Degassing of stratified magma by compositional convection

A new model is proposed for passive degassing from sub-volcanic magma chamb ers. The water content in stably stratified shallow magma chamber will be e quated to its solubility at the upper boundary by convection. Water from a lower layer high in water content can enrich the contact zone of the upper layer and lead to further convective overturn of this boundary layer. A com plete set of equations describing convection with bubble formation and diss olution is reduced to a simplified form by assuming a small bubble content. The development and pattern of flow driven by vesiculation is modeled nume rically in a 2D magma chamber for relatively low Raleigh numbers (5 x 10(5) ). Bubbles rising from the magma will collect near the roof in a layer of 8 -10 vol% and then escape upward to fumaroles. The Stokes flux of bubbles es caping from an andesitic magma with viscosity 10(4) P and a top surface of about 500 x 500 m corresponds with observed total magmatic water fluxes of 35 kg/s. Pressure within the chamber is buffered by elastic (and local visc o-elastic) deformations in the solid rocks bounding the chamber to the rang e between ambient ansi close to lithostatic values. In a chamber closed to fresh magma inputs, the decrease in volume due to such gentle volatile esca pe lowers the reference pressure. Bubbles flux from the lower layer induced by variation of the saturation level around stratification boundary may be efficient mechanism for the water transport between layers. (C) 2001 Elsev ier Science B.V. All rights reserved.