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Kinetics of deuteration in pyrope

Laboratoire Mécanismes de Transfert en Géologie, U. M. R. 5563, Equipe de Minéralogie, CNRS – Université Paul Sabatier, 39 allées Jules Guesde, F-31000 Toulouse, France

^* Corresponding author, e-mail: blanchar{at}cict.fr

Hydrogen-deuterium exchange experiments were performed at temperatures between 973 and 1223 K on a Dora Maira pyrope and on two pyrope samples from mantle xenoliths. The Fourier transform infrared (FTIR) spectra of hydrogen in the natural Dora Maira and xenolith pyropes are different, with several OH bands between 3600 and 3650 cm-1 and between 3510 and 3575 cm^-1, respectively. The self-diffusion of deuterium in Dora Maira pyrope is given by D = D₀ exp [-140 ± 38 kJmol^-1/RT] with log D₀ (m²/s) = -5.8 ± 1.9. This activation energy for diffusion is identical to those measured in diopside and olivine, but the pre-exponential term D₀ is one to two orders of magnitude smaller. This suggests that the mechanism of hydrogen self-diffusion in the pyrope structure is similar to that for other upper mantle mineral structures, such as olivine and pyroxene. In contrast to the Dora Maira pyrope, for the two pyrope crystals extracted from xenoliths, the total concentration of hydrous species is not preserved during H-D exchange experiments; dehydration occurs concurrently with deuteration. This renders the analysis of the kinetics of H-D exchange difficult. In the pyropes from mantle xenoliths, the kinetics of dehydration under reducing conditions is close to that for hydrogen dehydrogenation from experiments performed in air by Wang et al. (1996). This suggests that hydrogen exchange in mantle pyropes may be independent of the oxygen fugacity.

Key-words: pyrope, hydrogen, water, upper mantle, diffusion.

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