Calcium self-diffusion in natural diopside single crystals

We have measured the diffusion coefficient of ⁴⁴Ca along and perpendicular to c direction in natural Fe-bearing (∼2 at.%) diopside single crystals. Specimens were annealed at temperatures ranging from 1000 to 1250°C, with controlled oxygen fugacity. Diffusion profiles were analysed by Rutherford Back-Scattering Spectrometry (RBS) of α-particles. The diffusion of Ca is isotropic along c and b directions. In addition, the results clearly show two distinct diffusional regimes for the natural diopside, revealed by silica precipitates occurrence in the diopside matrix when T ≥ 1150°C. In this case the oxygen partial pressure pO₂ does not influence the self-diffusion coefficient which is characterized by the activation energy E = 396 ± 38 kJ/mol. For T ≤ 1100°C the diffusional process has a lower activation energy (E = 264 ± 33 kJ/mol) and varies as (pO₂)^-0.14±0.01 in the investigated range (from 10^-16 atm to 10^-6 atm). These results are consistent with previously reported results on electrical conductivity (Huebner and Voigt, 1988) and high temperature plastic deformation of natural diopside single crystals (Jaoul and Raterron, 1994). According to the point defects model, elaborated by Jaoul and Raterron (1994), the diffusional mechanism of Ca should be essentially interstitial. Furthermore, this mechanism should be the same for different diopside samples with iron content ranging from 0.4 to 2.42 at.%. Indeed, for Ca diffusion in synthetic diopside (0.4 at.% Fe) the activation enthalpy is very similar (281 ± 26 kJ/mol, Dimanov and Ingrin, 1995). On the other hand, the Fe content indoubtly influences the preexponential factor. The present paper reports Ca self-diffusion in diopside as a function of T, pO₂ crystallographic orientation, and Fe content. In fact, among all diffusion coefficients previously reported in diopside, but Si, D_Ca is the lowest. Thereby, Ca should be a kinetically limiting species for diffusion-controlled processes such as plastic deformation and cation exchanges. For instance, Ca self-diffusion controls Ca-Mg exchanges between pyroxenes. Then, our results could be helpful to better understand the closure behaviour (Dodson, 1973, 1976) of the Clinopyroxene-Orthopyroxene geothermometer.