¹Devin L.Schrader,¹Jemma Davidson
Earth and Planetary Science Letters 589. 117552 Link to Article [https://doi.org/10.1016/j.epsl.2022.117552]
¹Buseck Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, 781 East Terrace Road, Tempe, AZ 85287, United States of America
Copyright Elsevier

The most abundant group of meteorites currently falling to Earth, ordinary chondrites, originate from S-type (Si-rich) asteroids and are thought to have originated in the inner Solar System. These asteroids typically underwent only minor aqueous alteration but experienced varying degrees of thermal metamorphism that altered their primary compositions and textures. However, some rare members remain unaltered and retain the pristine compositions they obtained in the protoplanetary disk prior to accretion of their parent asteroids. In contrast, comets formed in the icy reaches of the outer Solar System. Here we report on silicate minerals in pristine ordinary chondrites that are compositionally distinct from those in all other known chondrites but show similarities to those found in comet samples returned from Comet Wild 2 by NASA’s Stardust mission and those sourced from an unknown number of comets represented by interplanetary dusty particles. The identification of this material suggests that comets may have formed from diverse far-flung Solar System materials, including grains that migrated from the inner Solar System to the comet-forming region between ∼1 Myr and potentially ⪆3 Myr after the first Solar System solids formed. This finding suggests that migration from the inner to the outer Solar System lasted for millions of years and that comets are composed of residual materials from the entire early Solar System.

¹S.Iannini Lelarge,^1,2L.Folco,¹M.Masotta,³R.C.Greenwood,⁴S.S.Russell,⁴H.C.Bates
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.05.003]
¹Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, Pisa, Italy
²CISUP, Centro per l’Integrazione della Strumentazione Università di Pisa, Lungarno Pacinotti 43 Pisa, Italy
³Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
⁴Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom
Copyright Elsevier

The cosmochemistry of meteorites provides unique clues on asteroids accretion, differentiation, collisional break-up and reassembly, processes of critical importance for understanding planet formation in the early solar system. Mesosiderites are a complex group of achondrites whose nearly 50:50 metal-silicate composition is interpreted in the literature as resulting from the mixing of core and crustal materials derived from differentiated asteroid(s). Because of their complex nature and contrasting geochemical, isotopic, and spectroscopic data, the formation mechanism of mesosiderites is still poorly understood and is open to a large variety of planetary differentiation scenarios and collisional histories. In this study, based on new petrographic and geochemical data of 16 mesosiderites, we investigate in detail the proposal that mesosiderites are related to the howardite-eucrite-diogenite (HED) meteorite group, whose parent body is widely considered to be asteroid 4 Vesta (∼500 km diameter), the target of the recent NASA’s Dawn mission. We present the first high precision oxygen isotope analyses on the matrix of a set of mesosiderite samples, coupled with new chemical and petrographic analyses of mesosiderites Um Hadid, Estherville, and Mount Padbury. Concordant Δ17O values between mesosiderites (–0.241 ± 0.015 (2σ)) and howardite-eucrite-diogenites (−0.241 ± 0.017‰ (2σ)) indicate that they derived from the same oxygen isotope reservoir, but petrological evidence, in particular the distinctly lower Fe/Mn ratios and the larger lithological diversity in mesosiderites, indicates that they formed within different parent bodies. This suggests that mesosiderites and howardite-eucrite-diogenites originated in distinct parent bodies that accreted in the 4 Vesta source region but experienced different geologic evolution in terms of crustal differentiation and impact history, which were more complex and catastrophic in the mesosiderite parent body.