equilibrium, any form of disequilibrium will yield errors. Results show that errors on estimates of temperature, pressure, and melt-water content increase systematically with increasing cooling rate (i.e. disequilibrium condition) depicting monotonic trends towards drastic overestimates. These trends are perfectly correlated with those of pl???liqKdCa???Na, pl???liqKdAb???An, and pl???liqKdFe???Mg, thus demonstrating their ability to test (dis)equilibrium conditions
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
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