Exploring the effects of crystallographic orientation on shock features in Martian meteorites: how does the orientation of a crystal affect how it bends or breaks?

Thurs 16 August, 2018 @14:00 PM, level 6 Opat Room
Dr Lucy Forman, Research Associate
Curtin University

Email:  lucy.forman[at]curtin.edu.au


Shock features within Martian samples are thought to result primarily from the impact that launched them from the surface of Mars, and so exploring the material response can help constrain shock parameters, related impact processes and locate candidate launch craters on the Martian surface. Different minerals have varied material responses to stress and, more specifically in this case, the stress applied by a propagating shockwave. Slip systems must be activated in each grain so that the crystal lattice can be deformed. However, often the dominant activated slip system is dependent upon the orientation the stress is applied in, with relation to the crystallographic orientation of the grain, and the physical conditions of the material at the time of impact. Here we explore the effect of crystallographic orientation on the quantifiable amount of crystal deformation that is generated in an impact scenario on the Martian surface.

The initial focus of this study is the lherzolitic Shergottite Roberts Massif (RBT) 04262, which comprises poikilitic pyroxenes amongst a pyroxene and olivine-rich mineralogy. We initially examined a large (10 x 7 mm) twinned pyroxene grain. At the macro scale, shock is heterogeneous but no mineral phase changes have been observed, therefore overall shock is limited. Electron backscatter diffraction (EBSD) techniques were used to determine pyroxene orientation to constrain spectral characteristics and understand the style of deformation within the grains. The data comprise crystallographic information from all mineral phases at a step size of 12.2 µm.

The twinned pyroxene grain is primarily pigeonite based on the composition, and was divided into twins A & B. Twin A shows very little internal deformation in the pigeonite region (<2 º), but a consistently greater amount of misorientation is present in the augite rim. However, twin B, which is twinned on the [001] plane with twin A, shows a variable amount of misorientation throughout the crystal, which appears to undulate in contrast to the radial trend in deformation in twin A.

This sample has a very low porosity, which would have also been true at the time of impact, and therefore heterogeneities in shock are not due to shockwave interactions arising from interaction with pores. We subsequently infer the crystallographic orientation of each grain dictated the degree of crystal-plastic deformation generated by the shock wave. Further EBSD microstructural analysis will be used to constrain the slip systems that have been activated in the pigeonite, and subsequently constrain the physical conditions at the time of impact. This approach may allow determination of the shockwave propagation direction with respect to the plane of the sample. Further Martian samples have been investigated using this analytical approach. This study will contribute directly to our understanding of impact-induced deformation in a suite of rocks ejected from the Martian surface at the same time, and potentially by the same impact event.