High-temperature diffusion of oxygen in synthetic diopside measured by nuclear reaction analysis

We have performed O sell-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 O-18 tracer diffusi on was imposed by solid/gas exchange between O-16 in diopside and O-18(2)-e nriched argon-hydrogen-H2O gas mixture. Diffusion profiles of O-18 were mea sured by Nuclear Reaction Analysis O-18 (p,alpha) N-15. The diffusion coeff icients are described by D = D-0 exp(- E/RT), with log D-0(m(2)/s) = -9.2 /- 1.0 and E = 310 +/- 30 kJ/mol. Our results are in agreement with Ryerson and McKeegan's (1994) data and Fa rver's (1989) data along a direction perpendicular to the c direction. Expe riments performed in a wide pO(2) range show that D is independent of pO(2) . 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 obser ved above 1523 K. We conclude that O diffusion in diopside is not affected by this premelting phenomenon.