Here's why:
* Bowen's Reaction Series: Olivine is at the top of the discontinuous series in Bowen's Reaction Series, which describes the order of mineral crystallization from cooling magma. Minerals at the top of the series crystallize at higher temperatures.
* Mafic Composition: Mafic rocks are rich in magnesium and iron, the primary components of olivine.
* Sills: Sills are tabular intrusions that solidify relatively quickly compared to larger magma chambers. This rapid cooling favors the formation of early-crystallizing minerals like olivine.
Therefore, as a mafic sill cools, olivine would be the first mineral to crystallize out of the liquid rock.
