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European Journal of Mineralogy; November, December 2006; v. 18; no. 6; p. 705-718; DOI: 10.1127/0935-1221/2006/0018-0705
© 2006 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
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Articles

(Fe,Mn)-Mg interdiffusion in natural diopside: effect of pO2

Alexandre Dimanov1,2,* and Michael Wiedenbeck1

1 GeoForschungsZentrum Potsdam, Telegrafenberg, Haus C, 14473 Potsdam, Germany
2 Laboratoire de Mécanique des Solides, UMR C 7649, Ecole Polytechnique, Route de Saclay, 91128 Palaiseau, France.

* Corresponding author, e-mail: dimanov{at}lms.polytechnique.fr

We investigated (Fe,Mn)-Mg interdiffusion along the [001] axis in single crystals of natural diopside at ambient pressure, at temperatures between 1000°C to 1200°C and at oxygen fugacities between 10–18.5 atm to 10–6 atm. Thin layers (~350–550 Å) of ferro-johannsenite deposited by RF plasma sputtering provided iron and manganese for exchange with magnesium from the diopside substrates. Interdiffusion profiles of Fe-Mg and Mn-Mg were analysed by secondary ion mass spectrometry in depth profiling mode. We found that the (Fe,Mn)-Mg interdiffusion coefficient is strongly pO2 sensitive. At 1100°C and between 10–7 atm and 10–15 atm the interdiffusion coefficient is proportional to ~(pO2)m, indicative of an extrinsic point defects regime. We obtained m = 0.22 ± 0.02, indicating a vacancy mechanism. The data at 1100°C further suggest that at pO2 x 10–15 atm and at pO2 > 10–7 atm the interdiffusion coefficient is pO2-independent, possibly in relation with the appearance of Early Partial Melting (EPM, Jaoul & Raterron, 1994). At constant oxygen fugacity and without EPM the interdiffusion coefficient has an activation energy of 297 ± 31 kJ/mol. Our data significantly improve the constraints on the activation energy beyond the preliminary results reported earlier (Dimanov & Sautter, 2000). This work contributes towards a better understanding of the point defects chemistry and diffusion mechanisms in Fe-bearing diopside.

Key-words: diopside, SIMS, oxygen fugacity, divalent cation diffusion, cation vacancies.






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