Dislocation recovery in fine-grained polycrystalline olivine |
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| Authors: | R J M Farla H Kokkonen J D Fitz Gerald A Barnhoorn U H Faul I Jackson |
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| Institution: | (1) Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia;(2) Present address: Department of Earth Sciences, Utrecht University, 3508 TA Utrecht, The Netherlands;(3) Department of Earth Sciences, Boston University, Boston, MA 02215, USA |
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| Abstract: | The rate of static dislocation recovery in Fo90 olivine has been studied under conditions of high temperature and controlled atmosphere in compressively deformed polycrystals
hot-pressed from synthetic (sol–gel) and natural (San Carlos) precursor powders. The sol–gel olivine, containing a small fraction
of orthopyroxene, was deformed to a final strain of 19% with a maximum differential stress of 266 MPa whereas the San Carlos
specimen was deformed to 15% strain and 260 MPa differential stress. Small samples cut from these deformed materials were
annealed under high-temperature, controlled atmosphere conditions, for different durations to allow partial recovery of the
dislocation sub-structures. Oxidative-decoration of the microstructural features, followed by backscattered electron imaging
at 5 kV and image analysis, was used to determine dislocation density. The variation of dislocation density ρ with time t at absolute temperature T was fitted to a second-order rate equation, in integral form, 1/ρ(t) − 1/ρ(0) = kt with k = k
0 exp(−E
a/RT). The activation energy E
a of the recovery process is 240 ± 43 and 355 ± 81 kJ mol−1 for sol–gel and San Carlos olivine polycrystals, respectively. The measured rates are one to two orders of magnitude lower
than those reported in previous studies on natural single crystal olivine. The difference may be explained by several factors
such as high dislocation densities measurable from large areas at high magnification for the SEM and the technique used to
estimate dislocation densities. Comparison between fine-grained sol–gel olivine and the coarser-grained San Carlos olivine
aggregate did not indicate that grain boundaries play an important role in dislocation recovery, but the absence of grain
boundaries might also have contributed to the high dislocation recovery rates previously measured for single crystals. |
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