Coordinated mineralogical and isotopic analyses of a cosmic symplectite discovered in a comet 81P/Wild 2 sample

1,2Ann N. Nguyen,2,3Eve L. Berger,2Keiko Nakamura-Messenger,2Scott Messenger,2Lindsay P. Keller
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12905]
1Jacobs, NASA Johnson Space Center, Houston, Texas, USA
2Robert M. Walker Laboratory for Space Science, EISD Directorate, Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, Texas, USA
3GeoControl Systems – Jacobs JETS Contract, NASA Johnson Space Center, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

We have discovered in a Stardust mission terminal particle a unique mineralogical assemblage of symplectically intergrown pentlandite ((Fe,Ni)9S8) and nanocrystalline maghemite (γ-Fe2O3). Mineralogically similar cosmic symplectites (COS) have only been found in the primitive carbonaceous chondrite Acfer 094 and are believed to have formed by aqueous alteration. The O and S isotopic compositions of the Wild 2 COS are indistinguishable from terrestrial values. The metal and sulfide precursors were thus oxidized by an isotopically equilibrated aqueous reservoir either inside the snow line, in the Wild 2 comet, or in a larger Kuiper Belt object. Close association of the Stardust COS with a Kool mineral assemblage (kosmochloric Ca-rich pyroxene, FeO-rich olivine, and albite) that likely originated in the solar nebula suggests the COS precursors also had a nebular origin and were transported from the inner solar system to the comet-forming region after they were altered.

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