A microstructural investigation of natural lamellar ringwoodite in olivine of the shocked Sixiangkou chondrite

Abstract

Olivine adjacent to the shock-produced veins of Sixiangkou meteorite contains several sets of ringwoodite lamellae nucleated and grown along planar fractures in olivine. These lamellae consist of two generations of crystallite aggregates of ringwoodite, and show features of incoherent nucleation and growth of ringwoodite along fractures of olivine. The lamellae are of zonal Fe–Mg distribution characterized as Fe-rich rim and Mg-rich interior. It indicates that the intracrystalline transformation of olivine to ringwoodite proceeds through Fe–Mg interdiffusion during growth of ringwoodite. The formation of the lamellae favors a fracture-involved phase transformation mechanism. Firstly, a thin layer of crushed or distorted olivine along a gliding plane of a fracture was produced and then transformed into the first generation of crystallite layer of ringwoodite; subsequently, the second generation of relatively coarse-grained ringwoodite grew on both interfaces of the firstly formed thin layer of ringwoodite. This study brings forward a new observation of intracrystalline olivine-ringwoodite transformation, in addition to other mechanisms. Since mechanical deformations resulting in planar fractures are common in olivine of terrestrial mantle rocks, this proposed mechanism could play an important role in the subducting lithospheric slabs.