Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp34a..04k&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #GP34A-04
Physics
1038 Mantle Processes (3621), 8121 Dynamics: Convection Currents, And Mantle Plumes, 8137 Hotspots, Large Igneous Provinces, And Flood Basalt Volcanism, 8180 Tomography (6982, 7270)
Scientific paper
Present-day seismic mantle imaging reveals complex structures below hotspots. The classical plume model of a mushroom-shaped thermal anomaly rising from a steady localized heat source is unable to explain key observations such as the patchy nature of the seismic tomography images. Here we present a series of laboratory experiments on a thermochemical plume generated from a thermal boundary layer which is stratified in composition. For quantitative analysis, we invented a new measurement method to visualize temperature, composition, and velocity fields simultaneously. For the description of the dynamics of a laminar thermochemical plume out of a thin dense layer, four dimensionless parameters (Rayleigh number, Buoyancy ratio, thickness ratio, and viscosity ratio) should be addressed, and here we systematically varied initial Buoyancy ratio B0 which is the ratio of the stabilizing chemical buoyancy to the destabilizing thermal buoyancy at the onset of convection. When B0=0, a purely thermal plume which has a large plume head and a narrower conduit is produced. For large B0 (>1), the thermal density anomaly cannot counterbalance the compositional anomaly and convection develops above the compositional interface. For intermediate B0, the interplay between the thermal and compositional effects generates complicated morphologies. Because all plumes cool by thermal diffusion as they rise in a cooler mantle, a chemically composite thermal plume will eventually attain a level of neutral buoyancy, at which it will begin to collapse. Separation within the plume will occur, whereby the chemically denser material will start to sink back while the heated surrounding mantle keeps rising. Experimental scaling laws predict maximum height of the chemically composite plumes and could well explain the time-dependence and morphology of a melting anomaly such as Iceland hotspot. Our laboratory experiments more generally imply that 1) mantle plumes are not necessarily narrow and continuous throughout the mantle but can be fat and patchy, and 2) a hot mantle region may not be buoyant and rising, but on contrary may be sinking.
Davaille Anne
Kumagai Ichiro
Kurita Kazuyoshi
Stutzmann Eléonore
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