COMPOSITION AND ORIGIN OF REFRACTORY-METAL-RICH ASSEMBLAGES IN A CA,AL-RICH ALLENDE INCLUSION

Metal-oxide-sulfide assemblages, henceforth opaque assemblages (OA) or Fremdlinge, with high contents of refractory metals (Ir, Os, Ru, Pt, etc.) were separated from the Allende Egg 6 Ca, Al-rich inclusion (Typ e B1). Instrumental neutron activation analysis (INAA) was applied to seven OAs including Zelda, described in an earlier paper by ARMSTRONG et al. (1987). After INAA, polished sections were prepared of four OAs for petrographic and mineralogical studies. In addition, several INAA analyses of the bulk Egg 6 inclusion and mineral separates were perfo rmed. The bulk inclusion shows enrichments in refractory lithophile el ements (Ca, Al, Sc, REEs, etc.) and refractory metals, typical of coar se-grained CAIs from Allende. The combined study of OAs and the parent Egg 6 inclusion indicates that the major host phases of refractory me tals in this inclusion, and probably in many others, are not the large isolated OAs analyzed here but numerous smaller submicroscopic refrac tory metal-rich alloys finely dispersed in the major minerals of the i nclusion. The Egg 6 OAs have refractory metal contents qualitatively c ompatible with an origin by condensation. Detailed calculations, howev er, demonstrate that the abundances of both refractory and non-refract ory metals in the OAs cannot be matched by simple condensation models. In addition, differences among OAs in absolute and relative abundance s of refractory metals require formation of individual OAs under sligh tly different conditions. The observed distribution of refractory meta ls within OAs reflects redistribution accompanying sulfurization and o xidation of the primary refractory metal-rich assemblages. Platinum an d Rh are primarily concentrated in NiFe, Os, Ru, and Re in tiny, micro n-sized OsRu-nuggets formed by exsolution from a more homogeneous allo y. Iridium partitions between FeNi and OsRu-nuggets, while Mo is exclu sively concentrated in MoS2. The two major sulfide phases, pyrrhotite and pentlandite, have less than 0.04% of any of the refractory metals, except for about 0.15% Mo. Tungsten is very low in the bulk OAs, alth ough condensation models predict high W. It is demonstrated that W-con tents in silicates surrounding OAs are unusually high, indicating redi stribution of W after incorporation of OAs in the inclusion in aggreem ent with laboratory simulation experiments. A model for the origin of the Fremdlinge must involve several stages: (1) Formation of refractor y metal alloys by condensation, (2) incorporation into existing CAIs a t elevated temperatures, and (3) oxidation and sulfurization of OAs an d extensive redistribution of elements including loss of volatile W-co mpounds and Mo-compounds.