Ryugu and Bennu preserve primordial isotopic heterogeneity absent from the meteorite record.

Ivuna-type (CI) carbonaceous chondrites are commonly used as proxies for the composition of the planet-forming disk because their chemical abundances resemble those of the solar photosphere. However, their isotopic uniformity may reflect pervasive alteration and isotopic averaging rather than preservation of the earliest Solar System reservoirs. Here we show that samples returned from the asteroids Ryugu and Bennu preserve isotopic heterogeneity in major planet-building elements, including magnesium, silicon, and iron, that is absent from CI chondrites and unresolved in other meteorites. These signatures reveal isotopically distinct components that are absent from, or homogenized in, known CI chondrites. We infer that Bennu and Ryugu retain a record of primordial isotopic diversity that was not completely homogenized during disk and parent-body processing. Thus, returned asteroid samples provide access to previously unsampled Solar System material that preserves early isotopic components not fully represented in the meteorite record.