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

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. La mineralogie, la petrologie et la geochimie d'un claste a texture ignee dans la meteorite chondrite Peace River L6 ont ete etudiees afin de determiner les roles des processus nebulaires, d'accretion et de metamorphisme quant a l'origine de l'amas parent. Le claste d'echelle centimetrique est gris et a grain fin; il est de plus en contact net avec la chondrite hote. Deux veines de taille inferieure a un millimetre recoupent le claste et la roche hote, indiquant que le claste s'est forme avant le(s) evenement(s) d'impact qui a(ont) produit les nombreuses veines presentes dans la meteorite Peace River. Le clase et son hote ont exactement les memes compositions minerales. Par contre, il existe des differences dans la mineralogie modale, la texture, les elements traces et la composition des isotopes d'oxygene entre le claste et la roche hote. Ces differences suggerent fortement que le claste ait ete forme par la fusion sous impact du materiel chondritique du groupe LL, impliquant la perte des composantes Fe-FeS et phosphatiques, la fusion a ete suivie d'un refroidissement relativement rapide et de l'incorporation dans la meteorite hote Peace River. Un metamorphisme subsequent sur l'amas parent Peace River a cause la recristallisation du claste et l'homogeneisation des compositions minerales et des abondances des elements labiles thermiques entre le claste et la roche hote. Un metamorphisme de choc, incluant la formation de veines sous le choc de la fusion, a eu lieu apres le metamorphisme, durant la fragmentation de l'amas parent chondrite L. Selon les resultats, la formation de l'astro'ide parent de la meteorite Peace River comprendrait l'incorporation de materiel d'autres astro'ides et le protolithe pre-metamorphique aurait ete une breche. Nous proposons donc que la meteorite Peace River soit reclassifiee en tant que breche polymictique. [Traduit par la Redaction]
Introduction The Peace River meteorite fell at approximately 0440 hours local time on March 31st, 1963, near the town of Peace River, Alberta, after a bright fireball that was observed [...]