Fluorine and chlorine diffusion in phonolitic melt

Abstract

The chemical diffusion of fluorine and chlorine in a Na-rich phonolitic melt of Montaña Blanca, Tenerife, was investigated experimentally at 1kbar in the temperature range of 800 to 1200°C, for anhydrous conditions and water contents of 2.1 to 2.4wt.%. Diffusion couple experiments were performed in rapid quench cold-seal pressure vessels and in an internally heated pressure vessel. Experimental durations were 3–24h for fluorine diffusion and 4–48h for chlorine diffusion. After the experiments, concentration profiles were measured by electron microprobe along the direction of diffusion and the diffusion coefficients were determined by assuming concentration independent diffusion.Fluorine and chlorine diffusion increases with increasing temperature and shows Arrhenian behaviour. In the temperature range 900–1200°C log Dfluorine ranges from about −12.3 to −11.4m2/s and log Dchlorine from about −13.7 to −12.3m2/s. Thus, fluorine diffusion is faster by about one order of magnitude than chlorine diffusion. Dissolving about 2wt.% water in the melt leads to a diffusivity increase by a factor of ~3 for fluorine (2.1±0.1wt.% H2O) and by a factor of ~7 for chlorine (2.4±0.3wt.% H2O). The activation energies for F and Cl diffusion are quite similar in dry and hydrous (2.1–2.4wt.% water) melts and were determined to be about 100±10 and 155±15kJ/mol, respectively.These results present the first data on halogen diffusion in phonolitic melts below 1200°C. The applied experimental conditions are directly relevant for magmatic processes of phonolite erupting volcanoes and the diffusivities do not need to be extrapolated from high temperatures.