Composition and formation of metal nodules and veins in ordinary chondrites

Five large metal nodules, a composite sample of five small metal nodules, one troilite nodule and two metal veins from five ordinary chondrites were analysed by electron microprobe and neutron activation analysis. Metal nodules and veins in H chondrites generally consist of large single crystals of kamacite, whereas L nodules contain significant taenite. Most nodules and veins are depleted by large factors ranging up to 240 in refractory siderophiles (Re, Os, Ir, Pt). Tungsten (normally a refractory siderophile) and Au, As, and Ga (volatile siderophiles) have abundance ratios similar to those of the common siderophiles Fe, Co and Ni. We propose that the metal with extremely low refractory-element contents was produced by shock-induced vaporization of chondritic material. The refractory elements condensed near the point of vaporization and were not transported with the vapor. Because the shock-generated gas was mildly oxidizing, W formed volatile oxides. The low Ni/Fe ratio and the resulting high kamacite contents may reflect dilution by Fe from dissociated FeS. Subsequent condensation of the refractory-depleted vapor into fractures and voids formed the metal veins and nodules. (author)