摘要:

We deformed polycrystalline Mg2GeO4olivine and spinel in a Griggs‐type solid medium apparatus. Flow of Mg2GeO4olivine can be represented by= 6.5×l07σ3.5exp(−105(kcal/mol)/ RT) where σ (kbar) is the differential stress and(s−1) is the natural strain rate. For Mg2GeO4spinel the flow law is= 2.6×104σ2exp(−73(kcal/ mol)/RT). The low stress exponent and activation enthalpy coupled with fine grain size (3 μm) suggest that Mg2GeO4spinel deformed by a superplastic mechanism. Flow parameters for the olivine phase suggest a dislocation creep mechanism. Comparison of theoretical superplastic flow laws for Mg2GeO4olivine with the spinel phase data suggests that strain rates in Mg2GeO4spinel are only about a factor of 3 lower than for Mg2GeO4olivine of the same grain size. A similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion. Transformation from Mg2GeO4olivine 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 transitio

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00389

年代:1981

数据来源: WILEY

摘要:

A large array of igneous and volcanic features has characteristics that are recognized in other disciplines as conclusive and direct evidence of convection in stratified and/or solidifying melts; e.g., macroscopic segregation (cryptic variation, zoning in magma chambers), mineral layering (in mafic intrusives), crescumulates (fingering) in the vertical and horizontal, banding (in pyroclastics), ‘rollover’ with attendant flashing of volatiles (explosive volcanism), etc. Some quantitative and qualitative aspects of convection in solidifying and/or stratified melts (e.g., mineral layer widths such as are observed in the Skaergaards) are examined to show consistency with field evidence. Convective fractionation does not possess the physical implausibilities of gravitational segregation (crystal settling). Neither is the field evidence as ambiguous if interpreted in terms of convective fractionation (which can explain amongst other things heavier material overlying lighter). Convective fractionation may operate on larger scales in the interior of plan

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00405

年代:1981

数据来源: WILEY