Wetting behavior of partial melts during crustal anatexis: the distribution of hydrous silicic melts in polycrystalline aggregates of quartz

The equilibrium distribution of hydrous silicic melts in polycrystalline aggregates of quartz was characterized in a series of partial melting and melt distribution experiments in the systems quartz-albite-orthoclase-H₂O and quartz-anorthite-H₂O, at 650 to 1000 MPa and 800 to 900° C. Near-equilibrium textures in these experiments are characterized by very low quartz-quartz-melt wetting angles, and by a substantial number of thin melt films along grain boundaries. Wetting angles in the H₂O-saturated experiments are as follows: 18° at 800° C-1000 MPa, and 12° at 900° C-1000 MPa in the granitic system; 18° at 850° C-650 MPa, 15° at 900° C-650 MPa, and 15° at 900° C-1000 MPa in the quartzanorthite system. In the granitic system at 900° C-1000 MPa, a decrease of H₂O content in melt from 17 wt% (at saturation) to 6 wt%, results in a slight increase of wetting angle from 12° to 16°. These low wetting angles — and the observation that many grain boundaries are wetted by melt films-indicate that the ratio of quartz-quartz to quartz-melt interfacial energies (_ss/_s1) is high: 2. Secondary electron imaging of fracture surfaces of melt-poor samples provided a three-dimensional insight into the geometry of melt; at low melt fraction, melt forms an interconnected network of channels along grain edges, as predicted for isotropic systems with wetting angles below 60°. This high-permeability geometry suggests that the segregation of granitic melts is not as sluggish as previously anticipated; simple compaction calculations for a permeability range of 10^-12 to 10^-9 m² indicate that segregation may operate at low to moderate melt fractions (below 30 vol. %), within relatively short time-scales, i.e., 10⁵ to 10⁶ years. Quartzmelt textures show significant deviations from the equilibrium geometries predicted for isotropic partially molten systems. The most consistent deviation is the pervasive development of crystallographically-controlled, planar faces of quartz; these faces provide definitive evidence for non-isotropic quartz-melt surface energy. For most silicates other than quartz, the grain-scale distribution of partial melts deviates even more significantly from equilibrium distributions in isotropic systems; accordingly, in order to describe adequately melt distributions in most natural source regions, the equilibrium model should be modified to account for anisotropy of solid-liquid interfacial energy.Contribution CNRS-INSU-DBT n^o 651