An igneous-textured clast in the Peace River meteorite: insights into accretion and metamorphism of asteroids in the early solar system

Abstract

The mineralogy, petrology, and geochemistry of an igneous-textured clast in the Peace River L6 chondrite meteorite was examined to determine the roles of nebular processes, accretion, and parent-body metamorphism in its origin. The centimetre-scale clast is grey and fine grained and is in sharp contact with the host chondrite. Two sub-millimetre veins cut across both the clast and host, indicating that the clast formed prior to the impact (shock) event(s) that produced the numerous veins present in the Peace River meteorite. The clast and host are indistinguishable in terms of mineral compositions. In contrast, there are differences in modal mineralogy, texture, as well as trace element and oxygen isotope composition between the clast and host. These differences strongly suggest that the clast was formed by impact melting of LL-group chondritic material involving loss of Fe-FeS and phosphate components, followed by relatively rapid cooling and incorporation into the Peace River host meteorite. Subsequent metamorphism on the Peace River parent body caused recrystallization of the clast and homogenization of mineral compositions and thermally labile element abundances between the clast and host. Shock metamorphism, including formation of shock melt veins, occurred post-metamorphism, during fragmentation of the L chondrite parent body. The results suggest that the formation of the Peace River parent asteroid included the incorporation of material from other asteroids and that the pre-metamorphic protolith was a breccia. Accordingly, we propose that the Peace River meteorite be reclassified as a polymict breccia.Résumé : La minéralogie, la pétrologie et la géochimie d'un claste à texture ignée dans la météorite chondrite Peace River L6 ont été étudiées afin de déterminer les rôles des processus nébulaires, d'accrétion et de métamorphisme quant à l'origine de l'amas parent. Le claste d'échelle centimétrique est gris et à grain fin; il est de plus en contact net avec la chondrite hôte. Deux veines de taille inférieure à un millimètre recoupent le claste et la roche hôte, indiquant que le claste s'est formé avant le(s) événement(s) d'impact qui a(ont) produit les nombreuses veines présentes dans la météorite Peace River. Le clase et son hôte ont exactement les mêmes compositions minérales. Par contre, il existe des différences dans la minéralogie modale, la texture, les éléments traces et la composition des isotopes d'oxygène entre le claste et la roche hôte. Ces différences suggèrent fortement que le claste ait été formé par la fusion sous impact du matériel chondritique du groupe LL, impliquant la perte des composantes Fe-FeS et phosphatiques, la fusion a été suivie d'un refroidissement relativement rapide et de l'incorporation dans la météorite hôte Peace River. Un métamorphisme subséquent sur l'amas parent Peace River a causé la recristallisation du claste et l'homogénéisation des compositions minérales et des abondances des éléments labiles thermiques entre le claste et la roche hôte. Un métamorphisme de choc, incl...

Petrology and oxygen isotopic composition of large igneous inclusions in ordinary chondrites: Early solar system igneous processes and oxygen reservoirs

“…A shift to higher Δ 17 O from inclusion to host now appears to be part of a pattern that continues from earlier reports (Rubin et al, 1981;Prinz et al, 1984;Mayeda et al, 1987;Hutchison et al, 1988;Ruzicka et al, 1995;Bridges and Hutchison, 1997;Almeida et al, 2017), although there are counterexamples (Rubin et al, 1983a;Casanova et al, 1987;Herd et al, 2013), as in this study. For the former case of inclusions with lower Δ 17 O than their hosts, previous work has shown: (1) two low-Δ 17 O inclusions (~0.35-0.47‰, similar to MET-I2 and MET-I3 of this study) in the L6 chondrite Y-793241 (Mayeda et al, 1987); (2) three inclusions with H-like oxygen isotopic composition in the Bovedy (L3), Y-75097 (L6), and Barwell (L5) chondrites (Rubin et al, 1981;Mayeda et al, 1987;Hutchison et al, 1988;Bridges and Hutchison, 1997); and 3inclusion Bo-1 with H-like Δ 17 O (~0.7‰) but much higher δ 18 O (Fig.…”

Cavezzo—The double face of a meteorite: Mineralogy, petrography, and geochemistry of a very unusual chondrite

“…Xenoliths of one type of ordinary chondrite within an isotopically distinct ordinary chondrite host have been documented in many other meteorites (e.g., Bridges and Hutchison 1997; Herd et al. 2013; Ruzicka et al. 2019).…”

“…These data suggest that specimen 1 is derived from isotopically distinct material compared to that of specimen 2. Xenoliths of one type of ordinary chondrite within an isotopically distinct ordinary chondrite host have been documented in many other meteorites (e.g., Bridges and Hutchison 1997;Herd et al 2013;Ruzicka et al 2019). The evidence from oxygen isotope analysis suggests that specimen 1, at least in part, consists of "xenolithic" material, with specimen 2 representing the enclosing host material.…”

Young asteroid mixing revealed in ordinary chondrites: The case of NWA 5764, a polymict LL breccia with L clasts

“…Such melted xenoliths were identified in (1) Ngawi, a regolith breccia LL chondrite with a H xenolith (Fodor and Keil ); (2) Paragould, a LL chondrite with a L xenolith (Fodor and Keil ); and (3) Peace River L chondrite (Herd et al. ) and possibly Cabezo de Mayo (Casanova et al. ).…”