Origin of olivine subgrain boundaries in mantle peridotites

Abstract

INVESTIGATION of the fabric, texture and dislocation micro-structure of olivine has given much insight into the deformation history of mantle peridotites¹. Optically visible (100) subgrain boundaries (tilt or kink-band boundaries) are particularly significant in this respect. Their existence is the principal argument favouring intracrystalline glide as the mechanism of deformation² and the spacing between them (d₁₀₀) is considered to be a measure of the stress conditions during deformation^3–6. Previous workers have argued that the sub-boundaries in olivine originated at the onset of deformation of periodotites^1,2, predating both the recrystallised strain-free secondary grains⁷ and grain boundary development in general¹. However, re-examination of the distribution of sub-boundaries in olivine reveals features which are quite inconsistent with this interpretation. It is suggested in the present work that (100) sub-boundaries are a late feature in these peridotites and most probably have no connection with hypothetical asthenosphere flow.