Influence of Fe on hydrogen diffusivity in orthopyroxene

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 MgSiO 3 – FeSiO 3 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.