1,721,151 research outputs found
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
Viscosity 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 of magmatic liquids: A model.
No full textThe viscosity of silicate melts controls magma transport dynamics, eruption style and rates of physicochemical processes (e.g., degassing, crystallization) in natural magmas. Thus a comprehensive viscosity model for magmatic liquids has long been a goal of earth scientists. Here we present a model that predicts the non-Arrhenian Newtonian viscosity of silicate melts as a function of T and melt composition, including the rheologically important volatile constituents H2O and F. Ourmodel is based on >1770 measurements of viscosity on multicomponent anhydrous and volatile-rich silicate melts. The non-Arrhenian T-dependence of viscosity is accounted for by the VFT equation [log ?=A+B/(T(K)-C)]. The optimization assumes a common, high-T limit (A) for silicatemelt viscosity and returns a value for this limit of -4.55 (+0.2) (e.g., log ?~10-4.6 Pa s). All compositional dependence is ascribed to the parameters B and C and is accounted for by an additional 17model coefficients. Our model is continuous in composition- and temperature-space and predicts the viscosity of natural volatile-bearing silicate melts (SiO2, Al2O3, TiO2, FeOtot, CaO, MgO,MnO, Na2O, K2O, P2O5, H2O, F2O-1) over fifteen log units of viscosity (10-1–1014 Pa s). The model for viscosity can also predict other transport properties including glasstransition temperatures (Tg) and melt fragility (m). We show strong systematic decreases in Tg and m with increasing volatile content. This pattern has implications for predicting styles of volcanic eruption and understanding silicate melt structure. Our model transforms a quarter-century of experimental study of melt viscosities, into a parameterisation having a predictive capacity thatmakes it relevant to diverse fields of research including: volcanology, geophysics, petrology and material sciences
The viscosity of trachytes, and comparison with basalts, phonolites, and rhyolites
No full textThe viscosity of natural liquids representative of the glassy portion of pumice collected from the deposits of the Campanian
Ignimbrite (IGC) and Monte Nuovo (MNV) eruption of Phlegrean Fields has been measured in the temperature range from 1770 K down to the glass transition, and for a dissolved water content range from dry to nearly 4 wt.%. Measurements were
performed by a combination of techniques involving concentric cylinder and micropenetration apparatuses, depending on the
specific viscosity range. These measurements, together with those made on samples from the Agnano Monte Spina (AMS)eruption of Phlegrean Fields presented in a companion paper, represent the first viscosity determinations for natural trachytic liquids. Liquid viscosities have been parameterized by means of a modified VFT equation that allows the calculation of
viscosity as a function of temperature and water content. Calculated viscosities are compared with those pertaining to natural
liquids of phonolitic, rhyolitic, and trachybasaltic composition, showing that trachytes are intermediate between rhyolites and
phonolites, consistent with the dominant eruptive style associated with the different magma compositions (mainly explosive for rhyolites and trachytes, either explosive or effusive for phonolites, mainly effusive for basalts). Compositional diversity among
the analyzed trachytes corresponds to liquid viscosity differences of one to two orders of magnitude, with higher viscosities
approaching that of rhyolite at the same temperature–water content conditions. All hydrous natural trachytes and phonolites become indistinguishable when isokom temperatures (i.e., temperatures corresponding to the same viscosity) are plotted versus a compositional parameter given by the molar ratio on an element basis (Si+Al)/(Na+K+H). In contrast, rhyolitic and basaltic liquids display distinct trends, with the more fragile basaltic liquids crossing the curves pertaining to all other compositions
Glass transition temperatures of natural hydrous melts: a relationship with shear viscosity and implications for the welding process.
No full textGlass transition temperatures (Tg) have been determined for natural multicomponent melts using differential scanning calorimetry. Trachytic, dacitic, phonolitic and basaltic base compositions have been analysed over a range of water contents up to 3.75 wt.%. For each sample Tg has been obtained over a range of cooling/heating rates using the extrapolated onset and the peak temperatures in heat capacity–temperature curves. Tg of all compositions are strongly reduced by increasing water content, particularly for the first 1 wt.% added. Base composition also has an effect, with the lowest Tg occurring in the peralkaline phonolite suite. For all samples a clear dependence on the cooling/heating rate has been recorded. These results have been compared with rheological investigations on the same samples. On the basis of the equivalence of the shear and enthalpic relaxation process timescales we provide a method to predict the shear viscosity at the glass transition for all the melts investigated in this study, both dry and hydrous. Our determinations of Tg provide a lower limit for the time–temperature envelope that gives rise to densely welded deposits and constraints on their emplacement temperature. Furthermore, by using the viscosity values predicted at the glass transition, we suggest that welding processes may occur over timescales on the order of tens of seconds to tens of minutes at Tg
The effect of iron on the rheological properties of silicate melts.
No full textAs the viscosity is probably one of the most important properties governing the dynamics of magmas at all the scales, its investigation is fundamental to provide importantconstraints on all the magmatic processes such as crystallization, magma differentiation and eruption dynamics on terrestrial planets. Igneous provinces have been found on Earth, Moon, Venus, Io and Mars. As matter of fact, it is generally accepted, that Martian rocks have a higher Fe content than rocks on Earth. Unfortunately, existing models of the physico-chemical properties of silicate melts are not calibrated for the high Fe contents. In order to adapt these models to high Fe-content compositions, we have developed a new experimental approach from which the physico-chemical properties of both iron components (FeO and Fe2O3) will be derived. Therefore, viscosity measurements have been conducted in this study over the whole temperature liquid range. High-T viscosities (1594-1275_C) have been measured in air using the concentric cylinder (CC) method. Low-T viscosities (817-711_C) have been measured under argon using the micro-penetration method (MP) for the melts that could be quenched to glasses. The oxidation state of Fe has been determined on quenched glasses at regular T steps by wet chemistry method. Two different simple Fe-bearing systems have been studied to date: (i) anorthite-diopside eutectic composition (AnDi) with variable amount of Fe (up to 30 wt%) as described in the literature as -a basalt analogue- and (ii) sodium disilicate (NS2 up to 20 wt% of Fe). In addition, the compositional range has been extended to include a Martian mantle composition based on the model of [1]. The high T viscosity data obtained for the AnDi-eutectic in this study are in good agreement with the data provided by previous authors [2] on the same composition. Since no previous studies have dealt with the low T viscosity of the AnDi-eutectic composition, our experimental data were compared with viscosity of Etna lava [3] taken as an example of natural basaltic composition. In fact, whereas the high T viscosities are similar to each other, this is not true at low temperature (i.e., the Etna basalt shows a much lower activation energy with respect to our AnDi-eutectic composition). High-T viscosities of SNC shows a good agreement with viscosities obtained for a natural peridotite from Balmuccia (Earth mantle analogue) [Dingwell - personal communication]. Low-T viscosities of SNC could not be measured due to the occurrence of crystallization. All the materials exhibit non-Arrhenian behaviour when observed within the complete range of the experimental determinations. Therefore, a Vogel Fulcher Tammann (VFT) form of equation was used to characterize the variation of the viscosity with temperature. In addition, the present results suggest that the effect of iron on decreasing the viscosity at isothermal temperatures (more effective at low T) is linear for AnDi samples containing up to 10 wt% of Fe, while this behaviour becomes non-near for higher Fe-content.[1] Dreibus, G., Waenke, H., (1985) Mars, A Volatile-Rich Planet, Meteoritics 20, 367-381. [2] Scarfe et al., (1983) Viscosity-tempearture relationship at 1 atm in the system diopside-anortite, Am.Mineral. 68, 1083-1088. [3] Giordano, D., Dingwell, D.B., (2003) Viscosity of hydrous Etna basalt: implications for Plinian-style basaltic eruptions, Bull.Volcanol. 65, 8-14
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 Þ ne 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 Þ vefold coordination, 40% [4]V5+60% [5]V5+ in proportion, and the
EXAFS-derived distances and coordination numbers are in agreement with this interpretation. No
signiÞ cant 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
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