High-Temperature Diffusion of Oxygen in Synthetic Diopside Measured by Nuclear Reaction Analysis

L. Pacaud, J. Ingrin and O. Jaoul
Laboratoire d'étude des mécanismes de transfert en géologie, UMR CNRS 5563, Minéralogie, 39 allées Jules Guesde, F-31000 Toulouse, France

Abstract: We have performed O self-diffusion experiments in synthetic diopside single crystals along the b-axis at temperatures ranging from 1473–1643 K, under controlled O partial pressure (10−11–10−2 atm). The 18O tracer diffusion was imposed by solid/gas exchange between 16O in diopside and 18O2-enriched argon-hydrogen-H2O gas mixture. Diffusion profiles of 18O were measured by Nuclear Reaction Analysis 18O (p,α) 15N. The diffusion coefficients are described by D=D0exp(−ERT), with log D0(m2/s)=−9.2 ± 1.0 and E 310 ± 30 kJ/mol.

Our results are in agreement with Ryerson and McKeegan's (1994) data and Farver's (1989) data along a direction perpendicular to the c direction. Experiments performed in a wide pO2 range show that D is independent of pO2.

We observe no change in the diffusion regime up to 1643 K (i.e. 22 K prior to melting temperature). This result differs from the diffusion study of Ca in diopside by Dimanov and Ingrin (1995), where a strong enhancement of Ca mobility, attributed to an excess disorder in the Ca-sublattice, was observed above 1523 K. We conclude that O diffusion in diopside is not affected by this premelting phenomenon.

Keywords: Nuclear Reaction Analysis • oxygen diffusion • diopside • premelting

Mineralogical Magazine; October 1999 v. 63; no. 5; p. 673-686; DOI: 10.1180/002646199548835
© 1999, The Mineralogical Society
Mineralogical Society (www.minersoc.org)