Differences in bulk Fe content and density between type I and type II ordinary chondrite chondrules: Implications for parent body heterogeneities in oxidation state and O-isotopic composition

Abstract

Type II chondrules have higher oxidation states than type I chondrules; in ordinary chondrites (OC), type II chondrules tend to be larger, richer in bulk Fe, and have higher densities than type I chondrules. Magnesian type IA chondrules tend to be richer in ¹⁶O than type II chondrules. Because the aerodynamic behavior of a particle is a function of the product of its size and density, type I and type II chondrules (or their precursors) were partly separated in the ordinary chondrite zone of the solar nebula prior to the accretion of OC parent asteroids. LL chondrites acquired a chondrule population with the highest type II/type I ratios, L chondrites acquired chondrules with an intermediate ratio, and H chondrites acquired chondrules with the lowest type II/type I ratios. This contributed to the observed differences among OC groups in oxidation state and O-isotopic composition: in going from H to L to LL, mean oxidation state increases and mean Δ¹⁷O values increase. Higher oxidation is marked by increases in the FeO contents of olivine, low-Ca pyroxene, chromite, and ilmenite; increases in the TiO₂ content of chromite; and increases in the Co content of kamacite.