Nickel and chromium stable isotopic composition of ureilites: implications for the Earth's core formation and differentiation of the ureilite parent body

We report the first Ni and Cr stable isotope data for ureilite meteorites that are the mantle residue of a carbon-rich differentiated planet. Ureilites have similar Ni stable isotope compositions as chondrites, suggesting the core-mantle differentiation of ureilite parent body did not fractionate Ni isotopes. Since the size of Earth is potentially larger than that of ureilite parent body (UPB; with diameter > 690 km), resulting in higher temperatures at the core-mantle boundary of Earth, it can be predicted that the terrestrial core formation may not directly cause Ni stable isotope fractionation. On the other hand, we also report high-precision Cr stable isotope composition of ureilites, including one ureilitic trachyandesite (ALM-A) that is enriched in lighter Cr stable isotopes relative to the main-group ureilites, which suggests the partial melting occurred on UPB. The globally heavy Cr in the UPB compared to chondrites can be caused by sulfur-rich core formation processes.