Chemical diffusion of fluorine in melts in the system Na 2O [sbnd]Al 2O 3[sbnd]SiO 2

Abstract

The volatilization of fluorine from three melts in the system Na 2O Al 2O 3 SiO 2 has been investigated at 1 atm pressure and 1200–1400°C. The melts chosen have base compositions corresponding to albite, jadeite and a peraluminous melt with 75 mole % SiO 2. Melt spheres were suspended from platinum loops in a vertical tube furnace in a flow of oxygen gas, then quenched, sectioned and analysed by electron microprobe. The microprobe scans indicate that transport of fluorine to the melt-vapor interface is by binary, concentration-independent interdiffusion of fluorine and oxygen. F O interdiffusivity increases in the order albite < peraluminous < jadeite. There is no simple reciprocal relationship between F O interdiffusivity and melt viscosity. Comparison with data on high-pressure interdiffusivity of fluorine and oxygen in jadeite melt indicates that F O interdiffusivity increases with pressure from 0.001 to 10 kbar while the activation energy remains unchanged. Fluorine chemical diffusivity in albite melt is substantially lower than H 2O chemical diffusivity in obsidian melts suggesting that different diffusive mechanisms are responsible for the transport of F and H 2O in igneous melts. Fluorine diffuses in albite melt via an anionic exchange with oxygen whereas water probably diffuses in obsidian melt via an alkali exchange mechanism.