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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Oral Presentation 2093
Continental growth and preservation of geochemical mantle heterogeneity
in spite of whole-mantle convection
Prof. Uwe Walzer
Institut f. Geowissenschaften Friedrich-Schiller-Universitaet Jena IASPEI
Roland Hendel, John. Baumgardner
The focus of this paper is the numerical simulation of the chemical differentiation of the Earth's mantle.
This differentiation induces the generation and growth of the continents and, as a complement, the
formation and augmentation of the depleted MORB mantle. Here, we present a solution of this problem
by an integrated theory in common with the problem of thermal convection in a 3-D compressible
spherical-shell mantle. The conservation of mass, momentum, energy, angular momentum, and of four
sums of the number of atoms of the pairs 238U-206Pb, 235U-207Pb,232Th-208Pb, 40K-40Ar is
guaranteed by the used equations. The whole coupled thermal and chemical evolution of mantle plus
crust was calculated starting with the formation of the solid-state primordial silicate mantle. No
restricting assumptions have been made regarding number, size, form, and distribution of the
continents. It was, however, implemented that moving oceanic plateaus touching a continent are to be
accreted to this continent at the corresponding place. The model contains a mantle-viscosity profile with
a usual asthenosphere beneath a lithosphere, a highly viscous transition zone and a second lowviscosity
layer below the 660-km mineral phase boundary. The central part of the lower mantle is highly
viscous. This explains the fact that there are, regarding the incompatible elements, chemically different
mantle reservoirs in spite of perpetual stirring during more than 4.49 billion years. The highly viscous
central part of the lower mantle also explains the relatively slow lateral movements of CMB-based
plumes, slow in comparison with the lateral movements of the lithospheric plates. The temperature- and
pressure-dependent viscosity of the model is complemented by a viscoplastic yield stress, Sy. The paper
includes a comprehensive variation of parameters, especially the variation of the viscosity-level
parameter, rn, the yield stress, Sy, and the temporal average of the Rayleigh number. In the rn-Sy plot,
a central area shows runs with realistic distributions and sizes of continents. This area is partly
overlapping with the rn-Sy areas of piecewise plate-like movements of the lithosphere and of realistic
values of the surface heat flow and Urey number.
Keywords: mantle convection, continental growth, chemical heterogeneity