REEVALUATION OF FORMATION OF METAL NODULES IN ORDINARY CHONDRITES

Compositions of four metal nodules from two ordinary chondrites, WIS91 627 (H3.7) and Jilin (H5), were determined by instrumental and radioch emical neutron activation analyses. Compared with bulk metal fractions , the metal nodules are characterized by strong and variable depletion of the refractory siderophile elements Re, Os, Ir, Ru, Pt and Rh but normal W and Mo, some fractionation of Co from Ni, and low Cu concentr ations. These characteristics are difficult to explain by shock-induce d vaporization followed by fractional condensation, a mechanism sugges ted by Widom et al. (1986). We propose formation of metal nodules duri ng metamorphism in the parent body. Refractory siderophile elements, s uch as Ir, Os, Rh, etc., are partly locked up in noble metal nuggets a nd cannot participate in kamacite formation. The occurrence of metal n odules in both equilibrated and unequilibrated ordinary chondrites sug gests that diffusion along grain boundaries was important in the devel opment of kamacite and taenite in ordinary chondrites.