Surface-energy considerations can be used to show that a magma body in chemical and thermal equilibrium with its surroundings will tend to disperse by infiltrating the dry grain edges of the host material. This prediction was tested in a series of “melt infiltration” experiments on a simplified basalt-peridotite analog system (forsterite-albite-anorthite). At 1290 °C and 10 kb, a melt of ∼ Fo23Ab38An39 composition penetrates the grain edges of poly-crystalline forsterite “rock” at a rate of 1 to 2 mm/day. The infiltration mechanism probably involves chemical solution of forsterite at grain edges and simultaneous precipitation of forsterite crystals within the melt reservoir. The term “chemical sloping” is proposed to describe the operation of this process in nature, and although it may have little influence on the major-element composition of magmas, it may be effective as a zone-refining mechanism.
The intergranular “mobility” of magmatic liquid in near-solidus country rock implies that (1) silicate melts may be important metasomatizing fluids under some circumstances in the upper mantle, and (2) small dikes and veins are unstable in hot surroundings and will disperse by infiltrating their wall rocks.