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- Creators: Barboni, Melanie
Description
Shock effects in meteorites provide important insights into impacts on their parent bodies. Eucrites are among the Howardite-Eucrite-Diogenite (HED) class of achondrites that likely originate from the intact, differentiated asteroid Vesta. Brecciated eucrites provide a record of the impact processes that occurred after the crustal formation of the parent body. Radiometric dating of HEDs has shown that they were affected by resetting events at 3.4 – 4.1 and 4.48 Ga. Therefore, shock effects in HEDs are windows into ancient impacts on asteroids early in solar system history. Northwest Africa (NWA) 8677 is a genomict eucrite with lithologies that are texturally different, but compositionally similar. The clasts in the breccia include strongly shocked (S5) gabbroic fragments and weakly shocked (S3) basaltic clasts. Coesite, a high-pressure polymorph of quartz, is preserved in the core of a large (~250 μm) silica grain, indicating the gabbro was strongly shocked. A large thermal overprint from the surrounding melt resulted in the transformation of coesite to low-pressure silica phases of quartz and cristobalite on the rims of this grain. The shock melt, interstitial to the breccia fragments, exhibits well-developed quench textures and contains a low-pressure mineral assemblage of plagioclase and pyroxene, implying that crystallization occurred after pressure release. The heterogeneity in shock features between the gabbroic and basaltic lithologies suggests that NWA 8677 experienced a variable impact history, which included at least two impact events. An initial impact strongly shocked and brecciated the gabbro and ejected both onto the regolith of the parent body where a more weakly shocked basalt was incorporated. A second impact produced the interstitial melt between the breccia matrix. The temperature of this shock melt remained high after pressure release, resulting in crystallization of a low-pressure assemblage of pyroxene and feldspar, as well as the transformation of quartz + cristobalite rims on coesite
ContributorsMarquardt, Madeline Claire (Co-author) / Sharp, Thomas (Co-author, Thesis director) / Fudge, Crystylynda (Co-author) / Irving, Tony (Co-author) / Barboni, Melanie (Committee member) / Desch, Steve (Committee member) / School of Earth and Space Exploration (Contributor) / Barrett, The Honors College (Contributor)
Created2019-05