Fine structure in H-group chondrites

Abstract

Small-scale features are described in chondrites of the H chemical group, by the technique of high-voltage (1 MV) transmission electron microscopy. The meteorites studied intensively are: Tieschitz (H3), Bremervorde (H3), Quenggouk (H4), Allegan (H5), and Butsura (H6). In none of them are the effects of shock deformation intense enough to hinder the description of other fine structure, such as the exsolution lamellae that occur in the calcic pyroxenes of Tieschitz and Bremervorde, the scattered exsolved particles in olivine in the H 4-6 meteorites, and the unequilibrated low-calcium pyroxene (comprising varying proportions of clinopyroxene and orthopyroxene) in H 3-5 chondrules. The highly unequilibrated meteorite Tieschitz has finely crystalline textures in the mesostasis of chondrules, and in non-clastic matrix (e.g. dark rims around chondrules). Interstitial to the non-clastic constituents is fragmental material, with an admixture of small olivine grains, which has not been shock-compacted. Bremervorde has less non-clastic matrix and has suffered some shock compaction. In Quenggouk and Allegan, while chondrule interiors are mostly undeformed, the porous matrix has been modified by repeated, mild shock deformation, mostly later than the high-temperature stages of the material’s evolution. Effects of fine-scale cataclasis (cracking) are inferred in small areas of grains where the shock effects were most concentrated. The deformations were not intense enough to cause recrystallization or veining, even on the finest scale; thus these porous rocks have not been indurated. Widespread mild deformation and very localized intense deformation in samples of Butsura show some high-temperature features.