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.

Plain Language Summary

The stable isotope fractionation of siderophile elements is robust to trace the planetary core formation processes. However, whether nickel (Ni) isotopes fractionate during the core formation is highly debated, since the origin of Ni stable isotope difference between bulk silicate Earth (BSE) and chondrites is not clear. Here, we report high-precision Ni stable isotope data (expressed as δ60/58Ni, the per mil deviation of 60Ni/58Ni ratios relative to NIST SRM 986) for ureilite meteorites that come from the mantle of a carbon-rich differentiated body. Ureilites have an average δ60/58Ni value of 0.26 ± 0.13‰ (2SD, N = 22) that is highly consistent with that of chondrites with δ60/58Ni = 0.23 ± 0.14‰ (2SD, N = 37), which suggests planetary core formation does not effectively fractionate Ni stable isotopes. There is a ureilite trachyandesite that enriches lighter Cr stable isotopes (δ53Cr = -0.11 ± 0.02‰; δ53Cr as the per-mil deviation of 53Cr/52Cr ratios relative to NIST SRM 979) relative to the main-group ureilites (δ53Cr = -0.05 ± 0.04‰; 2SD, N = 10), which suggests the partial melting occurred on ureilite parent body (UPB). The globally heavy Cr in the UPB compared to chondrites can be caused by sulfur-rich core formation processes.