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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
An expanded non Arrhenian model for silicate melt viscosity: A treatment for metaluminous, peraluminous and peralkaline liquids
No full textWe present new viscosity measurements for melts spanning a wide range of anhydrous compositions including: rhyolite, trachyte, moldavite, andesite, latite, pantellerite, basalt and basanite. Micropenetration and concentric cylinder viscometry measurements cover a viscosity range of 10−1 to 1012 Pas and a temperature range from 700 to 1650 °C. These new measurements,
combined with other published data, provide a high-quality database comprising ∼800 experimental data on 44 well-characterized melt compositions. This database is used to recalibrate the model proposed by Giordano and Dingwell [Giordano, D., Dingwell, D. B., 2003a. Non-Arrhenian multicomponent melt viscosity: a model. Earth Planet. Sci. Lett. 208, 337–349] for predicting the
viscosity of natural silicate melts. The present contribution clearly shows that: (1) the viscosity (η)–temperature relationship of natural silicate liquids is very well represented by the VFT equation [log η=A+B/ (T−C)] over the full range of viscosity considered here, (2) the use of a constant high-T limiting value of melt viscosity (e.g., A) is fully consistent with the experimental data, (3) there are 3 different compositional suites (peralkaline, metaluminous and peraluminous) that exhibit different patterns in
viscosity, (4) the viscosity of metaluminous liquids is well described by a simple mathematical expression involving the
compositional parameter (SM) but the compositional dependence of viscosity for peralkaline and peraluminous melts is not fully
controlled by SM. For these extreme compositions we refitted the model using a temperature-dependent parameter based on the
excess of alkalies relative to alumina (e.g., AE/SM). The recalibrated model reproduces the entire database to within 5% relative error (e.g., RMSE of 0.45 logunits)
Viscosity and glass transition temperature of hydrous melts in the system CaAl2Si2O8-CaMgSi2O6
No full textThe Newtonian shear viscosity and glass transition temperature (Tg) of hydrous melts in the system Anorthite
(An)–Diopside (Di)–H2O have been experimentally obtained. Viscosity data on hydrous samples with up to ca.
3wt.%H2O have beenmeasured using a micropenetration technique in the interval between 108.3 and 1013.1 Pa s
and temperatures up to 880 °C at ambient pressure. Measurements of the calorimetric Tg valueswere performed by using differential scanning calorimetry (DSC). For each sample the cooling rate dependence of Tg was
characterised at three different temperatures corresponding to the onset, the peak and the stable liquid regions of the heat capacity curves. These results show strong correlations between these temperatures that can probably be extrapolated to other unequivocally defined metrics of the glass transition interval. Comparison with viscosity
data obtained on the same samples shows that glass transition temperatures at each single heating/cooling rate reflect constant viscosity values for these hydrous liquids. Thus observed relationship between calorimetric Tg and
viscosity is independent of composition and water content (c.f., [Giordano, D., Nichols, A.R., Dingwell, D.B. (2005). Glass transition temperatures of natural hydrous melts: a relationship with shear viscosity and implications for
the welding process. J. Volcanol. Geotherm. Res. 142, 105–118.]) Measured and calculated viscosities and glass transition temperatures for melts in the An–Di–H2O system show substantial differences with those of basaltic composition, suggesting that, despitewhat commonly assumed, An–Di is not a good rheological proxy for basaltic compositions. The observed differences are reduced at high temperature in the low viscosity range and are significantly more pronounced at low temperature. We infer that such an effect is due to the different contributions to the configurational entropy provided by the simplemelts in the An–Di–H2O system compared to the multicomponent basaltic melt investigated. Some implications about the role of water in influencing melt
properties are discussed. The results provided here demonstrated that, in some instances, extrapolating the
physical properties of simple systems to those of natural multicomponentmelts is not appropriate andmay result
in erroneous evaluation of petrological and volcanological scenarios which require knowledge of those properties
The effect of Fe on the viscosity of silicate melts by PD-titration and XANES spectroscopy
No full textThe oxidation state of iron can exert considerable influence on magma phase equilibria and rheology. It is typically recognized that ferrous iron behaves as a network modifier in most silicate melts. Ferric iron, on the other hand, occurs both as a network former (coordination IV) and as a modifier. Despite of a wide range of spectroscopic investigations, the efficiency of iron and its oxidation state on modifying the viscosity of silicate melts is still unclear and under debate, mainly due to a lack of data on the melt structure. Here we present new viscosity data for a wide range of natural rhyolitic, trachytic, moldavitic, andesitic, latitic, pantelleritic, basaltic and basanitic compositions as well as wet chemistry and synchrotron analysis. Dry Newtonian shear viscosities were investigated at high temperature (1050-1600°C) and low temperature (616-860°C) using the concentric cylinder apparatus and the micropenetration technique, respectively. The glasses obtained by fast quenching (from different temperature) the high-T melts were measured for determining, via potassium dichromate (PD) titration, the Fe2+/Fe3+ ratio. The structural role of iron species was also investigated by Fe K-edge XANES spectroscopy performed at ESRF (Grenoble). Wet chemistry analyses were found consistent with high-T prediction of [1] and [2] empirical models. Fe K-edge XANES spectra allowed to interpret the rheological measurements in terms of structural played by iron species.[1] Kress and Carmichael 1991, Contrib. Min. Petrol. 108, 82; [2] Ottonello et al. 2001, Chem. Geol. 174, 157 Geophysical Research Abstracts, Vol. 7, 07275, 2005 SRef-ID: 1607-7962/gra/EGU05-A-07275 © European Geosciences Union 2005
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
Fe K-Edge XANES of Synthetic and Natural Silicate Glasses: Composition and fO2 Dependent Structural Properties
No full textHeat capacity of hydrous trachybasalt from Mt Etna: comparison with CaAl2Si2O8 (An)–CaMgSi2O6 (Di) as basaltic proxy compositions
Full text linkThe specific heat capacity (Cp) of six variably hydrated (~3.5 wt% H2O) iron-bearing Etna trachybasaltic glasses and liquids has been measured using differential scanning calorimetry from room temperature across the glass transition region. These data are compared to heat capacity measurements on thirteen melt compositions in the iron-free anorthite (An)–diopside (Di) system over a similar range of H2O contents. These data extend considerably the published Cp measurements for hydrous melts and glasses. The results for the Etna trachybasalts show nonlinear variations in, both, the heat capacity of the glass at the onset of the glass transition (i.e., Cpg) and the fully relaxed liquid (i.e., Cpl) with increasing H2O content. Similarly, the “configurational heat capacity” (i.e., Cpc = Cpl − Cpg) varies nonlinearly with H2O content. The An–Di hydrous compositions investigated show similar trends, with Cp values varying as a function of melt composition and H2O content. The results show that values in hydrous Cpg, Cpl and Cpc in the depolymerized glasses and liquids are substantially different from those observed for more polymerized hydrous albitic, leucogranitic, trachytic and phonolitic multicomponent compositions previously investigated. Polymerized melts have lower Cpl and Cpc and higher Cpg with respect to more depolymerized compositions. The covariation between Cp values and the degree of polymerization in glasses and melts is well described in terms of SMhydrous and NBO/Thydrous. Values of Cpc increase sharply with increasing depolymerization up to SMhydrous ~ 30–35 mol% (NBO/Thydrous ~ 0.5) and then stabilize to an almost constant value. The partial molar heat capacity of H2O for both glasses (Cgp H2O) and liquids (C1p H2O) appears to be independent of composition and, assuming ideal mixing, we obtain a value for C1p H2O of 79 J mol−1 K−1. However, we note that a range of values for C1p H2O (i.e., ~78–87 J mol−1 K−1) proposed by previous workers will reproduce the extended data to within experimental uncertainty. Our analysis supgests that more data are required in order to ascribe a compositional dependence (i.e., nonideal mixing) to C1p H2Ol
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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