Shear-Induced ringwoodite formation in the Martian shergottite Dar al Gani 670

Abstract

Shock-produced melt veins in the Martian shergottite Dar al Gani 670 crosscut large olivine crystals. The upper part of one of these crystals appears to be sheared off and displaced along the shock vein. From the olivine–vein interface small lamellae of ringwoodite grow into the host crystal. The ≤1–3 mm wide and up to20 mm long lamellae consist of small bands and blocks and are orientated along specific crystallographic orientations. Texture and composition, i.e., more Fe-rich than the host olivine, indicate that lamellae formed via incoherent diffusion-controlled growth. It is suggested that a combination of high particle velocities and shock-induced defects lead to enhanced diffusion rates. In addition, shearing caused grain size reduction allowing rapid Fe–Mg interchange and induced lattice defects serving as nucleation sites for ringwoodite. Crystallographic orientation of ringwoodite lamellae indicates that during shock deformation the [001]{hk0} slip system was activated in olivine. Natural high-pressure phases in Martian meteorite allow to constrain phase transitions taking place in the inaccessible Earth's mantle. High pressure shear instabilities of olivine at subduction zones in 400–700 km depth are considered being responsible for deep earthquakes. At such p–T-conditions, breakdown of olivine results in formation of ringwoodite filled micro-anti cracks which interact with eachother finally leading to catastrophic shear failure. Our results strongly suggest that shearing itself contributes to a runaway process of enhanced ringwoodite formation and, thus, reinforces catastrophic material failure that may result in deep earthquakes.