Nucleosynthetic isotope anomalies show that the first few million years of solar system history were characterized by two distinct cosmochemical reservoirs, CC (carbonaceous chondrites and related differentiated meteorites) and NC (the terrestrial planets and all other groups of chondrites and differentiated meteorites), widely interpreted to correspond to the outer and inner solar system, respectively. At some point, however, bulk CC and NC materials became mixed, and several dynamical models offer explanations for how and when this occurred. We use xenoliths of CC materials in polymict ureilite (NC) breccias to test the applicability of such models. Polymict ureilites represent regolith on ureilitic asteroids but contain carbonaceous chondrite-like xenoliths. We present the first ⁵⁴Cr isotope data for such clasts, which, combined with oxygen and hydrogen isotopes, show that they are unique CC materials that became mixed with NC materials in these breccias. It has been suggested that such xenoliths were implanted into ureilites by outer solar system bodies migrating into the inner solar system during the gaseous disk phase ∼3–5 Myr after CAI, as in the “Grand Tack” model. However, combined textural, petrologic, and spectroscopic observations suggest that they were added to ureilitic regolith at ∼50–60 Myr after CAI, along with ordinary, enstatite, and Rumuruti-type chondrites, as a result of the breakup of multiple parent bodies in the asteroid belt at this time. This is consistent with models for an early instability of the giant planets. The C-type asteroids from which the xenoliths were derived were already present in inner solar system orbits.

核合成同位素异常表明，太阳系历史的最初几百万年的特征是两个不同的宇宙化学库，CC（碳质球粒陨石和相关的分化陨石）和NC（类地行星和所有其他类球粒陨石和分化陨石），被广泛解释分别对应外太阳系和内太阳系。然而，在某些时候，散装 CC 和 NC 材料会混合在一起，并且几个动力学模型可以解释这种情况如何以及何时发生。我们使用多聚脲岩 (NC) 角砾岩中的 CC 材料捕虏体来测试此类模型的适用性。多聚脲岩代表脲岩小行星上的风化层，但含有碳质球粒陨石状捕虏体。我们提供了此类碎屑的第一个⁵⁴ Cr 同位素数据，该数据与氧和氢同位素相结合，表明它们是独特的 CC 材料，与这些角砾岩中的 NC 材料混合。有人认为，这些捕虏体是在 CAI 之后约 3-5 Myr 的气态盘阶段，由外太阳系天体迁移到内太阳系而被植入到尿素岩中的，如“Grand Tack”模型中所示。然而，综合纹理、岩石学和光谱观察表明，它们在 CAI 后与普通、顽火辉石和 Rumuruti 型球粒陨石一起被添加到约 50-60 Myr 的脲质风化层中，这是由于多个母体在 CAI 中破碎的结果。此时的小行星带。这与巨行星早期不稳定的模型是一致的。捕虏体所源自的 C 型小行星已经存在于太阳系内部轨道中。