Chemical structure and evolution of the mantle and continents determined by inversion of Nd and Sr isotopic data,II. Numerical experiments and discussion |
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| Authors: | CJ Allègre SR Hart J-F Minster |
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| Institution: | 1. Laboratoire Géochimie et Cosmochimie, Institut de Physique du Globe, Université de Paris VII, Paris France;2. Center for Geoalchemy, Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 U.S.A. |
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| Abstract: | Application of the inversion technique described in Part I 1] to models in which the depleted (MORB) mantle is created by extraction of continental crust results in a stable and self-consistent set of model data, including estimates of uncertainties on all the parameters. The inversion shows that the mean age of the continents (and of the depleted mantle) is 2.14–2.46 b.y., and that the lower crust must be depleted in Rb by a factor of ～ 4 relative to the upper crust. The fraction of total mantle which has been depleted ranges over wide limits (30–90%) depending on: (a) whether the lower mantle is considered to be virgin, or somewhat depleted; (b) whether the depleted reservoir is represented in Sm/Nd and Rb/Sr systematics by average values of MORB, or by extreme values of MORB, and (c) whether the bulk earth Nd content is presumed to be similar to E or H chondrites, or is higher than all known meteorite classes. The usefulness of crust-mantle budget models in constraining such questions as whole mantle versus two-layer mantle convection will be enhanced by better data and understanding in these three key areas of geochemistry. |
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