The Kapoeta howardite: Implications for the regolith evolution of the howardite‐eucrite‐diogenite parent body

Abstract

Abstract— A large hand sample and numerous polished thin sections, made from the hand sample, of the Kapoeta howardite and its many diverse lithic clasts were studied in detail by optical microscopy and electron microprobe techniques in an attempt to understand the surface processes that operated on the howardite‐eucrite‐diogenite (HED) parent body (most likely the asteroid 4 Vesta). Four unique, unusually large clasts, designated A (mafic breccia), B (granoblastic eucrite), D (howardite) and H (melt‐coated breccia), were selected for detailed study (modal analysis, mineral microprobe analysis, and noble gas measurements). Petrographic studies reveal that Kapoeta consists of a fine‐grained matrix made mostly of minute pyroxene and plagioclase fragments, into which are embedded numerous different lithic and mineral clasts of highly variable sizes. The lithic clasts include pyroxene‐plagioclase (eucrite), orthopyroxenite (diogenite), howardite, impact‐melt, metal‐sulfide‐rich, and carbonaceous chondrite clasts. The howardite clasts include examples of lithic clasts that constitute breccias‐within‐breccias, suggesting that at least two regolith generations are represented in the Kapoeta sample we studied. The clast assemblage suggests that repeated shock lithification was an important process during regolith evolution. Noble gas analyses of clast samples fall into two populations: (a) solar‐gas‐rich clasts H (rim only) and D and (b) clasts A and B, which are essentially free of solar gases. The concentrations of solar noble gases in the two matrix samples differ by a factor of ∼40. It appears that clast D is a true regolith breccia within the Kapoeta howardite (breccia‐within‐breccia), while clast H is a regolith breccia that has been significantly impact reworked. Our data indicate that the Kapoeta howardite is an extraordinarily heterogeneous rock in modal mineral and lithic clast abundances, grain size distributions, solar‐wind noble gas concentrations and cosmic‐ray exposure ages. These results illustrate the repetitive nature of impact comminution and lithification in the regolith of the HED parent body.