Compositional convection effects during the degassing of a stratified magma

The magma degassing associated with a volcanic process can occur during bot h the magma ascent through a conduit toward the surface, followed by erupti on, and its long-term existence in a chamber beneath a volcano. In the firs t case, the degassing can reach catastrophic rates leading to disastrous co nsequences., in the second case, volatiles can be released for a long time, moving through cracks toward the surface at nearly magmatic temperatures. The currently existing models are based on the assumption that magma chambe rs beneath volcanoes should have the form of a bottle, i.e., a main chamber extends upward as a rather narrow vertical channel, and the pressure decre ase accompanying the ascent in this channel favors its degassing. The degas sed and cooled magma sinks into the main chamber. However, the assumption o f a long-lived narrow molten channel is not valid for all volcanoes. This p aper presents a somewhat different possible model of gradual degassing in t he magma chamber, which is associated with compositional convection in the chamber itself and does not require the existence of a vertical channel. If the chamber is sufficiently close to the surface and an appropriate amount of cracks are present in the overlying layer (including the solidified con duit through which the previous eruption occurred), the pressure at the cha mber boundary can be sufficiently small, and degassing can occur in the cha mber itself. This effect is modeled by a fixed pressure (smaller than its l ithostatic values) at the top of the chamber, taken as a boundary condition . The magma degassing in the chamber with convective mixing of the melt and bubbles is calculated. Models with different initial depth distributions o f concentrations of water dissolved in the melt and with different composit ions of magma layers are considered. The two-phase convective flow velociti es are found. The method of markers is used for solving the transport equat ions of the fluid phase and melt. The evolution of the melt degassing and m ixing is calculated for the models considered. The evolution of convective flow velocities, as well as vertical and lateral distributions of concentra tions of the gaseous phase and dissolved water in the chamber, is calculate d. it is shown that the dormant fumarole regime of volcanoes is possible in a chamber of an arbitrary shape without assuming the existence of a narrow vertical channel above the chamber.