"Average" interdiffusion of (Fe,Mn)-Mg in natural diopside

(Fe,Mn)-Mg interdiffusion has been investigated in single crystals of natur al diopside at ambient pressure, high temperatures (900 degrees C to 1240 d egrees C) and low oxygen fugacity (pO(2) approximate to 10(-18) atm to 10(- 13) atm). Interdiffusion couples were made with polycrystalline ferro-johan nsenite thin film (similar to 250 Angstrom to similar to 500 Angstrom) as t he iron and manganese reservoir (Fe/(Fe + Mn + Mg) similar to 48 at. %) and with diopside single crystals as the semi-infinite calcium- and magnesium- rich medium. The diopside specimens contain Fe as major impurity (Fe/(Fe Mg) approximate to 6 at. %). Diffusion profiles of Fe and Mn ranging from s imilar to 250 Angstrom to similar to 3500 Angstrom were analysed by Rutherf ord back-scattering spectrometry (RBS). This technique is well adapted to m easurements of concentration profiles of heavy elements in silicates, but i t can not resolve the signals from elements with nearly same mass (Fe and M n). Therefore Fe and Mn concentration profiles overlap, but the characteris tic shapes of the convoluted diffusion profiles indicate that Fe and Mn dif fuse at similar rates. This is also confirmed by secondary-ion mass spectro scopy (SIMS), which allows one to obtain separately Fe and Mn concentration profiles. We obtained an "average" inter-diffusion coefficient characteris ed by an apparent activation energy of 406 kJ/mol and a pre-exponential fac tor of 0.955 cm(2)/s. Such a result is surprisingly low for a diffusion pro cess involving Fe and Mg exchanges.