Microstructural constraints on the formation of the Seymchan pallasite

Abstract

Pallasites are meteorites that consist of olivine grains in a FeNi-metal matrix. Although research on pallasites started decades ago, no consensus exists on their formation. Pallasites containing rounded olivine grains are often explained as having formed at the core-mantle boundary of their parent body. Pallasites containing angular olivines however, are explained by one or more impacts fracturing the olivine grains. Difficulty unravelling the pallasites’ origin is added by pallasite samples that contain both rounded and angular olivine: mixed-type pallasites. These alternative hypotheses will be tested by analysing a sample of the mixed-type Seymchan pallasite. The analysis combines oxidation decoration, electron backscatter diffraction (EBSD), and, for the first time on pallasites, high-angular resolution electron backscatter diffraction (HR-EBSD). The olivine grains reside in a kamacite matrix and display both angular and round shapes, although most grains contain rounded corners. Oxidation decoration yields a low dislocation density and EBSD lattice misorientation maps display clear subgrain structures in one olivine grain, and their absence in other grains. HR-EBSD analyses quantify the GND density, and residual stress and elastic strain heterogeneities. They reveal both subgrain boundaries and overprinting slip bands in one grain, while a different grain displays no microstructures. The olivine grains are therefore subdivided into two types. One type displays no intragranular deformation microstructures. The other grain-type contains large fractures, subgrain boundaries, and slip bands. These types of olivine grains imply two different deformation histories. The contrasting histories suggest that the types resided at different depths within the parent body and were subsequently mixed. Altogether, these observations suggest the following sequence of events: first, a hot planetary body was impacted, deforming olivine close to the surface. Because of the high temperatures the dislocations restructured into subgrain boundaries. The body was again impacted when it was colder, so that the slip bands were preserved. This impact also mixed the olivine grains with the body’s molten metal core. Afterwards, the planetary body cooled.