| Summary: | Recent debate has questioned the efficiency of magma differentiation in basaltic magma chambers (Marsh, 2013). As a result, petrologists are increasingly inclined to interpret olivine cumulate layers in small intrusive bodies as separately injected slurries, as opposed to the result of in-situ differentiation. Two sills with basal olivine layers, the 0601 sill, Victoria Island, NWT, Canada, and the Palisades Sill, Fort Lee, NJ have been examined in detail. First, it can be shown that the 0601 sill could have formed by the emplacement of a single phenocryst bearing magma, followed by settling of the initial phenocryst load, and in-situ differentiation. Second, a high-resolution analysis of the internal ordering of the Palisades Sill olivine horizon, widely recognized as the result of an olivine slurry, is described in order to create a generalizable model of basal olivine layers formed via slurry emplacement (Husch, 1990; Gorring, 1995). Both sills were analyzed for whole rock major and trace element compositions, mineral compositions, crystal size distributions and modal mineralogy. Parent-Daughter modeling, Pearce Element Ratio Analysis, trace element modeling, and MELTS (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) were then used to model formation mechanisms. Finally, the 0601 sill olivine cumulate layer is com-pared with the Palisades Sill olivine horizon. While both olivine cumulate zones look superficially similar, detailed investigation reveals that a well developed trend of progressively increasing olivine upward from the base, as observed in the 0601 sill is best explained by crystal settling within an initial phenocryst-phyric magma, followed by in-situ differentiation, and cannot be explained by a slurry emplacement process.
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