Solidification Front Fractionation in Phenocryst-Free Sheet-Like Magma Bodies

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Abstract
Basal cumulate zones are common in sills formed of magma-carrying phenocrysts upon emplacement, and the process leading to these cumulates and the corresponding roofward depleted zones are well understood. Many large sills which seem to have been emplaced with phenocryst-free magma, are nearly uniform in composition and show little tendency to differentiate by crystal settling. We examine whether, for an initially crystal-free sill solidifying inwardly from top and bottom, crystals nucleated and grown along the roof can escape incorporation, survive resorption in settling through the hotter central region, and reach the basal solidification front to form a cumulate. We call this process solidification front fractionation and build a relatively simple model to examine the rheologic, dynamic, and crystal growth rate conditions leading to the downward escape of newly grown, single crystals from the roof of a sheet-like body of mafic magma cooling by conduction. Our results suggest that, whereas a thin micro-cumulate may form near the center of the body, it is highly unlikely that solidification front fractionation will lead to significant basal accumulation of crystals. This finding is consistent with findings from various basaltic sills.