F and Cl diffusion in phonolitic melts: Influence of the Na/K ratio

Abstract

Fluorine and chlorine diffusion were measured in two natural phonolitic melts, from Vesuvius (Italy) and from Laacher See (Germany), at 0.5 and 1.0 GPa, between 1250 and 1450 °C at anhydrous conditions and with about 2 and 5 wt.% of dissolvedwater. The two different startingmaterials allowus to investigate the alkali effect,Na vs. K, on halogen diffusion.One compositionwas a K-rich (~10wt.%) phonoliticmelt corresponding to thewhite pumice phase of the 79ADeruption of Vesuvius, and the other aNa-rich (~10 wt.%) phonoliticmelt corresponding tomost differentiated melt of the 12,000 BC eruption of Laacher See. The diffusion-couple technique in a piston cylinder was used for the experiments. Experiments were performed with only one halogen diffusing and with the simultaneous diffusion of a halogenmixture (F, Cl, Br) in order to evaluate the interactions between the halogens during diffusion. Diffusion coefficients for F range between 2×10−11m2/s at 1250 °C and 7×10−11m2/s at 1450 °C for the Na-rich melt and between 1×10−11 m2/s at 1250 °C and 8×10−11 m2/s at 1450 °C for the K-rich melt at anhydrous conditions. Diffusion coefficients for Cl range between 2×10−12 m2/s at 1250 °C and 1×10−11 m2/s at 1450 °C for theNa-richmelt and between 7×10−12m2/s at 1250 °C and 2×10−11m2/s at 1450 °C for the K-richmelt at anhydrous conditions. Fluorine diffusivity is higher than Cl in the Na-rich-phonolitic melt by one order of magnitude,whereas in the K-rich-phonoliticmelt F and Cl diffusivities are similar. The effect ofwater is significant for Cl in both Na-rich and K-rich melts: the addition of water enhances Cl diffusivity by up to one order of magnitude, butwater does not significantly affect F diffusion. F and Cl diffusivities always differ fromone another in the same phonoliticmelt composition. F diffusivities are similar in both compositions. Conversely, Cl diffusion depends upon the dominant alkali. These results evidence that halogen diffusivitymay represent a limiting factor for their degassing during rapid syneruptive decompression and vesiculation of H2O-rich-phonolitic melts. The contrasting volatile diffusivities of F and Cl in silicate melts duringmagma vesiculation may be a key, controlling factor of the composition of the vapour phase (bubbles) produced. Such diffusion controlled degassingmodelmay explain the absence of F and Cl degassing observed during the 79AD eruption of Vesuvius.