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The effects of undercooling and deformation rates on the crystallization kinetics of Stromboli and Etna basalts
No full text""We have investigated the effect of undercooling. and deformation on the evolution of the texture and the. crystallization kinetics of remelted basaltic material from. Stromboli (pumice from the March 15, 2007 paroxysmal. eruption) and Etna (1992 lava flow). Isothermal crystallization. experiments were conducted at different degrees of. undercooling and different applied strain rate (T = 1,157–. 1,187 C and _ci = 4.26 s-1 for Stromboli; T = 1,131–. 1,182 C and _ci = 0.53 s-1 for Etna). Melt viscosity. increased due to the decrease in temperature and the increase. in crystal content. The mineralogical assemblage comprises. Sp ? Plg (dominant) ± Cpx with an overall crystal fraction. (\\\/) between 0.06 and 0.27, increasing with undercooling and. flow conditions. Both degree of undercooling and deformation. rate deeply affect the kinetics of the crystallization. process. Plagioclase nucleation incubation time strongly. decreases with increasing DT and flow, while slow diffusionlimited. growth characterizes low DT—low deformation rate. experiments. Both Stromboli (high strain rate) and Etna (low. strain rate) plagioclase growth rates (G) display relative. small variations with Stromboli showing higher values. (4.8 ± 1.9 9 10-9 m s-1) compared to Etna (2.1 ± 1.6 9. 10-9 m s-1). Plagioclase average nucleation rates J continuously. increase with undercooling from 1.4 9 106 to 6.7 9. 106 m-3 s-1 for Stromboli and from 3.6 9 104 to 4.0 9 106. m-3 s-1 for Etna. The extremely low value of 3.69104m-3. s-1 recorded at the lowest undercooling experiment for Etna. (DT = 20 C) indicates that the crystallization process is. growth-dominated and that possible effects of textural. coarsening occur. G values obtained in this paper are generally. one or two orders of magnitude higher compared to. those obtained in the literature for equivalent undercooling. conditions. Stirring of the melt, simulating magma flow or. convective conditions, facilitates nucleation and growth of. crystals via mechanical transportation of matter, resulting in. the higher J and G observed. Any modeling pertaining to. magma dynamics in the conduit (e.g., ascent rate) and lava. flow emplacement (e.g., flow rate, pa ̄hoehoe–‘a‘a ̄ transition). should therefore take the effects of dynamic crystallization. into account."
The effect of undercooling on the crystallization kinetics of Stromboli and Etna basalts at dynamic conditions
No full textCrystallization kinetics and rheology of leucite-bearing tephriphonolite magmas from the Colli Albani volcano (Italy)
No full textWe have investigated the rheology of liquid and crystal-bearing tephriphonolite magmas from the Colli Albani volcanic district. High (1124-1569°C) and low (690-800°C) temperature anhydrous liquid viscosities were determined by a combination of concentric cylinder (101.0 to 103.6Pas) and micropenetration (109.2 to 1012.1Pas) viscometry. Comparison with literature data reveals that at high temperatures, viscosity seems to be related to the melts degree of polymerization (NBO/T), while at low temperatures the dependency is not linear with values of viscosity higher than expected. Subliquidus isothermal crystallization experiments and viscosity determinations were carried out at high temperature (1150-1240°C) in air using a concentric cylinder apparatus at constant shear strain rate (γ ̇=0.1s-1). The overall crystal fraction varies between φ=0.06 at 1240°C (leucite) and φ=0.34 at 1150°C (leucite φ=0.32+plagioclase φ=0.02), with a direct linear increase of crystal content with decreasing temperature which parallels the viscosity increase. The inspection of products quenched at the end of the crystallization stage, defined when viscosity reaches a constant value, reveals strong evidence of leucite clustering. After the first segment of the experiment, performed at a constant shear rate, a second stage of experiments at variable shear rate was performed, comprised of an up-ramp (γ ̇=0.1s-1-0.9s-1) and a down-ramp (γ ̇=0.9s-1-0.1s-1) segment. At the end of the down-ramp, leucite crystals appear sub-spherical and unclustered. For the same applied shear rate, the viscosity values of the up-ramp are not recovered within the experimental time-scale, indicating strain and strain-rate dependent rheology for these suspensions. While the down-ramp viscosity data are shown to be in perfect agreement with literature models, discrepancies between the up-ramp data and pre-existing predicting models have been observed. We suggest that this complex behavior is related to the clustering of leucite crystals during the crystallization process, and their breakage during the higher shear rate viscosity measurements. The kinetics of crystallization (in terms of nucleation and growth rates) seems to be strongly affected by the degree of undercooling and dynamic stirring conditions. We conclude that any modeling of tephriphonolite magma flow (either in conduit or subaerial) should take into account the effects of strain, strain-rate and internal textures (i.e. clustering)
The rheology of crystal-bearing basaltic magmas from the paroxymal eruption of 15 March 2007 at Stromboli.
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The multiphase rheology of crystal- and vesicle-bearing magma from Monte Nuovo (Campi Flegrei, Italy)
No full textConfort 15 model of conduit dynamics: applications to Pantelleria Green Tuff and Etna 122 BC eruptions
No full textNumerical simulations are useful tools to illustrate how flow parameters and physical processes may affect eruption dynamics of volcanoes. In this paper, we present an updated version of the Conflow model, an open-source numerical model for flow in eruptive conduits during steady-state pyroclastic eruptions (Mastin and Ghiorso in A numerical program for steady-state flow of magma-gas mixtures through vertical eruptive conduits. U.S. Geological Survey Open File Report 00-209, 2000). In the modified version, called Confort 15, the rheological constraints are improved, incorporating the most recent constitutive equations of both the liquid viscosity and crystal-bearing rheology. This allows all natural magma compositions, including the peralkaline melts excluded in the original version, to be investigated. The crystal-bearing rheology is improved by computing the effect of strain rate and crystal shape on the rheology of natural magmatic suspensions and expanding the crystal content range in which rheology can be modeled compared to the original version (Conflow is applicable to magmatic mixtures with up to 30 vol% crystal content). Moreover, volcanological studies of the juvenile products (crystal and vesicle size distribution) of the investigated eruption are directly incorporated into the modeling procedure. Vesicle number densities derived from textural analyses are used to calculate, through Toramaru equations, maximum decompression rates experienced during ascent. Finally, both degassing under equilibrium and disequilibrium conditions are considered. This allows considerations on the effect of different fragmentation criteria on the conduit flow analyses, the maximum volume fraction criterion (“porosity criterion”), the brittle fragmentation criterion and the overpressure fragmentation criterion. Simulations of the pantelleritic and trachytic phases of the Green Tuff (Pantelleria) and of the Plinian Etna 122 BC eruptions are performed to test the upgrades in the Confort 15 modeling. Conflow and Confort 15 numerical results are compared analyzing the effect of viscosity, decompression rate, temperature, fragmentation criteria (critical strain rate, porosity and overpressure criteria) and equilibrium versus disequilibrium degassing in the magma flow along volcanic conduits. The equilibrium simulation results indicate that an increase in viscosity, a faster decompression rate, a decrease in temperature or the application of the porosity criterion in place of the strain rate one produces a deepening in fragmentation depth. Initial velocity and mass flux of the mixture are directly correlated with each other, inversely proportional to an increase in viscosity, except for the case in which a faster decompression rate is assumed. Taking into account up-to-date viscosity parameterization or input faster decompression rate, a much larger decrease in the average pressure along the conduit compared to previous studies is recorded, enhancing water exsolution and degassing. Disequilibrium degassing initiates only at very shallow conditions near the surface. Brittle fragmentation (i.e., depending on the strain rate criterion) in the pantelleritic Green Tuff eruption simulations is mainly a function of the initial temperature. In the case of the Etna 122 BC Plinian eruption, the viscosity strongly affects the magma ascent dynamics along the conduit. Using Confort 15, and therefore incorporating the most recent constitutive rheological parameterizations, we could calculate the mixture viscosity increase due to the presence of microlites. Results show that these seemingly low-viscosity magmas can explosively fragment in a brittle manner. Mass fluxes resulting from simulations which better represent the natural case (i.e., microlite-bearing) are consistent with values found in the literature for Plinian eruptions (~106 kg/s). The disequilibrium simulations, both for Green Tuff and Etna 122 BC eruptions, indicate that overpressure sufficient for fragmentation (if present) occurs only at very shallow conditions near the surface
The combined effect of crystals and bubbles on the rheology of Monte Nuovo trachytic magma (Campi Flegrei)
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