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Influence of Fe on hydrogen diffusivity in orthopyroxene

¹ Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Angewandte und Experimentelle Mineralogie, Goldschmidtstraße 1, 37077 Göttingen, Germany
² Naturhistoriska riksmuseet, Sektionen för mineralogi, Box 50007, 10405 Stockholm, Sweden

^* Corresponding author, e-mail: rstalde{at}gwdg.de

The presence of iron has important consequences for hydrogen diffusion in many minerals, allowing charge compensation during hydrogen extraction by a redox mechanism. Here we present new experimentally determined diffusion data for hydrogen in synthetic orthopyroxenes in the system MgSiO₃ – FeSiO₃ derived from hydrogen extraction experiments. Results confirm the previously made assumption that hydrogen extraction is rate limited by hydrogen self diffusion, if a threshold value for x_Fe is exceeded. Our best estimate for this threshold value is x_Fe = 0.12 (±0.02). Natural orthopyroxenes from ultramafic rocks exhibit small, but significantly higher diffusivities than the synthetic samples, leading to the conclusion that other factors such as the concentration of M-site vacancies or the presence of other cations also may influence the rate of hydrogen extraction.

Key-words: enstatite, ferrosilite, hydrogen diffusion, IR spectroscopy.