Ubiquitous brecciation after metamorphism in equilibrated ordinary chondrites

Abstract

Our petrographic study of 42 H and L chondrites of types 4–6 revealed 8 that contain rare chondrules or clasts with olivines and pyroxenes having aberrant Fe/(Fe + Mg) ratios. For example, a chondrule in Slovak (H5) contains olivine with Fa 17–22; elsewhere olivine contains Fa 17–18. Although these chondrites are not obviously brecciated, their ingredients could not have experienced the same degree of metamorphism; these rocks must be fragmental breccias that were lithified after peak metamorphism. Regolith breccias and postmetamorphic, fragmental breccias, both polymict and monomict, may constitute a majority of ordinary chondrites. Comparison of chondritic and achondritic breccias indicates that chondritic, fragmental breccias are often very difficult to recognize because they resemble the fine‐grained mixtures of chondrules, and mineral and chondrule fragments from which they formed. In addition, the chemical and textural uniformity throughout most of the ordinary chondrite bodies ensures that mixtures of materials from different regions are well disguised. Contrary to the authors' claim, the Van Schmus‐Wood classification of ordinary chondrites does not identify rocks with similar thermal histories; the petrologic type of a chondrite is only a measure of the average degree of metamorphism experienced by its ingredients. Thus attempts to correlate physical properties with petrologic types must fail unless breccia and shock properties are considered. The extent of brecciation in ordinary chondrites and most achondrites indicates that meter‐sized unbrecciated rocks must be rare on the surfaces of their parent bodies.