Consolidating the isotopic trichotomy of planetary materials with new evidence

Nucleosynthetic isotope anomalies in planetary materials reveal three reservoirs in the protoplanetary disk: one in the inner Solar System, represented by non-carbonaceous chondrites (NC), and two in the outer Solar System, represented by Ivuna-type carbonaceous chondrites (CI) and other carbonaceous chondrites (CC). Separation of the CC and CI reservoirs has been debated, with some suggesting that certain carbonaceous achondrites might bridge the isotopic gap between these two groups. Our new Fe isotope data for NC and CC achondrites together with so far uncharacterised chondrite groups show that, while three ungrouped CC achondrites share O, Ti and Cr isotopic similarities with CI chondrites, their Fe isotopic compositions differ, spanning the range observed in CC meteorites. These findings reaffirm the existence of a distinct isotopic reservoir represented by CI chondrites, supporting their formation in a distinct region of the protoplanetary disk or alteration of the constituents of CC and CI parent bodies by nebular processes. The absence of CI-like differentiated bodies could be due to their parent bodies incorporating ice during formation, which limited heating.