Wetting Angles, Equilibrium Melt Geometry, and the Permeability Threshold of Partially Molten Crustal Protoliths

Abstract

The permeability of a partially molten rock is one of the primary factors controlling the melt segregation rate. With decreasing melt percentage, permeability becomes increasingly sensitive to the grain-scale geometry of partial melt. At low melt percentages, the ratio of grain-boundary energy to solid-melt interfacial energy, [γ _ss ]/[γ _sl ], is the fundamental physical property that determines the equilibrium melt geometry, including the wetting angle θ at a solid-solid-melt triple junction, the area-tovolume ratio s/v of melt pockets at grain corners, and the permeability threshold φ _c (φ _c , is the volume melt percentage at which melt interconnection is established). The trends of increasing θ and φ _c or decreasing s/v with decreasing [γ _ss ]/[γ _sl ] are well established in the case of a monomineralic system with isotropic interfacial energies. We argue that these general trends must apply as well in natural systems where solid-melt interfacial energies are essentially anisotropic.

Measurements of wetting angles at quartz-quartz-melt, feldspar-feldspar-melt and amphibole-amphibole-melt triple junctions consistently yielded low to very low median values, in the range 10°–60°. These low angles result from high values of [γ _ss ]/[γ _sl ], up to 2.0 for the lowest angles. They indicate that the permeability thresholds of partially molten crustal protoliths should be very low: < 1 vol. % to a few vol. %. This result is confirmed in a series of melt-infiltration experiments in which a hydrous granitic melt was placed in contact with a texturally-equilibrated, polycrystalline aggregate of quartz, at 900 °C-1 GPa. Secondary electron imaging of the quartzite close to the melt reservoir revealed the presence of an interconnected network of grain-edge melt channels at a melt percentage < 0.04 vol. %.

The major implication of the experimental studies is to show that melt segregation may potentially operate at very low degrees of melting (theoretically, any value ≥ φ _c ). Because of the high viscosities of granitic melts, melt segregation is presumed to be inefficient at such low degrees of melting. There may exist therefore a range of melt percentages above (φ _c over which the partial melt is interconnected but nearly stagnant.