An Investigation of Dislocation in Olivine Phenocrysts from the Hawaiian Basalts

Abstract

Intracrystalline distortions (like undulose extinction, dislocations, and subgrain boundaries) in olivine from naturally-deformed peridotites are generally taken as signs of dislocation creep. However, similar features in olivine phenocrysts that have been found in basaltic magmas are still not well understood. In particular, whether subgrain boundaries in olivine phenocrysts arise from plastic deformation or grain growth is still debated (in the latter case, they are essentially grain boundaries but not subgrain boundaries. Therefore, we used hereinafter subgrain-boundary-like structures instead of subgrain boundaries to name this kind of intracrystalline distortion). Here we carried out a detailed study on dislocations and subgrain-boundary-like (SG-like) structures in olivine phenocrysts from two Hawaiian basaltic lavas by means of petrographic microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). Abundant and complex dislocation substructures (free dislocations, dislocation walls, and dislocation tangles) were observed in the decorated olivine grains, similar to those in olivine from peridotite xenoliths entrained by the Hawaiian basalts. The measured average dislocation density is (2.9±1.3)×10¹¹ m⁻², and is three to five orders of magnitude higher than that in laboratory-synthesized, undeformed olivine. TEM observations on samples cut across the SG-like structures by FIB (focused ion beam) demonstrated that this kind of structures is made of an array of dislocations. These observations clearly indicate that these structures are real subgrain boundaries rather than grain boundaries. These facts suggest that the observed high dislocation densities and subgrain boundaries cannot result from crystal crystallization/growth, but can be formed by plastic deformation. These deformation features do not prove that the olivine phenocrysts (and implicitly mantle xenoliths) were deformed after their capture by the basaltic magmas, but can be ascribed to a former deformation event in a dunitic cumulate, which was formed by magmatic fractionation, then plastically deformed, and finally disaggregated and captured by the basaltic magma that brought them to the surface.

Keywords

olivine phenocryst / dislocation / subgrain boundary / deformation / Hawaiian basalts

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