When a magma containing phenocrysts in concentrations greater than about 8 percent intrudes rock to form a dike or sill, mechanical interactions between the phenocrysts create a grain dispersive pressure. To maintain a balance in the dispersive pressure, the phenocryst concentration must decrease toward the walls of the intrusion because of the increase in the velocity gradient and the fluid viscosity. This mechanism offers an explanation for the observed rapid, but gradational, increases in content from phenocryst-poor margins to a phenocryst-rich center, especially in picritic dikes and sills. Using semi-empirical equations for the dispersive pressure, I found that a plug flow velocity profile yields a phenocryst distribution very similar to those observed in the field.
If the phenocryst interactions are sufficiently intense, then the grain dispersive pressure would be also dependent upon the size of the phenocrysts. This dependence would cause the larger phenocrysts to migrate preferentially to the center of the intrusion and the smaller phenocrysts to the walls, giving a size variation as well as a concentration variation. It is uncertain whether the interactions are sufficiently intense to give size sorting by this mechanism.
At the phenocryst concentrations normally encountered, a review of experimental studies indicates that the Magnus-effect and similar single particle forces are small in magnitude in comparison to the forces of grain interaction. They would have only a modifying effect on the phenocryst distribution produced by the grain dispersive pressure.
Modifications of the phenocryst distribution produced by gravity during and after intrusion are examined. During intrusion, gravity settling of the phenocrysts would be balanced by a gradient of the grain dispersive pressure, causing the phenocryst distribution to be asymmetrical with respect to the center of the intrusion. Phenocryst interactions during gravity settling following intrusion, in which the faster settling larger grains capture smaller grains within their boundary layers, may be responsible for sharp discontinuities observed in the distributions of phenocrysts.