Mesosiderite formation: redox from the metal perspective

Abstract

While mesosiderite formation is not well understood, it is widely believed that redox reactions occurred during metal-silicate mixing. Previous studies focused on evaluating redox reactions by studying the silicates within mesosiderites, but little attention has been given to the metal for complementary evidence of such processes. Here, the evidence for redox reactions in the metal portion of mesosiderites is evaluated. Five mesosiderites were chosen that span the range of petrologic classes (A, B, C, Anomalous) and that have experienced the lowest grade of metamorphism (subgroup 1). The metal compositions of both matrix metal and clast (nodule) metal were compared and examined for evidence of redox reactions. A deletion of P in the matrix metal relative to clast metal was observed, which along with the FeO reduction in silicates noted by previous workers, indicates that redox reactions did occur during metal-silicate mixing. However, depletions of Cu and Mo in the matrix metal cannot be explained by these reactions and require other processes, such as sulfidization, to have occurred. Metal clast compositions produce a trend that is suggestive of fractional crystallization; however, it is unlikely that the whole metallic portion sampled by these meteorites experienced fractional crystallization. It appears to be constrained to the anomalous samples measured here. This trend could be explained by isolated metallic masses that underwent fractional crystallization before mixing or by hit-and-run collisions that produced metallic masses that ranged in size.