This is the third of a series of papers which investigates the evolution of a binary alloy that is cooled from above and releases buoyant residual fluid as one component of the alloy is preferentially incorporated within the solid. This paper focuses on the compositional zonation that is produced when the melt is completely solidified. Parts 1 and 2 considered the temperature of the cooled boundary to be greater than the eutectic temperature of the alloy so that only partial solidification of the alloy could occur. Here we extend the study by investigating the effects that arise when the cooling temperature is less than the eutectic temperature. The formation of a completely solid layer results, which extends from the cooling plate down to a mushy zone of dendritic crystals and interstitial melt. The melt below this mushy layer is convectively unstable because it is cooled from above. This generates vigorous thermal convection. Eventually the melt becomes completely solidified and a compositionally zoned solid is formed. The solid below the cooling plate is shown to be of fixed bulk composition until it reaches the depth that the interface between the mushy layer and melt occupied when the melt first became saturated. Below this level, the composition of the solid decreases with depth until it merges into the solid growing from the floor, whose composition is nearly equal to that of the heavier component of the alloy. Results of laboratory experiments with aqueous solutions of sodium sulphate are in good agreement with our quantitative predictions. The model is used to give an interpretation of profiles of the composition of magnesium oxide found within solidified komatiite lavas.
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