Constraints on the Melt Distribution in Anisotropic Polycrystalline Aggregates Undergoing Grain Growth

Abstract

It has long been recognised that at elevated temperatures surface energy is the driving force for the distribution of melts and fluids among crystalline grains. While for ideal isotropic systems only two parameters, the dihedral angle (the ratio of grain boundary energy to solid-liquid surface energy) and the melt fraction are needed to completely constrain the melt distribution, anisotropic systems present a more complex problem. Surface energy minimisation includes, in addition to surface (or interface) area reduction, also interface rotation. Grain growth, driven by surface area reduction of the aggregate as a whole, means that locally interfaces constantly have to readjust their orientation, a feature not present in isotropic systems. In contrast to isotropic systems, where the geometry of the melt network is the same at all melt fractions, no unique link exists between melt fraction and melt geometry for anisotropic systems. This link is the basis for the high permeability calculated for isotropic aggregates. The degree of anisotropy and therefore the deviation from the ideal isotropic model depends not only on the solid but also on the melt or fluid involved. For the system olivine + basaltic melt the differences to the isotropic model are substantial. Since for anisotropic systems no model exists which can predict the texture of a partially molten aggregate, experimentally produced samples are evaluated in order to determine bulk physical properties of partial melts. Due to the high permeability predicted by the isotropic model, the in situ melt fraction in partially molten regions in the upper mantle would be so small that seismic velocities or the dynamic behaviour would remain essentially unaffected by the presence of melt. In contrast, the experimentally observed melt distribution indicates that a finite melt fraction is needed before efficient segregation can begin, which will affect seismic velocities and influence the dynamic behaviour of partially molten regions.