Origin of metallic Fe-Ni in Renazzo and related chondrites

Abstract

The “anomalous” positive Co-Ni correlation among metal grains from the Renazzo and Al Rais chondrites reported in previous studies has two components: 1. (1) a chondrule trend that extends up to 120–145 mg/g Ni probably reflects the mixing of Fe-rich and Fe-poor nebular metal; and 2. (2) a weak extrachondrule trend that spans a Ni range from 40–70 mg/g reflects Fe introduction during mild metamorphism. To assess the formation of this metal, we determined Ni and Co zoning profiles in metallic Fe-Ni from different petrographic sites (chondrule interiors, chondrule margins, chondrule rims, and matrix) in Renazzo, Al Rais, and the related chondrite, MacAlpine Hills 87320. Metal from chondrule interiors shows flat Ni and Co concentrations and profiles, moderately large grain-to-grain compositional variations (even within chondrules), and generally high Ni and Co. Nickel concentrations extend above the kamacite stability limit; etching such “martensite” shows high-Ni domains in some cases, but observed Ni concentrations do not exceed 190 mg/g. Metal from chondrule margins adjacent to matrix shows convex (inverted U-shaped) Ni and Co zoning profiles; the highest Ni and Co concentrations are at grain centers, although the mean central Ni and Co concentrations in margin grains are much lower than those in metal from chondrule interiors. About half of the matrix metal shows flat Co and Ni profiles like those from chondrule interiors; the remainder are convex. The differences in mean Co and Ni contents between chondrules and extrachondrule materials may reflect differential sampling of nebular metal grains having compositions that varied with particle size. We interpret the low Co and Ni contents at the edges of grains in chondrule margins to reflect dilution by Fe produced by FeO reduction. The reduction seems not to have occurred in the nebula but rather during a brief period of parent-body metamorphism; reduction of fine-grained FeO produced metallic Fe that plated onto nearby metal grains. Although diffusive exchange between pentlandite and metal could produce the observed Ni gradient, it is doubtful that it could produce the similar gradient of Co. Metal from chondrule interiors was inaccessible during the reduction event and'retained the flat profiles established during chondrule formation.