Crystallographic preferred orientations and misorientations in some olivine rocks deformed by diffusion or dislocation creep

The development of crystallographic preferred orientations (CPO) and grain misorientation distributions (MOD) in fine-grained (0.5-30 mu m) olivine ro cks, experimentally deformed by diffusion creep and dislocation creep has b een investigated. The use of electron back-scattered diffraction (EBSD), in a scanning electron microscope (SEM), has enabled the measurement of CPO i n rocks which are too fine-grained to be measured by conventional U-stage m ethods. Our objective is to study the influence of deformation and recrysta llisation mechanisms on the CPO and MOD. The olivine rocks studied were def ormed in uni-axial compression, in a gas-medium apparatus, to 17-24% strain at temperatures of 1200-1300 degrees C and 300 MPa confining pressures. Th e samples show a trend of weaker CPO with lower flow stress which may be re lated to an increasing component of grain boundary sliding and diffusion cr eep. In the diffusion creep regime the CPO and MOD are weak to random, wher eas in the dislocation creep regime the CPO and MOD are non-random but the MOD is principally controlled by the CPO. These results confirm the idea, b ased on studies from metals, that the CPO and MOD in olivine are characteri stic of the deformation mechanism. Dynamic recrystallisation during disloca tion creep results in the occurrence of more intermediate-angle (10-40 degr ees) grain boundaries than expected from the CPO. In local areas of complet e recrystallisation the MOD is controlled by the CPO which implies that the statistical MOD retains no signature of the initial recrystallisation proc ess. In the dislocation creep regime small grains have a weaker CPO compare d to large grains. This result is consistent with predictions from deformat ion mechanism maps which indicate that the fine recrystallised grains defor m by a combination of dislocation creep and grain boundary sliding. The gra in boundaries found in the deformed olivine polycrystals are predominately high-angle boundaries with misorientations between 60 and 117 degrees. No o bvious evidence has been found for the occurrence of preferred misorientati on, or special, grain boundaries. (C) 1999 Elsevier Science B.V. All rights reserved.