Calcium self-diffusion in diopside at high temperature: Implications for transport properties

We have investigated Ca-44 self-diffusion in natural diopside single crysta ls (containing similar to 2 atomic % Fe) at temperatures up to 1320 degrees C (i.e. 30 degrees C below the nominal melting point). Oxygen fugacity was controlled by gaseous mixtures. Diffusion profiles ranging from similar to 50 to 500 nm were analysed by Rutherford Back-Scattering Spectrometry (RBS ). The present results are complementary to previous studies, and show that in both synthetic (Fe-poor) and natural (Fe-rich) diopside, there are two different diffusion regimes for Ca with a transition at similar to 1230+/-1 5 degrees C. Below this temperature diffusion is characterised by an activa tion enthalpy of similar to 284+/-10 kJ/mol, while at higher temperatures i t increases up to similar to 1006+/-75 kJ/mol. These regimes are proposed t o be respectively extrinsic and intrinsic. For the intrinsic regime Ca self -diffusion may involve Ca-Frenkel point defects. These are pairs of a vacan cy on a M-2 site and a calcium cation on an interstitial (normally unoccupi ed) site. The concentration of such point defects depends only on temperatu re, and it is especially important at very high temperatures. The activatio n enthalpy for intrinsic diffusion may represent the half defect formation enthalpy plus the migration enthalpy for movement through interstitial site s. For the extrinsic regime we propose Ca self-diffusion to involve extrins ic interstitial point defects with concentration proportional to (P-O2)(-0. 19+/-0.03). We suggest that for both regimes, Ca diffusion involves the wel l known M-3 sites in the octahedral layers, as well as sites in the tetrahe dral layers, that we call M-4. These sites are especially convenient to exp lain the observed isotropic diffusion. Increasing concentration of Ca-Frenk el point defects may be related to the onset of premelting, which affects t he thermodynamic properties of Fe-"free" diopside above 1250 degrees C. In the light of the present results, premelting is also expected to occur in n atural Fe-bearing diopside and it could strongly influence its thermodynami c and transport properties. Subsequently, in deep upper mantle conditions ( T approximate to 1250 degrees C-1300 degrees C) where premelting could occu r, diffusional cation exchanges with surrounding phases and diffusion contr olled creep might be facilitated. Finally, our diffusion data support a pre vious suggestion that electrical conductivity may be electronic rather than ionic.