Iron diffusion in single-crystal diopside

Iron tracer diffusion experiments in diopside have been performed using nat ural and synthetic single crystals of diopside, and stable iron tracers enr iched in Fe-54, at temperatures in the range 950-1100 degreesC, total press ure 1 atm, for times up to 29 days. Iron isotope diffusion profiles were de termined with an ion microprobe. For experiments performed at log pO(2) = - 13, in directions parallel to the c axis and the b axis of two natural, low iron (Fe similar to 1.8 at %) diopsides, the data obey a single Arrhenius relationship of the form D = 6.22(-5.9)(+49.6) x 10(-15) exp(-161.5 +/- 35. 0 kJ mol(-1)/RT) m(2) s(-1). A single datum for iron diffusion in iron-free , single-crystal diopside at 1050 degreesC, is approximately 1 order of mag nitude slower than in the natural crystals. The pO(2) dependence of iron di ffusion in natural crystals at 1050 degreesC (power exponent = 0.229 +/- 0. 036) indicates a vacancy mechanism; this is consistent with the results of unpublished atomistic simulation studies. There is no evidence of anisotrop y for iron diffusion in diopside.