This paper shows how to construct the Onsager tensor of ''phenomenological'' coefficients which is the key element in the Thermodynamics of Irreversible Processes. In a diffusion controlled deformation process, these coefficients couple the forces involved during diffusional creep to the fluxes of individual species, which are simultaneously diffusing from places in compression (sources of atoms) to places in relative extension (sinks). During such processes an electrical field is generated and couples the different moving species. Global stoichiometry must be preserved between sources and sinks to avoid local accumulation of some species and precipitation of extra phases. The diffusion coefficient Di of all the major constituents of forsterite and olivine (10% Fe), with general formula Me2SiO4, are now quite well known in the temperature (T~1373--1873 K) and oxygen partial pressure (pO2) ranges which are also accessible to experimental creep studies. It is found that DSi≪DOx≪DMg, DFe. The activation energies and pO2 dependences are equally well constrained, both for diffusion and for high temperature creep.

The main result of this model is that the diffusional creep rate &egr;˙ is naturally controlled by the diffusion rate of the slowest species DSi, but with an important enhancement due to the presence of the most abundant and mobile defects, the octahedral vacancies, VMe, with concentration <VMe>, which are ''pulling'' ther slow silicons. The model yields an activation energy for creep which is the sum of that of silicon diffusion and of the VMe formation. The exponents of the (pO2)n power law dependences are additive: n(&egr;˙)=n(DSi)+n(<VMe>). A similar relation is also deduced for the activation volumes. Comparisons between the available creep and diffusion data show that these relations are reasonably obeyed. Finally, an attempt to compute &egr;˙ by using the above model for an experimentally deformed olivine specimen yields a value within 1 order of magnitude of the &egr;˙ which has been measured, and for which the dislocation microstructure clearly shows that climb is the rate controlling mechanism. ¿1990 American Geophysical Union