Lessons from Dynamic Heds: Diagonite Microstructures Suggest Solid-State Deformation, Annealing and Incipient Differentiation

Abstract:

Diagonite meteorites are commonly thought to represent the deep crust or mantle material of Vesta-like bodies. Here, we present textural analysis by neutron diffraction and microstructural and microchemical data on diagonite Yamato 74013. The meteorite is composed mainly of orthopyroxene with inclusions and interstitial grains of troilite and chromite. A coarse grained orthopyroxene – troilite vein transects the sample. Neutron diffraction data shows a distinct crystallographic preferred orientation (CPO) of the orthopyroxene grains. Such strong CPO is unusual for meteorites such as chondrites and stony meteorites.

Detailed electron backscatter diffraction analysis confirms that the origin of the preferred orientation is solid state crystal plastic deformation. All phases in the matrix of the rock show crystal plastic deformation, chromite shows distinct substructures, with inclusions of troilite. Orthopyrxone grains are between 50-120 microns in diameter and exhibit irregular interlocking grain boundaries, subgrain boundaries and continuous crystal lattice bending. Grain boundary morphology suggests post-deformation recrystallization with a dominance of grain boundary migration. Orthopyroxene within the vein have traces of S and show straight grain boundaries to each other. Vein troilite grains are interstitial and reach sizes up to 50 times larger than in the matrix.

Our results suggest that deep in Yamato’s parent body, orthopyroxene is deformed in a crystal plastic regime and experienced significant post-deformational recrystallization. The irregular distribution of interstitial troilite and chromite suggests that segregation of core material has been incipient retaining significant amounts of metal. One possible conclusion is that within small bodies like Vesta, even though deformation and differentiation were concomitant within a dynamic environment, differentiation was not efficient.