Physical controls of nucleation, growth and migration of vapor bubbles in partially molten cumulates
The interstitial melt in partially molten cumulate piles in layered intrusions must at some point reach saturation with a volatile phase such as water vapor or hydrosaline melt. A number of models have been proposed in which orthomagmatic fluids migrate through partially solidified cumulates and participate in the formation of ore deposits. Here I examine the topology of the crystal—melt—vapor system in a cumulate and relate this to the role of capillary forces in governing the size and mobility of individual vapor bubbles. Capillary forces will play a dominant role in setting the number density and sizes of bubbles. In any cumulate rock with crystals smaller than several cm in diameter, bubbles of the postcumulus aqueous phase will be unable to migrate away from their sites of nucleation and growth. Although bubble stranding does not preclude the eventual loss of volatile constituents from the cumulate after complete solidification, it does prevent such constituents from migrating while the system remains partially molten and therefore prohibits them from participation in magmatic-hydrothermal interactions within the magma chamber.