Partially molten plumes and magma fingers: two modes of melt transport through the mantle in terrestrial bodies

Abstract The dynamics of partially molten mantle are central to the thermochemical evolution of terrestrial bodies, and partially molten plumes driven by melt buoyancy have been recognized as a crucial ingredient. Recent numerical models of the lunar mantle, however, have revealed a distinct finger-like structure, “magma finger”, that forms through a mechanism different from these plumes. To clarify the magma finger phenomenon and its relation to partially molten plumes, we performed a linear perturbation analysis and two-dimensional simulations of magma–matrix flow in a horizontal layer, where melt percolates through the convecting matrix. Our study shows that the upward migration of magma takes place in two regimes, depending on the competition between the two kinds of flows both driven by melt buoyancy: the Stokes flow of matrix and the percolation flow of melt. When the retention number Rₓ R t, defined by the ratio of the velocity of the former flow to the latter, is large, the magmatism–mantle upwelling (MMUb) feedback dominates the convective flow: buoyancy of melt generated by decompression during matrix upwelling boosts the upwelling itself. When a solid layer overlies the partially molten layer, the MMUb feedback induces partially molten plumes that ascend through the solid layer. At lower Rₓ R t, in contrast, a perturbation in the melt content in the partially molten layer propagates upward by a melt percolation-dominated instability: the perturbation induces a spatial variation in the rate of matrix expansion/contraction due to upward magma migration, shifting the perturbation upward. When a solid layer is overlaid, the melt percolation-dominated instability causes an instability along the layer boundary, generating magma fingers that extend upward into the solid layer. The calculated threshold in Rₓ R t for the onset of the MMUb feedback suggests that the feedback is responsible for the volcanism that generated the Large Igneous Provinces while being unimportant for hotspot volcanism on the Earth. Since Rₓ R t increases with decreasing matrix viscosity, volcanism caused by the MMUb feedback is likely more important in earlier terrestrial planets with hotter, softer mantles. Magma fingers are, in contrast, expected to have developed in the lunar mantle if a partially molten layer has developed at its base. Graphical Abstract