Mass-independent Fe, Cr, and Ti isotope heterogeneities between CM1 and CM2 chondrites induced by refractory inclusions and terrestrial weathering

CM chondrites record significant fluid–rock interaction in the early Solar System, yet whether their petrologic range reflects progressive hydration within a single parent body or sampling of multiple bodies remains debated. We present high-precision mass-independent Fe, Cr, and Ti isotope data (µ ⁵⁴ Fe, µ ⁵⁴ Cr, µ ⁵⁰ Ti) for ten bulk CM1, CM2, and CM1/2 chondrites to constrain their formation and alteration history. Variability in µ ⁵⁰ Ti values record heterogeneous distribution of refractory inclusions at sub-centimetre scales with a common chondrule–matrix reservoir. Bulk µ ⁵⁴ Fe compositions are generally uniform, also supporting derivation from a common isotopic reservoir. However, several samples, independent of their degree of aqueous alteration, exhibit resolvable µ ⁵⁴ Fe depletions. These shifts are best explained by two terrestrial weathering processes of preferential removal via leaching of chondritic alteration products and metals and the exchange of aqueous Fe in Antarctic fluids with chondritic Fe. While these results support derivation from a common isotopic reservoir for all CM chondrites, it remains unclear whether the CM chondrites sample a single or multiple CM parent bodie(s). This study highlights the need to account for terrestrial weathering effects when interpreting subtle µ ⁵⁴ Fe variations in carbonaceous chondrites and additional mass-independent, refractory element isotopic analyses of CM1 chondrites.