CALCIUM SELF-DIFFUSION IN NATURAL DIOPSIDE SINGLE-CRYSTALS

We have measured the diffusion coefficient of Ca-44 along and perpendi cular to c direction in natural Fe-bearing (similar to 2 at.%) diopsid e single crystals. Specimens were annealed at temperatures ranging fro m 1000 to 1250 degrees C, with controlled oxygen fugacity. Diffusion p rofiles were analysed by Rutherford Back-Scattering Spectrometry (RBS) of alpha-particles. The diffusion of Ca is isotropic along c and b di rections. In addition, the results clearly show two distinct diffusion al regimes for the natural diopside, revealed by silica precipitates o ccurrence in the diopside matrix when T greater than or equal to 1150 degrees C. In this case the oxygen partial pressure pO(2) does not inf luence the self-diffusion coefficient which is characterized by the ac tivation energy E = 396 +/- 38 kJ/mol. For T less than or equal to 110 0 degrees C the diffusional process has a lower activation energy (E = 264 +/- 33 kJ/mol) and varies as (pO(2))(-0.14+/-0.01) in the investi gated range (from 10(-16) atm to 10(-6) atm). These results are consis tent with previously reported results on electrical conductivity (Hueb ner and Voigt, 1988) and high temperature plastic deformation of natur al diopside single crystals (Jaoul and Raterrob, 1994). According to t he point defects model, elaborated by Jaoul and Raterron (1994), the d iffusional mechanism of Ca should be essentially interstitial. Further more, this mechanism should be the same for different diopside samples with iron content ranging from 0.4 to 2.42 at.%. Indeed, for Ca diffu sion in synthetic diopside (0.4 at.% Fe) the activation enthalpy is ve ry 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, pO(2) crystallographic orientation, and Fe content. In fact, amo ng all diffusion coefficients previously reported in diopside, but Si, Dc, is the lowest. Thereby, Ca should be a kinetically limiting speci es for diffusion-controlled processes such as plastic deformation and cation exchanges. For instance, Ca self-diffusion controls Ca-Mg excha nges between pyroxenes. Then, our results could be helpful to better u nderstand the closure behaviour (Dodson, 1973, 1976) of the Clinopyrox ene-Orthopyroxene geothermometer.