Effects of the gravitational field on liquid distribution in partial melts within the upper mantle

Abstract

The gravitational field has a major effect on the geometrical distribution of liquid in partial melts occurring within the earth, provided that local conditions and kinetic factors favor stabilization of the melt in contiguous association with individual crystalline grains as opposed to vertical melt segregation. As a result of gravity and the density contrast between the liquid and the crystalline phases, the differential pressure ΔP existing between the two will increase with increasing elevation above the base of a partially molten zone whose fluid phase is connected vertically. This differential pressure must be balanced by the effect of surface tension acting along the curved liquid‐crystal interfaces. The changes in ΔP imposed by gravity can be mechanically compensated for only by increased positive curvature of the interfaces with increased elevation. This forces closure of liquid pathways when ΔP exceeds a critical value which is dependent on vertical distance over the base of the zone. Above the elevation at which the fluid becomes unconnected, the pressure difference, due to buoyancy, can no longer be transmitted from below, and melt will once more be interconnected upward through another limited column until excess fluid pressure again forces closure of connected channels. The result is vertical stratification of liquid phase connectivity into horizontal layers of partial melt with high connectivity separated by regions in which the melt occurs as isolated pockets. Stratification heights are estimated for the cases of (1) homogeneously distributed crystalline phases and (2) a rhythmically zoned occurrence of a minor, nonwetting crystalline phase. They are a few centimeters to a few tens of centimeters and a few hundred meters to a few kilometers, respectively, for the two cases. If, however, the melt fraction exceeds a critical value within the range of 5–22%, the liquid phase must interconnect vertically, and some fraction or all of the liquid may collect in sills or migrate vertically out of the system. Possible implications for the seismic and electrical properties of the mantle are also considered.