Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmdi12a..03k&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #DI12A-03
Mathematics
Logic
8120 Dynamics Of Lithosphere And Mantle: General (1213), 8121 Dynamics: Convection Currents, And Mantle Plumes
Scientific paper
To understand the origin and evolution of mantle heterogeneity, a multidisciplinary approach is required. A successful model must satisfy the constraints provided by seismology, which reveals the current state of the mantle, and geochemistry, which reflects the time-integrated history of the planet. One of the most important observations is the prominence of the degree-2 structure in the lower mantle. The large structures in the lower mantle beneath Africa and the Pacific contribute to the degree-2 signal, but their origin is not yet understood. A number of mantle models have been proposed containing multiple reservoirs in various configurations to produce a depleted source of mid-ocean ridge basalts along with the range of long-lived sources inferred from the geochemistry of ocean islands. One important constraint is provided by the Earth's total heat loss. Heat is produced by radioactive decay in the depleted mantle and continental crust; an additional radioactive source is required to make up the heat production of the bulk silicate earth known from as revealed by cosmochemistry and geodynamical models. This heat source may be located in the lower mantle, possibly associated with the superplume structures. This requires a tradeoff between the radiogenic heat productivity of the reservoir and its size: a small volume must contain a higher concentration of heat producing elements to balance the global heat budget. A very small reservoir at the base of the mantle can likely be ruled out because it would become extremely hot (resulting in velocities that are incompatible with seismic models) or buoyant (and therefore short-lived.) The dynamical implications of a dense layer can be assessed using the criteria that the layer must be stable through time, shows topography at its interface, has an effective density profile compatible with seismic models, and has an appropriate heat flux across the core- mantle boundary. A reservoir that is enriched in radiogenic elements compared to the depleted source of mid-ocean ridge basalts would be responsible for the peculiar geochemical signature in some ocean island basalts, but would be rarely sampled at the surface. The geochemical, seismological, and geodynamical evidence together suggest the presence of a hot, abyssal layer that stabilizes mantle plumes and produces the seismic signature of the superplumes.
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