Abstract
The structures of igneous cumulates can provide valuable clues to inaccessible magmatic processes. Many mafic layers in the Rhum Intrusion have ‘finger structures’1–3 that protrude into the overlying felsic rocks. These structures are attributable to melting of the overlying olivine–feldspar rocks by hot mafic magma emplaced as sills4. The bases of mafic layers contain smooth-bottomed olivine cumulates. Here I show that the heat needed to melt the roof of the layer is carried by rapidly rising light solute released by growth of basal olivine crystals that were deposited by two phase-convection. Compositional convection therefore drives a heat pump transfering heat of fusion from remote growing crystals to those that are melting. This action contrasts with cooling under a refractory roof, which is conduction-limited. The heat pumping principle shows why ultramafic magmas may melt their roofs whereas felsic magmas tend to form upper border zones.
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