Mineralogy of carbonaceous chondrite clasts in HED achondrites and the Moon

Abstract

Abstract 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 found on the Moon and called the Bench Crater meteorite is apparently shocked CM1 material. The CM2 clasts are matrix supported mixtures of olivine‐pyroxene‐phyllosilicate‐sulfide bearing aggregates, loose olivines and pyroxenes, sulfides, carbonates, and sinuous spinel‐phyllosilicate‐diopside calcium‐aluminum‐rich inclusions (CAIs). Magnetite and metal are rare. Some aggregates have fine‐grained rims of material resembling matrix. The opaque, fine‐grained matrix consists predominantly of serpentine of extremely variable composition and sulfides; tochilinite is occasionally present. The trace element data for one Jodzie clast from this study and the average of similar clasts from Kapoeta support a CM classification; volatiles are depleted relative to CI and enriched relative to CR material.The CR2 clasts are found (in small numbers) in only four howardites: Bholghati, Jodzie, Kapoeta and Y793497. Petrographically, they are matrix‐supported mixtures of olivine aggregates (sometimes containing sulfides), loose olivines, pyrrhotite, pentlandite, low‐Ca pyroxene (minor), hedenbergite (rare), kamacite (rare and only found within olivine), Ca‐carbonates and abundant magnetite framboids and plaquets. Phyllosilicates are fine‐grained and largely confined to matrix; they are mixtures of serpentine and saponite. The matrix of CR2 clasts also contains pyrrhotite, pentlandite, chromite and a significant fraction of poorly‐crystalline material with the same bulk composition as matrix phyllosilicate.There is evidence of heating in a substantial number of clasts, both CM2 and CR2, including: (1) corrugated serpentine flakes, (2) pseudomorphs of anhydrous ferromagnesian material after flaky phyllosilicates, and (3) hedenbergite rims on calcite. While the timing of the hedenbergite rims is debatable, the destruction of phyllosilicates clearly occurred at a late stage, plausibly during impact onto the HED asteroid(s) and Moon, and required peak heating temperatures on the order of 400 °C.We note that in general, CM2 material was the most common carbonaceous chondrite lithology impacting the HED asteroids (with howardites and eucrites taken together), as it is for the Earth today. A total of 61 out of 75 carbonaceous chondrite clasts from HED meteorites belong to the CM clan, petrologic grade 2. This is also supported by published siderophile and volatile element data on howardites, eucrites and diogenites that are taken 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 that the ratio of disaggregated CM2 to CR2 asteroidal material has been approximately constant through the history of the solar system. Finally, our results are also compatible with type‐2 carbonaceous chondrites being equivalent to or from the same source as the material that originally accreted to form the HED asteroid.