Magma degassing process during Plinian eruptions

To assess magma degassing processes during Plinian eruption, noble gas abundances and isotopic ratios have been determined in pumices by crushing under vacuum. Samples come from Plinian eruptions, which occur along different subduction zones. In addition, two pumice samples come from the Azores hotspot. All samples are characterized by a systematic enrichment in neon over argon, a depletion of krypton relative to argon and an isotopically fractioned 38Ar/36Ar ratio. These features do not depend on geological setting, or on pumice age, or eruption intensity. However, they are similar for pumices from the same eruption. A correlation is observed between 84Kr/36Ar and the 38Ar/36Ar ratio. This illustrates that only one physical process is at the origin of the fractionation. We propose here a model of kinetic magma degassing before fragmentation to explain the elemental and isotopic fractionation. Noble gases diffuse in a magma shell surrounding a preexisting bubble. The model put forward an alternative to the model of Pinti et al. (1999), in which diffusion occurs after pumice quenching in Plinian plume. The model explains measurements and shows the rapidity of the magma degassing process in the conduit (few hundreds of seconds).