The NC-CC dichotomy explained by significant addition of CAI-like dust to the Bulk Molecular Cloud (BMC) composition

Abstract

Nucleosynthetic isotope anomalies of planetary materials provide insight into their genetic ties, informing our understanding of early Solar System isotopic architecture and evolution. Isotope anomalies of non‑carbonaceous (NC) and carbonaceous (CC) materials in multi-element space suggests their variability primarily emerged from mixing between several primordial nebular source regions in the nascent protoplanetary disk. In particular, it has been suggested that the elemental and isotopic compositions of CC meteorites reflect admixtures of NC-like, CI-like, and CAI-like components. Despite the plethora of elements for which isotope anomalies have been characterized, no mixing model has quantitatively reproduced CC meteorite compositions for more than two elements.In this paper, we leverage the recent characterization of Fe isotope anomalies in NC and CC materials, as well as CAIs, to place new constraints on the evolution of the early Solar System and the origin of the CC chondrites. We first respond to the recent proposal, based on Fe isotope analyses of returned samples from Cb-type asteroid Ryugu, that Ryugu and CI chondrites are genetically distinct from NC and CC bodies, originating from a third “CI reservoir” beyond the location of the CC reservoir. Namely, we propose that the appearance of such a trichotomy in meteoritic heritages arises from the current lack of Fe isotope data for CC achondrites. We go on to present a self-consistent mixing model that explains the Ti, Cr, Fe, and Ca concentrations and isotope anomalies of the CM, CV, CO, CK, and CR chondrite groups via admixing of (i) elementally OC-like material, (ii) CI/Ryugu-like material, (iii) isotopically CAI-like dust, and (iv) CAIs sensu stricto. We find that the CAI-like dust constitutes a major and broadly constant fraction (∼36%) of all CC chondrites, and identify the CI-like component with the bulk composition of the Solar System's parent molecular cloud, denoting it BMC for “Bulk Molecular Cloud.” We interpret our results in the context of a qualitative model for early Solar System isotopic evolution.