Segregation processes in vesiculating magmas

Abstract

Vesiculation of crystallising magma can produce either a mobile vesicular magma or a rigid network of crystals containing vesicular liquid. Where partially crystallized rigid mush underlies less-crystallized magma, such as near the base of a lava flow or in the cumulus pile of a magma chamber, evolved interstitial melt and/or gas may escape into the main body of magma. The consequences of this may include contamination of the overlying liquid with gas and interstitial melt, or intrusion of diapirs of vesicular evolved liquids to form vertical vesicle cylinders and other segregation features found in many basaltic lava flows and sills. Analog experiments were used to investigate some of the phenomena that can arise during vesiculation within a crystal mush, which was simulated by pumping air through a porous plate that formed the floor of a container filled with a viscous liquid floored with a layer of glass beads. Experiments used either a single liquid or two stably stratified liquids with a liquid interface either coincident with the top of the porous layer of beads or slightly above the porous layer. For a range of liquid viscosities and air flow rates (vesiculation rates), individual bubbles emerged from the top of the porous layer of beads and carried a thin trail of interstitial liquid into the overlying liquid. The number of bubble trains leaving the surface of the porous bed increased with decreasing liquid viscosity and flow rate, and with increasing bead size (and, hence, with increasing permeability). Analog vesicle cylinders, composed of diapirs of bubbly interstitial liquid, were produced only when a layer of buoyant bubbly liquid lay above the surface of the porous layer. The relative size of the bubbles and constrictions within the porous layer are argued to control whether individual bubbles (leading to bubble trains) or vesicular liquid (leading to vesicle cylinders) leaves the porous layer and hence whether vesicle cylinders can form.