HR: 12:05h
AN: V52A-08    [Abstracts]
TI: The Influence of Volatiles on the Glass Transition
AU: * Nichols, A R
EM: nichols@min.uni-muenchen.de
AF: Dept. of Earth & Eviron. Sciences, University of Munich, Munich, 80333 Germany
AU: Giordano, D
AF: Dept. of Earth & Ocean Sciences, University of British Columbia, Vancouver, V6T 1Z4 Canada
AU: Morizet, Y
AF: IGMR, ETH, Zurich, 8092 Switzerland
AU: Dingwell, D B
AF: Dept. of Earth & Eviron. Sciences, University of Munich, Munich, 80333 Germany
AU: Kohn, S C
AF: Dept. of Earth Sciences, University of Bristol, Bristol, BS8 1RJ United Kingdom
AU: Brooker, R A
AF: Dept. of Earth Sciences, University of Bristol, Bristol, BS8 1RJ United Kingdom
AB: The glass transition is the temperature interval across which melt relaxation times change appreciably, from instantaneous at high temperatures (super-cooled liquid) to infinite at low temperatures (glass). It has several important implications, being where the speciation of water is locked into the melt structure, viscous flow and welding stop and elastic response to mechanical stress commences. It depends on composition, thermal history and the timescale of the experimental method being used. In this study we use the variation of heat capacity with temperature measured by differential scanning calorimetry to define glass transition temperatures ({\it T$_{g}$}) for a series of synthetic and natural samples with a range of volatile contents. Two methods have been used to define {\it T$_{g}$}: a) the temperature of the onset of the glass transition, and b) the temperature of the peak in heat capacity across the glass transition. For each sample the thermal history (cooling and heating at 10 K/min) is the same, allowing direct comparison of {\it T$_{g}$}. The samples include natural glasses with a wide range of compositions (trachyte, dacite, phonolite, basalt) that have been doped with varying amounts of water, basaltic glasses from the Hawaiian drill core that contain naturally occurring amounts of water and jadeite glasses synthesised with varying amounts of water and carbon dioxide. We will show that for each natural composition water affects {\it T$_{g}$} with the same overall pattern. Increasing water contents cause {\it T$_{g}$} to decrease (2 wt.% water causes drops of 190 to $280\deg$C), with the largest effect occurring with the addition of the first 1 wt.% of water (drops of 110 to $200\deg$C). The drop varies depending on the glass composition. The effect of carbon dioxide on the {\it T$_{g}$} of jadeite glass is less clear, no systematic variation with concentration occurs. Water affects the {\it T$_{g}$} of jadeite glass in exactly the same way as in the natural compositions. We will also examine how the width of the glass transition (in terms of temperature) and the height of the heat capacity peak are affected by composition and volatile content.
DE: 8400 VOLCANOLOGY
DE: 8439 Physics and chemistry of magma bodies
DE: 5134 Thermal properties
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting