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Science 15 January 1999:
Vol. 283. no. 5400, pp. 362 - 365
DOI: 10.1126/science.283.5400.362
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Reports

Enhancement of Cation Diffusion Rates Across the 410-Kilometer Discontinuity in Earth's Mantle

S. Chakraborty, *dagger R. Knoche, H. Schulze, D. C. Rubie, D. Dobson, N. L. Ross, R. J. Angel

Rates of cation diffusion (magnesium, iron, and nickel) have been determined in olivine and its high-pressure polymorph, wadsleyite, at 9 to 15 gigapascals and 1100° to 1400°C for compositions that are relevant to Earth's mantle. Diffusion in olivine becomes strongly dependent on composition at high pressure. In wadsleyite, diffusion is one to two orders of magnitude faster than in olivine, depending on temperature. Homogenization of mantle heterogeneities (chemical mixing) and mineral transformations involving a magnesium-iron exchange will therefore occur considerably faster in the transition zone than at depths of less than 410 kilometers.

S. Chakraborty, Institut für Mineralogie und Geochemie, Universität zu Köln, Zülpicher Strasse 49B, 50674 Köln, Germany. R. Knoche, Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52, A-6020, Innsbruck, Austria. H. Schulze, D. C. Rubie, R. J. Angel, Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany. D. Dobson and N. L. Ross, Mantle Petrology Group, Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK.
*   To whom correspondence should be addressed. E-mail: Sumit.Chakraborty{at}rz.ruhr-uni-bochum.de

dagger    Present address: Institut für Mineralogie, Ruhr-Universität Bochum, D-44780 Bochum, Germany.

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