# Diffusion-limited REE uptake by eclogite garnets and its consequences for Lu–Hf and Sm–Nd geochronology

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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 ^{176}Lu/^{177}Hf and ^{147}Sm/^{144}Nd 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.

## Keywords

147Sm Isochron Light Rare Earth Element Garnet Growth Diffusion Halo## Notes

### Acknowledgments

This study was funded by Swiss National Science Foundation grant SNF2100-066996 to LPB and U.S. National Science Foundation grant EAR-0309853 to CMJ. Many thanks to A.W. Hofmann for granting time access to the ionprobe, as well as T. Lapen for fruitful discussions. The constructive reviews of J. Van Orman and R. Dohmen and the editorial handling of J. Hoefs significantly improved the manuscript and were greatly appreciated.

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