HR: 17:15h
AN: U34A-06    [Abstracts]
TI: Complexity in the Thermal Structure of Deep Mantle Plumes
AU: * Houseman, G A
EM: greg@earth.leeds.ac.uk
AF: University of Leeds, School of Earth and Environment, Leeds, LS2 9JT United Kingdom
AB: Thermal plumes are an intrinsic part of any convection system in which the lower boundary is heated. Recent analyses of the core dynamo conclude that heat flow out of the core could be a significant fraction of total surface heat flow. In this case hot thermal plumes should be an important element of the mantle convection system. Thermal plumes arise because fluid in the lower thermal boundary layer detaches from the boundary and moves upwards, either steadily or as transient bursts of hot fluid. Recent seismic images of plumes by Montelli et al. (Science, 203, 338-343, 2004) show considerable internal structure and depth variation of plume velocity anomalies (interpreted usually as thermal anomalies). Some plumes appear to originate from the base of the mantle, others perhaps from shallower depths. I have carried out simplified numerical experiments of constant-viscosity convection at high Rayleigh number (Ra) in order to obtain model plume structures that can be compared with the seismic images. At Ra = 5,000,000, with isothermal boundaries and a plausible rate of internal heating, complex plume structures are obtained. In general a narrow core of hot fluid extends from lower to upper surface, with the thermal anomaly decreasing with height above the lower boundary (contrary to the seismic images). Plume locations at mid-mantle depths are determined by an irregular network of hot sheets, just above the lower boundary. Upward flow is focussed into plumes at the nodes where these hot sheets are joined. Cross-sections of model plumes at mid-mantle depths typically reveal an elongated thermal anomaly, possibly with two or more plume cores in close proximity. The flow pattern has long-term persistence, but in detail has variable time dependence. In particular, boundary layer instabilities that produce new plumes are most likely to occur close to existing plumes, creating new plume conduits that rise parallel to existing conduits. The overall geometrical similarity between seismic and model plume images inspires confidence in both the seismic images and the numerical simulations. Differences between model and seismic images may yet be exploited to obtain improved models of mantle convection properties.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8130 Heat generation and transport
DE: 3230 Numerical solutions
SC: Union [U]
MN: 2004 AGU Fall Meeting