Comet 81P/Wild 2 dust impactors of Stardust turnip-like tracks analogous to cluster IDPs

We measured oxygen isotope ratios of 16 silicate fragments from seven aerogel tracks (turnip -like type B tracks 77, 149, 172, 191, and 220; carrot -like type A tracks 22 and 175) of the comet 81P/Wild 2 collector from NASA's Stardust mission using secondary ion mass spectrometry. Thirteen were prepared by ultramicrotomy; three from track 220 were prepared by sputtering resin blocks using a SIMS Kohler beam, a new procedure aiming to mine as many cometary particles encased in aerogel/resin as possible. Combining new and literature results, we recognized that most silicate fragments of individual type B tracks have diverse mineralogy but consistent massindependent fractionation of oxygen isotopes ( Delta 17 O = delta 17 O - 0.52 x delta 18 O). These observations suggest that their impactors are loosely bound aggregates of unequilibrated materials originating mainly from similar protoplanetary disk regions, resembling the cluster IDP U2-20-GCA. Furthermore, silicate fragments from type A track 22 have almost identical mineralogy and Delta 17 O values, confirming that its impactor is a single chondrule-like fragment. The terminal particle of type A track 175 is pure forsterite with Delta 17 O of - -23 %o. Six iron -rich fragments of this study have positive oxygen isotope ratios ( Delta 17 O- +2 %o) and ordinary chondrite chondrule-like olivine compositions. Together with five similar fragments in the literature, a unique population (Mg# <= 86) of Wild 2 fragments that resemble chondrules from the inner solar system (O -E -R) chondrites or the outer solar system CH -CB chondrites was identified. The remaining 16 O -poor Wild 2 fragments are Mg# >= 79 silicates with Delta 17 O - -2 %o and a small amount of Mg# <= 79 silicates with Delta 17 O - 0 %o, which are most consistent with CR chondrite chondrules. Thus, we conclude that in addition to the possible major source of CR chondrite chondrule-like materials, the inner solar system or CH -CB chondrule-like materials are a minor component of comet Wild 2, like the cluster IDP U2-20-GCA.