No abstract

The kinetics of hydrogen diffusion in diopside single crystals have been investigated through dehydration experiments performed in air between 700 ø and 1000øC. Samples oriented along [001], [100]*, and [010] crystallographic directions were cut in thin slices of less than 1 mm thickness and about 4 x 4 mm 2 across. The decrease, with time of heating, of the OHconcentration was measured from the integral of the OH-absorption bands of infrared spectra taken on a few millimeter per millimeter area at the center of the sample slices. The data show no significant effect of crystallographic orientation on the rates of dehydration, suggesting that hydrogen diffusion in diopside is isotropic. The simultaneous fit of diffusion data measured in the three directions to an Arrhenius law follows: In D = In DO -exp[-136 + 27 kJ/mol/RT] with In DO = -14.5 + 2.8. The rates of dehydration of diopside are slower than the rates of dehydration of olivine. However, if olivine undergoes extensive dehydration during ascent within magma, dehyß ..,•,o.. of a;,,,o;a• may also be •ig-it'ic•nt if we. ta__ke into account the smaller •m'ain size of this mineral, as observed in nodules.