THE KAPOETA HOWARDITE - IMPLICATIONS FOR THE REGOLITH EVOLUTION OF THE HOWARDITE-EUCRITE-DIOGENITE PARENT BODY

A large hand sample and numerous polished thin sections, made from the hand sample, of the Kapoeta howardite and its many diverse lithic cla sts were studied in detail by optical microscopy and electron micropro be techniques in an attempt to understand the surface processes that o perated on the howardite-eucrite-diogenite (HED) parent body (most lik ely the asteroid 4 Vesta). Four unique, unusually large clasts, design ated A (mafic breccia), B (granoblastic eucrite), D (howardite) and H (melt-coated breccia), were selected for detailed study (modal analysi s, mineral microprobe analysis, and noble gas measurements). Petrograp hic studies reveal that Kapoeta consists of a fine-grained matrix made mostly of minute pyroxene and plagioclase fragments, into which are e mbedded numerous different lithic and mineral clasts of highly variabl e sizes. The lithic clasts include pyroxene-plagioclase (eucrite), ort hopyroxenite (diogenite), howardite, impact-melt, metal-sulfide-rich, and carbonaceous chondrite clasts. The howardite clasts include exampl es of lithic clasts that constitute breccias-within-breccias, suggesti ng that at least two regolith generations are represented in the Kapoe ta sample we studied. The clast assemblage suggests that repeated shoc k lithification was an important process during regolith evolution. No ble 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 gas es in the two matrix samples differ by a factor of similar to 40. It a ppears that clast D is a true regolith breccia within the Kapoeta howa rdite (breccia-within-breccia), while clast H is a regolith breccia th at has been significantly impact reworked. Our data indicate that the Kapoeta howardite is an extraordinarily heterogeneous rock in modal mi neral and lithic clast abundances, grain size distributions, solar-win d noble gas concentrations and cosmic-ray exposure ages. These results illustrate the repetitive nature of impact comminution and lithificat ion in the regolith of the HED parent body.