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Carbonaceous and non-carbonaceous lithic fragments in the Plainview, Texas, chondrite: origin and history

Authors
Journal
Geochimica et Cosmochimica Acta
0016-7037
Publisher
Elsevier
Publication Date
Volume
40
Issue
2
Identifiers
DOI: 10.1016/0016-7037(76)90175-7

Abstract

Abstract The Plainview. Texas, meteorite is a polymict-brecciated H-group chondrite composed of recrystallized light-colored portions embedded in a well-compacted, dense, somewhat recrystallized, dark-colored matrix. Both portions consist of equilibrated silicates (H5 classification), but a small number of silicate grains and unequilibrated lithic fragments not compatible with equilibrated ordinary H-group material are present in the dark-colored matrix. Lithic fragments include: (i) dark-colored, more or less altered, type II carbonaceous chondrites. (ii) unequilibrated ordinary chondrites and (iii) light-colored, unequilibrated and equilibrated fragments, some of which are compositionally similar to the host. Also present are fragment-like dark areas that are highly-shocked host material and not true lithic fragments (pseudo-fragments). Conclusions: Plainview represents a complex regolith breccia formed by repeated impact episodes. Recrystallized, light-colored portions represent surface or near-surface material of a small (asteroidal-sized) parent body. Impacts broke up this material to form fine-grained, dark material which enclosed light-colored protolith. Lithic fragments (i-iii) and some unequilibrated silicate grains and chondrules (apparently derived from unequilibrated chondrites) were embedded in the dark matrix during these repeated impacts. Xenolitlils of carbonaceous and unequilibrated ordinary chondrites are either residues of projectiles that impacted the Plainview parent body, or material from coexisting regoliths impact-splashed into Plainview regolith. Chondrules and silicate grains in the dark matrix which differ from H-group material are likely related to these xenoliths and their regoliths. Light-colored lithic fragments may represent shock-melted chondritic material, sometimes compositionally-modified, or new, achondritic meteoritic types. Unequilibrated and carbonaceous lithic fragments in the dark-colored host matrix indicate that equilibration of the host occurred before incorporation of the fragments and that compaction and lithification of the Plainview regolith to form a coherent meteorite must have occurred at temperatures below 300°C and/or on a short time scale.

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