Metal in mesosiderites: Evidence for redox and fractional crystallization

Mesosiderites are an amalgamation of crustal silicates and molten metal, and their formational history is not well understood. It is widely believed that redox reactions occurred in the mesosiderites during metal-silicate mixing. Previous studies evaluated 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 within the metal portion of five mesosiderites is documented, most notably lower P content in the matrix metal relative to clast metal (nodule). These observations, together with the noted FeO reduction in silicates, provide further support for redox reactions occurring during metal-silicate mixing. Samples with differing Ir concentrations, such as Chaunskij and RKP A70015, have been previously classified as anomalous. However, the marked variation in highly siderophile element concentrations in all of these mesosiderites is consistent with fractional crystallization. These compositional trends 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.