27 cm
/mol).
In contrast, Bussod et al. (1993) obtained a very small
activation volume (
5-10 cm
/mol). One way to reconcile
these apparently inconsistent results is to assume a large
decrease in activation volume with pressure, although
experimental measurements of pressure dependence of
activation volume are difficult.
The activation volume for diffusion-controlled dislocation
climb in olivine was determined up to 10 GPa using a
multi-anvil apparatus. The result show a small activation volume
(
6 cm
/mol; Karato et al., 1993). Based on this result,
Karato and Wu (1993) discussed the possible change in
deformation mechanisms in the upper mantle. The cation
diffusion coefficients in Mg
SiO
have been
determined in the pressure range where this mineral
transforms from olivine to 
-phase and to
-phase (Farber et al., 1994). They found much higher chemical
diffusion coefficients in 
-phase and in 
-phase
as compared to olivine. Small activation volumes (3 to 7
cm
/mol) were found.