Dehydration-melting of solid amphibolite at 10 kbar: Textural development, liquid interconnectivity and applications to the segregation of magmas

Abstract

Anisotropic crystal structures and rock texture control liquid morphology and distribution during dehydration-melting at 10 kbar in solid cylinders of lineated amphibolite (mode: hornblende 70%, plagioclase 30%), sealed in gold capsules, in piston-cylinder runs ranging from 21 days at 850 °C to 4 days at 1000 °C. The shapes of most liquid pockets are crystallographically-controlled, with many corners having angles greater than 60°. Few crystal/liquid triple junctions develop the interfacial energy-controlled dihedral angles (θ), which form in experiments using finely-ground powders of minerals with poor cleavage. Liquid interconnectivity probably is attained at 875 °C with only 2% liquid, indicating that dihedral angles less than 60° may not be necessary to achieve interconnectivity in partially melted metamorphic rocks. The surfaces between elongated grains in lineated rocks can become pathways for the migration of liquid or the diffusion of components. By 850 °C, hornblende begins to dehydrate at internal nucleation sites, producing a texture of hornblende rims and clinopyroxene cores (generally attributed to hydration of clinopyroxene). Within the temperature interval of 850–900 °C, transient vapor generates layers of low viscosity, H_2O-saturated, granitoid liquid between hornblende and plagiocase crystal faces, potentially capable of segregation if time-temperature relationships are suitable. At higher temperatures the increased liquid fraction is H2O-undersaturated, with viscosity too high to permit segregation. There is a prospect that segregation of initially hydrous liquids could contribute to the dehydration of low-potassium amphibolites and effectively remove incompatible trace elements during the transition from amphibolite-facies to granulite-facies. Further experiments are needed to study the effects of time and temperature on textures in anisotropic rocks, particularly lineated amphibolites.

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