HR: 1340h
AN: V53A-1530    [Abstracts]
TI: Gas Segregation in Dykes and Sills
AU: * Menand, T
EM: T.Menand@bristol.ac.uk
AF: University of Bristol, Centre for Environmental and Geophysical Flows, Department of Earth Sciences, Bristol, BS8 1RJ United Kingdom
AU: Phillips, J C
EM: J.C.Phillips@bristol.ac.uk
AF: University of Bristol, Centre for Environmental and Geophysical Flows, Department of Earth Sciences, Bristol, BS8 1RJ United Kingdom
AB: Many basaltic volcanoes emit a substantial amount of gas over long periods of time, whilst erupting relatively little degassed lava, implying that gas segregation must have occured in the magmatic systems. The geometry and degree of connectivity of the plumbing system of a volcano control the movement of magma in that system and could therefore provide an important control on gas segregation in basaltic magmas. We investigate gas segregation in a simple geometry consisting of a vertical conduit connected to a horizontal dyke or sill by means of analogue experiments. Degassing is simulated by electrolysis, producing micrometric bubbles in viscous mixtures of water and golden syrup. The presence of exsolved bubbles induces a buoyancy-driven exchange flow between the conduit and the intrusion that leads to gas segregation. Bubbles segregate from the fluid by rising and accumulating as a foam at the top of the intrusion, coupled with the accumulation of denser degassed fluid at the base of the intrusion. Steady-state influx of bubbly fluid from the conduit into the intrusion is balanced by outward flux of lighter foam and denser degassed fluid. The length and time scales of this gas segregation are controlled by the rise of bubbles in the horizontal intrusion. Comparison of the gas segregation time scales with that of the cooling and solidification of the intrusion suggests that segregation is more efficient in sills than in horizontal dykes, and that it could be efficient in intermediate as well as basaltic magmas. Gas segregation has also implications for the generation of gas-rich and gas-poor magmas in persistent basaltic volcanoes. At low magma supply rates, very efficient gas segregation is expected, inducing episodic degassing activity that erupts relatively gas-poor magmas whereas at higher magma supply rates, gas segregation is expected to be less effective leading to stronger explosions erupting gas-rich as well as gas-poor magmas. These general physical principles have been applied to Stromboli. Their implications are consistent with independent field data. Gas segregation at Stromboli is likely to occur in a shallow reservoir of sill-like geometry at 3.5 km depth with exsolved gas bubbles 0.1-1 mm in diameter. Transition between Strombolian activity erupting gas-poor, highly porphyritic magmas and violent explosions that erupt also gas-rich, low porphyritic magmas would occur at a critical magma supply rate of 0.1-1 m³/s.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8430 Volcanic gases
DE: 8439 Physics and chemistry of magma bodies
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005