Modelling the dynamics and thermodynamics of volcanic degassing

The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas i n a vertical conduit linking a shallow degassing zone with a deep magma cha mber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO2-saturated deep chamber and adiabatic cooling of ascending magma. We find that overtu rn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission fate i s proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power. and inversely proportional to the degassed mag ma viscosity) and th e mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this damp ens co nvection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved w ater, the similar to 700kg s(-1) magmatic water flux can be modelled with a 4-10 m radius conduit, degassing 20-100% of the available water and all of the 1 to 4 vol.% CO2 chamber gas. At Mount St. Helens in June 1980, assumi ng a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissol ved water, the similar to 500 kg s(-1) magmatic water flux can be modelled with a 22-60 m radius conduit, degassing similar to 2-90% of the available water and all of the 0.1 to 3 vol.% CO2 chamber gas. The range of these res ults is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles fro m deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes.