Experimental samples and methods

Diffusion annealings were performed in a quartz tube under an oxygen atmosphere enriched to 70% with the lsO isotope at a pressure of 0.21 X 105 Pa. The isotope composition of the gaseous phase was maintained constant throughout 

A typical concentration profile of lsO oxygen is shown in Fig. 1. It is characterized by some specific features as compared to a simple case of the diffusion equation for a constant source. Firstly, it is seen that the concentration cs on the surface of the sample is lower than the equilibrium concentration in the test samples. Therefore, a solution of the diffusion equation depending on the boundary conditions must be used [18]. Secondly, the concentration cj of the lsO isotope was almost constant at a large depth x and was much higher than the initial concentration of the lsO isotope in samples. It is not improbable that this 

From general considerations, the use of equation (2) seems to be more reasonable. However, we used both approximations (1) and (2) to understand how the choice of boundary conditions affected the D value. The data in Fig. 2 (these data were sufficiently typical) and in Table 1 show that boundary conditions had a weak effect on the diffusion coefficients. Therefore, results obtained with the approximation (2) will be discussed below. It may be noted, however, that the approximation 

Calculated from equation (1). Calculated from equation (2). 

---