Composition and mineralogy of refractory-metal-rich assemblages from a Ca,Al-rich inclusion in the Allende meteorite

Abstract

We have separated four refractory metal-rich samples (10 μg to 190 μg) from a single Ca,Al-inclusion of the Allende meteorite. Chemical analyses were performed by INAA. From the analysed samples polished sections were prepared. Mineral phases from six large Fremdlinge and the surrounding inclusion were analysed by EDS. Three of the four separated samples have variable absolute but similar relative abundances of refactory metals. The pattern is controlled by volatility. The most refractory metals W, Os, and Re are enriched relative to the less refractory metals Ir, Mo, Ru, and Pt. This pattern can be reproduced by condensation calculations. A metal alloy in equilibrium with a gas of solar composition at a temperature of 1672 K, a pressure of 10 −3 atm, and an H 2O H 2− ratio of 5 · 10 −3 would have the same abundance pattern. All six Fremdlinge have similar texture and mineralogy. They are composed of Ni-rich metal containing 2–9% Ir, surrounded by a fine-grained intergrowth of V-magnetite and FeS. The Ni-Fe metal poikilitically encloses tiny Os-particles, domains of a W- and Mo-rich solid solution (Ca(W,Mo)O 4), and V-magnetite. It is shown that the major fraction of Mo and W is now present as oxides, although both elements must originally have condensed as metals. The oxidation process requires an increase in ƒ o 2 by several orders of magnitude. Based on chemical and textural information obtained in this study the formation of these aggregates in four steps is suggested: 1. 1) Condensation of a refractory metal alloy (W, Os, Re, Ir, Mo, Ru); 2. 2) Oxidation of the refractory metal alloy, allowing formation of Ca(W,Mo)O 4; 3. 3) Condensation of Ni-rich metal and V-magnetite at high oxygen fugacity (relative to the ƒ o 2 in a solar gas); 4. 4) Reaction of the existing assemblage with S to form FeS at lower temperatures.