STRUCTURE OF THE UPPER BOUNDARY LAYER OF A SOLIDIFYING INTRUSION WITH CRYSTAL SEDIMENTATION

Abstract

Cooling of magmatic intrusions within the upper crust is associated with convection and crystallization. A one-dimensional crystallization model, with sedimentation, in the upper boundary layer is presented. The sedimentation velocity is assumed to be constant. A two-component, two-phase system is considered, assuming thermodynamic equilibrium. A steady-state solution for the system of equations of conservation of energy and mass is found. The structure of the boundary layer is characterized by the distribution of the crystals which depends strongly on the ratio, j, of the sedimentation velocity to the velocity of the solidification front. For ratios greater than 1, the crystal distribution produces a stable density gradient within the boundary layer, contrary to the widespread intuition that the appearance of crystals will produce a strong hydrodynamic instability. A special Stefan-like condition for crystal content on the front of solidification is proposed. It is found that a jump in the distribution of crystals exists at the boundary, providing conservation of mass in the whole system. Numerical modelling of the time-dependent problem shows that, in the transient period, an unstable stratification may develop which may possibly lead to two-phase plume convection within the whole magma chamber. At the same time the upper part of the boundary layer becomes depleted with crystals and may not take place in the convection. This stable sublayer may be attributed to the occurrence of light acidic caps in rhyolitic magma chambers under the volcanos.