The motion of negatively buoyant particles embedded within kinematically prescribed convective velocity fields is used to study the evolution of crystal settling in magma chambers. We consider the effects of Stokesian settling velocity, crystal growth, aspect ratio of the convective domain and the site of crystal nucleation on mixing and transport of heavy crystals in this system. The percentage of particles retained in a convecting fluid is very sensitive to the site of nucleation. Particles nucleating along the cold roof or within the wall boundary layers are generally not retained. As a consequence, crystal settling can occur even in rapidly convecting magma chambers. Equilibrium crystal densities are reached relatively quickly, within one overturn, and the distribution of particles deposited on the bottom of the chamber is always asymmetric. This feature may facilitate the formation of primary sedimentary structures in plutons. Laboratory experiments using small, negatively buoyant spheres in a highly viscous fluid with axisymmetric thermal convection confirm the basic predictions of this model.
Bibliographical noteFunding Information:
and a grant from the Minnesota Supercomputer Institute. We thank U.S. Hansen, B.D. Marsh, G. Bergantz, and M. Mangan for many helpful comments.
This research has been supported by NSF EAR-8511200, EAR-8579303 and EAR 86-08479