Hydrogen in diopside: Diffusion profiles

The kinetics of diffusion for hydrogen in diopside single crystals From Jai pur, India, were determined by performing room pressure dehydration experim ents at temperatures li um 700-850 degrees C and an oxygen fugacity of 10(- 14) bar. The hydrogen diffusivities were determined for the [100], [010], a nd [001]* directions either from concentration profiles for hydroxyl in sam ples after annealing or from bulk hydroxyl concentrations as a function of anneal time for sequential dehydration experiments. The rate of diffusion i s anisotropic, with fastest transport along the [100] and [001]* axes and s lowest along the [010] axis. Fits of the data to an Arrhenius law yield act ivation energies and preexponential terms of 181 +/- 38 kJ/mol and 10(-2.1/-1.9) m(2)/s For diffusion parallel to [100], and 153 +/- 32 kJ/mol and 10 (-3.4+/-1.6) m(2)/s for diffusion parallel to [001]*. For diffusion paralle l to [010], the data were measured over an insufficient temperature range t o calculate the activation energy For diffusion. However, these diffusiviti es were approximately an order of magnitude slower than those fur diffusion parallel to [100] or [001]*. The measured rates and anisotropy for self-di ffusion of hydrogen in diopside are consistent with those determined from h ydrogen-deuterium exchange in Russian diopside (Hercule and Ingrin 1999). T he hydrogen diffusivities are also similar in magnitude to those for olivin e (Mackwell and Kohlstedt 1990) and are large enough that the hydrogen cont ent of millimeter-size diopside grains with compositions near Jaipur diopsi de will adjust to changing environmental conditions in time scales of hours at temperatures as low as 800 degrees C. As xenoliths ascending from the m antle remain at high temperatures (i.e., >1000 degrees C) but experience a rapid decrease in pressure, diopside grains may dehydrate during ascent. Th us, low water contents for diopside crystals from xenoliths cannot be taken as indicative of low water contents in the mantle.