Creep mechanism in Mg2GeO4: Effects of a phase transition

We deformed polycrystalline Mg2GeO4 olivine and spinel in a Griggs‐type solid medium apparatus. Flow of Mg2GeO4 olivine can be represented by = 6.5×l07 σ3.5 exp(−105(kcal/mol)/ RT) where σ (kbar) is the differential stress and (s−1) is the natural strain rate. For Mg2GeO4 spinel the flow law is = 2.6×104 σ2exp(−73(kcal/ mol)/RT). The low stress exponent and activation enthalpy coupled with fine grain size (3 μm) suggest that Mg2GeO4 spinel deformed by a superplastic mechanism. Flow parameters for the olivine phase suggest a dislocation creep mechanism. Comparison of theoretical superplastic flow laws for Mg2GeO4 olivine with the spinel phase data suggests that strain rates in Mg2GeO4 spinel are only about a factor of 3 lower than for Mg2GeO4 olivine of the same grain size. A similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion. Transformation from Mg2GeO4 olivine to spinel reduced grain size to approximately 3 μm. Thus we might expect a similar reduction in grain size in the earth's transition zone which could result in superplastic deformation of the transformed phase and cause a weak ‘decoupling’ zone at the transition boundary in the mantle. Superplasticity brought about by transformation‐induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle.