Hydrogen diffusing out of pyroxene (and olivine) can be descirbed as two separate processes. The first process, which in Fe-rich samples is rapid and rate limited by hydrogen self-diffusion, includes polaron-proton exchange where protons diffuse out of the lattice and the Fe3+/Fe2+ ratio increses by a redox reaction.
The second process involves loss of hydrogen defect associates, such as metal vacancies and charge-deficient substitutions, rate limited by cation diffusion with reaction kinetics which in many cases is two orders of magnitude slower compared to the iron redox reaction. Depending on iron content, the first process allows everything from complete to very small partial dehydrogenation. Nevertheless, the frequent observations of hydrogen in mantle xenocryst pyroxene suggest that complete loss of hydrogen requires the second, slower process.
网友评论