Lithic components in the paired howardites EET 87503 and EET 87513: Characterization of the regolith of 4 Vesta

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The matrix and twenty four clasts from the paired howardites EET 87503 and EET 87513 were studied by petrographic observation, instrumental neutron activation analysis (INAA), and electron microprobe analysis. Materials that presumably were affected by moderate to high degrees of shock include impact-melt breccias and eucritic clasts with blackened pyroxenes. Eucritic clasts were derived from cumulate eucrites, main-group eucrites, and evolved eucrites that are more Fe-rich than Nuevo Laredo, and include unequilibrated and equilibrated materials. Several of the diogenitic clasts are orthopyroxenite breccias that are much finer-grained than typical diogenites. Bulk compositions and mineral compositions of matrix and clasts indicate that EET 87513 can be modeled as a mixture, by weight, of 30% diogenite, 17% cumulate eucrite, 25% main-group eucrite, 25% Nuevo Laredo-trend eucrite, and 3% CM2 chondrite. The character of this mixture is probably related to the depth of excavation by impact processes on 4 Vesta and these meteorites may be appropriate as approximate analogues for portions of the surface regolith. The relatively large proportion of CM2 material in this mixture, compared with the relatively small number of carbonaceous chondrite clasts observed in thin sections, suggests that much of this CM2 material is present as sub-microscopic particles between individual mineral fragments and clasts.
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