Refractory inclusions in Bennu samples indicative of outer Solar System accretion.

Abstract

NASA's OSIRIS-REx mission returned material from the carbonaceous asteroid Bennu, which is dominated by aqueous-alteration products but preserves rare high-temperature components. Here we present an integrated petrographic, oxygen-isotopic and ²⁶Al-²⁶Mg chronological characterization of refractory inclusions in Bennu samples. All identified inclusions are small (<~100 µm) and commonly preserve mineralogical evidence of in situ aqueous alteration, indicating incorporation into Bennu's parent body prior to hydration. Their mineralogical characteristics and oxygen-isotopic and ²⁶Al-²⁶Mg systematics demonstrate formation during the earliest stage of Solar System evolution and genetic affinity with refractory inclusions in asteroid Ryugu samples and chondritic meteorites. The shared refractory-inclusion signatures across diverse chondritic materials are consistent with widespread outward transport and mixing of early-formed inner-disk solids. In Bennu samples analyzed thus far, large (>submillimeter) refractory inclusions are absent, in contrast to most carbonaceous chondrites. This size distribution implies that Bennu's constituents accreted in the outer Solar System beyond the pressure bump associated with proto-Jupiter, which effectively filtered distinct dust populations.