Diffusion of halogens (F, Cl, Br, I) in silicic melt

Abstract

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 behaviour 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 with the largest increase being 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 diffusion consistently range from ∼200 to 390 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, however, this correlation is attenuated in hydrous melt. While in anhydrous melt, halogen diffusion is closely related to ionic porosity, in hydrous experiments, the process of ionic detachment becomes more important as a rate-limiting diffusion mechanism, e.g., comparable to the case of diffusion of divalent/trivalent cations. 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, which may be applied as a monitoring tool for volcanic unrest on actively degassing volcanoes.