MINERALOGY OF CARBONACEOUS CHONDRITE CLASTS IN HED ACHONDRITES AND THE MOON

The majority of the carbonaceous chondrite clasts found in howardites, eucrites and diogenites are CM2 material, a lesser proportion is CR2 material, and other rare types are present. A single clast that was fo und on the Moon and called the Bench Crater meteorite is apparently sh ocked CM1 material. The CM2 clasts are matrix supported mixtures of ol ivine-pyroxene-phyllosilicate-sulfide bearing aggregates, loose olivin es and pyroxenes, sulfides, carbonates, and sinuous spinel-phyllosilic ate-diopside calcium-aluminum-rich inclusions (CAIs). Magnetite and me tal are rare. Some aggregates have fine-grained rims of material resem bling matrix. The opaque, fine-grained matrix consists predominantly o f serpentine of extremely variable composition and sulfides; tochilini te is occasionally present. The trace element data for one Jodzie clas t from this study and the average of similar clasts from Kapoeta suppo rt a CM classification; volatiles are depleted relative to CI and enri ched relative to CR material. The CR2 clasts are found (in small numbe rs) in only four howardites: Bholghati, Jodzie, Kapoeta and Y793497. P etrographically, they are matrix-supported mixtures of olivine aggrega tes (sometimes containing sulfides), loose olivines, pyrrhotite, pentl andite, low-Ca pyroxene (minor), hedenbergite (rare), kamacite (rare a nd only found within olivine), Ca-carbonates and abundant magnetite fr amboids and plaquets. Phyllosilicates are fine-grained and largely con fined to matrix; they are mixtures of serpentine and saponite. The mat rix of CR2 clasts also contains pyrrhotite, pentlandite, chromite and a significant fraction of poorly-crystalline material with the same bu lk composition as matrix phyllosilicate. There is evidence of heating in a substantial number of clasts, both CM2 and CR2, including: (1) co rrugated serpentine flakes, (2) pseudomorphs of anhydrous ferromagnesi an material after flaky phyllosilicates, and (3) hedenbergite rims on calcite. While the timing of the hedenbergite rims is debatable, the d estruction of phyllosilicates clearly occurred at a late stage, plausi bly during impact onto the HED asteroid(s) and Moon, and required peak heating temperatures on the order of 400 degrees C. We note that in g eneral, CM2 material was the most common carbonaceous chondrite lithol ogy impacting the HED asteroids (with howardites and eucrites taken to gether), as it is for the Earth today. A total of 61 out of 75 carbona ceous chondrite clasts from HED meteorites belong to the CM clan, petr ologic grade 2. This is also supported by published siderophile and vo latile element data on howardites, eucrites and diogenites that are ta ken to indicate that CM-like materials were the most common impactors on the HED asteroid(s). The ratio of CR/CM clasts in HED asteroids is essentially the same as for modern falls at Earth. This may indicate t hat the ratio of disaggregated CM2 to CR2 asteroidal material has been approximately constant through the history of the solar system. Final ly, our results are also compatible with type-2 carbonaceous chondrite s being equivalent to or from the same source as the material that ori ginally accreted to form the HED asteroid.