Ureilites are carbon-rich ultramafic achondrites that display unique metal–graphite textures including abundant strips of metal and graphite situated along grain boundaries, fracture fillings, and graphite pockets. Shock metamorphism suggest an episode of brittle deformation caused by impact disruption of the ureilite parent body. The origin of carbon and metal in ureilites has long been debated; numerous arguments suggest that the carbon is endogenous, while some or all of the grain boundary metal may be exogenous. We conducted experiments to simulate the formation of the metal–graphite textures in ureilite evolution.

Deformation experiments employing a model system of olivine, FeS melt, and graphite were performed with the SAPHiR three-axis press of BGI that is situated at the MLZ in Garching, Germany. Two scenarios were simulated with different sample setups: (I) intrusion of FeS melt into a polycrystalline olivine matrix containing dispersed graphite and (II) intrusion of graphite into an olivine matrix containing dispersed FeS melt pockets. The intruding material was contained in a cavity at the centre of the samples that partially collapsed during deformation. The samples were compressed and heated to 1 GPa and 1300 °C, statically annealed, and subsequently deformed at high strain rates of up to 1 × 10-2 s-1 to simulate an impact event.

The recovered samples revealed that the microstructures of scenario I (FeS intrusion) most closely simulate the textures observed in ureilites, which supports an endogenous origin of graphite and a largely exogenous origin of the grain boundary metal, consistent with geochemical arguments. During brittle deformation FeS melt intruded along grain boundaries and mixed with locally mobilized graphite thereby forming microstructures that closely resemble metal-graphite textures observed in ureilite meteorites. Hence, our results suggest a direct connection between the grain-scale metal-graphite textures in ureilites and the large-scale collision history of their parent body.