Composition and origin of refractory-metal-rich assemblages in a Ca,Al-rich Allende inclusion

Abstract

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 (Type 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 performed. The bulk inclusion shows enrichments in refractory lithophile elements (Ca, Al, Se, REEs, etc.) and refractory metals, typical of coarse-grained CAIs from Allende. The combined study of OAs and the parent Egg 6 inclusion indicates that the major host phases of refractory metals in this inclusion, and probably in many others, are not the large isolated OAs analyzed here but numerous smaller submicroscopic refractory metal-rich alloys finely dispersed in the major minerals of the inclusion. The Egg 6 OAs have refractory metal contents qualitatively compatible with an origin by condensation. Detailed calculations, however, demonstrate that the abundances of both refractory and non-refractory metals in the OAs cannot be matched by simple condensation models. In addition, differences among OAs in absolute and relative abundances of refractory metals require formation of individual OAs under slightly different conditions. The observed distribution of refractory metals within OAs reflects redistribution accompanying sulfurization and oxidation of the primary refractory metal-rich assemblages. Platinum and Rh are primarily concentrated in NiFe, Os, Ru, and Re in tiny, micron-sized OsRu-nuggets formed by exsolution from a more homogeneous alloy. Iridium partitions between FeNi and OsRu-nuggets, while Mo is exclusively concentrated in MoS 2. 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, although condensation models predict high W. It is demonstrated that W-contents in silicates surrounding OAs are unusually high, indicating redistribution of W after incorporation of OAs in the inclusion in aggreement with laboratory simulation experiments. A model for the origin of the Fremdlinge must involve several stages: (1) Formation of refractory metal alloys by condensation, (2) incorporation into existing CAIs at elevated temperatures, and (3) oxidation and sulfurization of OAs and extensive redistribution of elements including loss of volatile W-compounds and Mo-compounds.