HR: 16:20h
AN: V44B-02    [Abstracts]
TI: Implications of Heat Flow in the Triaxial Earth on Layered Convection and Plume Formation
AU: Criss, R E
EM: criss@wustl.edu
AF: Washington U., Dept. Earth and Planet. Sci., 1 Brookings Dr., St. Louis, MO 63130 United States
AU: * Hofmeister, A M
EM: hofmeist@wustl.edu
AF: Washington U., Dept. Earth and Planet. Sci., 1 Brookings Dr., St. Louis, MO 63130 United States
AB: Perception of Earth as vigorous arises from the discrepancy between model-dependent estimates of global heat flux (Q) and bulk radiogenic content, which necessitate additional sources and large secular delay. Weak, layered mantle convection is instead indicated by downward revision of these parameters, and by new theoretical models and measurements on the variation of thermal conductivity (k) with temperature. Hydrothermal circulation has been used to justify Q=44 TW derived from the half-space cooling model, rather than 31 TW obtained directly from measurements, yet MOR magmatism provides at most 4 TW. The half-space cooling model assumes inappropriate 1-D boundary conditions, resulting in infinite flux along the ridge centers over all time. Geological observations, inferred mantle overturn rates, estimated mantle cooling rates, and recent geodynamic models independently suggest that neither delayed secular cooling nor primordal heat are currently significant sources, necessitating that current heat production predominately originates in radioactive decay and is quasi-steady-state. Models of Earth's bulk composition based on enstatite chondrites are sufficiently radioactive to supply Q=31 TW, contain enough iron metal to account for Earth's huge core, and have the same oxygen isotopic ratios as the bulk Earth. That Earth is now quasi-steady state is further supported by nearly uniform release of heat over the entire surface. Weak mantle convection, suggested by quasi-steady state, is compatible with circulation within a chemically distinct mantle layers, as thinner layers mean lower Rayleigh numbers. Different dynamical styles above and below 670 km are required by k(T) variations and a change from vibrational to radiative transport mechanisms. Finally, the surface expression of mantle convection is compatible with layering: Geodesic and tomographic studies indicate that lower mantle flow is dominated by a double torus. We propose that the upper mantle system is organized in response to the non-hydrostatic triaxial stress field arising from convective motions of the lower mantle. Simple conjugate shears in the lithosphere that result from triaxial deformation are occupied by oceanic ridges and make a striking "X" pattern in polar projection. Their orientation creates alternating thermal and mechanical couplings between the upper and lower mantle systems, leading to largely E-W continental drift, and to longitudinal concentration of continents and subducting slabs. Hot-spot and ridge magmatism is attributed to thermal runaway and near-solidus temperatures, rather than to material exchange with lower mantle, which is strongly impeded.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8130 Heat generation and transport
DE: 5139 Transport properties
DE: 3999 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting