Formation of large metal nodules in ordinary chondrites

Literature data show that many large (2-30 mm) metallic Fe-Ni nodules in or dinary chondrites are depleted in refractory siderophiles (e.g., Re, Os, Ir , Ru, Pt, and Ph) and Cu relative to fine-grained metal but are not deplete d in (normally refractory) W and Mo, common siderophiles (e.g., Ni and Co) and volatile siderophiles (e.g., Au, As, and Sb). It was recently proposed that metamorphic diffusion could form the metal nodules and that partitioni ng of siderophile elements between kamacite and taenite could account for t he nodules' unusual compositions. However, the nodules have the same compos itional trends as metal nodules and veins in shocked meteorites that clearl y formed by impact-related processes. If large metal nodules formed during metamorphism, type 6 chondrites should contain abundant large nodules. Inst ead, the mean metal grain size in H6 chondrites is 0.12+/-0.15 mm, far smal ler than any of the nodules. It seems likely that the metal nodules formed by impact vaporization, oxidation of W and Mo to form volatile oxides, conc omitant partial oxidation of Re and Os, rapid fractional condensation of re fractory siderophiles, transport of the residual vapor, condensation of Cu, S, and Se into vugs and fractures to form sulfide, and condensation of the remaining siderophiles to form metal nodules and veins. During vapor cooli ng and condensation (or during subsequent parent body annealing) W, Mo, Re, and Os oxides were reduced.