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Laboratoire Mécanismes de Transferts en Géologie, CNRS, UMR 5563, Université Paul Sabatier, 14 avenue Edouard Belin, 31400 Toulouse, France
* E-mail: hertweck{at}lmtg.obs-mip.fr
Single crystals of Mg2GeO4 spinel, a stable analogue to mantle ringwoodite at atmospheric pressure up to 800°C, were synthesized from Mg2GeO4 olivine in hydrous conditions at 1.9 GPa and ~1000°C. Infrared (IR) spectra show OH-stretching peaks at 3531 and 3502 cm1. Quantification of IR spectra reveals hydrogen in the order of 5 to 10 ppm wt. H2O. A complete hydrogen/deuterium exchange was achieved at 700°C leading to a diffusion coefficient of deuterium of ~3x1014 m2/s. This value is of the same order as diffusion coefficients observed in other mantle minerals, such as garnet and diopside.
The OO distance of ~ 2.9 Å calculated from the spectra of Mg2GeO4 spinel is consistent with the GeO4 tetrahedra edge length known from crystal-structure data. Whereas the hydration mechanism and the site occupancies involved are still in discussion for mantle spinel, the IR measurements on Mg2GeO4 confirm the idea of protonation at the tetrahedral edges.
KEYWORDS: Mg2GeO4 spinel, ringwoodite, IR spectroscopy, hydrogen, diffusion coefficient
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A. Beran and E. Libowitzky Water in Natural Mantle Minerals II: Olivine, Garnet and Accessory Minerals Reviews in Mineralogy and Geochemistry, January 1, 2006; 62(1): 169 - 191. [Full Text] [PDF] |
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