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Experimental study of crystallization kinetics and eruption dynamics in Campi Flegrei trachytic melts
No full textThe aims of this work are to estimate timescales of phlegraean eruptions and the eruptive dynamics of Campi Flegrei volcanoes through crystallization kinetic experiments. Cooling and decompression experiments have been carried out on trachytic composition melt in order to investigate crystallization kinetics of alkali feldspar and the implication for magma dynamics during the ascent towards the surface. Experiments have been conducted using Cold Seal Pressure Vessel apparatus, at pressure between 30 and 200 MPa, temperature between 750 and 860 °C, time between 7,200 and 57,600 seconds and fO2 NNO +0.8, thereby reproducing syn-eruptive conditions of the Campi Flegrei volcanoes. Alkali feldspar is the main phase present in this trachyte and its abundance can strongly vary with small changes in pressure, temperature and water content in the melt, implying appreciable variations in the textures and in the eruptive style. Texture analysis allow us to quantify the real sizes and number of alkali feldspars for each sample and consequently to calculate nucleation density, volume fraction, nucleation and growth rate of alkali feldspar. Results obtained show that these variables are strictly related to degree of undercooling(DeltaT = Tliquidus - Texperimental), time, water content in the melt, viscosity and element diffusivity. DeltaT is the driving force for crystallization and it has a strong infuence on nucleation and growth processes. In fact, generally at small DeltaT growth process dominates crystallization, whereas at large DeltaT nucleation dominates crystallization. Time also is important variable during crystallization process, because long experiment durations involve more nucleation events of alkali feldspar than short experiment durations. This is an important aspect to understand magma evolution in the magma chamber and in the conduit and to estimate growth rate (YL) and nucleation rate (Im). YL and Im of alkali feldspar are significant variables to assess the timescales of phlegraean eruptions. In fact, the magma ascent time from the magma chamber to the surface was constrained for phlegraean volcanoes, combining the growth rates (YL) estimated for alkali feldspars and the crystal size distribution (CSD) theory. The order of magnitude of alkali feldspar growth rates obtained in this study vary from 10-7 to 10-8 cm/s, implying changes in residence time into the conduit before the eruption from several hours to around two week, respectively. Water content in the melt is another important variable to study crystallization kinetics of alkali feldspar in trachytic melt. It can enhance or inhibit the nucleation and growth processes of alkali feldspar. Generally, the results obtained in this work show that low water content and high polymerization degree facilitate nucleation, whereas higher water content and lower polymerization degree enhance growth. This behavior is not appliable to all melts, indeed it is only relative to alkali feldspars and trachytic melts. It actually means that a mineral phase may have different crystallization kinetics as a function of the melt bulk composition. Partition coefficients are an important tool to understand magma evolution in a volcanic system. Therefore, in order to better understand major and trace elements partition coefficient of alkali feldspar during growth experiments, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) measurements have been carried out. Barium and strontium are strongly compatible elements in alkali feldspar and their partition coefficients are influenced by potassium content, because Ba2+ and Sr2+ can enter in the K+ site. Furthermore, combining textural data and trace elements analysis obtained in this work and other studies, it is possible to conclude that crystallization processes strongly depend on starting melt composition
Crystallization kinetics of alkali feldspars in cooling and decompression-induced crystallization experiments in trachytic melt
No full textAbstract Cooling and decompression experiments have been carried out on trachytic melts
in order to investigate crystallization kinetics of alkali feldspar, the effect of the degree of
undercooling (ΔT= T liquidus− T experimental) and time on nucleation and crystal growth
process. This experimental work gives us new data about crystallization kinetics of trachytic
melts, and it that will be useful to better understand the natural system of Campi Flegrei
volcanoes. Experiments have been conducted using cold seal pressure vessel apparatus, ..
Natural and laboratory compaction band in porous carbonates: a 3D characterization using synchrotron X-ray microtomography
No full textPorous carbonates form important reservoirs for water and hydrocarbons. Post-depositional processes (e.g.
mechanical) are important to quantify because they may affect the fluid flow properties of reservoirs. Field-based
studies (Tondi et al., 2006; Rustichelli et al., 2012) described bed-parallel compaction bands (CBs) within carbonates
with a wide range of porosities. These CBs are burial-related structures, which accommodate volumetric strain by grain
rotation, grain translation, pore collapse and pressure solution. Cilona et al. (2012) performed triaxial compression
experiments, under dry conditions on the porous cretaceous grainstones (the Orfento Formation, in Majella Mountain,
Abruzzi), reproducing for the first time CBs in laboratory. In this work, the authors defined the pressure conditions at
which natural CBs form and documented the role of Hertzian cracks for grain size and porosity reduction within the
CBs.
Here we use a new methodology to characterize the pore networks of natural and laboratory CBs and compare them
with the host rock one.
Data were collected using the synchrotron X-ray microtomography technique at the SYRMEP beamline of the
Elettra-Sincrotrone Trieste Laboratory (Basovizza (Trieste), Italy). Quantitative analyses of the samples were carried
out using the Pore3D software library (Brun et al., 2010). The porosity was calculated from segmented 3D images of
deformed and pristine rocks. The process of skeletonization, which provides the number of connected pores within a
rock volume, was applied. By analyzing the skeletons we were able to highlight the differences between natural and
laboratory CBs, and to investigate how pore connectivity evolves as a function of the deformation.
Preliminary results show that within compaction bands both pore volume and connectivity are reduced in
comparison with the undeformed host rock. Natural CB has a lower porosity with respect to the laboratory one. In
natural CBs, the contact among granules seem be welded, whereas in the laboratory CBs it shows pores with irregular
shape
Spherulites growth in trachytic melts: a textural quantitative study from synchrotron X-ray microtomography and SEM data
Full text linkThis study shows the first textural data on synthetic alkali-feldspar spherulites grown in trachytic melts during
cooling and decompression experiments with water-saturated conditions. Previous textural studies have shown
the shape evolution and the growth process of spherulites as a function of undercooling (deltaT) and water content,
although just in basaltic and rhyolitic melts [1-3]. Spherulites are spherical clusters of polycrystalline aggregates
that occur commonly in rhyolitic melts under highly non-equilibrium conditions [3-4].
Cooling and decompression experiments have been carried out on trachytic melts in order to investigate crys-
tallization kinetics of alkali feldspars and the implications for magma dynamics during the ascent towards the
surface. Experiments have been conducted using cold seal pressure vessel apparatus at pressure range of 30 - 200
MPa, temperature of 750 - 850 degrees C and time of 2 - 16 hours, thereby reproducing pre- and syn-eruptive conditions of the Campi Flegrei volcanoes.
This study presents quantitative data on spherulite morphologies obtained both by scanning electron microscopy
(SEM) and synchrotron X-ray microtomography. Size, aspect ratio, number and crystallographic misorientation of
alkali feldspar crystals will be measured. Furthermore, experiments performed at different durations could allow
us to follow the growth and the evolution of spherulites.
The shape of spherulites changes as a function of delta T and experimental durations. Two kind of spherulites occured during experiments: open spherulites and close spherulites. The open spherulites are characterized by an structure with large (generally rectangular prismatic), widely spaced fibers with main axis converging towards a central
nucleus, in agreement with previous observations [5-6]. Instead, the close spherulites consist of acicular and tiny
fibers radially aggregated around a nucleus and single crystals are hardly distinguishable.
First preliminary results show:
a) spherulites grow between 70-200 MPa, thus the nucleation process was favored at higher water contents;
b) open spherulites seem to be favored at low deltaT, whereas close spherulites were favored in experiments at higher
delta T and long durations;
c) estimated growth rates of spherulites were of 10-7 cm/s
The Grizzly Lake complex (Yellowstone Volcano, USA): Mixing between basalt and rhyolite unraveled by microanalysis and X-ray microtomography
No full textMagma mixing is a widespread petrogenetic process. It has long been suspected to operate in concert with fractional crystallization and assimilation to produce chemical and temperature gradients in magmas. In particular, the injection of mafic magmas into felsic magma chambers is widely regarded as a key driver in the sudden triggering of what often become highly explosive volcanic eruptions. Understanding the mechanistic event chain leading to such hazardous events is a scientific goal of high priority. Here we investigate a mingling event via the evidence preserved in mingled lavas using a combination of X-ray computed microtomographic and electron microprobe analyses, to unravel the complex textures and attendant chemical heterogeneities of the mixed basaltic and rhyolitic eruption of Grizzly Lake in the Norris-Mammoth corridor of the Yellowstone Plateau volcanic field (YVF). We observe evidence that both magmatic viscous inter-fingering of magmas and disequilibrium crystallization/dissolution processes occur. Furthermore, these processes constrain the timescale of interaction between the two magmatic components prior to their eruption. X-ray microtomography images show variegated textural features, involving vesicle and crystal distributions, filament morphology, the distribution of enclaves, and further textural features otherwise obscured in conventional 2D observations and analyses. Although our central effort was applied to the determination of mixing end members, analysis of the hybrid portion has led to the discovery that mixing in the Grizzly Lake system was also characterized by the disintegration and dissolution of mafic crystals in the rhyolitic magma. The presence of mineral phases in both end member, for example, forsteritic olivine, sanidine, and quartz and their transport throughout the magmatic mass, by a combination of both mixing dynamics and flow imposed by ascent of the magmatic mass and its eruption, might have acted as a “geometric perturbation” of flow fields further fuelling mass exchange between magmas in terms of both chemical diffusion and crystal transfer. These results illuminate the complexity of mixing in natural magmatic systems, identifying several reaction-related textural factors that must be understood more deeply in order to advance our understanding of this igneous process
SO<sub>2</sub> emissions, plume heights and magmatic processes inferred from satellite data: The 2015 Calbuco eruptions
Full text linkQuantifying time-series of sulfur dioxide (SO2) emissions during explosive eruptions provides insight into volcanic processes, assists in volcanic hazard mitigation, and permits quantification of the climatic impact of major eruptions. While volcanic SO2 is routinely detected from space during eruptions, the retrieval of plume injection height and SO2 flux time-series remains challenging. Here we present a new numerical method based on forward- and backward-trajectory analysis which enable such time-series to be determined.Using this method applied to GOME-2 satellite imagery we investigate the SO2 emissions from two sub-Plinian eruptions of Calbuco, Chile, produced in April 2015. Our results show a mean injection height of 15 km for the two eruptions, with overshooting tops reaching 20 km. We calculate a total of 0.295 ± 0.045 Tg of SO2 emitted, with 0.140 ± 0.033 Tg produced by the first eruption and 0.155 ± 0.031 Tg by the second one. Using standard models we convert plume heights to mass eruption rates (MER). Comparing gas flux and MER we discover quite different volcanic processes driving the two eruptions, with the first eruption producing an SO2 flux three times higher than the second one, while they both had similar MERs. We propose that this difference reflects different exsolved volatile contents before the onset of the two eruptions, with the first eruption richer in pre-exsolved gas than the second one. This hypothesis is supported by melt inclusion measurements of sulfur concentrations in plagioclase phenocrysts and groundmass glass of tephra samples through electron microprobe analysis. Combining the satellite and petrological analysis, we propose that the overpressure caused by the pre-exsolved volatile phase (not only SO2, but also probably H2O and CO2) may have triggered the eruption.These results demonstrate that our new methodology produces constraints on SO2 flux and plume height time-series permitting new insights into sub-surface processes using satellite SO2 data
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
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