Diffusion-limited REE uptake by eclogite garnets and its consequences for Lu–Hf and Sm–Nd geochronology |
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| Authors: | Susanne Skora Lukas P Baumgartner Nancy J Mahlen Clark M Johnson Sébastien Pilet Eric Hellebrand |
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| Institution: | (1) Institute of Mineralogy and Geochemistry, University of Lausanne, Anthropole, 1015 Lausanne, Switzerland;(2) Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706, USA;(3) Max-Planck Institute for Chemistry, Postfach 3060, 55020 Mainz, Germany;(4) Present address: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA |
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| Abstract: | Garnets from the Zermatt-Saas Fee eclogites contain narrow central peaks for Lu + Yb + Tm ± Er and at least one additional small peak towards the rim. The REE Sm + Eu + Gd + Tb ± Dy are depleted in the cores but show one prominent peak close to the rim. These patterns cannot be modeled using Rayleigh fractionation accompanied by mineral breakdown reactions. Instead, the patterns are well explained using a transient matrix diffusion model where REE uptake is limited by diffusion in the matrix surrounding the porphyroblast. Observed profiles are well matched if a roughly linear radius growth rate is used. The secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism. The model predicts anomalously low 176Lu/177Hf and 147Sm/144Nd ratios in garnets where growth rates are fast compared to diffusion of the REE, and these results have important implications for Lu–Hf and Sm–Nd geochronology using garnet. |
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