An unidentified reservoir of comet 81P/Wild 2 chondrules.

Comets, which have been stored in the cryogenic regions for billions of years, preserve key records of the solar system's formation and the dynamic evolution of planets and other small bodies. Twenty years ago, NASA Stardust mission returned samples from a Jupiter-family comet, 81P/Wild 2, for laboratory analysis, and uncovered an unexpected truth that this comet is not entirely made of interstellar grains but contains significant amounts of high-temperature silicate fragments. However, the sources of these fragments remain ambiguous. Here, we report the petrography and oxygen three-isotopes of ~100 silicate fragments from track 227, one of the largest impact tracks in the Stardust aerogel collector. The fragments are mostly pieces of small chondrules (typically <60 µm) with many exhibiting igneous textures such as microporphyritic olivine and, occasionally, nonporphyritic barred olivine and cryptocrystalline. Approximately 90% of the fragments define a modest negative correlation between the mass-independent fractionation of oxygen isotopes (Δ17O = δ17O -0.52 × δ18O, -4 to 0‰) and the redox state [Mg# = mol% Mg/(Mg + Fe), 100 to 62]. The remaining 10% are markedly FeO-rich (Mg# <60) and 16O-depleted (Δ17O up to 7‰). These characteristics distinguish them from chondrules in any known chondrite groups, nor can they be reproduced by mixing chondrule populations in any proportion. Instead, these fragments are from an unidentified chondrule reservoir in a more distant outer disk region. These chondrules were possibly formed in a highly oxidized, ice-rich environment ≥3 Ma after solar system formation.