HR: 09:24h
AN: U11B-07    [Abstracts]
TI: Multi-scale Convection in a Mantle with Strongly Temperature-dependent Rheology
AU: * Androvandi, S
EM: androvandi@ipgp.jussieu.fr
AF: Institut de Physique du Globe, 4 Place Jussieu, PARIS cedex 05, 75 252, France
AU: Davaille, A
EM: davaille@ipgp.jussieu.fr
AF: Institut de Physique du Globe, 4 Place Jussieu, PARIS cedex 05, 75 252, France
AB: Owing to the heterogeneity and complex rheology of mantle material, several scales of convection coexist in the Earth's mantle, producing cold linear slabs, mid-ocean ridges, 3D superswells and hot spots. However, if those features have individually been generated and studied in numerical and laboratory experiments, the exact conditions for their coexistence in a self-consistent convective model have remained elusive. We studied the characteristics of thermal instabilities developping when a layer of sugar syrup, a fluid with a strongly temperature-dependent viscosity and high Prandtl number (> 7000), is heated from below and cooled from above. A new visualization technique allowed to determine both the temperature and velocity fields inside the experimental tank. We focuss on high Rayleigh numbers (1.7x10⁶ to 3.3x10⁷) and intermediate viscosity ratios (7 to 4100). For this parameter range, "sluggish lid" convection occurs, in which three different scales of convection develop. Owing to the viscosity increase with decreasing temperature, the tank thermal structure becomes asymmetric: thermal boundary layer (TBL) instabilities, typical of high Rayleigh number convection, develop under the coldest, therefore most viscous, part of the upper thermal boundary layer which cannot move as fast as the less viscous fluid. The largest convective scale is therefore cellular, with cold downwelling sheets of viscous fluid encasing hotter parts of the tank. Within each of those cells develop several (typically 3 to 7) hot 3D upwelling plumes. Upon impinging under the cold TBL, each plume in turn generates locally a small ring of cold material which does not reach the bottom of the tank. The introduction of a denser layer at the bottom of the tank can vary the morphology of the hot instabilities but has no influence on the existence of the large-scale cold circulation. Hence high Rayleigh thermal convection in the sluggish lid regim can produce large-scale cells delimited by cold subducting slabs, within which several 3D plumes develop. On Earth, two of such cells exist, the Pacific and the Indo-Atlantic boxes. Our experiments further suggest that what has been named the two "hot superplumes", i.e. the two seismically slow regions encased within the subduction rings, are in fact each constitued of several hot instabilities.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7270 Tomography (6982, 8180)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8410 Geochemical modeling (1009, 3610)
SC: Union [U]
MN: 2007 Fall Meeting