Hydrogen diffusion in natural and synthetic orthopyroxene

Abstract

Hydrogen diffusion coefficients in natural orthopyroxenes and synthetic enstatite were determined by dehydration and hydration experiments at 700 and 900 degreesC. In natural Opx (approximately En(90)Fs(10)) small but significant differences in diffusivities along the three crystallographic axes were observed, [001] being the fastest direction, followed by [100] and [010]. Hydrogen diffusion in pure enstatite proved to be about 2 orders of magnitude slower and isotropic. The activation energy for hydrogen diffusion in pure enstatite was determined to be -295 (+/-55) kJmol(-1), and -213 (+/-47) kJmol(-1) for orthopyroxene from Kilbourne Hole. Long-term hydration experiments did not lead to saturation in hydrogen. Instead, after an initial increase in hydrogen concentration, a slow but continuing decrease could be observed in all cases. It is suggested that the investigated samples lose their ability to store hydrogen even when heated in a hydrogen atmosphere. This loss in storage ability can itself be described by a diffusion equation, its diffusion coefficients being more than 1 order of magnitude slower than the diffusion of hydrogen.