Geophysical Journal | 2006 volume 28 3

Micro-structural transformations of some garnets under conditions of high pressure and temperature

© Korchin V.A.

  Residual impact of high uniform pressure up to 75 kbar and temperature up to 700oC on the transformations of microstructure of garnet mono-crystal has been studied. A method of width measurement of the line (ΔH) of ferromagnetic resonance of synthetic ferrite-garnet considerably dependent on distribution of dislocations and admixture atoms within a crystal was used. Threshold pressure (Pth) was found to be within the interval 30-55 kbar depending on the temperature (T=20÷700oC) with residual alterations of ΔH revealed after their action, caused by the number of dislocations, admixture ions and configuration of their localizations. Micro-structural alterations are dependent on time of pressure impact, initial density of dislocations and other crystal defects. It has been found that short-term pressure impact (some higher than Pth) promotes perfection of single crystal microstructure - decreases the density of defects, simplifies the character of their ensembles and localizations. Sustained pressure (P>Pth) and temperature impact produces within the garnet structure such alterations, which are specific in case of plastic deformation of fragile crystals. On the base of two effects revealed, which are opposite in their essence an assumption has been made that in mineral substance with high garnet content (for example, the Earth's mantle) under conditions, which are gradient by pressure (lithostatic pressure in the depths) some interface has to exist, above which (P<Pth) the medium is conventionally "perfect" by its structure, and lower it is deformed, discompacted with decreased crystalline structural stability. This gives us a right to suppose that seismic interface within the Earth's mantle is thermodynamic structural by its nature and reflects the beginning of velocity inversion zone at the depths of 70-150 km.

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