Equilibrium fluid distribution in an ultramafic partial melt under hydrostatic stress conditions

Abstract

The geometrical distribution of the fluid phase in partially molten mafic material has been studied under conditions providing close approach to textural equilibrium. Mechanical mixtures of natural dunite and basalt were held at temperatures slightly above solidus and at 10‐kbar confining pressure for time durations exceeding 160 hours. For melt fractions of 1–2% fluid phase observed as glass in quenched specimens occurs as a completely interconnected network of channels along intergranular edge intersections. The intergranular facial contacts appear dry, and the grains appear to be in mechanical contact across these faces within the 200‐Å resolution of a scanning electron microscope. Grain‐grain intersection angles near 120° occur between the solid phases, suggesting that crystalline anisotropy plays a minor role in defining surface energies. These results confirm that surface energy plays a dominant role in determining fluid distribution in equilibrium partial melts of silicate composition.