Liquid-distribution and attainment of textural equilibrium in a partially-molten crystalline system with a high-dihedral-angle liquid phase

The evolution of a high-dihedral-angle liquid in a crystalline matrix, such as Fe-S melt in an olivine matrix, is of vital importance for a range of questions including planetary core-mantle differentiation. We performed hydrostatic in situ analogue experiments on norcamphor-H2O liquid, and high-temperature experiments on olivine-FeS melt to investigate the evolution of the liquid and the attainment of textural equilibrium in the liquid and the solid. The liquid distribution was dominated by large unconnected liquid pools, smaller melt lenses, and the regular occurrence of dry three-grain triple junctions. Compared to low-dihedral-angle systems, the liquid pockets had a very low mobility during grain growth and disequilibrium features, such as elongated liquid pools, only reequilibrated slowly. The results have several consequences for the evolving microstructure: (1) Immobile liquid pockets hinder grain boundary migration and promote abnormal grain growth; (2) The attainment of textural equilibrium in solid-liquid regions is delayed with respect to liquid-free regions; (3) Grain growth does not promote liquid pocket growth, as in low-dihedral-angle systems, and liquid pockets do not directly interact with each other during annealing; (4) An existing liquid network is unstable and pinches off in the long-term, even if the liquid-fraction is above the theoretical percolation threshold. With regard to this final point, percolative core-mantle differentiation through an interconnected melt network may not be a viable mechanism, even above the suggested percolation threshold of ∼ 5 vol.%.