The 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 amicropenetration 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 showstrong correlations between these temperatures that canprobably 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 andwater 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 systemcompared 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 naturalmulticomponentmelts is not appropriate andmay result in erroneous evaluation of petrological and volcanological scenarioswhich require knowledge of those properties.

Viscosity and glass transition temperature of hydrous melts in the system CaAl2Si2O8–CaMgSi2O6

GIORDANO, Daniele;
2008

Abstract

The 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 amicropenetration 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 showstrong correlations between these temperatures that canprobably 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 andwater 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 systemcompared 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 naturalmulticomponentmelts is not appropriate andmay result in erroneous evaluation of petrological and volcanological scenarioswhich require knowledge of those properties.
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http://www.sciencedirect.com/science/article/pii/S0009254108002532
GIORDANO D; M. POTUZÀK; C. ROMANO; D.B. DINGWELL; M. NOWAK
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2318/95225
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