An experimental and numerical study of surface tension-driven melt flow

To determine the role of surface tension-driven melt migration in planetary bodies, we investigated the effect of static annealing on the evolution of melt-rich bands in partially molten samples. In shear deformation experiments, deviatoric stress causes melt to segregate; when the stress is removed, surface tension causes the melt to relax back to a homogeneous distribution. Samples composed of 76 vol.% olivine  +  20 vol.% chromite  +  4 vol.% MORB were deformed to shear strains of ~ 3.5 at 1523 K, 300 MPa and shear stresses of 20 to 55 MPa. After deformation, the samples were statically annealed for 0, 10, or 100 h. During annealing, melt transport driven by surface tension occurs, but takes place much more slowly than flow driven by deviatoric stress. Finite difference numerical simulations were performed of surface tension-driven melt flow resisted by viscous deformation of the olivine matrix. These models best reproduce the distribution of melt in the annealed samples when the solid viscosity ηs = 1.7 ± 0.5 × 1012 Pa s with n = 2.4 ± 0.3 and b = 9000 ± 1900 in the expression for permeability κ = ϕnd2 / b where d is grain size. The large value of b compared with estimates from geometrical models is probably due to clogging of the melt tubes by the secondary solid phase (chromite). Redistribution of melt by surface tension is likely to be the dominant process in small (~ 10 km radius) planetesimals in the absence of convection or impact-induced deformation. However, this redistribution process is sufficiently slow that large bodies of localized melt (magma chambers) are likely to develop.