Planetesimal formation in the inner and outer disk: Evidence from ungrouped iron meteorites

Isotope anomalies in meteorites reveal a fundamental dichotomy between non-carbonaceous (NC) and carbonaceous (CC) materials [1]. This dichotomy is recognized in both early- and late-formed planetesimals, but whether these formed from the same or distinct materials remains uncertain. To address this, we analyzed Fe and Ni isotopes in a large suite of ungrouped iron meteorites representing ~22 parent bodies. These samples, previously characterized for Mo and W isotopes, derive from early-formed differentiated bodies of NC (9) or CC (16) affinity [2].

Our new Fe and Ni isotope data show that early- and late-formed NC and CC planetesimals span similar isotopic ranges as their respective chondrites. This indicates that bodies in each reservoir accreted from similar dust mixtures, either in long-lived disk structures or in different sub-reservoirs containing the same materials. Among CC bodies, two findings stand out. First, many ungrouped irons match the isotopic composition of late-formed CR chondrites, suggesting the CR reservoir formed early and remained isolated throughout disk evolution. Second, CI chondrites are the only CC meteorites whose isotopic composition is absent among differentiated meteorites, implying planetesimals with this signature formed late by a distinct mechanism [3] and/or in a separate reservoir [4].

References: [1] Kleine et al. (2020) Space Sci. Rev. 216, 55. [2] Spitzer et al. (2025) Geochim. Cosmochim. Acta, in press. [3] Spitzer et al. (2024) Sci. Adv. eadp2426. [4] Hopp et al. (2022) Sci. Adv. eadd8141.