Formation of large metal nodules in ordinary chondrites

Abstract

Literature data show that many large (2–30 mm) metallic Fe‐Ni nodules in ordinary chondrites are depleted in refractory siderophiles (e.g., Re, Os, Ir, Ru, Pt, and Rh) and Cu relative to fine‐grained metal but are not depleted 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 partitioning of siderophile elements between kamacite and taenite could account for the nodules' unusual compositions. However, the nodules have the same compositional trends as metal nodules and veins in shocked meteorites that clearly formed by impact‐related processes. If large metal nodules formed during metamorphism, type 6 chondrites should contain abundant large nodules. Instead, the mean metal grain size in H6 chondrites is 0.12±0.15 mm, far smaller 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, concomitant partial oxidation of Re and Os, rapid fractional condensation of refractory 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 cooling and condensation (or during subsequent parent body annealing) W, Mo, Re, and Os oxides were reduced.