Yves Feisel, Jonathan M. Castro, Christoph Helo, Anne-Sophie Bouvier, Thomas Ludwig, Donald B. Dingwell

Chemical diffusion of the halogens F, Cl, Br, and I in silica-rich natural melts was experimentally investigated by the diffusion couple technique. Experiments were conducted under anhydrous conditions at atmospheric pressure and hydrous conditions (~1.5 wt.% H2O) at 160 MPa, over a temperature range of 750–1000 °C and 1000–1200 °C, respectively. Quenched trachytic melt samples were analyzed using an electron microprobe (EPMA) and secondary ion mass spectrometry (SIMS).
All halogens exhibit Arrhenian behavior during diffusion in the investigated melt compositions with F always diffusing fastest. The other halogens show progressively slower diffusion (F > Cl > Br > I) correlated to their ionic radii. In anhydrous melt a diffusivity range of 3–4 orders of magnitude is covered among the halogens with DF(1000 °C) ~510-13 m2s-1 and DI(1000 °C) ~110-16 m2s-1. The diffusivities of all halogens increase in hydrous melt yielding for example DF(1000 °C) ~ 310-12 m2s-1, which represents an increase of <1 order of magnitude. However, the largest increase is observed for the slowest-diffusing halogens, e.g., resulting in an increase of up to 2 orders of magnitude for iodine at 1000 °C compared to the anhydrous case. This behavior yields a narrower overall diffusive range of only 1–2 orders of magnitude among all halogens. Activation energies (EA) of all halogens consistently range from ~200–290 kJ mol-1 in anhydrous melts. In hydrous melt EA generally decreases, with the highest decrease determined for F (~131 kJ mol-1) and only slight changes for the other halogens (~201–222 kJ mol-1).
Our diffusivity data of the anhydrous series exhibit a pronounced correlation of diffusivity with the ionic radii, suggesting that halogen diffusion in highly polymerized melt is closely related to the melt’s ionic porosity. The correlation between diffusivity and ionic radius is only weakly observed in the hydrous experiments indicating that the ionic porosity is sufficiently large to weaken the rate-limiting effect of the ionic radius due to the more depolymerized melt structure in the hydrous case. In hydrous experiments, the process of ionic detachment becomes more important as a rate-limiting diffusion mechanism, comparable to the case of diffusion of divalent/trivalent cations or halogen diffusion in basaltic melt.
The results of this study provide the first consistent diffusion dataset including all halogens under naturally relevant magmatic conditions and highlight the pronounced compositional effect of both, major element and dissolved H2O on halogen diffusion. The derived diffusion parameters may be readily used for modelling of diffusive fractionation in silicic melts or determining the timescales of natural silicic volcanic processes based on halogen concentration measurements. Furthermore, these data emphasize the potential of diffusive fractionation among the halogens, especially in a melt of low water content, which may be applied as a monitoring tool for volcanic unrest on actively degassing volcanoes.