1,721,042 research outputs found
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
The rheology of crystal-bearing basaltic magmas from the paroxymal eruption of 15 March 2007 at Stromboli.
No full text
The combined effects of water and fluorine on the viscosity of silicic magmas
No full textThe Newtonian viscosity of water-plus-fluorine-bearing silicate melt of haplogranitic composition
(HPG8) has been determined. Viscosities of HPG8 melt with addition of 3.11 and 4.25 wt.% of F and up to
3 wt.% H2O have been obtained using a micropenetration technique in the interval 109.74 to 1011.84 Pa s and
temperatures varying from 370 to 700°C, at ambient pressure. Determination of the temperature dependence
of viscosity from this and previous studies permits the parameterization of the viscosity of melts containing
water and fluorine, having similar composition, within a 0.3 log units standard error. The viscosity of water-bearing, F-rich haplogranitic samples is represented by a modified Vogel-Fulcher-Tammann (VFT)
equation which provides a non-Arrhenian description of the temperature dependence of the viscosity. The results of this study indicate that, taken individually or together, both H2O and F- have a strong and similar
effect on the viscosity of SiO2-rich compositions. This similarity between F2O-1 and H2O greatly simplifies
the task of predicting viscosity for volatile-rich, highly silicic magmas. The low viscosities of hydrous
fluorine-bearing granitic melts favour efficient crystallization-fractionaction paths for these liquids, controlling
degassing paths and consequently the eruptive behaviour. Numerical simulations of eruptive events normally do not take into account the contribution of fluorine; this may introduce a significant error in the description of the fluid-dynamic properties of magma and, therefore, in the accurate prediction of eruptive scenarios, as
well as in hazard assessment studies. Fluorine, unlike water, remains dissolved in the melt at high concentrations
and low confining pressures. The incorporation of fluorine data and the modelling of fluorine-bearing
viscosity data are therefore of fundamental importance for simulations of magma dynamics and prediction of eruptive scenarios
V oxidation state and coordination number in silicate glasses by XAS
No full textThe local structure of vanadium (V) in synthetic glasses of basaltic composition has been studied
by means of high-resolution V K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy to obtain quantitative data on distances, V coordination number (CN), and oxidation state. The compositions and experimental conditions were
chosen so as to verify the effect of bulk-glass composition (using a diopside-anorthite composition,
a sodium disilicate glass, and an iron-titanium-bearing basaltic glass) and V content (from 0.1 to 5 V2O5 wt%) on the structural role of V in these glasses.
The combined analysis of high-resolution XANES spectra and EXAFS data indicate that on average, the Fe-free glasses synthesized in air show vanadium in the V5+ state, mainly in tetrahedral
coordination (less than 20% [5]V5+) and with distances of 1.697 (± 0.020) Å, in agreement with the values found for tetrahedral V in minerals. In contrast, the Fe-bearing basaltic glasses display a mixture of V5+ in fourfold and fivefold coordination, 40% [4]V5+-60% [5]V5+ in proportion, and the
EXAFS-derived distances and coordination numbers are in agreement with this interpretation. No
significant changes in the V local structure were found in the glasses analyzed as a function of V-contents
in the 0.1 to 5 V2O5 wt% range. The data obtained suggest that the structural role of vanadium in these melts is rather insensitive to bulk composition, in terms of V and alkali content, but can be strongly affected by the presence of other transition elements, e.g., Fe3+ competing with V to enter
the tetrahedral framework
Deformation of foamed rhyolites under internal and external stresses: an experimental investigation
Full text linkThe style of magma eruption depends strongly on the character of melt degassing and foaming. Depending on the kinetics of these processes the result can be either explosive or effusive volcanism. In this study the kinetics of foaming due to the internal stresses of gas expansion of two types of obsidian have been investigated in time series experiments (2 min-24 h) followed by quenching the samples. The volumetric gas-melt ratio has been estimated through the density measurements of foamed samples.
The variation of gas volume (per unit or rhyolite melt volume) with time may be described by superposition of two exponentials responsible for gas generation and gas release processes respectively. An observed difference in foaming style in this study is interpreted as the result of variations in initial contents of microlites that serve as bubble nucleation centers during devolatilization of the melts. Quantitatively the values of the gas generation rate constants (k g) are more than an order of magnitude higher in microlite-rich obsidian than in microlite-free obsidian. Possible origins of differences in the degassing style of natural magmas are discussed in the light of bubble nucleation kinetics in melts during foaming. In a complementary set of experiments the mechanical response of vesicular melt to external shear stress has been determined in a concentric cylinder viscometer. The response of vesicular melt to the pulse of shear deformation depends on the volume fraction of bubbles. The obtained response function can be qualitatively described by a Burgers body model. The experimental shear stress response function for bubble-bearing melt has an overshoot due to the strain-dependent rheology of a twophase liquid with viscously deformable inclusions
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