Compositional convection and layering in a rock melt

Abstract

When a crystal grows (or dissolves) in a solution it becomes surrounded by a thin boundary zone of liquid that is relatively depleted (or enriched) in crystal components. The compositional difference and, hence, the horizontal density gradient that exist in this zone, may cause it to convect away from the crystal1. This process of natural convection is a possible cause of fractionation and liquid layering in crystallizing magmas2–9. Experiments with aqueous solutions have demonstrated the efficacy of the process in model magma chambers4,10, and recent growth and dissolution experiments involving minerals in rock melts have suggested that compositional convection indeed occurs in silicate systems11–15, but the process has not been demonstrated unequivocally for rock melts. Here we report on simple experiments involving silica dissolution in a superheated mafic rock melt in which compositional layering evidently occurred. The results are consistent with the operation of compositional convection during dissolution, and suggest that more rigorous experiments will be useful in testing some of the new principles of fluid motion recently introduced to petrology.