Crystallization against inclined floors in magma chambers commonly releases a light depleted fluid which drives compositional convection. We describe the fundamental fluid mechanics of this process and derive equations describing the buoyanr compositional boundary layer adjacent to the top of the crystal pile. The question of whether magma chambers become zoned as a result of compositional convection from sloping floors, as has been previously suggested, is addressed in a laboratory study. Because crystal sizes in magma chambers are usually smaller than the thickness of the compositional boundary layer, an appropriate laboratory model is thermal convection from a smooth, sloping heat exchanger. We investigate the circumstances under which this configuration leads to thermal stratification and find that only when the experimental tank has an extension with an unheated floor does a filling box process lead to significant stratification. This geometry, however, is geologically unrealistic, requiring that crystallization be confined to only part of the floor.

We conclude that compositional convection from inclined floors will not lead to compositional stratification in magma chambers. Experiments investigating the same problem by cooling and crystallizing aqueous solutions are misleading partly because the size of crystals grown from aqueous solutions is extremely large compared with the thickness of the compositional boundary layer, whereas this is not usually true in magma chambers. This difference has a major influence on the nature of convection and we suggest that caution should be applied in extending the results of aqueous solution experiments involving crystallization to magma chambers. The petrological consequences of this work are (1) the fluid mechanics of crystallization at an inclined floor and horizontal floor are similar and suggestions, made elsewhere, concerning the geological implications in the latter case apply equally to the sloping floor case, (20 large volumes of homogeneous magma can exist in a magma chamber when crystallization is taken place against an inclined floor, and (3) models describing the crystallization of layered intrusions which involve the interaction of crystallization against an inclined floor with compositionally stratified magma require the stratification to be established by an external process. ¿ American Geophysical Union 1988