Viscosity and glass transition temperature of hydrous melts in the system CaAl 2 Si 2 O 8 –CaMgSi 2 O 6 D. Giordano a, ⁎, M. Potuzak b,d , C. Romano a , D.B. Dingwell b , M. Nowak c a Department of Geological Sciences, Terza Università degli Studi di Roma, largo San Leonardo Murialdo 1, 00146 Roma, Italy b Department for Earth and Environmental Sciences, University of Munich, Theresienstraße 41/III, 80333 Munich, Germany c Institut für Geowissenschaften, Eberhard Karls Universität Tübingen, Wilhelmstraße 56, 72074 Tübingen, Germany d Corning Incorporated, SP-FR-04, Corning, NY 14831, USA ABSTRACT ARTICLE INFO Article history: Accepted 18 June 2008 Keywords: Melt Glass Glass transition Viscosity Basalt The Newtonian shear viscosity and glass transition temperature (T g ) of hydrous melts in the system Anorthite (An)–Diopside (Di)–H 2 O have been experimentally obtained. Viscosity data on hydrous samples with up to ca. 3 wt.% H 2 O have been measured using a micropenetration technique in the interval between 10 8.3 and 10 13.1 Pa s and temperatures up to 880 °C at ambient pressure. Measurements of the calorimetric T g values were performed by using differential scanning calorimetry (DSC). For each sample the cooling rate dependence of T g 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 T g 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–H 2 O system show substantial differences with those of basaltic composition, suggesting that, despite what 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 simple melts in the An–Di–H 2 O 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 multicomponent melts is not appropriate and may result in erroneous evaluation of petrological and volcanological scenarios which require knowledge of those properties. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The thermodynamic and transport properties of hydrous silicate melts are of fundamental importance for the characterisation of magma dynamics within the Earth. Yet, the information concerning the transport and calorimetric properties of hydrous silicate melts remains scarce. The viscosity of magma is a parameter of paramount importance to describe flow and textural properties observed at volcanic sites, depending strongly on composition, temperature, pressure, dissolved volatile content (e.g., Polacci et al., 2004; Richet et al., 1996; Whittington et al., 2000, 2001; Romano et al., 2003; Hui and Zhang, 2007; Giordano et al., 2008) as well as crystal and bubble content, and, it is generally a function of the deformation rate applied to the melt (e.g., Krieger, 1972; Pinkerton and Stevenson, 1992; Pinkerton and Norton, 1995; Lejeune and Richet, 1995; Costa, 2005; Sato, 2005; Ishibashi and Sato, 2007; Caricchi et al., 2007, 2008-this issue). Nevertheless, pressure up to about 20 kbar and solid content within 30 vol.% do not influence viscosity as much as temperature, composition, or dissolved water content (Marsh, 1981; Dingwell, 1989; Pinkerton and Stevenson, 1992; Lejeune and Richet, 1995). In particular, the melt phase may become non-Newtonian near the glass transition. The glass transition interval represents a kinetic boundary between the viscous flow and the brittle failure of a magma. The crossing of this rheological boundary can strongly affect the eruptive scenario of a magma rising toward the surface, determining whether effusion or explosion occurs (e.g., Dingwell, 1996; Papale, 1999). Here, the viscosity (η) and the glass transition temperatures (T g ) of melts in the An–Di–H 2 O system containing up to 3 wt.% of dissolved H 2 O have been investigated. The An–Di system is of interest as it has been applied as a model for basalt petrogenesis (e.g., Bowen, 1915; Osborn and Chemical Geology 256 (2008) 203–215 ⁎ Corresponding author. Tel.: +39 06 57338018; fax: +39 06 57338209. E-mail address: dgiordan@uniroma3.it (D. Giordano). 0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2008.06.027 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo