GAS MIGRATION AND THE FORMATION OF PATHWAYS THROUGH HIGH-CRYSTALLINITY MAGMAS

Many ore-bearing minerals are deposited from magmatic vapors. The formation of these minerals requires that volatiles be transported through the solidification front of crystallizing plutons. We study the physical processes by which a gas phase migrates through crystal-rich magmas, using analog experiments and theory. Two cases can be distinguished, depending on the ability of solid particles to move in the melt phase. In the first case, ascending gas bubbles can move particles aside, allowing bubble pathways to widen. This occurs when particle contents are lower than those at which particles form a rigid framework. We study the formation of pathways in this case with analog experiments, using corn syrup and plastic beads or sand grains (to simulate magma with crystals). The velocity of rising bubbles is dependent on their size and on the local geometry of the particle network. Because of the random distribution and orientation of particles, the rise velocity of a single bubble is highly variable. In addition, particles can cause bubbles to coalesce or split up into smaller bubbles, with different rise velocities. Small bubbles can stay trapped beneath particles for long time periods, allowing them to accumulate relative to larger ones. In the second case, the crystals form a rigid framework with the melt in the pore space between the crystals. The critical crystallinity at which this lock-up point is reached depends on particle shape and size distributions. Bubble migration through such systems is generally approached by modeling bubble rise through a network of connected capillary tubes (the pore space). We develop an equation describing the rise velocity of a large bubble in a capillary tube, where the liquid is allowed to flow out the top of the tube and in through the bottom. We use this to describe bubble migration through high-crystallinity magma. The migration velocity of bubbles calculated in this way is higher than predicted from capillary tube models in which bubble velocity is constrained by the backflow of liquid through the fluid film between the bubble and the tube walls. Our results extend two-phase (gas-liquid) models for volatile transport in magma to crystallinities at which the particles play an active role in hindering bubble rise.