Partially Molten Plumes and Melt-Fingers: Two Modes of Magma-Transport through the Mantle in Terrestrial Bodies

Abstract

To understand the dynamics of partially molten mantle in terrestrial bodies, we carried out a linear perturbation analysis and 2-D numerical simulations of magma-matrix flow in a horizontal layer, where decompression melting generates magma that percolates through the convecting matrix. Our study shows that there are two regimes for the upward migration of magma, depending on the melt-buoyancy parameter B_m, which is the ratio of the Stokes velocity of matrix to the percolation velocity of melt, both driven by the melt-buoyancy. At large B_m, the magmatism-mantle upwelling (MMUb) feedback dominates the convective flow in the layer: decompression melting during upwelling enhances magma buoyancy, which further strengthens the upwelling. When a solid layer is overlaid on the partially molten layer, the MMUb feedback induces partially molten plumes that ascend through the solid layer by their melt-buoyancy. At lower B_m, in contrast, a perturbation in the melt-content in the partially molten layer propagates upward as a porosity wave: the perturbation induces a spatial variation in the rate of expansion or contraction of matrix caused by magma migration, leading to an upward shift of the perturbation. When a solid layer is overlaid, the porosity wave develops also along the layer boundary to induce a finger-like magma structure, or melt-finger, that extends upward into the solid layer. The threshold value of B_m for MMUb feedback suggests that it can explain volcanism forming Large Igneous Provinces, but not hotspot volcanism on Earth. Since B_m increases with decreasing matrix viscosity, volcanism caused by the MMUb feedback is likely to have been more important in earlier terrestrial planets where the mantle was hotter and softer. Melt-fingers are, in contrast, expected to have developed in the lunar mantle if a partially molten layer has developed at its base in the history of the Moon.