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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 textViscosity of a Teide phonolite in the welding interval
No full textThe viscosity of a natural phonolitic composition with variable amounts of H2O has been experimentally determined. The
starting materials were crystal-free phonolitic glasses from Montana Blanca, situated within the Las Canadas caldera of Teide.
Dry phonolitic melt viscosities were determined using concentric cylinder viscometry in the low viscosity range. The glassy
quench products of these runs were then hydrated by high pressure synthesis in a piston cylinder apparatus to generate a suite of samples with water contents ranging from 0.02 to 3.75 wt%. Samples thus hydrated were quenched rapidly and prepared (cut and polished) for the determination of water contents by infrared spectroscopy before and after experimental viscometry. The
viscosities of the melts (dry and hydrated) were determined at 1 bar using a micropenetration technique. Samples were stable under the measurement conditions up to 3.75 wt% H2O. Homogeneity of water content was confirmed by infrared spectroscopy
and total water contents were calculated using absorptivity coef®cients for compositions extremely close to that investigated here. The variation of viscosity as a function of water content and temperature can be described in the high viscosity interval of relevance to many welding processes by the non-Arrhenian expression:
log10 visc = -5.900-0.286 ln (H2O)+ (10775.4 - 394.8(H2O))/(T-148.7 + 21.65 ln(H2O))
whereas the high viscosity range alone is adequately described by the Arrhenian expression
log10 visc = 10.622 - 0.738 ln(H2O) + (17114.3 - 590.4(H2O))x 1/T
where visc is the viscosity in Pa s, H2O is the water content in wt% and T is the temperature in K.
These results are particularly useful for the scaling of conditions extant during the welding of phonolitic products of Montana
Blanca. The welding of glassy phonolitic rocks is enhanced by the lower viscosity of these melts with respect to calcalkaline rhyolites. The ratio of viscosities of phonolitic to calcalkaline rhyolitic melts is a complex function of temperature and water content and reaches up to 104.5 at 0.1 wt% H2O and 500°C. Abundant evidence of welding and remobilisation of pyroclastic and spatter products of Teide system volcanism are consistent with these experimental observations
Viscosity data for hydrous peraluminous granitic melts: comparison with a metaluminous model
No full textWe performed 27 viscosity determinations on dry and water-bearing peraluminous haplogranitic melts. The dry melt compositions cover the range of normative corundum to be expected in peraluminous granitic melts in nature. The compositions are based on addition
of Al2O3 to a haplogranitic melt (HPG8) whose composition is near that of the projection
of the 2 kbar H2O-saturated minimum melt composition into the system NaAlSi3O8-
KAlSi3O8-SiO2. The H2O contents of the hydrous melts were analyzed using Karl Fischer titration ranging from 1 to 3 wt%. The viscosity determinations were performed using a
modified micropenetration method in the viscosity range of 1010 to 1011 Pa·s, at 1 atm
pressure, and in the temperature ranges of 880–940 °C and 470–640 °C for the dry and wet melts, respectively. For the dry peraluminous melts in this high viscosity range, addition of the first few percent of normative corundum to a metaluminous granitic melt increases the viscosity, which remains nearly constant despite further addition of Al2O3.
Thus a viscosity maximum is inferred for dry slightly peraluminous granitic melts. The hydrous melt viscosity data were compared with the recent calculational model of Hess and Dingwell (1996), which was based on and designed for metaluminous melt viscosities.
That model is capable of describing the viscosities of hydrous peraluminous granitic melts within the uncertainties stated for its application in metaluminous melts
Pre-eruptive volatile (H2O, F, Cl, and S)contents of phonolitic magmas feeding the 3600-year-old Avellino eruption from Vesuvius, southern Italy
No full textThe behaviour of H2O, F, Cl and S prior to and during the Plinian phase of the 3550-year old Avellino eruption was determined by the study of unheated primary glass inclusions and matrix glasses. The fall deposit can be divided into two sub-units on the basis of an abrupt change in colour: a phonolitic white layer at the base and a tephriphonolitic banded grey pumice layer at the top. The original tephriphonolitic magma is always found mixed with the phonolitic magma. Primary glass inclusions were only found in feldspar, amphibole and garnet associated with the white pumice. Glass inclusions in the products of the Avellino Plinian eruption contain a mean of 3.10 +/- 0.74 wt.% H2O, 0.95 +/- 0.15 wt.% F, 0.59 +/- 0.02 wt.% Cl, and 560 +/- 55 ppm S. The concentration of CO2 was below the detection Limit. Coexisting matrix glasses contain similar amounts of halogens but significantly less water (similar to 0.5 wt.%) and sulphur (similar to 100 ppm), suggesting that during the eruption the halogens did not behave as volatiles. Owing to the lack of primary glass inclusions and its hybrid nature, similar conclusions can only be hypothesised for the tephriphonolitic magma on the basis of volatile element distribution in the matrix glasses. Taking into account both our data and data from parental mafic melt inclusions by a previous study by P. Marianelli and co-workers, it has been possible to reconstruct the behaviour of volatiles in the Avellino magmatic reservoir in two stages: (1) variation of volatiles during crystal fractionation; and (2) exsolution of an "excess" gas phase when volatiles reach their solubility limit in silicate melts. The agreement between our data and the solubility limit of chlorine, sulphur and probably water in phonolitic melts at the postulated pressure and temperature (i.e., 100 MPa and 850 degrees C), suggests that such a phase exists. Compared to the other volatiles, fluorine does not exhibit a behaviour (i.e., flat distribution) consistent with the attainment of its solubility limit in phonolites. Using the petrologic method, the minimum output of volatiles during the Plinian phase of the Avellino eruption was hypothesised to be about 2 x 10 6 tons of H2SO4 and 25 x 10 6 tons of H2O. Due to both the presence of an "excess" vapour phase prior to eruption and the assumptions inherent in the petrologic method, the total calculated amounts should be considered as very conservative estimates of volatile outputs during the eruption
The influence of trace amount of water on the viscosity of rhyolites
No full textAs a major volatile in volcanic systems, water has a significant influence on the rheological properties of silicic magmas. This is especially so at minor water contents relevant to the emplacement of silicic lavas. To investigate the influence of water on the viscosity of natural rhyolitic obsidians, a novel strategy has been adopted employing parallel-plate and micropenetration techniques. Viscosities have been determined on three types of material: (a) raw water-bearing obsidians; (b) remelted (1650 degrees C, 1 atm) degassed glasses of the obsidians; and (c) hydrothermally hydrated (1300 degrees C, 3 kbar) obsidians. Ten natural rhyolitic obsidians (peraluminous, calc-alkaline and peralkaline) were employed: seven originated from lava flows and contained <0.2 wt.% H2O, two samples were F-rich from pyroclastic successions, and one was an obsidian cobble with 1.5 wt.% water also associated with pyroclastic units. Melt compositions and water contents were stable during viscometry. The measured decreases in activation energies of viscous flow and viscosity with small amounts of water are much greater than the Shaw calculation scheme predicts. In addition, a marked nonlinear decrease in eta exists with increasing water content. In contrast to the case for peralkaline rhyolites, 0.1-0.2 wt.% water decreases activation energies significantly (up to 30%) for calc-alkaline compositions. These results have important implications for the ease of near-surface degassing of silicic magmas during emplacement and permit the testing of calculational models for viscosity, largely based on synthetic systems
Proprietà reologiche dei fusi naturali: i liquidi magmatici
No full textRiassunto: Questo contributo fornisce un aggiornamento sui più recenti studi
sperimentali e sui modelli di calcolo relativi allo studio della reologia dei fusi silicatici naturali
The temperature dependence of the speciation of water in NaAlSi3O8-KAlSi3O8 melts using fictive temperature derived from synthetic fluid-inclusions
No full textThe speciation of water dissolved in glasses along the join NaAlSi3O8-KAlSi3O8 has been investigated using infrared spectroscopy. Hydrous melts have been hydrothermally synthesized by chemical equilibration of cylinders of bubble-free anhydrous start glasses with water at 1040 ° C and 2 kbar. These melts have been isobarically and rapidly (200 degrees C/s) ''drop''-quenched to room temperature and then subsequently depressurized. The speciation of water in the quenched glasses reflects the state of water speciation at a temperature (the so-called fictive temperature) where the quenched-in structure of the glasses closely corresponds to the melt structure at equilibrium. This fictive temperature is detectable as the macroscopically measureable glass transition temperature of these melt compositions. A separate set of experiments using vesicular samples of the same chemistry has precisely defined the glass transition temperature of these melts (+/-5 degrees C) on the basis of homogenization temperatures for water-filled fluid inclusions (Romano et al. 1994). The spectroscopic data on the speciation of water in these quenched glasses has been quantified using experimentally determined absorptivities for OH and H2O for each individual melt composition. The knowledge of glass transition temperatures, together with quantitative speciation data permits an analysis of the temperature dependence of the water speciation over the 113° C range of fictive temperatures obtained for these water-saturated melts.
The variation of water speciation, cast as the equilibrium constant K where
K = [H2O][O-m]/[OH](2)
is plotted versus the fictive temperature of the melt to obtain the temperature dependence of speciation. Such a plot describes a single linear trend of the logarithm of the equilibrium constant versus reciprocal temperature, implying that the exchange of K for Na has little influence on melt speciation of water. The enthalpy derived from temperature dependence is 36.5 (+/-5) kJ/mol. The results indicate a large variation in speciation with temperature and an insensitivity of the speciation to the K-Na exchange
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