Eur. J. Mineral. 2007, 19, 657–669 Published online October 2007 This paper was presented at the EMPG XI symposium in Bristol, U.K. (September 2006) The influence of H 2 O and CO 2 on the glass transition temperature: insights into the effects of volatiles on magma viscosity Y MORIZET 1, *, A R.L. NICHOLS 2 ,S C. KOHN 3 , R A. BROOKER 4 and D B. DINGWELL 5 1 Dept of Planetology and Geodynamic, University of Nantes, 2 rue de la Houssinière, 44300 Nantes, France *Corresponding author, e-mail: yann.morizet@univ-nantes.fr 2 Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan 3 Dept of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK 4 Department of Earth Sciences, University College London Gower Street, London WC1E 6BT, UK 5 Department for Earth and Environmental Sciences, University of Munich Thersienstraße 41/III, 80333 Munich, Germany Abstract: CO 2 can play an important role in eruptive processes; in particular, it has the potential to reach saturation at lower concentrations than H 2 O and initiate degassing. The effect of such CO 2 loss on magma viscosity is not well constrained, especially compared to the established effects of H 2 O loss. In terms of understanding the CO 2 solubility mechanism, recent spectroscopic studies have shown that CO 2 speciation is strongly temperature dependent and that CO 2 speciation preserved in quenched glasses below T g is different from the true CO 2 speciation observed in the melts. However, the effect of CO 2 on the glass transition temperature, and by inference the viscosity, has not been previously established. In this study, calorimetric measurements were conducted on synthetic H 2 O- and CO 2 -bearing phonolite and jadeite glasses in order to investigate the volatile’s effect on the glass transition interval, by defining a single glass transition temperature (T onset g ). The samples were synthesised in a piston-cylinder apparatus between 1300 and 1550 ◦ C, at 1.0 to 2.5 GPa, and contained up to 2.29 wt.% CO 2 and up to 5.49 wt.% H 2 O. For both compositions, H 2 O has a large effect in reducing T onset g , but CO 2 appears to have little or no effect. For the entire range of H 2 O contents, T onset g decreases exponentially with H 2 O content from 870 to 523 K and 1036 to 636 K for phonolite and jadeite, respectively, regardless of the CO 2 content. No measurable effect of CO 2 on T onset g was observed. These results suggest that compared to H 2 O, CO 2 contributes little to changes in the physical properties of the melt. They also provide strong evidence for the decoupling of CO 2 speciation from the bulk silicate melt structural relaxation process at T g . Key-words: CO 2 ,H 2 O, glass transition temperature, silicate melts. 1. Introduction 1.1. Volatiles in magmatic systems H 2 O and CO 2 are the most important volatiles in mag- matic systems, having dramatic effects processes such as source melting (Kushiro et al., 1968; Brey & Green, 1975; Holloway, 1976; Hirose & Kawamoto, 1995), mag- matic degassing (Symmonds et al., 1994), magma rheol- ogy (Richet et al., 1996; Whittington et al., 2000; Giordano et al., 2005), and resulting magma transport (Spera, 1981, 1984) or eruption style (Dingwell, 1996). An understand- ing of how H 2 O and CO 2 interact with silicate melts is therefore necessary to develop models that predict solubil- ity (e.g. Dixon et al., 1995; Dixon, 1997; Papale, 1999; Newman & Lowenstern, 2002; Brooker et al., 2001a;b), speciation (e.g. Stolper et al., 1987; Ihinger et al. 1999; Brooker et al., 1999; 2001a,b; Morizet et al., 2001; Nowak et al., 2003), and the physical properties of volatile-bearing silicate melts (e.g. Lange, 1994; Dingwell et al., 1996; Bourgue & Richet, 2001). 1.2. Effect of volatiles on viscosity It is recognised that H 2 O significantly influences the physical properties (e.g. viscosity and density) of silicate melts (e.g. Friedman et al., 1963; Richet et al., 1996; Schulze et al., 1996, 1999; Giordano et al., 2005; Bouhifd et al., 2006; Vetere et al., 2006). The addition of H 2 O has long been recognised to reduce the viscosity of a melt, with catastrophic consequences as H 2 O is degassed from magma to give an increase in viscosity that con- tributes to explosive volcanic eruptions (Hess & Dingwell, 0935-1221/07/0019-1751 $ 5.85 DOI: 10.1127/0935-1221/2007/0019-1751 c  2007 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart