Villemant, B.

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.

The eruptive history of Mt. Somma–Vesuvius is characterised by large explosive events: Pomici di Base eruption (22,030±175 yr cal BP), Mercato (8890±90 yr cal BP), Avellino (3945±10 yr cal BP) and Pompeii (79 AD). Pre-eruptive conditions and sin-eruptive degassing processes of the Avellino eruption, the highest-magnitude Plinian event, have been investigated, using volatile contents (F, Cl, H2O) in melt inclusions and residual glass, and textural characteristics of pumice clasts of the 9 fallout layers sampled in detail in a representative sequence. The sequence displays an up-section sharp colour change from white to grey, corresponding to variations in both magma composition and textural characteristics. The pre-eruptive conditions have been constrained by systematic measurements of Cl content in both melt inclusions and matrix glass of pumice clasts. The pumice glass composition varies from Na-rich phonolite (white pumice) to Krich phonolite (grey pumice). The measured Cl values constantly cluster at 5200±400 ppm (buffer value), whatever the composition of the melt, suggesting that the entire magma body was saturated with subcritical fluids. This Cl saturation constrains the pre-eruptive pressures and maximum H2O contents at 200±10 MPa and 6.3±0.2 wt.% H2O for the white pumice melt and 195±15 MPa and 5.2±0.2 wt.% H2O for the grey pumice melt. The fluid phase, mainly composed of a H2O-rich vapour phase and brine, probably accumulated at the top of the reservoir and generated an overpressure able to trigger the onset of the eruption. Magma degassing was rather homogeneous for the white and grey eruptive units, mostly occurring through closed-system processes, leading to a typical Plinian eruptive style. A steady-state withdrawal of an H2O-saturated magma may explain the establishment of a sustained Plinian column. Variation from white to grey pumice is accompanied by decrease of mean vesicularity and increase of mean microcrystallinity and permeability related to significant vesicle coalescence. Despite this, the ascending magma column still evolves under closed-system degassing, without significant gas loss through conduit walls. The Avellino eruption shows numerous similarities with the 79 AD Pompeii eruption in pre-eruptive conditions, degassing processes and eruptive style which are discussed here.