We report an experimental study of total water diffusion (irrespective of water speciation) in two different
phonolite melts, which are representative of Monta?a Blanca, Tenerife, Spain (MBP) and Laacher See, East Eifel, Germany (LSP-II). Both
phonolites have Na-rich compositions, but differ in their alumina saturation index, with MBP being peralkaline and LSP-II being slightly peraluminous. Diffusion couple experiments for MBP were performed at 200-250 MPa in the temperature range of 800-1050 ¡ãC and water contents between 1 and 6.5 wt. % . Due to higher liquidus temperatures of LSP-II, the accessible temperature and water concentration range was reduced to 875-1050 ¡ãC and 3-6.5 wt. % water. All experiments were performed in rapid quench cold-seal pressure vessels, which enabled rapid heating and quenching of the samples within seconds. Compared to the run durations of 30-90 min, these short heating and cooling periods can be neglected and no corrections needed to be applied for the calculation of the diffusion coefficients. Water diffusion profiles were determined by FT-IR micro-spectroscopy on doubly polished glass sections and the diffusion coefficients were determined by Boltzmann-Matano analysis. Water diffusion increases with increasing water content and temperature and follows the empirical relations:logD(MBP)[m2/s]=-6.225-0.346¡¤cwater+0.0246¡¤cwater2-5871.7-673.7¡¤cwater+30.9¡¤cwater2/T,forT=1073-1323K,cwater=2-6wt. % ,P=200MPaandlogD(LSP-II)[m2/s]=(-6.001-0.277¡¤cwater)-(6281-565.6¡¤cwater)/T,forT=1148-1323K,cwater=3-6wt. % ,P=200MPawherecwateris the total water concentration in wt. % andTthe temperature in Kelvin.
In the experimental range all diffusion coefficients range between 10?11.1 and 10?9.6 m2/s. The dependence on water concentration is similar for both melt compositions and ranges between 0.15 and 0.25 log D [m2/s] per wt. % water. The temperature dependence of water diffusion seems to increase with decreasing water content, which is reflected in increasing activation energy: 56-80 kJ/mol in MBP and 55-88 kJ/mol in LSP-II for water contents of 6-3 wt. % . The compositional dependence for the investigated phonolite melts is small. The diffusion coefficients for the peralkaline MBP melt are about 0.20 ¡À 0.05 log units higher than those for peraluminous LSP-II melt, which corresponds to a factor of 1.5 in diffusion speed. The diffusivity data were determined for P, T and cwater conditions that are relevant for storage and ascent of phonolite magmas and can therefore directly be applied for modelling of the eruptive dynamics of such magmas.
