KAPOETA


Howardite
Regolith breccia
standby for kapoeta photo
Fell April 22, 1942
31° 10' S., 127° 45' E.

An 11.355 kg stone ~18 cm in diameter fell in Equatoria, South Sudan, at 7:00 in the evening. The mass was recovered on the Kapoeta–Nathalani Road. The main mass currently resides with the Sudan Geological Survey in Khartoum.

Kapoeta has a composition of a microbreccia formed through impact gardening of the compacted regolith of its parent body, probably the asteroid/planetoid 4 Vesta. Kapoeta consists of mineral and rock fragments, primarily eucrite and diogenite sourced, having a mixing ratio of 2 parts eucritic to 1 part diogenitic material, and which spans the known range of Fe/(Fe+Mg) ratios of HED samples. This mixing ratio corresponds to an Al-oxide content near 8–9 wt%, which some have defined as a compositional cluster to which the regolithic howardites belong. Kapoeta has a cumulate/noncumulate eucrite ratio commensurate with that of known eucrites. Unlike most howardites which are fragmental breccias, Kapoeta is one of a small number of howardites that are distinguished as regolithic breccias (e.g., LEW 85313, MET 00423, PRA 04401, SCO 06040, and EET 87513) (Warren et al., 2009; Cartwright et al., 2012). Consistent with this category, Kapoeta is enriched in solar-wind noble gases, contains high siderophile abundances (e.g., >300 PPM Ni; high Ir), contains an abundance of glasses (mostly spheroidal or turbid-brown), and incorporates xenoliths of black, carbonaceous chondrite fragments and microclasts. The carbonaceous (CM) fragments are probably a late addition to the regolith breccia, and contain planetary gases in addition to solar-wind implanted gases (Cartwright et al., 2011).

In a study of regolithic howardites conducted by Cartwright et al. (2013), it was concluded that noble gas analysis is the most important indicator among those used for determining a regolithic origin. They resolved two trends that exist among regolithic howardites: 1) the presence of a trapped solar wind or fractionated solar wind noble gas component, and/or 2) the presence of a planetary noble gas component (such as the dominant Q-gases or the exotic HL-noble gases), known to be associated with xenolithic carbonaceous chondrite clasts; the presence of only cosmogenic noble gases is not prognostic of a regolithic origin.

It is proposed by Warren et al. (2009) that the regolithic howardites represent a rare ancient phase of regolith gardening that has since been destroyed within the last 1 b.y. in the large impact which produced the Vestoids. The tight clustering of the Al-oxide composition near 8–9 wt% and the consistency of the ~1:2 mixing ratio of eucrite to diogenite for the regolithic howardites infers a single large impact excavation into shallow eucrite and deeper diogenite layers; these layers were proportionately equal with respect to asteroid radius. This was followed by shallow lateral mixing and homogenization to produce a specific regolith grade/maturity consistent with a high diversity of components. The high siderophile content of the regolithic howardites compared to other HED breccia types is consistent with an extended period of impact gardening that homogenized impactor and regolith materials.

The larger carbonaceous impactor fragments are composed of fine-grained matrix material with embedded chondrules resembling those of the CM2 type, while the carbonaceous chondrite microclasts (CCMs) are smaller than 1 mm and comprise three different types: 1) tochilinite-rich, consistent with CM2 chondrite material; 2) magnetite-rich, olivine-poor; and 3) magnetite-rich, olivine-rich, the latter two types having no representative meteorites in our collections (Gounelle et al., 2005). In common with Antarctic micrometeorites, these three CCM types have a combined D/H ratio nearly identical to that of the Earth, and it is argued that CCMs might represent the same material which delivered water to the accreting Earth, a theory consistent with research results obtained by the Carnegie Institution for Science and published in Science Express, July 12, 2012. In their analysis of platinum group elements to discern the impactor types present in howardites, Wee et al. (2010) identified a wide variety of material including various subgroups of carbonaceous, ordinary, and enstatite chondrites. Kapoeta exhibits high abundances of impactor material equivalent to ~10% chondritic debris, with 2–3% comprising CM chondrite material.

Also present in the Kapoeta regolith breccia are impact-melt clasts of howardite material, as well as glass spherules which have been shown to be a product of impact rather than fire-fountaining (Boesenberg and Mandeville, 2007). All of these disparate components were mixed together over eons of continuous bombardment to the surface of the HED-parent asteroid. Based on Hf–W systematics of a limited number of howardites, an isochron was gleaned which is consistent with ages previously determined for eucrites and diogenites (Lee and Fukuyama, 2009). A breccia compaction age of 2 b.y. has been calculated by investigators. As evidenced by the presence of solar-wind-implanted noble gases, Kapoeta was part of a regolith on its parent body for at least a time, and a high Ni content has been found to be diagnostic of such a regolithic origin. This regolith residence was followed by a period of burial lasting several millions of years which drove 26Al to extinction. Based on data from the cosmogenic radionuclide 36Cl, a cosmic-ray exposure age of ~3 m.y. was derived. This data also suggests that the meteoroid had a pre-atmospheric diameter of ~40 cm, and that the Kapoeta meteorite had been located several cm deep within this meteoroid.

Studies of orthopyroxene crystals from selected howardites indicate a magnesium concentration in Kapoeta consistent with that of diogenites (Domeneghetti et al., 2000, 2004). The presence of exsolved augite, along with a slow cooling rate, suggests an origin of Kapoeta within a deep, plutonic, diogenitic source region, which experienced an ejection by impact. A more detailed scenario for the formation of the HED clan can be found on the Millbillillie page.

The basaltic achondrite group is a complicated one to classify due to the diversity in the structural and mineralogical relationships among its members. This group is composed of brecciated and unbrecciated, monomict and polymict eucrites, diogenites, and howardites, and has recently undergone a redefinition.

The monomict subgroup containing eucrites, cumulate eucrites, and diogenites is further subdivided into brecciated and unbrecciated members. The polymict subgroup samples a compositional and textural continuum of regolith and surface breccias consisting of eucrites, cumulate eucrites, diogenites, and howardites. Those meteorites containing more than 90% of a single component are given the prefix 'polymict' attached to their present description (e.g. polymict eucrites contain less than 10% non-eucritic material; polymict diogenites contain more than 90% orthopyroxenite). Those meteorites that contain less than 90% of any single component are defined as howardites. While this 10% level is still an arbitrary dividing line based simply on mineral mixing proportions, it represents an amount of orthopyroxene that can easily be detected by X-ray diffraction techniques. An additional tool to distinguish polymict eucrites from howardites involves pyroxenes in the basaltic clasts—in howardites they are mostly unzoned, whereas in polymict eucrites they are usually zoned.

The number of howardites in our collections is about one-third the number of eucrites and slightly less than the number of diogenites; however, the compositionally distinct subset of regolithic howardites is much more poorly sampled (Warren et al., 2009). A transmitted light view of a petrographic thin section of Kapoeta can be seen on J. Kashuba's page. The Kapoeta specimen pictured above is a 2.4 g partial slice.