Lewis Cliff 86211 and 86498: Metal-sulfide liquid segregates from a carbonaceous chondrite impact melt

Abstract

Abstract The petrogenesis of the ungrouped iron meteorite Lewis Cliffs (LEW) 86211 and its proposed pair LEW 86498 has remained elusive in the decades since their discovery in Antarctica. Wasson (1990) and Kracher et al. (1998) noted the enrichment in the siderophile refractory elements, fine-grained texture, and high abundances of sulfides in LEW 86211 as features that are both difficult to explain and that set it apart from other iron meteorites. In this work, we investigate the pairing and formation of these two ungrouped iron meteorites using a combination of petrography, electron microprobe analyses, and secondary ion probe analyses of oxygen-three isotope of olivine. Similarities in petrographic features and phase compositions further support the initial pairing of LEW 86211 and 86498. The bulk composition of LEW 86211 (Wasson, 1990) closely resembles those of separated chondritic metallic components (e.g., Kong and Ebihara, 1997), which indicates this pairing group formed directly from this portion of a chondrite. The metal-sulfide cellular textures and mineral compositional trends are consistent with LEW 86211 and 86498 forming by rapid cooling of the FeNiS immiscible liquid of a larger chondritic impact melt unit. Previous bulk oxygen-three isotope analyses (Clayton and Mayeda, 1996) combined with the in situ oxygen-three isotope analyses from this work are consistent with LEW 86211 having a carbonaceous chondrite provenance. LEW 86211 is most similar to CR chondrites in its oxygen-three isotope signatures, but may not be from an established carbonaceous chondrite group. The silicate inclusions in LEW 86211 and 86498 record evidence of pre-impact metamorphism and later reduction related to contact with the metal-sulfide impact melt liquid. The silicate inclusions appear to have been engulfed by metal-sulfide liquids rather than part of the impact melted unit. Additionally, the size of this sulfide-dominated pairing group compared to the volume of sulfides and metal in unmelted CR chondrite suggests that these meteorites originated from a much larger carbonaceous chondrite impact melt body than has been previously recognized (e.g., Lunning et al., 2016).